环境工程概论（英文）：环境工程概论

,
?Part I Wastewater Engineering
?1 Wastewater Engineering,An Overview
Fig,1-1 Schematic diagram of a wastewater management infrastructure
1-1 Terminology
Table 1-1 Terminology commonly used in the field of wastewater engineering
1-2 Impact of Regulations on Wastewater Engineering
1-3 Health and Environmental Concerns in Wastewater
Management
Fig,1-2 Covered treatment plant facilities for the control of odor emissions
1-4 Wastewater Characteristics
Tab 1-2 Levels of wastewater treatment
Tab 1-3 Unit Operations and Processes used to Remove Constituents Found in
Wastewater
1-5 Wastewater Treatment
Fig 1-3 Facilities used for chemical treatment of odors from treatment facilities
Fig,1-4 UV lamps used for the disinfection of wastewater
1-6 Wastewater Reclamation and Reuse
1-7 Biosolids and Residuals Management
Fig,1-5 Egg-shaped digesters used for the anaerobic treatment of biosolids
2 Constituents in Wastewater
An understanding of the nature of wastewater is essential in the
design and operation of wastewater collection,treatment,and
reuse facilities,and in the engineering management of
environmental quality,
(1) the constituents found in wastewater,
(2) sampling and analytical procedures,
(3) physical characteristics,
(4) inorganic nonmetallic constituents,
(5) metallic constituents,
(6) aggregate organic constituents,
(7) individual organic constituents and compounds,and
(8) biological characteristics,
2-1 Wastewater Constituents
Constituents Found in Wastewater
Table 2-1 Common analyses used to assess
the constituents found in wastewater
Many of the physical properties and chemical and
biological characteristics are interrelated,For example,
temperature,a physical property,affects both the
amounts of gases dissolved in the wastewater and the
biological activity in the wastewater…
Constituents of Concern in Wastewater Treatment
Tab 2-2 Principal constituents of concern in wastewater
treatment
Suspended solids can lead to sludge deposition and
anaerobic conditions;
Biodegradable organics composed of proteins;
carbohydrates and fats,and are measured most commonly
in terms of BOD and COD;
Pathogens can transmite Communicable diseases;
Priority pollutants include carcinogenicity,metogenicity,
teratogenicity or high acute toxicity;
Nutrients lead to the growth of undesirable aquatic life;
Refractory organics include surfactants,phenols,and
agricultural pesticides,
2-2 Sampling and Analytical Procedures
Sampling
(1) routine operating data for accessing overall plant
performance
(2) data that can be used to document the performance of a
given treatment operation or process
(3) data that can be used to implement proposed new
programs
(4) data needed for reporting regulatory compliance,
The data collected must be,
(1),Representative,
(2).Reproducible.The data must be reproducible
by others following the same sampling and
analytical protocols,
(3),Defensible,The data must have a known
degree of accuracy and precision,
(4),Useful,The data can be used to meet the
objectives of the monitoring plan,
There are no universal procedures for sampling,
Sampling programs must be tailored individually to fit
each situation,
Quality assurance project plan (QAPP) (known
previously quality assurance/quality control,QA/QC),
(1).Sampling plan,
(2).Sample types and size,
(3).Sample labeling and chain of custody,
(4).Sampling methods,
(5).Sampling storage and preservation,
(6).Sample constituents,
(7).Analytical methods,
If the physical,chemical,and/or biological integrity of
the samples is not maintained during interim periods
between sample collection and sample analysis,a
carefully performed sampling program will become
worthless,
Prompt analysis is undoubtedly the most positive
assurance against error due to sample deterioration,
Probable errors due to deterioration of the sample
should be noted in reporting analytical data,
Methods of Analysis
The analyses used to characterize wastewater vary from
precise quantitative chemical determinations to the more
qualitative biological and physical determinations,
The quantitative methods of analysis are either gravimetric,
volumetric,or physicochemical,
Turbidimetry,colorimetry,potentiometry,polarography,
adsorption spectrometry,fluorometry,and nuclear
radiation are representative of the physicochemical
analyses,
Units of Measurement for Physical and
Chemical Parameters
kg/m3,%(by volume or by mass),pg/L,ng/L,μg/L,mg/L,
g/L,ppb,ppm,mol/L,eq/L,meq/L,
The concentration of trace constituents is usually
expressed as micrograms per liter (μg/L) or nanograms per
liter (ng/L),
For dilute systems,one liter of sample weighs
approximately one kilogram,the units of mg/L or g/m3 are
interchangeable with ppm,
"parts per billion" (ppb) and "parts per trillion"(ppt) are
used interchangeably with μg/L and ng/L
2-3 Physical Characteristics
The most important physical characteristic of
wastewater is its total solids content,which is
composed of floating matter,settleable matter,
colloidal matter,and matter in solution,Other
important physical characteristics include particle
size distribution,turbidity,color,transntittance,
temperature,conductivity,and density,specific
gravity and specific weight,Odor,sometimes
considered a physical factor,is discussed in the
following section,
Solids
In the characterization of wastewater,coarse materials are usually
removed before the sample is analyzed for solids,
Tab 2-3 Definitions of solids found in wastewater
Fig,2-1 Interrelationships of solids found in water and wastewater,
In much of the water quality literature,the solids passing through
the filter are called dissolved solids,
A filtration step is used to separate the total suspended solids (TSS)
from the total dissolved solids (TDS),Filters with nominal pore sizes
varying from 0.45 μm to about 2.0 μm have been used for the TSS test,
Fig,2-2 Size ranges of organic contaminants in wastewater and size
separation and measurement techniques used for their quantification
Particle Size Distribution
Information on particle size is of importance in assessing
the effectiveness of treatment processes (e.g.,secondary
sedimentation,effluent filtration and effluent disinfection),
The biological conversion rate of these particles is
dependent on size,
The methods used most commonly to study and quantify
the particles in wastewater are serial filtration,electronic
particle counting,and microscopic observation,
Serial Filtration
2.0 μm as specified in Standard Methods for the TSS test,
More than 20 mg/L of additional TSS would have been
measured,
Electronic Particle Size Counting
Diluted sample through a calibrated orifice, The
conductivity is correlated to the size of an equivalent
sphere,
Fig,2-5 Volume fraction of particle sizes found in the
effluent from two activated-sludge plants with clarifiers
having different side water depths
However,the particle size data for the large particles are
quite different,owing primarily to the design and operation
of the secondary clarification,
Microscopic Observation
Various types of stains can be used on a routine basis,to
qualitatively assess microscope slide,
Video camera attached to a microscope and transmitted to a
computer are dependent on the computer software,the
mean,minimum,and maximum diameter,length to width
ratio,the circumference,the surface area,the volume,and
the centroid, Particle imaging greatly reduces the time
required but the cost of the software and equipment is
often prohibitive for many small laboratories,
Turbidity
Turbidity,a measure of the light-transmitting properties of
water,is another test used to indicate the quality of waste
discharges and natural waters with respect to colloidal and
residual suspended matter,
The results of turbidity measurements are reported as
nephelometric turbidity units (NTU),
It should be noted that the presence of air bubbles in the
fluid will cause erroneous turbidity readings,In general,
there is no relationship between turbidity and the
concentration of total suspended solids in untreated
wastewater,
There is,however,a reasonable relationship between
turbidity and total suspended solids for the settled and
filtered secondary effluent from the activated sludge
process,
The conservation factors for settled secondary effluent and
for secondary effluent filtered with a granular-medium
depth filter will typically vary from 2.3 to 2.4 and 1.3 to
1.6,respectively,
However,turbidity readings at a given facility can be used
for process control,Some on line turbidity meters used to
monitor the performance of microfiltration units are
affected by the air used to clean the membranes,
Color
Condition refers to the age of the wastewater,which is
determined qualitatively by its color and odor,Fresh
wastewater is usually a brownish-gray color,sequentially
from gray to dark gray,and ultimately to black that is
often described as septic,
In most cases,the gray,dark gray,and black color of the
wastewater is due to the formation of metallic sulfides,
which form as the sulfide produced under anaerobic
conditions reacts with the metals in the wastewater,
Absorption/Transmittance
The absorbance measured using a spectrophotometer and a
fixed path length (usually 1.0 cm) is a measure of the
amount of light,of a specified wave-length,
The factors that affect the percent transmission include
selected inorganic compounds (e.g.,copper; iron,etc.),
organic compounds (e.g.,organic dyes,humic substances,
and conjugated ring compounds such as benzene and
toluene),and TSS,
Iron is considered to be the most important with respect to
UV absorbance because dissolved iron can absorb UV
light directly and because iron will adsorb onto suspended
solids,bacterial clumps and other organic compounds,
The sorbed iron can prevent the UV light from penetrating
the particle and inactivating organisms that may be
embedded within the particle,Dosage control is extremely
important when UV disinfection is to be used,
Organic constituents,are compounds with six conjugated
carbons or a five- or six-member conjugated ring,The
reduction in transmittance observed during storm events is
often ascribed to the presence of humic substances from
stormwater flows,
Fig,2-6 Transmittance measured at various wavelengths
for activated-sludge effluents and lagoon effluents
Unfiltered and filtered transmittance are mearsured in
wastewater in connection with the evaluation and design of
UV disinfection systems,
Temperature
The temperature of wastewater is commonly higher than
that of the local water supply,because of the addition of
warm water from households and industrial activities,As
the specific heat of water is much greater than that of air,
the observed wastewater temperatures are lower only
during the hottest summer months,
Effects of Temperature
The temperature of water is a very important parameter
because of its effect on chemical reactions and reaction
rates,aquatic life,and the suitability of the water for
beneficial uses,
Industrial establishments that use surface water for cooling
water purposes are particularly concerned with the
temperature of the intake water,
In addition,oxygen is less soluble in warm water than in
cold water,The the rate of biochemical reactions that
accompanies an increase in temperature,combined with
the decrease in the quantity of oxygen present in surface
waters,can often cause serious depletions in dissolved
oxygen concentrations in the summer months,
A sudden change in temperature can result in a high rate of
mortality of aquatic life,Moreover,abnormally high
temperatures can foster the growth of undesirable water
plants and wastewater fungus,
Optimum Temperatures for Biological Activity
Optimum temperatures are in the range from 25 to 35℃,
Aerobic digestion and nitrification stops at50℃, At 15℃,
methane-producing bacteria become quite inactive,and at
about 5℃,the autotrophic-nitrifying bacteria practically
cease functioning,At 2℃,even the chemoheterotrophic
bacteria dormant,
Conductivity
The conductivity increases as the concentration of ions
increases,In effect,the measured EC value is used as a
surrogate measure of TDS concentration,
2-4 Inorganic Nonmetallic Constituents
Inorganic chemical constituents of concern include
nutrients,nonmetallic constituents,metals,and gases,
Inorganic nonmetallic and metallic constituents derive from
the background levels in the water supply and from the
additions resulting from domestic use,from the addition of
highly mineralized water from private wells and
groundwater,and from industrial use,
Inorganic nonmetallic consfituents considered in this
section include pH,nitrogen,phosphorus,alkalinity,
chlorides,sulfur,other inorganic constituents,gases,and
odors,
Chlorides
Chlorides in natural water result from the leaching of
chloride-containing rocks and soils with which the water
comes in contact,and in coastal areas from saltwater
intrusion,
Human excreta contain about 6 g of chlorides per person
per day,If hardness of water is high,home regeneration
type water softeners will also add large quantities of
chlorides,
Because conventional methods of waste treatment do not
remove chloride to any significant extent,higher than usual
chloride concentrations can be taken as an indication that a
body of water is being used for waste disposal,
Alkalinity
Alkalinity in wastewater results from the presence of the
hydroxides [OH-],carbonates [CO32-],and bicarbonates
[HCO3- ] of elements such as calcium,magnesium,sodium,
potassium,and ammonia,Of these,calcium and magnesium
bicarbonates are most common,
The alkalinity in wastewater helps to resist changes in pH
caused by the addition of acids,Alkalinity is determined by
titrating against a standard acid,
In practice,alkalinity is expressed in terms of calcium
carbonate,3 meq/L of alkalinity would be expressed as 150
mg/L as CaCO3,
Nitrogen
Nitrogen is an essential building block in the synthesis of
protein, Insufficient nitrogen can necessitate the addition
of nitrogen to make the waste treatable,
Sources of Nitrogen
(1) plant and animal origin,
(2) sodium nitrate,
(3)atomspheric nitrogen,
Forms of Nitrogen
The chemistry of nitrogen is complex because of the
several oxidation states,
The most common and important forms of nitrogen are
ammonia,ammonium,nitrogen gas,nitrite ion,and
nitrate ion, The oxidation state of nitrogen in most organic
compounds is -III,
Tab 2-5 Definition of the various terms used to define
various nitrogen species
Urea readily converted to ammonium carbonate,is seldom
found in untreated municipal wastewaters,Organic
nitrogen is determined analytically using the Kjeldald
method,
Total Kjeldabl nitrogen is the total of the organic and
ammonia nitrogen,
Ammonia nitrogen exists in aqueous solution as either the
ammonium ion (NH4+) or ammonia gas (NH3),depending
on the pH of the solution,
Nitrite nitrogen,determined colorimetrically,is relatively
unstable oxidized to the nitrate form,Nitrite can be very
important in wastewater or water pollution studies because
it is extremely toxic to most fish and other aquatic species,
Nitrites present in wastewater effluents are oxidized by
chlorine and thus increase the chlorine dosage
requirements and the cost of disinfection,
The U.S,EPA primary drinking water standards limit
nitrogen to 45 mg/L as NO3-,because of its serious and
occasionally fatal effects on infants,Nitrates may vary in
concentration from 0 to 20 mg/L as N in wastewater
effluents,
Nitrogen Pathways in Nature
Fig,2-7 Generalized nitrogen cycle in the aquatic and
soil environment
Decomposition by bacteria readily changes the organic
form to ammonia,The age of wastewater is indicated by
the relative amount of ammonia that is present,
The predominance of nitrate nitrogen in wastewater
indicates that the waste has been stabilized with respect to
oxygen demand,Nitrates,however,can be used by plants
and animals to form protein,
Phosphorus
Municipal wastewaters may contain from 4 to 16 mg/L of
phosphorus as P,
The usual forms of phosphorus that are found in aqueous
solutions include the orthophosphate,polyphosphate,and
organic phosphate,
Polyphosphates undergo hydrolysis in aqueous solutions
and revert to the orthophosphate forms; however,this
hydrolysis is usually quite slow,The organically bound
phosphorus can be an important constituent of industrial
wastes and wastewater sludge,
The polyphosphates and organic phosphates must be
converted to orthophosphates using an acid digestion step,
Sulfur
The sulfate ion occurs naturally in most water supplies and
is present in wastewater as well,Sulfur is required in the
synthesis of proteins and is released in their degradation,
Hydrogen sulfide gas,which will diffuse into the
headspace above the wastewater sewers that are not
flowing full,tends to collect at the crown of the pipe,The
accumulated H2S can then be oxidized biologically to
sulfuric acid,which is corrosive to concrete sewer pipes,
This corrosive effect,known as "crown rot," can seriously
threatened the structural integrity of the sewer pipe,
Sulfates may upset the biological process if the sulfide
concentration exceeds 200 mg/L,If burned in gas engines,
the products of combustion can damage the engine,
Gases
Gases commonly found in untreated wastewater include
nitrogen (N2),oxygen (O2),carbon dioxide (CO2),
hydrogen sulfide (H2S),ammonia (NH3),and
methane(CH4),The latter three are derived from the
decomposition of the organic matter present in wastewater
and are of concern with respect to worker health and safety,
Environmental engineer must be familiar include chlorine
(Cl2) and ozone (O3),and the oxides of sulfur and
nitrogen,
It will be useful to review the ideal gas law and to consider
the solubility of gases in water and Henry's law as applied
to the gases of interest,
Solubility of Gases in Water
The actual quantity of a gas that can be present in solution
is governed by,
(1) the solubility of the gas as defined by Henry's law,
(2) the partial pressure of the gas in the atmosphere,
(3) the temperature,and
(4) the concentration of the impurities in the water (e.g.,
salinity,suspended solids,etc.),
The Ideal Gas Law
Boyle's law (volume of a gas is inversely proportional to
pressure at constant temperature)
Charles' law (volume of a gas is directly proportional to
temperature at constant pressure)
Dissolved Oxygen
Oxygen is only slightly soluble in water,Dissolved oxygen
levels tend to be more critical in the summer months,The
presence of dissolved oxygen in wastewater is desirable
because it prevents the formation of noxious odors,
Hydrogen Sulfide
Hydrogen sulfide is formed from the reduction of mineral
sulfites and sulfates, This gas is a colorless,inflammable
compound having the characteristic odor of rotten eggs,
The loss of smell can lead to a false sense of security that
is very dangerous,The blackening of wastewater and
sludge usually results from the formation of hydrogen
sulfide,Various other metallic sulfides are also formed,
Methane
Methane is a colorless,odorless,combustible hydrocarbon
of high fuel value,
Because methane is highly combustible and the expolosion
hazard is high,access ports (manholes) and sewer
junctions or junction chambers where there is an
opportunity for gas to collect should be ventilated with a
portable blower during and before the time required for
operating personnel to work in them for inspection
renewals,or repairs,
Odors
Odors in domestic wastewater usually are caused by gases
produced by the decomposition of organic matter or by
substances added to the wastewater,
Industrial wastewater may contain either odorous
compounds or compounds that produce odors during the
process of wastewater treatment,
Odors have been rated as the foremost concern of the public
relative to the implementation of wastewater treatment
facilities,
Within the past few years,the control of odors has become a
major consideration in the design and operation of
wastewater collection,treatment,and disposal facilities,
especially with respect to the public acceptance of these
facilities,
In many areas,projects have been rejected because of the
concern over the potential for odors,
Effects of Odors
Effects of odors is related primarily to the psychological
stress rather than to the harm they do to the body,
Offensive odors can cause poor appetite for food,lowered
water consumption,impaired respiration,nausea and
vomiting,and mental perturbation,and can lead to the
deterioration of personal and community pride,interfere
with human relations,discourage capital investment,lower
socioeconomic status,and deter growth,
These problems can result in a decline in market and rental
property values,tax revenues,payrolls,and sales,
Detection of Odors
One of the difficulties in developing a universal theory has
been the inadequate explanation of why compounds with
similar structures may have different odors and why
compounds with very different structures may have similar
odors,
Tab 2-7 Major categories of odorous compounds
associated with untreated wastewater
Odor Characterization and Measurement
It has been suggested that four independent factors are
required for the complete characterization of all odor,
intensity,character,hedonics,and detectability,
Odor can be measured by sensory methods,and specific
odorant concentrations can be measured by instrumental
methods,
MDTOC ----minimum detectable threshold odor
concentration
The ED50 value represents the number of times all odorous
air sample must be diluted before the average person(50
percentile) can barely detect an odor in the diluted sample,
TON---- threshold odor number
2-5 Metallic Constituents
Many of these metals are also classified as priority
pollutants,
The presence of any of these metals in excessive quantities
will interfere with many beneficial uses of the water
because of their toxicity; therefore,it is frequently
desirable to measure and control the concentrations of
these substances,
Importance of Metals
All living organisms require varying amounts (macro and
micro) of metallic elements,such as iron,chromium,
copper,zinc,and cobalt,for proper growth,but the same
metals can be toxic when present in elevated
concentrations,
Where composted sludge is applied in agricultural
applications,arsenic,cadmium,copper,lead,mercury,
molybdenum,nickel,selenium,and zinc must be
determined,
Sources of Metals
These are found particularly in metal-plating wastes and
should be removed by pretreatment at the site of the
industry rather than be mixed with the municipal
wastewater,Fluoride,a toxic anion,is found commonly in
wastewater from electronics manufacturing facilities,
Tab 2-9 Typical waste compounds produced by
commercial,industrial,and agricultural activities that
have been classified as priority pollutants
Sampling and Methods of Analysis
Metals are determined typically by flame atomic absorption,
electrothermal atomic absorption,inductively coupled
plasma,or IPC/mass spectrometry,
2-6 Aggregate Organic Constituents
The organic matter in wastewater typically consists of
proteins (40 to 60 percent),carbohydrates (25 to 50 per
cent),and oils and fats (8 to 12 percent),Because urea
decomposes rapidly it is seldom found in other than very
fresh wastewater,
In general,the analyses may be classified into those used to
measure aggregate organic matter comprising a number of
organic constituents with similar characteristics that cannot
be distinguished separately,
Measurement of Organic Content
Gross concentrations of organic matter are greater than
about 1.0 mg/L and trace concentrations are in the range
of l0 –12 to 100 mg/L,
Laboratory methods include,
(1) biochemical oxygen demand(BOD),
(2) chemical oxygen demand (COD),
(3) total organic carbon (TOC),
Complementing these laboratory tests is the theoretical
oxygen demand (ThOD),which is determined from the
chemical formula of the organic matter,
Trace organics in the range of 10–12 to 10-13 mg/L are
determined using instrumental methods including gas
chromotography and mass spectroscopy,
Biochemical Oxygen Demand (BOD)
Despite the widespread use of the BOD test,it has a
number of limitations,BOD test results are now used
(1) to determine the approximate quantity of oxygen that
will be required to biologically stabilize the organic matter
present,
(2) to determine the size of waste treatment facilities,
(3) to measure the efficiency of some treatment processes,
(4) to determine compliance with wastewater discharge
permits,
Basis for BOD Test
The term COHNS(which represents the elements carbon,
oxygen,hydrogen,nitrogen,and sulfur) represent the
organic waste and the term C5H7NO2 (first proposed by
Hoover and Porges) to represent cell tissue,
If only the oxidation of the organic carbon,The oxygen
demand is known as the ultimate carbonaceous or first-
stage BOD,and is usually denoted as UBOD,
BOD Test Procedure
To ensure that meaningful results are obtained,the samples
must be suitably diluted with a specially prepared dilution
water so that adequate nutrients and oxygen will be
available during the incubation period,Normally,several
dilutions are prepared to cover the complete range of
possible values,
Tab 10 Measurable of BOD using various dilutions of
samples
The BOD of the sample is the difference in the dissolved
oxygen concentration values,expressed in milligrams per
liter,divided by the decimal fraction of sample used,
When testing waters with low concentrations of
microorganisms,a seeded BOD test is conducted,
Fig,2-8 Procedure for settling up BOD test bottles,
(a)with unseeded dilution water and (b) with seeded
dilution water
The organisms contained in the effluent from primary
sedimentation facilities are used commonly as the seed for
the BOD test,Seed organisms can also be obtained
commercially,When the sample contains a large
population of microorganisms (e.g.,untreated wastewater),
seeding is not necessary,
Longer time periods (typically 7 days),which correspond
to work schedules,are often used,especially in small
plants where the laboratory staff is not available on the
weekends,
The 20℃ temperature used is an average value for slow-
moving streams in temperate climates and is easily
duplicated in an incubator,
After incubation,the dissolved oxygen of the sample is
measured and the BOD is calculated using the following
Eq.,
BOD,mg/L = (D1-D2)/P
When diluted water is seeded,
BOD,mg/L = [(D1-D2)-(B1-B2)]f/P
Within a 20-day period,the oxidation of the carbonaceous
organic matter is about 95 to 99 percent complete,and in
the 5-day period used for the BOD test,oxidation is from
60 to 70 percent complete,
Effect of particle size on BOD Reaction Rates
The observed BOD reaction rate coefficients are affected
significantly by the size of the particles in wastewater,
Limitations in the BOD test
The limitations of the BOD test are as followings
(1)a high concentration of active,acclimated seed bacteria
is required;
(2)pretreatment is needed when dealing with toxic wastes,
and the effects of nitrifying organisms must be reduced;
(3)only the biodegradable organics are measured;
(4)the relatively long period of time required to obtain test
results,
Of the above,perhaps the most serious limitation is that the
5-day period may or may not correspond to the point
where the soluble organic matter that is present has been
used,
Total and soluble chemical oxygen demand(COD
and SCOD)
The COD test is used to measure the oxygen equivalent of
the organic material in wastewater that can be oxidized
chemically using dichromate in an acid solution,
CnHaObNc + dCr2O72- + (8d+c)H+ → NCO 2 + [(a+8d-3c)/2]
H2O + cNH4+ + 2dCr3+
Some of the reasons for the observed difference are as
follows:(1)many organic substances which are difficult to
oxidize biologically,such as lignin,can be oxidized
chemically,(2)inorganic substances that are oxidized by the
dichromate increase the apparent organic content of the
sample,(3)certain organic substances may be toxic to the
microorganisms used in the BOD test
One of the main advantages of the COD test is that it can
be completed in about 2.5 h,To reduce the time further,a
rapid COD test that takes only about 15 min has been
developed,
The principal fractions are particulate and soluble COD,
Fractions that have been used include,
(1)readily biodegradable soluble COD,
(2)slowly biodegradable colloidal and particulate COD,
(3)nonbiodegradable soluble COD,and
(4)nonbiodegradable colloidal and particulate COD,The
readily biodegradable soluble COD is often fractionated
further into complex COD that can be fermented to volatile
fatty acids(VFAs) and short chain VFAs,Unfortunately,as
noted previous
Oil and Grease
The term oil and grease,as commonly used,includes the
fats,oils,waxes,and other related constituents found in
wastewater,
The oil and grease content of a wastewater is determined
by extraction of the waste sample with
trichlorotrifluoroethane
Other extractable substances include mineral oils,such as
kerosene and lubricating and road oils,Oil and grease are
quite similar chemically; they are compounds (esters) of
alcohol or glycerol(glycerin) with fatty acids,The
glycerides of fatty acids that are liquid at ordinary
temperatures are called oils,and those that are solids are
called grease (or fats),
If grease is not removed before discharge of treated
wastewater,it can interfere with the biological life in the
surface waters and create films,
The low solubility of fats and oils reduces their rate of
microbial degradation,Mineral acids attack them,however,
resulting in formation of glycerin and fatty acid,In the
presence of alkalis,such as sodium hydroxide,glycerin is
liberated,and alkali salts of the fatty acids are formed,
These alkali salts are known as soaps,
They are soluble in water,but in the presence of hardness
constituents,the sodium salts are changed to calcium and
magnesium salts of the fatty acids,or so-called mineral
soaps,These are insoluble and are precipitated,
Kerosene,lubricating,and road oils are derived from
petroleum and coal tars and contain essentially carbon and
hydrogen,These oils sometimes reach the sewers in
considerable volume from shops,garages,and streets,
To an even greater extent than fats,oils,and soaps,the
mineral oils tend to coat surfaces,The particles interfere
with biological action and cause maintenance problems,
Surfactants
Surfactants,or surface-active agents cause foaming in
wastewater treatment plants and in the surface waters into
which the waste effluent is discharged,Surfactants are
most commonly composed of a strongly hydrophobic
group combined with a strongly hydrophilic group,
In the United States,ionic surfactants amount to about
two-thirds of the total surfactants,
Surfactants tend to collect at the air-water interface with
the hydrophilic in the water and the hydrophobic group in
the air
During aeration of wastewater,these compounds collect on
the surface of the air bubbles and thus create a very stable
foam,
ABS was especially troublesome because it resisted
breakdown by biological means.As a result of legislation
in 1965,ABS has been replaced in detergents by lineax-
alkyl-sulfonate (LAS),which is biodegradable, So-called
"hard" synthetic detergents axe still used extensively in
many foreign countries,
Two tests are now used to determine the presence of
surfactants in water and wastewater,The MBAS
(methylene blue active substances) test is used for anionic
surfactans,Nonionic surfactants are measured using the
CTAS (cobalt thiocyanate active substances) test,
2-7 Individual Organic Compounds
Individual organic compounds are determined to assess the
presence of priority pollutants identified by the U.S,
Environmental Protection Agency (U.S,EPA) and a
number of new emerging compounds of concern,
Priority pollutants (both inorganic and organic) have been
and are continuing to be selected on the basis of their
known or suspected carcinogenicity,mutagenicity,
teratogenicity,or high acute toxicity,
Priority Pollutants
The Environmental Protection Agency has identified
approximately 129 priority pollutants in 65 classes to be
regulated by categorical discharge standards,
Two types of standard are used to control pollutant
discharges to publicly owned treatment works (POTWs),
One is prohibited discharge standards,the other is
Categorical standards
Disinfection Byproducts
It has been found that when chlorine is added to water
containing organic matter a variety of organic compounds
containing chlorine are formed,Collectively,these
compounds,along with others,are known as disinfection
byproducts (DBPs),Although general present in low
concentrations,they are of concern because many of them
are known as suspected potential human carcinogens,
Typical classes of compounds include trihalomethanes
(THMs),haloacetic acids (HAAs),trichlorophenol,and
aldehydes,
More recently,N-nitrosodimethylamine (NDMA) has been
found in the effluent from wastewater-treatment plants,
Nitrosamines are among the most powerful carcinogens
known,The U.S,EPA action limit for NDMA is 2 parts
per trillion,NDMA appears to be formed during the
chlorination process,in treated effluent,the nitrite ion can
react with hydrochloric acid,
Dimethylamine is also part of polymers used for water
treatment and for ion-exchange resins,
Considerable attention has been focused over the past five
years on the use of ultraviolet disinfection as a possible
replacement for chlorine,
2-8 Biological Characteristics
The biological characteristics of wastewater are of
fundamental importance in the control of diseases caused
by pathogenic organisms of human origin,and because of
the extensive and fundamental role played by bacteria and
other microorganisms in the decomposition and
stabilization of organic matter,both in nature and in
wastewater treatment plants,
Microorganisms Found in Surface Waters and
Wastewater
Organisms found in surface water and wastewater include
bacteria,fungi,algae,protozoa,plants and animals,and
viruses,
Living single-cell microorganisms that can only be seen
with a microscope are responsible for the activity in
biological wastewater treatment,
The prokaryotes have the simplest cell structure and
include bacteria,blue green algae (cyanobacter),and
archaea,
Many archaea are bacteria that can grow under extreme
conditions of temperature and salinity,and also include
methanogethc methane-producing bacteria,important in
anaerobic treatment processes,
The absence of a nuclear membrane to contain the cell
DNA is also a distinguishing feature of the prokaryota
organisms,
Viruses are obligate intracellular parasites that require the
machinery of a host cell to support their growth,Although
viruses contain the genetic information (either DNA or
RNA) needed to replicate themselves,they are unable to
reproduce outside of a host cell,Viruses are classified
separately according to the host infected,Bacteriophage,as
the name implies,are viruses that infect bacteria,
3 Analysis and Selection of Wastewater Flowrates and
Constituent Loadings
Reliable data for existing and projected flowrates affect the
hydraulic characteristics,sizing,and operational
considerations of the treatment system components,
Constituent mass 1oading,the product of constituent
concentration and flowrate,is necessary to determine
capacity and operational characteristics of the treatment
facilities and ancillary equipment to ensure that treatment
objectives are met,
3-1 Components of Wastewater Flows
1,Domestic (also called sanitary) wastewater,
2,Industrial wastewater,
3,Infiltration/inflow (I/I)
4,Stormwater,Infiltration is extraneous water that enters
the collection system through leaking joints,cracks and
breaks,or porous walls,
Wastewater flows in sanitary collection systems consist of
three major components,
(1) domestic wastewater,
(2) industrial wastewater,and
(3) infiltration/inflow,
3-2 Wastewater Sources and Flowrates
Domestic Wastewater Sources and Flowrates
The principal sources of domestic wastewater in a
community are the residential areas and commercial
districts,Other important sources include institutional and
recreational facilities,
For areas now served with collection systems,wastewater
flowrates are commonly determined from existing records
or by direct field measurements,
For new developments,wastewater flowrates are derived
from an analysis of population data and estimates of per
capita wastewater flowrates from similar communities,
on the average about 60 to 90 percent of the per capita
water consumption becomes wastewater,
The higher percentages(90%) apply to the northern states
during cold weather; the lower percentages(60%) are
applicable to the semiarid region of the southwestern
United States where landscape irrigation is used
extensively,
Residential Areas
Wastewater flowrates can vary depending on
various situations such as economic,social,and
other characteristics of the community,
Tab 3-1 Typical wastewater flowrates from
urban residential sources in the U.S,
Reduced household water use changes not only
the quantity of wastewater generated but also the
characteristics of wastewater as well,
Commercial Districts
Depending on the function and activity,unit flowrates for
commercial facilities can vary widely,Because of the wide
variations that have been observed,every effort should be
made to obtain records from actual or similar facilities,
Unit----passenger,bedroom,vehicle serviced,employee,
seat,guest,
Flowrates were generally expressed in terms of quantity of
flow per unit area,
Institutional Facilities
Tab 3-2 Typical wastewater flowrates from institutional
sources in the U.S,
Recreational Facilities
Wastewater flowrates from many recreational facilities are
highly subject to seasonal variations,
Tab 3-3 Typical wastewater flowrates from recreational
facilities in the U.S,
Strategies for Reducing Interior Water Use and
Wastewater Flowrates
Tab 3-4 Typical rates of water use for various devices
and appliances in the U.S,
Tab 3-5 Flow-reduction devices and appliances in the
U.S,
Water Use in Developing Countries
Water use and,consequently,wastewater-generation rates
in developing countries,however,are significantly lower,
In some cases,the water supply is only available for
limited periods of the day,
Sources and Rates of Industrial (Nondomestic)
Wastewater Flows
Extremely high peak flowrates may be reduced by the use
of onsite detention tanks and equalization basins,
Typical design values are 7.5 to 14 m3/ha·d for light
industrial developments and 14 to 28 m3/ha·d for medium
industrial developments,For industries without internal
water recycling or reuse programs,it can be assumed that
about 85 to 95 percent of the water used in the various
operations and processes will become wastewater,
Average domestic (sanitary) wastewater contributed from
industrial facilities may vary from 30 to 95 L/capita·d,
Infiltration/Inflow
Fig,3-1 Graphic identification of infiltration/inflow
Infiltration,Water entering a collection system
from a variety of entry points including service
connections and from the ground through such
means as defective pipes,pipe joints,connections,
or access port (manhole) walls,
Steady inflow,Water discharged from cellar and
foundation drains,cooling-water discharges,and
drains from springs and swampy areas,This type
of inflow is steady and is identified and measured
along with infiltration,
Direct inflow,Those types of inflow that
have a direct stormwater runoff' connection
to the sanitary collection system and cause
an almost immediate increase in wastewater
flowrates,Possible sources are roof leaders,
yard and areaway drains,access port covers,
cross connections from storm drains and
catch basins,and combined systems,
Infiltration into Collection Systems
The proportion of the rainfall that percolates into the
ground depends on the character of the surface and soil
formation and on the rate and distribution of the
precipitation,Any reduction in permeability,such as that
due to buildings,pavements,or frost,decreases the
opportunity for precipitation to become groundwater and
increases the surface runoff correspondingly,
The presence of high groundwater results in leakage into
the collection systems and in an increase in the quantity of
wastewater and the expense of disposing of it,
Infiltration/inflow is a variable part of the wastewater,
depending on the quality of the material and workmanship
in constructing the collection systems and building
connections,the character of the maintenance,and the
elevation of the groundwater compared with that of the
collection system,
The rate and quantity of infiltration depend on the length of
the collection system,the area served,the soil and
topographic conditions,and,to a certain extent,the
population density (which affects the number and total
length of house connections),
The use of high quality pipe with dense
walls,pre-cast access port sections,and
joints sealed with rubber or synthetic
gaskets is standard practice in modern
collection-system design to reduce
infiltration,
Exfiltration from Collection Systems
Collection systems that have high infiltration rates
and are in need of rehabilitation also may exhibit
high exfiltration,
If the piping and joints are in poor condition,
significant quantities of wastewater may seep into
the ground,travel through the gravel bedding of
the piping system,or even surface in extreme
cases,
Combined System Flowrates
During a rainfall event,the amount of storm
flow is normally much larger than the dry-
weather wastewater flow,and the observed
flows during wet weather can mask
completely the dry weather flow patterns,
When the collection system capacity is
exceeded,a portion of the flow may be
discharged directly into a receiving body
through overflows(maybe intentionally
most of times),or routed to a special
combined sewer overflow (CSO) treatment
facility,
In some cases where the combined system
is undersized,flooding or surcharging may
occur at various upstream locations within
the system,
In contrast,the hydrograph at the treatment
plant shows less distinct flow peaks and a
lag time of several hours for flows to return
to normal dry-weather levels following
rainfall cessation,
The peak flowrates and accompanying
mass loadings,however,must be accounted
for in the hydraulic design of the treatment
plant and in the selection of appropriate unit
operations and processes,
3-3 Analysis of Wastewater Flowrate Data
In cases where only flowrate data in the collection system
is available,it must be recognized that the flowrates may
differ somewhat from the flowrate entering the treatment
plant because of the flow-dampening effect of the sewer
system,
Peak hourly flowrates may also be attenuated by the
available storage capacity in the sewer system,
Definition of Terms
Tab 3-6 Terminology used to quantify observed
variations in flowrate and constituent concentrations
Wastewater flowrates vary during the time of day,day of
the week,season of the year,or depending upon the nature
of the dischargers to the collection system,
Short-Term Variations
Fig,3-3 Typical hourly variations in domestic wastewater
flowrates
Minimum flows occur during the early morning hours
when water consumption is lowest and when the base flow
consists of infiltration and small quantities of sanitary
wastewater,
A second peak flow generally occurs in the early evening
between 7 and 9 P.M,The time of occurrence and the
amplitude of the peak flowrates vary with the size of the
community and the length of the collection system,
As the community size increases,the variations between
the high and low flows decrease due to (1) the increased
storage in the collection system of large communities that
tends to equalize flowrates and (2) changes in the
economic and social makeup of the community,When
extraneous flows are minimal,wastewater discharge
curves resemble water consumption curves,but with a lag
of several hours,
Seasonal Variations
Seasonal variations in domestic wastewater flows are
commonly observed at resort areas,in small communities
with college campuses,and in communities that have
seasonal commercial and industrial activities,
Industrial Variations
Industrial wastewater discharges are difficult to predict,
Many manufacturing facilities generate relatively constant
flowrates during production,but the flowrates change
markedly during cleanup and shutdown,
Industrial discharges are most troublesome in smaller
wastewater-treatment plants where there is limited capacity
to absorb shock loadings,
Wastewater Flowrate Factors
Peaking factors can be developed based on
maximum hour,maximum day,maximum
month,or other time periods,and are applied
most frequently to determine the peak
hourly flowrate,
The most common method of determining
the peaking factor is from the analysis of
flowrate data,
3-5 Analysis of Constituent Mass Loading Data
From the standpoint of treatment processes,one of the
most serious deficiencies results when the design of a
treatment plant is based on average flowrates and average
BOD and TSS loadings,with little or no recognition of
peak conditions,
In many communities,peak influent flowrates and BOD
and TSS loadings can reach two or more times average
values,it must also be emphasized that,in nearly all cases,
peak flowrates and BOD and TSS mass-loading rates do
not occur at the same time,
Wastewater Constituent Concentrations
Composite samples made up of portions of samples collected
at regular intervals during a day are used,
Quantity of Waste Discharged by Individuals in the
United States,
Tab,3-7 Quantity of waste discharged by individual on a
dry weight basis
If one or more members of a family are ill and shedding
pathogens,the number of measured organisms can increase by
several orders of magnitude,
Composition of Wastewater in Collection Systems
Tab,3-8 Typical composition of untreated domestic
wastewater
Because there is no "typical" wastewater,it must be
emphasized that the typical data presented in Table 3-8
should only be used as a guide,
Mineral Increase Resulting from Water Use
Tab,3-9 Typical mineral increase from domestic water
use
Increases in the mineral content of wastewater may be due
in part to addition of highly mineralized water from private
wells and groundwater and from industrial use,Domestic
and industrial water softeners also contribute significantly
to the increase in mineral content,
Variations in Constituent Concentrations
Short-Term Variation in Constituent Values
Fig,3-4 Typical hourly variations in flow and strength of
domestic wastewater
The BOD variation generally follows the flow,The peak
BOD (organic matter) concentration often occurs in the
evening,
Seasonal Variation in Constituent Values,
Variations in Industrial Wastewater
In some cases,flow values and water quality measurements
may vary by several orders of magnitude over a period of a
year,
For example,the BOD and TSS concentrations contributed
from vegetable-processing facilities during the noon wash-
up period may far exceed those contributed during working
hours,Problems with high short-term loadings most
commonly occur in small treatment plants that have
limited reserve capacity to handle "shock loadings." The
seasonal impact of industrial wastes such as canneries can
cause both the flow and BOD loadings to increase from
two to five times average conditions,
If industrial wastes are to be discharged to the collection
system for treatment in a municipal wastewater facility,it
will be necessary to characterize the wastes adequately to
identify the ranges in constituent concentrations and mass
loadings,Such characterization is also needed to determine
if pretreatment is required before the waste is permitted to
be discharged into the collection system,
With sufficient characterization of the wastewater from
industrial discharges,suitable pretreatment facilities can be
provided and plant upsets can be avoided,
Variations in Constituent Values in Combined
Collection Systems,
Tab,3-10 Typical factors influencing the characteristics
of combined wastewater
Fig,3-5 Typical variations of flowrate,BOD,TSS,and
fecal coliform in a combined collection system during a
storm event
As shown,the BOD and fecal coliform bacteria
concentrations are low during the storm when runoff flows
are high,After the storm,when runoff subsides and the
flow consists primarily of wastewater,concentrations rise
significantly,
Unlike BOD and fecal coliform bacteria,TSS
concentrations rise slightly during the storm,and remain
unchanged after the storm,
The slight rise in the TSS concentration during the peak
flow may be due to a phenomenon common to many
combined sewer systems known as the "first flush”,
Much of the accumulated surface contaminants are washed
into the combined system,In combined collection systems,
the increased flows may be capable of resuspending
material deposited previously during low-flow periods,
Factors known to contribute to the magnitude and
frequency of the first-flush effect include combined sewer
slopes; street and catch basin cleaning frequency and
design; rainfall intensity and duration; and surface buildup
of debris and contaminants,
Wastewater from combined collection systems usually
contains more inorganic matter than wastewater from
sanitary collection systems because of the larger quantities
of storm drainage that enter the combined sewer system,
Effect of Mass Loading Variability on Treatment
Plant Performance
Fig,3-6 Illustration of diurnal Wastewater flow,BOD and
Mass loading variability
The variations are more pronounced in small collection
systems where the collection system storage capacity does
not provide a significant dampening effect,The impact of
these load variations is seen most dramatically in the
effects on biological treatment operating conditions,
Fig,3-7 Example variations of TSS and BOD
concentrations and mass loadings over a monthly period
3-6 Selection of Design Flowrates and Mass Loadings
The rated capacity of wastewater-treatment plants is
normally based on the average annual daily flowrate at the
design year plus an allowance for future growth,
Conditions that must be considered include peak and
minimum hydraulic flowrates and the maximum,minimum
and sustained process constituent mass loading rates,
Additionally,periods of initial operation and low flows
and loads must be taken into consideration in design,
Tab,3-11 Typical flowrates and mass loading factors
used for the design and operation of wastewater-
treatment facilities
The overall objective of wastewater treatment is to provide
a wastewater-treatment system that is capable of coping
with a wide range of probable wastewater conditions while
complying with the overall performance requirements,
Design Flowrates
Flowrates need to be developed both for the initial period
of operation and for the future (design) period,
Consideration of the flowrates during the early years of
operation is often overlooked,and oversizing of equipment
and inefficient operation can result,
Rationale for the Selection of Flowrates
The process units and hydraulic conduits must be sized to
accommodate the anticipated peak flowrates, Provisions
have to be made to ensure bypassing of wastewater does
not occur either in the collection system or at the treatment
plant,
Forecasting Flowrates
A yardstick by which total dry-weather base flow can be
measured is 460 L/capita·d,established by the U.S,EPA as
a historical average where infiltration is not excessive,
Minimum Flowrate
In cases where very low nighttime flow is expected,
provisions for recycling treated effluent may have to be
included to sustain the process,
In the absence of measured flowrate data,minimum daily
flowrates may be assumed to range from 30 to 70 percent
of average flowrates for medium- to large size
communities,respectively,
Peak Flowrate Factors
Tab,3-11 Typical flowrates and mass loading factors
used for the design and operation of wastewater-
treatment facilities
Peak hourly flowrates are used to size the hydraulic
conveyance system and other facilities such as
sedimentation tanks and chlorine contact tanks where little
volume is available for flow dampening,
Other peaking factors such as maximum week or maximum
month may be used for treatment facilities such as pond
systems that have long detention times or for sizing solids
and biosolids processing facilities that also have long
detention times or ample storage,
4 Introduction to Process Analysis and Selection
Treatment methods in which the application of physical
forces predominate are known as physical unit operations,
Examples of physical unit operations include screening,
mixing,sedimentation,gas transfer,filtration,and
adsorption,Treatment methods in which the removal or
conversion of constituents is brought about by the addition
of chemicals or by other chemical reactions are known as
chemical unit processes,
Examples of chemical unit processes include disinfection,
oxidation,and precipitation,Treatment methods in which
the removal of constituents is brought about by biological
activity are known as biological unit processes,
Unit operations and processes occur in a variety of
combinations in treatment flow diagrams,
The rate at which reactions and conversions occur,and the
degree of their completion,is generally a function of the
constituents involved,the temperature,and the type of
reactor (i.e.,container or tank in which the reactions take
place),
The fundamental basis for the analysis of the physical,
chemical,and biological unit operations and processes
used for wastewater treatment is the materials mass
balance principle in which an accounting of mass is made
before and after reactions and conversions have taken
place,
Fig,4-1 Overview of a biological nutrient removal(BNR)
wastewater-treatment plant
4-1 Reactors used for the Treatment of Wastewater
Types of Reactors
Fig,4-2 Definition sketch for various types of reactors
used for wastewater treatment
The principal types of reactors used for the treatment of
wastewater,are
(1) the batch reactor,
(2) the complete-mix reactor (also known as the continuous-
flow stirred-tank reactor (CFSTR) in the chemical
engineering literature),
(3) the plug-flow reactor (also known as a tubular-flow
reactor),
(4) complete-mix reactors in series,
(5) the packed-bed reactor,
(6) the fluidized-bed reactor,
Batch Reactor
In the batch reactor,flow is neither entering nor leaving the
reactor (i.e,flow enters,is treated,and then is discharged,
and the cycle repeats),
The BOD test is carried out in a batch reactor that are
often used to blend chemicals or to dilute concentrated
chemicals,
Complete-Mix Reactor
Fluid particles leave the reactor in proportion to
their statistical population,Complete mixing can
be accomplished in round or square reactors if the
contents of the reactor are uniformly and
continuously redistributed,The actual time
required to achieve completely mixed conditions
will depend on the reactor geometry and the power
input,
Plug-Flow Reactor
Fluid particles pass through the reactor with little
or no longitudinal mixing and exit from the reactor
in the same sequence in which they entered,
The particles retain their identity and remain in the
reactor for a time equal to the theoretical detention
time,
Complete-Mix Reactors in Series
The series of complete-mix reactors is used to
model the flow regime that exists between the
ideal hydraulic flow patterns corresponding to the
complete-mix and plug-flow reactors,If the series
is composed of one reactor,the complete-mix
regime prevails,If the series consists of an infinite
number of reactors in series,the plug-flow regime
prevails,
Packed-Bed Reactors
Dosing can be continuous or intermittent (e.g.,trickling
filter),The packing material in packed-bed reactors can be
continuous or arranged in multiple stages,with flow from
one stage to another,
Fluidized-Bed Reactor
The fluidized-bed reactor is similar to the packed-bed
reactor in many respects,but the packing material is
expanded by the upward movement of fluid (air or water)
through the bed,The expanded porosity of the fluidized-
bed packing material can be varied by controlling the
flowrate of the fluid,
Application of Reactors
Tab,4-1 Principal applications of reactor types used for
wastewater treatment
Operational factors that must be considered in the selection
of the type of reactor or reactors to be used in the treatment
process include
(1) the nature of the wastewater to be treated,
(2)the nature of the reaction (i.e.,homogeneous or
heterogeneous),
(3) the reaction kinetics governing the treatment process,
(4) the process performance requirements,
(5)local environmental conditions,In practice,the
construction costs and operation and maintenance costs
also affect reactor selection,
Ideal Flow in Complete-Mix and Plug-Flow
Reactors
The ideal hydraulic flow characteristics of complete-mix
and plug-flow reactors are illustrated on Fig,4-3 in which
dye tracer response craves are presented for pulse (slug-
dose) and step inputs (continuous injection),
Fig,4-3 Output tracer response curves from reactors
subject to pulse and step inputs of a tracer
Nonideal Flow in Complete-Mix and Plug-Flow
Reactors,
In practice the flow in complete-mix and plug-
flow reactors is seldom ideal,
It is the precautions taken to minimize these
effects that are important,
4-2 Mass-balance Analysis
The fundamental approach used to study the hydraulic flow
characteristics of reactors and to delineate the changes that
take place when a reaction is occurring in a reactor (e.g.,a
container),or in some definable portion of a body of liquid,
is the mass-balance analysis,
The Mass-Balance Principle
The mass-balance analysis is based on the principle that
mass is neither created nor destroyed,but the form of the
mass can be altered (e.g.,liquid to a gas),The mass-
balance analysis affords a convenient way of defining what
occurs within treatment reactors as a function of time,
Fig,4-4 Definition sketch for the application of materials
mass-balance analysis for a complete-mix reactor with
inflow and outflow,The presence of a mixer is used to
represent symbolically the fact the contents of the reactor
are mixed completely,The photo is of a typical complete-
mix activated sludge reactor used for the biological
treatment of wastewater,
The system boundary is drawn to identify all of the liquid
and constituent flows into and out of the system
For a given reactant,the general mass-balance analysis is
given by
1,General word statement
2,The corresponding simplified word statement
A positive sign is used for the rate-of-generation term
because the necessary sign for the operative process is past
of the rate expression (e.g.,rc = -kC for a decrease in the
reactant or rc = + kC for all increase in the reactant),
Preparation of Mass Balances
1,Prepare a simplified schematic or flow diagram of the
system or process
2,Draw a system or control volume boundary to define the
limits, Proper selection of the system or control volume
boundary is extremely important because,in many
situations,it may be possible to simplify the mass-balance
computations,
3,List all of the pertinent data and assumptions
4,List all of the rate expressions for the biological or
chemical reactions
5,Select a convenient basis
It is recommended that the above steps be followed routinely,
Application of the Mass-Balance Analysis
To apply a mass-balance analysis to the liquid contents of
the reactor shown on Fig,4-4,it will be assumed that,
1,The volumetric flowrate into and out of the control volume
is constant,
2,The liquid within the control volume is not subject to
evaporation (constant volume),
3,The liquid within the control volume is mixed completely,
4,A chemical reaction involving a reactant A is occurring
within the reactor,
5,The rate of change in the concentration of the reactant A
that is occurring within the control volume is governed by
a first-order reaction (rc = -kC ),
Using the above assumptions,the mass balance
can be formulated as follows
1,Simplified word statement
2,Symbolic representation
Before attempting to solve any mass-balance
expression,a unit check should always be made to
assure that units of the individual quantities are
consistent,
Steady-State Simplification
Fortunately,in most applications in the field of wastewater
treatment,the solution of mass-balance equations,such as
the one given by the equations,can be simplified by noting
that the steady-state(i.e.,long-term) concentration is of
principal concern,If it is assumed that only the steady-state
effluent concentration is desired,then above equation can
be simplified by noting that,under steady-state conditions,
the rate accumulation is zero (dC/dt = 0),
4-3 Analysis of Nonideal Flow in Reactors Using
Tracers
Because of a lack of appreciation for the
hydraulics of reactors,many of the treatment
plants that have been built do not perform
hydraulically as designed,
Fig,4-5 Definition sketch for short circuiting caused by (a)density currents caused by temperature differences; (b)wind circulation patterns;
(c)inadequate mixing; (d)fluid advection(平流 ) and dispersion
Factors leading to nonideal flow in reactors include,
1,Temperature differences,In complete-mix and plug-flow
reactors,nonideal flow (short circuiting) can be caused by
density currents due to temperature differences,When the
water entering the reactor is colder or warmer than the
water in the tank,a portion of the water can travel to the
outlet along the bottom of or across the top of the reactor
without mixing completely (see Fig,4-5a),
2,Wind-driven circulation patterns,In shallow reactors,
wind-circulation patterns can be set up that will transport a
portion of the incoming water to the outlet in a fraction of
the actual detention time (see Fig,4-5b),
3,Inadequate mixing,Without sufficient energy input,
portions of the reactor contents may not mix with the
incoming water (see Fig,4-5c),
4,Poor design,Depending on the design of the inlet and
outlet of the reactor relative to the reactor aspect ratio,
dead zones may develop within the reactor that will not
mix with the incoming water (see Fig,4-5d),
5,Axial dispersion in plug-flow reactors,In plug-flow
reactors the forward movement of the tracer is due to
advection and dispersion,Advection is the term used to
describe the movement of dissolved or colloidal material
with the current velocity,Dispersion is the term used to
describe the axial and longitudinal transport of material
brought about by velocity differences,turbulent eddies,
and molecular diffusion,
Need for Tracer Analysis
The use of dyes and tracers for measuring
the residence time distribution curves is one
of the simplest and most successful methods
now used to assess the hydraulic
performance of full-scale reactors,
Important applications of tracer studies
include
(1) the assessment of short circuiting in
sedimentation tanks and biological reactors,
(2) the assessment of the contact time in
chlorine contact basins,
(3) the assessment of the hydraulic approach
conditions in UV reactors,and
(4) the assessment of flow patterns in
constructed wetlands and other natural
treatment systems,
Tracer studies are also of critical
importance in assessing the degree of
success that has been achieved with
corrective measures,
Types of Tracers
Over the years,a number of tracers have
been used to evaluate the hydraulic
performance of reactors,Important
characteristics for a tracer include,
(1)The tracer should not affect the flow
(should have essentially the same density
as water when diluted),
(2)The tracer must be conservative so that
a mass balance can be performed,
(3)It must be possible to inject the tracer
over a short time period,
(4)The tracer should be able to be analyzed
conveniently,
(5)The molecular diffusivity of the tracer
should be low,
(6)The tracer should not be absorbed on or
react with the exposed reactor surfaces,
(7)The tracer should not be absorbed on or
react with the particles in wastewater,
Dyes and chemicals that have been used
successfully in tracer studies include congo-
red,fluorescein,fluorosilicic acid (H2SiF6),
hexafluoride gas (SF6),lithium chloride
(LiCl),Pontacyl Brilliant Pink B,potassium,
potassium permanganate,rhodamine WT,
and sodium chloride (NaCl),Pontacyl
Brilliant Pink B (the acid form of
rhodamine WT) is especially useful in the
conduct of dispersion studies because it is
not readily adsorbed onto surfaces,
? Because fluorescein,rhodamine WT,and Pontacyl
Brilliant Pink B can be detected at very low
concentrations using a fluorometer,they are the
dye tracers used most commonly in the evaluation
of wastewater-treatment facilities,Lithium
chloride is commonly used for the study of natural
systems,Sodium chloride,used commonly in the
past,has a tendency to form density currents
unless mixed,Hexafluoride gas (SF6) is used most
commonly for tracing the movement of
groundwater,
Conduct of Tracer Tests
Fig 4-6 Schematic of setup used to control tracer studies of plug-flow
reactors
(a)slug of tracers added to flow; (b)continuous input of tracer added to
flow,Tracer response curve is measured continuously,
Analysis of Tracer Response Curves
Fig,4-8 Typical tracer response curves,two different types of circular
clarifiers and open channel UV disinfection system
4-4 Reactions,Reaction Rates,and Reaction Rate
Coefficients
The stoichiometry of reaction refers to the definition of the
quantities of chemical compounds involved in a reaction,
The rate expressions discussed in this section will be
integrated with the hydraulic characteristics of the reactors,
discussed previously,to define treatment kinetics,
Types of Reactions
The two principal types of reactions that occur in
wastewater treatment are classified as homogeneous and
heterogeneous (non-homogeneous),
Homogeneous Reactions
In homogeneous reactions,the reactants
are distributed uniformly throughout the
fluid so that the potential for reaction at
any point within the fluid is the same,
Homogeneous reactions are usually
carried out in the batch,complete-mix,
and plug-flow reactors,
Examples of irreversible reactions are
a,Simple reactions
A——>B
A + A——> C
aA + bB ——> C
b,Parallel reactions
A + B ——> C
A + B ——>D
c,Consecutive reactions
A + B ——> C
A + C——> D
Examples of reversible reactions are
A <——> B
A + B <——> C + D
Heterogeneous Reactions
Heterogeneous reactions occur between one
or more constituents that can be identified
with specific sites,such as those on an ion
exchange resin,
These reactions are more difficult to study
because a number of interrelated steps may
be involved,The typical sequence of these
steps is as follows,
1,Transport of reactants from the bulk fluid to
the fluid-solid interface (external surface of
catalyst particle)
2,Intraparticle transport of reactants into the
catalyst particle (if it is porous)
3,Adsorption of reactants at interior sites of
the catalyst particle
4,Chemical reaction of adsorbed reactants to
adsorbed products (surface reaction)
5,Desorption of adsorbed products
6,Transport of products from the interior sites
to the outer surface of the catalyst particle
Rate Expressions Used in Environmental Modeling
Tab,4-2 Constituent transformation and removal
processes(i.e.,fate processes) in the environment
Process
Comments
Constituents
affected
Adsorption/
desorption
Many chemical constituents tend to attach or sorb onto solids,The
implication for wastewater
discharges is that a substantial
fraction of some toxic chemicals
is associated with the suspended
solids in the effluent,Adsorption
combined with solids settling
results in the removal from the
water column of constituents that
might not otherwise decay
Metal,trace organics,NH
4+,
PO43-
Algal synthesis
The synthesis of algal cell tissue using the
nutrients found in wastewater
NH4+,NO3-,PO43-,pH,etc
Bacterial
conversion
Bacterial conversion (both aerobic and anaerobic) is the most important process
in the transformation of constituents
released to the environment,The
exertion of BOD and NOD is the most
common example of bacterial conversion
encountered in waterquality
management,The depletion of oxygen in
the aerobic conversion of organic wastes
is also known as deoxygenation,Solids
discharged with treated wastewater are
partly organic,Upon settling to the
bottom,they decompose bacterially
either anaerobically or aerobically,
depending on local conditions,The
bacterial transformation of toxic organic
compounds is also of great significance,
BOD5,nitrification,denitrification,
sulfate reduction,
anaerobic
fermentation (in
bottom sediments),
conversion of
priority organic
pollutants,etc,
Chemical
reactions
Important chemical reactions that occur in the environment include hydrolysis,
photochemical,and oxidation-reduction
reactions,Hydrolysis reactions occur
between contaminants and water,
Chemical disinfection,
decomposition of
organic compounds,
specific ion exchange,
element substitution,
Filtration
Removal of suspended and colloidal
solids by straining (mechanical and
chance contact),sedimentation,
interception,impaction,and adsorption,
TSS,colloidal particles
Flocculation
Flocculation is the term used to describe
the aggregation of smaller particles into
larger particles that can be removed by
sedimentation and filtration,
Flocculation is brought about by
Brownian motion,differential velocity
gradients,and differential settling in
which large particles overtake smaller
particles and form larger particles,
Colloidal and small particles
Gas
absorption/desor
ption
The process whereby a gas is taken up by a liquid is known as absorption,For example,
when the dissolved oxygen concentration in
a body of water with a free surface is below
the saturation concentration in the water,a
net transfer of oxygen occurs from the
atmosphere to the water,The rate of
transfer (mass per unit time per unit
surface area) is proportional to the amount
by which the dissolved oxygen is below
saturation,The addition of oxygen to water
is also known as reaeration,Desorption
occurs when the concentration of the gas in
the liquid exceeds the saturation value,
and there is a transfer from the liquid to
the atmosphere,
O2,CO2,CH4,NH3,H2S
4-5 Treatment Processes Involving Mass Transfer
Unlike mechanical separations,the driving force for the
transfer of material is a pressure or concentration gradient),
The separation operations are sometimes identified as
equilibrium phase separations or equilibrium contact
separations,because the transfer of a component will cease
when equilibrium conditions prevail,
Tab,4-4 Principal applications of mass transfer
operations and processes in wastewater treatment
The most important mass transfer operations in
wastewater treatment involve
(1) the transfer of material across gas-liquid
interfaces as in aeration and in the removal of
unwanted gaseous constituents found in
wastewater by air stripping,
(2) the removal of unwanted constituents from
wastewater by adsorption onto solid surfaces such
as activated carbon and ion exchange,
Gas-Liquid Mass Transfer
The simplest and most commonly used is the two-film
theory proposed by Lewis and Whitman (1924),
The two-film theory remains popular because,in more than
95 percent of the situations encountered,the results
obtained are essentially the same as those obtained with the
more complex theories,
The Two-Film Theory
The two-film theory is based on a physical model in which
two films exist at the gas-liquid interface,as shown on Fig,
4-9,
Fig,4-9 Definition sketch for the two-film theory of gas transfer,
(a)absorption;(b)desorption
(a) "absorption," in which a gas is transferred from the gas
phase to the liquid phase,
(b) "desorption," in which a gas is transferred out of the liquid
phase into the gas phase,The two films,one liquid and one
gas,provide the resistance to the passage of gas molecules
between the bulk-liquid and the bulk-gaseous phases,
It is assumed that the concentration and partial pressure in
both the bulk liquid and bulk-gas phase are uniform (i.e.,
mixed completely)
Using Fick's first law,the mass flux for each phase for
absorption (gas addition) is written as follows,
r= kG(PG-Pi)=kL(Ci-CL)
where r=rate of mass transferred per unit area per unit time
kG =gas film mass transfer coefficient
PG =partial pressure of constituent A in the bulk of the gas
phase
Pi =partial pressure of constituent A at the interface in
equilibrium with concentration Ci of constituent A in liquid
It should be noted that the gas and liquid film mass transfer
coefficients depend on the conditions at the interface,The
terms (PG - Pi) and (Ci-CL) represent the driving force
causing transfer in the gas and liquid phase,respectively,
Thus,the degree of mass transport can be enhanced by
reducing the thickness of the film,depending on which is
the controlling film,
If it is assumed that all of the resistance to mass transfer is
caused by the liquid film,then the rate of mass transfer can
be defined as follows in terms of the overall liquid mass
transfer coefficient,
r= KL(CS-CL)
Absorption of Gases
Fig,4-10 Definition sketch for the absorption of a gas,
(1)under turbulent conditions of gas in the gaseous and
liquid phases is uniform; (b)under quiescent conditions,
Liquid-Solid Mass Transfer
Adsorption
The adsorbate is the substance that is being removed from
liquid phase at the interface,The adsorbent is the solid,
liquid,or gas phase onto which the adsorbate accumulates,
The adsorption process takes place in three steps,
macrotransport,microtransport,and sorption,
Although adsorption also occurs on the surface of the solid
adsorbent and in the macropores and mesopores,the
surface area of these parts of most solid adsorbents is so
extremely small compared with the surface area of the
micropores,
Two important characteristics of the solid adsorbent are
(1) its extremely large surface area to volume ratio
(2) its preferential affinity for certain constituents in the
liquid phase,
Granular or powdered activated carbon (GAC or PAC) is
used most commonly for the removal of selected
constituents from wastewater,
A common adsorption isotherm is the Freundlich isotherm,
4-6 Introduction to Process Selection
The purpose of process analysis is to select the most suitable
unit operations and processes and the optimum operational
criteria,
Important Factors in Process Selection
The first factor,‘process applicability,’ stands out above all
others and reflects directly upon the skill and experience of
the design engineer,
Available resources include performance data from
operating installations,published information in technical
journals,manuals of practice published by the Water
Environment Federation,process design manuals published
by EPA,and the results of pilot-plant studies,
Tab,4-5 Important factors that must be considered when
evaluating and selecting unit operations and processes
Factor
Comment
1.Process applicability
The applicability of a process is evaluated on the basis of past
experience,data from full-scale plants,published data,and from
pilot-plant studies,If new or unusual conditions are encountered,
pilot-plant studies are essential,
2.Applicable flow
range
The process should be matched to the expected range of flowrate,For example,stabilization ponds are not suitable for extremely
large flowrates in highly populated areas,
3.Applicable flow
variation
Most unit operations and processes have to be designed to operate over a wide range of flowrates,Most processes work best
at a relatively constant flowrate,If the flow variation is too great,
flow equalization may be necessary,
4.Influent wastewater
characteristics
The characteristics of the influent wastewater affect the types of
processes to be used (e.g.,chemical or biological) and the
requirements for their proper operation,
5.Inhibiting and
unaffected constituents
What constituents are present and may be inhibitory to the treatment processes?
What constituents are not affected during treatment?
6.Climatic constraints
Temperature affects the rate of reaction of most chemical and
biological processes,
Temperature may also affect the physical operation of the facilities,
Temperatures may accelerate odor generation and also limit
atmospheric dispersion,
7.Process sizing based
on reaction kinetics or
process loading criteria
Reactor sizing is based on the governing reaction kinetics and kinetic coefficients,If kinetic expressions are not available,
process loading criteria are used,Data for kinetic expressions and
process loading criteria usually are derived from experience,
published literature,and the results of pilot-plant studies
8.Process sizing based
on mass transfer rates or
process loading criteria
Reactor sizing is based on mass transfer coefficients,If mass transfer rates are not available,process loading criteria are used,
Data for mass transfer coefficients and process loading criteria
usually are derived from experience,published literature,and the
results of pilot-plant studies,
9.Performance
Performance is usually measured in terms of effluent quality and its
variability,which must be consistent with the effluent discharge
requirements
10.Treatment residuals
The types and amounts of solid,liquid,and gaseous residuals
produced must be known or estimated,Often,pilot-plant studies are
used to identify and quantify residuals,
11.Sludge processing
Are there any constraints that would make sludge processing and
disposal infeasible or expensive? How might recycle loads from
sludge processing affect the liquid unit operation or processes? The
selection of the sludge processing system should go hand in hand
with the selection of the liquid treatment system
12.Environmental
constraints
Environment factors,such as prevailing wind directions and proximity to residential areas,may restrict or affect the use of
certain processes,especially where odors may be produced,Noise
and traffic may affect selection of a plant site,Receiving waters
may have special limitations,requiring the removal of specific
constituents such as nutrients,
13.Chemical
requirements
What resources and what amounts must be committed for a long period of time for the successful operation of the unit operation or
process? What effects might the addition of chemicals have on the
characteristics of the treatment residuals and the cost of treatment?
14.Energy requirements
The energy requirements,as well as probable future energy cost,
must be known if cost-effective treatment systems are to be
designed,
Selection of Reactor Types
A complete-mix reactor might be selected over a plug-flow
reactor,because of its dilution capacity,if the influent
wastewater is known to contain toxic constituents that
cannot be removed by pretreatment,Alternatively,a plug-
flow or multistage reactor might be selected over a
complete-mix reactor to control the growth of filamentous
microorganisms,
Process Selection Based on Mass Transfer
The principal operations in wastewater treatment involving
mass transfer are aeration,especially the addition of
oxygen to water; the drying of biosolids and sludge; the
removal of volatile organics from wastewater; the stripping
of dissolved constituents such as ammonia from digested
supernatant; and the exchange of dissolved constituents as
in ion exchange,
Process Design Based on Loading Criteria
If appropriate reaction rate expressions and/or mass
transfer coefficients cannot be developed,generalized
loading criteria are frequently used,Early design loading
criteria for activated sludge biological treatment systems
were based on aeration tank capacity [e.g.,kg of BOD/m3
(lb BOD/103 ft3)],
Bench Tests and Pilot-Plant Studies
The purpose of conducting pilot-plant studies is to establish
the suitability of the process in the treatment of a specific
wastewater under specific environmental conditions and to
obtain the necessary data on which to base a full-scale
design,
Tab,4-6 Considerations in setting up pilot-plant testing
programs
For example,testing of UV disinfection systems is
typically done,
(1) to verify manufacturers' performance claims,
(2) to quantify effects of effluent water quality constituents
on UV performance,
(3) to assess the effect(s) of system and reactor hydraulics
on UV performance,
(4) to assess the effect(s) of effluent filtration on UV
performance,
(5) to investigate photo reactivation and impacts,
Bench-scale tests are conducted in the laboratory with
small quantities of the wastewater in question,Pilot-scale
tests are typically conducted with flows that are 5 to 10
percent of the design flows,
Reliability Considerations in Process Selection
Because waste water treatment effluent quality is variable
for a number of reasons (varying organic loads,changing
environmental conditions,etc.),it is necessary to ensure
that the treatment system is designed to produce effluent
concentrations equal to or less than the permit limits,
5 Physical Unit Operation
Because physical unit operations were derived originally
from observations of the physical world,they were the first
treatment methods to be used,
The unit operations most commonly used in wastewater
treatment include (1) screening,(2) coarse solids reduction
(comminution,maceration,and screenings grinding),(3)
flow equalization,(4) mixing and flocculation,(5) grit
removal,(6) sedimentation,(7) high-rate clarification,(8)
accelerated gravity separation (vortex separators),(9)
flotation,(10) oxygen transfer,(11)packed-bed filtration,
membrane separation,(12 ) aeration,(13)biosolid
dewatering,and (14) volatilization and stripping of volatile
organic compounds (VOCs),
5-1 Screening
A screen is a device with openings,generally of
uniform size,that is used to retain solids found in
the influent wastewater
The coarse materials could
(1) damage subsequent process equipment,
(2) reduce overall treatment process reliability and
effectiveness,
(3) contaminate waterways,
All aspects of screenings removal,transport,and disposal
must be considered in the application of screening devices,
including
(1) the degree of screenings removal required because of
potential effects on downstream processes,
(2) health and safety of the operators as screenings contain
pathogenic organisms and attract insects,
(3) odor potential
(4) requirements for handling,transport,and disposal,
(5) disposal options,
Classification of Screens
Coarse screens have clear openings ranging from 6 to 150
mm; fine screens have clear openings less than 6 mm,
Micro screens generally have screen openings less than 50
μm,
The screening element may consist of parallel bars,rods or
wires,grating,wire mesh,or perforated plate,and the
openings may be of any shape but generally are circular or
rectangular slots,
Coarse Screens (Bar Racks)
In wastewater treatment,coarse screens are used to protect
pumps,valves,pipelines and other appurtenances from
damage or clogging by rags and large objects,
Hand-Cleaned Coarse Screens
Hand-cleaned coarse screens are used frequently ahead of
pumps in small wastewater pumping stations and
sometimes used at the headworks of small- to medium-
sized wastewater-treatment plants,Often they are used for
standby screening in bypass channels for service during
high-flow periods,when mechanically cleaned screens are
being repaired,or in the event of a power failure,
A perforated drainage plate should be provided at
the top of the rack where the raking may be stored
temporarily for drainage,
The screen channel should be designed to prevent
the accumulation of grit and other heavy materials
in the channel ahead of the screen and following it,
The channel floor should be level or should slope
downward through the screen without pockets to
trap solids,
Mechanically Cleaned Bar Screens
The design of mechanically cleaned bar screens has
evolved over the years to reduce the operating and
maintenance problems and to improve the screenings
removal capabilities,Many of the newer designs include
extensive use of corrosion-resistant materials including
stainless steel and plastics(ABS,etc),
Reciprocating Rake (Climber) Screen
The reciprocating-rake-type bar screen (see Fig,5-3b)
imitates the movements of a person raking the screen,
A major advantage is that all parts requiring maintenance
are above the waterline and can be easily inspected and
maintained without dewatering the channel,The front
cleaned,front return feature minimizes solids carryover,
As a result,the reciprocating rake screen may have limited
capacity in handling heavy screenings loads,particularly in
deep channels where a long ‘reach’ is necessary,
Fig 5-3 Typical mechanically cleaned coarse screens,
(a)front clean,front return chain-driven;
(b)reciprocating rake,(c)catenary,(d)continuous belt
Catenary Screen
In the catenary screen (see Fig,5-3c),the rake is held
against the rack by the weight of the chain,If heavy
objects become jammed in the bars,the rakes pass over
them instead of jamming,The screen,however,has a
relatively large ‘footprint’ and thus requires greater space
for installation,
Continuous Belt Screen
The continuous belt screen is a relatively new development
for use in screening applications,A large number of
screening elements (rakes) are attached to the drive chains,
Hooks protruding from the belt elements are provided to
capture large solids such as cans,sticks,and rags,
Design of Coarse Screen Installations
Considerations in the design of screening
installations include
(1) location;
(2)approach velocity;
(3)clear openings between bars or mesh size;
(4) headloss through the screens;
(5) screenings handling processing,and disposal;
(6) controls,
In nearly all cases,coarse screen should be installed ahead
of the grit chambers,If grit chambers are placed before
screens,rags and other stringy material could foul the grit
chamber collector mechanisms,wrap around air piping,
and settle with the grit,
In hand-cleaned installations,it is essential that the velocity
of approach be limited to approximately 0.45 m/s at
average flow to provide adequate screen area for
accumulation of screenings between raking operations,
Additional area to limit the velocity may be obtained by
widening the channel at the screen and by placing the
screen at a flatter angle to increase the submerged area,As
screenings accumulate,partially plugging the screen,the
upstream head will increase,submerging new areas for the
flow to pass through,
Two or more units should be installed so that one unit may
be taken out of service for maintenance,
The unit can be dewatered for screen maintenance and
repair,If only one unit is installed,it is absolutely essential
that a bypass channel with a manually cleaned bar screen
be provided for emergency use,
An approach velocity of at least 0.4 m/s is recommended
to minimize solids deposition in the channel,To prevent
the pass-through of debris at peak flowrates,the velocity
through the bar screen should not exceed 0.9 m/s,Headloss
through mechanically cleaned coarse screens is typically
limited to about 150 mm,
Hydraulic losses through bar screens are a function of
approach velocity and the velocity through the bars,
For installations with multiple units,the screenings may be
discharged onto a conveyor or into a pneumatic ejector
system and transported to a common screenings storage
hopper,As an alterative,screenings grinders may be used
to grind and shred the screenings,Ground screenings are
then returned to the wastewater,however,ground
screenings may adversely affect operation and
maintenance of down stream equipment such as clogging
weir openings on sedimentation tanks or wrapping around
air diffusers,
Fine Screens
The applications for fine screens range over a broad
spectrum; uses include preliminary treatment (following
coarse bar screens),primary treatment (as a substitute for
primary clarifiers),and treatment of combined sewer
overflows,Fine screens can also be used to remove solids
from primary effluent that could cause clogging problems
in trickling filters,
Screens for Preliminary and Primary Treatment
Fine screens used for preliminary treatment are of the
(l) static (fixed),
(2) rotary drum,
(3) step type,
Typically,the openings vary from 0.2 to 6 mm,Examples
of line screens are illustrated on Fig,5-4,
Fig,5-4 Typical fine screens,(a)static wedge-wire,
(b)drum,(c)step,In step screens,screenings are moved
up the screen by means of movable and fixed vertical
plates,
Stainless-steel mesh or special wedge-
shaped bars are used as the screening
medium,Provision is made for the
continuous removal of the collected solids,
supplemented by water sprays to keep the
screening medium clean,
Drum Screens
The wastewater flows either into one end of the drum and
outward through the screen with the solids collection on
the interior surface,or into the top of the unit and passing
through to the interior with solids collection on the exterior,
Internally fed screens with applicable for flow ranges of
0.03 to 0.8 m3/s per screen,while externally fed screens
are applicable for flowrates less than 0.13 m3/s,Drum
screens are available n various sizes from 0.9 to 2 m in
diameter and from 1.2 to 4 m in length,
Step Screens
The design consists of two step-shaped sets of thin vertical
plates,one fixed and one movable,
The fixed and movable step plates alternate across the
width of an open channel and together form a single screen
face,The movable plates rotate in a vertical motion,
Through this motion solids captured on the screen face are
automatically lifted up to the next fixed step landing,and
are eventually transported to the top of the screen where
they are discharged to a collection hopper,The circular
pattern of the moving plates provides a self-cleaning
feature for each step,
Solids trapped on the screen also create a "filter mat" that
enhances solids removal performance,
Design of Fine-Screen Installations
An installation should have a minimum of two
screens,each with the capability of handling peak
flowrates,In colder climates,hot water or steam is
more effective for grease removal,
Headloss depends on the size and amount of solids
in the wastewater,the size of the apertures,and
the method and frequency of cleaning,
Microscreens
Microscreening involves the use of variable low-speed (up
to 4 r/min),continuously backwashed,rotating-drum
screens operating under gravity-flow conditions,
The wastewater enters the open end of the drum and flows
outward through the rotating-drum screening cloth,The
collected solids are backwashed by high-pressure jets into
a trough located within the drum at the highest point of the
drum,
Reducing the rotating speed of the drum and less frequent
flushing of the screen have resulted in increased removal
efficiencies but reduced capacity,
Screenings Retained on Coarse Screens
Coarse screenings,collected on coarse screens of about 12
mm or greater spacing,consist of debris such as rocks,
branches,pieces of lumber,leaves,paper,tree roots,
plastics,and rags,
Tab,5-4 Typical information on the characteristics and
quantities of screenings removed from wastewater with
coarse screens
Combined storm and sanitary collection systems may
produce volumes of screenings several times the amounts
produced by separate systems,
Screenings Retained on Fine Screens
The materials retained on fine screens include small rags,
paper,plastic materials of various types razor blades,grit,
undecomposed food waste,feces,etc,Compared to coarse
screenings,the specific weight of the fine screenings is
slightly lower and the moisture content is slightly higher,
Fine screenings contain substantial grease and scum,which
require similar care,especially if odors are to be avoided,
Screenings Handling,Processing,and Disposal
Screenings are discharged from the screening unit directly
into a screenings grinder,a pneumatic ejector,or a
container for disposal; or onto a conveyor for transport to a
screenings compactor or collection hopper,
Screenings compactors can be used to dewater and reduce
the volume of screenings,
Fig,5-5 Typical device used for compacting screenings
Compactors can reduce the water content of the screenings
by up to 50 percent and the volume by up to 75 percent,As
with pneumatic ejectors,large objects can cause jamming,
but automatic controls can sense jams,automatically
reverse the mechanism,and actuate alarms and shut down
equipment,
Means of disposal of screenings include (1) removal by
hauling to disposal areas (landfill) including co-disposal
with municipal solid wastes,(2) disposal by burial on the
plant site (small installations only),(3) incineration either
alone or in combination with sludge and grit (large
installations only),and (4) discharge to grinders or
macerators where they are ground and returned to the
wastewater,
In some cases,screenings are required to be lime stabilized
for the control of pathogenic organisms before disposal in
landfills,
5-2 Coarse Solids Reduction
There is a wide divergence of views,however,on the
suitability of using devices that grind and shred screenings
at wastewater-treatment plants,One school of thought
maintains that once coarse solids have been removed from
wastewater,they should not be returned,regardless of the
form,The other school of thought maintains that once cut
up,the solids are more easily handled in the downstream
processes,Shredded solids often present downstream
problems,particularly with rags and plastic bags,as they
tend to form ropelike strands,Rag and plastic strands can
have a number of adverse impacts,such as clogging pump
impellers,sludge pipelines,and heat exchangers,and
accumulating on air diffusers and clarifier mechanisms,
Plastics and other non-biodegradable material may also
adversely affect the quality of bio-solids that are to be
beneficially reused,
5-3 Flow Equalization
Flow equalization is a method used to overcome the
operational problems caused by flowrate variations,to
improve the performance of the downstream processes,
and to reduce the size and cost of down- stream treatment
facilities,
Description/Application
Fig,5-8 Typical wastewater treatment plant flow
diagram incorporating flow equalization,(a)in-line
equalization ;(b)off-line equalization,Flow equalization
can be applied after grit removal,after primary
sedimentation,and after secondary treatment where
advanced treatment is used
Off-line equalization is sometimes used to capture the ‘first
flush’ from combined collection systems,
The principal benefits are,
(1) biological treatment is enhanced,because shock
loadings are eliminated or can be minimized,inhibiting
substances can be diluted and pH can be stabilized;
(2) the effluent quality and thickening performance of
secondly sedimentation tanks is improved through
improved consistency in solids loading;
(3) effluent filtration surface area requirements are reduced,
and more uniform filter-backwash cycles are possible;
(4) in chemical treatment,damping of mass loading
improves chemical feed control and process reliability,
Disadvantages of flow equalization include,
(1) relatively large land areas or sites are needed;
(2) equalization facilities may have to be covered for odor
control near residential areas;
(3) additional operation and maintenance is required;
(4) capital cost is increased,
Location of Equalization Facilities
In some cases,equalization after primary treatment and
before biological treatment may be appropriate,
Equalization after primary treatment causes fewer
problems with solids deposits and scum accumulation,If
flow-equalization systems are to be located ahead of
primary settling and biological systems,the design must
provide for sufficient mixing to prevent solids deposition
and concentration variations,and aeration to prevent odor
problems,
In-Line or Off-Line Equalization
As shown on Fig,5-8,it is possible to achieve considerable
damping of constituent mass loadings to the downstream
processes with in-line equalization,but only slight
damping is achieved with off-line equalization,
Volume Requirements for the Equalization Basin
The volume required for flowrate equalization is
determined by using an inflow cumulative volume diagram
in which the cumulative inflow volume is plotted versus
the time of day,The average daily flowrate,also plotted on
the same diagram,is the straight line drawn from the origin
to the endpoint of the diagram,
Fig,5-9 Schematic mass diagrams for the determination of the required
equalization basin storage volume for two typical flowrate patterns
The required volume is then equal to the vertical distance
from the point of tangency to the straight line representing
the average flowrate,
The required volume is then equal to the vertical distance
between the two lines,
In practice,the volume of the equalization basin will be
larger than that theoretically determined to account for the
following factors,
(1)continuous operation of aeration and mixing equipment
will not allow complete drawdown,
(2) volume must be provided to accommodate the
concentrated plant recycle streams that are expected,if
such flows are returned to the equalization basin
Basin Geometry
If in-line equalization is used to dampen both the flow and
the mass loadings,it is important to use a geometry that
allows the basin to function as a continuous-flow stirred-
tank reactor insofar as possible,
Therefore,elongated designs should be avoided,and the
inlet and outlet configurationally should be arranged to
minimize short circuiting,
Basin Construction
New basins may be of earthen,concrete,or steel
construction; earthen basins are generally the least
expensive,
In most installations,a liner is required to prevent ground-
water contamination,Basin depths will vary depending on
land availability,ground-water level,and topography,if a
liner is used in areas of high groundwater,the effects of
hydraulic uplift on the liner must be considered,
If a floating aerator is used to provide mixing and prevent
septicity and odor formation,a minimum operating level is
needed to protect the aerator,
To prevent wind-induced erosion in the upper portions of
the basin,it may be necessary to protect the slopes with
riprap,soil cement,or a partial concrete layer,Fencing
should also be provided to prevent public access to the
basins,
Mixing and Air Requirements
To minimize mixing requirements,grit-removal facilities
should precede equalization basins where possible,Mixing
requirements for blending a medium-strength municipal
wastewater,having a suspended solids concentration of
approximately 210 mg/L,range from 0.004 to 0.008
kW/m3 of storage,In equalization basins that follow
primary sedimentation and have short detention times (less
than 2 h),aeration may not be required,
To protect the aerators in the event of excessive level
drawdown,low-level shutoff controls should be provided,
Various types of diffused air systems may also be used for
mixing and aeration including static tube,jet,and
aspirating aerators,
Operational Appurtenances
Among the appurtenances that should be included in the
design of equalization basins are (1) facilities for flushing
any solids and grease that may tend o accumulate on the
basin walls; (2) a high water takeoff for the removal of
floating material and foam; (3) water sprays to prevent the
accumulation of foam on the sides of the basin and to aid
in scum removal; and (4) separate odor control facilities
where covered equalization basins must be used,
Pumps and Pump Control
An automatically controlled flow-regulating device will be
required where gravity discharge from the basin is used,
5-4 Mixing and Flocculation
Mixing is an important unit operation in many phases of
wastewater treatment including,
(1) mixing of one substance completely with another,
(2) blending of miscible liquids,
(3) flocculation of wastewater particles,
(4) continuous mixing of liquid suspensions,
(5) heat transfer,
Continuous Rapid Mixing in Wastewater
Treatment
Continuous rapid mixing is used,most often,where one
substance is to be mixed with another,The principal
applications of continuous rapid mixing are in
(1) the blending of chemicals with wastewater (e.g.,the
addition of alum or iron salts prior to flocculation and
settling or for dispersing chlorine and hypochlorite into
wastewater for disinfection),
(2) the blending of miscible liquids,
(3) the addition of chemicals to sludge and biosolids to
improve their dewatering characteristics,
Continuous Mixing in Wastewater Treatment
Continuous mixing is used where the contents of a
reactor or holding tank or basin must be kept in
suspension such as in equalization basins,
flocculation basins,suspended-growth biological
treatment processes,aerated lagoons,and aerobic
digesters,
Flocculation in Wastewater Treatment
The purpose of wastewater flocculation is to form
aggregates or flocs from finely divided particles
and from chemically destabilized particles,
Flocculation is a transport step that brings about
the collisions between the destabilized particles
needed to form larger particles that can be
removed readily by settling or filtration,
Maintaining Material in Suspension
In biological treatment systems the mixing device is also
used to provide the oxygen needed for the process,Thus,
the aeration equipment must be able to provide the oxygen
needed for the process and must be able to deliver the
energy needed to maintain mixed conditions within the
reactor,
In both aerobic and anaerobic digestion,mixing is used to
homogenize the contents of the digester to accelerate the
biological conversion process,and to distribute uniformly
the heat generated from biological conversion reactions,
Types of Mixers Used for Rapid Mixing in
Wastewater Treatment
The principal devices used for rapid mixing in wastewater-
treatment applications include static in-line mixers,high-
speed induction mixers,pressurized water jets,and
propeller and turbine mixers,Mixing can also be
accomplished in pumps and with the aid of hydraulic
devices such as hydraulic jumps,Parshall flumes,or weirs,
Static Mixers
Static in-line mixers contain internal vanes or orifice plates
that bring about sudden changes in the velocity patterns as
well as momentum reversals,Static mixers are principally
identified by their lack of moving parts,
In-line mixers are available in sizes varying from about 12
mm to 3 m× 3 m open channels,Low-pressure-drop round,
square,and rectangular in-line static mixers have been
developed for chlorine mixing in open channels and
tunnels
For static in-line mixers with vanes,the longer the mixing
elements,the better the mixing; however,the pressure loss
increases,
In-line mixers are similar to static mixers but contain a
rotating mixing element to enhance the mixing process,
In the in-line mixer shown on Fig,5-12c,the power
required for mixing is supplied by an external source,For
the mixer shown on Fig,5-12d,the power for mixing is
supplied by the energy dissipation caused by the orifice
plate and by the power input to the propeller mixer,
High-Speed Induction Mixer
A proprietary device,consists of a motor-driven open
propeller that creates a vacuum in the chamber directly
above the propeller,
Pressurized Water Jets
With pressurized water jet mixers,the power for mixing is
provided by an external source (i.e.,the solution feed
pump),
Turbine and Propeller Mixers
Turbine or propeller mixers are usually constructed with a
vertical shaft driven by a speed reducer and electric motor,
Two types of impellers are used for mixing,(1) radial-flow
impellers and (2) axial-flow impellers,Radial-flow
impellers generally have flat or curved blades located
parallel to the axis of the shaft,The vertical flat-blade
turbine impeller is a typical example of a radial-flow
impeller,Axial-flow impellers make an angle of less than
90o with the drive shaft,
Types of Mixers Used for Flocculation in
Wastewater Treatment
Static Mixers
Static mixers can be comprised of over and under narrow
flow channels,such as shown on Fig,5-12a,or the narrow
flow channels can be laid out horizontally,
In some designs,the channel spacing is varied to provide a
decreasing energy gradient so that the large floc particles
formed toward the end of the flocculation basin will not be
broken apart,
Paddle Mixers
Paddle mixers are used as flocculation devices when
coagulants,such as aluminum or ferric sulfate,and
coagulant aids,such as polyelectrolytes and lime,are
added to wastewater or solids (sludge),
Increased particle contact promotes floc growth,but,if the
mixing is too vigorous,the increased shear forces will
break up the floc into smaller particles,
Types of Mixers Used for Continuous Mixing in
Wastewater Treatment
Horizontal,submersible propeller mixers are often used to
maintain channel velocities in oxidation ditches,mix the
contents of anoxic reactors (see Fig,5-15),and aid in the
destratification of reclaimed water storage reservoirs,
Pneumatic Mixing
In pneumatic mixing,a gas (usually air or oxygen) is
injected into the bottom of mixing or activated-sludge
tanks,and the turbulence caused by the rising gas bubbles
serves to mix the fluid contents of the tank,
Fig,5-15 Submerged propeller mixers used to mix the contents of an
anoxic reactor
5-5 Gravity Separation Theory
The terms sedimentation and settling are used
interchangeably,
Sedimentation is also used for solids concentration
in sludge thickeners,
Particle Settling Theory
The settling of discrete,nonflocculating particles
can be analyzed by means of the classic laws of
sedimentation formed by Newton and Stokes,
Newton's law yields the terminal particle velocity
by equating the gravitational force of the particle to
the frictional resistance,or drag,
Settling in the Laminar Region
For Reynolds numbers less than about 1.0,viscosity is the
predominant force governing the settling process,
Flocculent Particle Settling
The extent to which flocculation occurs is dependent on the
opportunity for contact,which varies with overflow rate,
depth of the basin,velocity gradients in the system,
concentration of particles,and range of particle sizes,The
effects of these variables can be determined only by
sedimentation tests,
The settling characteristics of a suspension of flocculent
particles can be obtained by using a settling column test,
Such a column can be of any diameter but should be equal
in height to the depth of the proposed tank,
Settling should take place under quiescent conditions,The
duration of the test should be equivalent to the settling time
in the proposed tank,
The more traditional method of determining settling
characteristics of a suspension is to use a column similar to
the one described above but with sampling ports inserted at
approximately 0.5 m intervals,At various time intervals,
samples are withdrawn from the ports and analyzed for
suspended solids,The percent removal is computed for
each sample analyzed and is plotted as a number against
time and depth,
Inclined Plate and Tube Settling
Inclined plate and tube settlers are shallow settling devices
consisting of stacked offset trays or bundles of small
plastic tubes of various geometries,
They are based on the theory that settling depends on the
settling area rather than detention time,Although they are
used predominantly in water-treatment applications,plate
and tube settlers are used in wastewater-treatment for
primary,secondary,and tertiary sedimentation,
Fig,5-17 Plate and tube settlers,(a)module of inclined tubes,(b)tubes
installed in a rectangular sedimentation tank,(c)operation,(d)definition
sketch for the analysis of settling in a tube settler
To be self-cleaning,plate ox tube settlers are
usually set at an angle between 45 and 60o above
the horizontal,When the angle is increased above
60o,the efficiency decreases,If the plates and
tubes are inclined at angles less than 45% solids
will tend to accumulate within the plates or tubes,
Nominal spacing between plates is 50 mm,with
an inclined length of 1 to 2 m,To control
biological growths and the production of odors
(the principal problems encountered with their
use),the accumulated solids must be flushed out
periodically
Inclined settling systems are generally
constructed for use in one of three ways
with respect to the direction of liquid flow
relative to the direction of particle
settlement,
(1) countercurrent,(2) co-current,and (3)
cross-flow,
Countercurrent Settling,
Fig,5-18 Lamella plate settler
Further thickening of the solids occurs in the
hopper due to compression in the quiescent
zone made possible by feeding the plates
from the side rather than from the bottom,
5-6 Grit Removal
Primary sedimentation tanks function for the
removal of the heavy organic solids,
Grit chambers are provided to (1) protect moving
mechanical equipment from abrasion and
accompanying abnormal wear; (2) reduce
formation of heavy deposits in pipelines,channels,
and conduits; and (3) reduce the frequency of
digester cleaning caused by excessive
accumulations of grit,
There are three general types of grit chambers,horizontal
flow,of either a rectangular or a square configuration;
aerated; or vortex type,
The aerated type consists of a spiral-flow aeration tank
where the spiral velocity is induced and controlled by the
tank dimensions and quantity of air supplied to the unit,
The vortex type consists of a cylindrical tank in which the
flow enters tangentially creating a vortex flow pattern;
centrifugal and gravitational forces cause the grit to
separate,
Rectangular Horizontal-Flow Grit Chambers
The design velocity will carry most organic particles
through the chamber and will tend to resuspend any
organic particles that settle but will permit the heavier grit
to settle out,
The cross-sectional area will be governed by the rate of
flow and by the number of channels,Allowance should be
made for inlet and outlet turbulence,
Grit removal from horizontal- flow grit chambers is
accomplished usually by a conveyor with scrapers,buckets,
or plows,Screw conveyors or bucket elevators are used to
elevate the removed grit for washing or disposal,
Square Horizontal-Flow Grit Chambers
They are nominally designed to remove 95 percent of the
0.15-mm-diameter (100- mesh) particles at peak flow,
In square grit chambers,the solids are removed by a
rotating raking mechanism to a sump at the side of the tank,
The concentrated grit then may be washed again in a
classifier using a submerged reciprocating rake or an
inclined-screw conveyor,By either method,organic solids
are separated from the grit and flow back into the basin,
resulting in a cleaner,dryer grit,
Aerated Grit Chambers
In aerated grit chambers,air is introduced along
one side of a rectangular tank to create a spiral
flow pattern perpendicular to the flow through the
tank,
If the velocity is too great,grit will be carried out
of the chamber; if it is too small,organic material
will be removed with the grit,
Aerated grit chambers are nominally designed to
remove 0.21-mm-diameter (65-mesh) or larger,
with 2- to 5-minute detention periods at the peak
hourly rate of flow,The cross section of the tank
is similar to that provided for spiral circulation in
activated-sludge aeration tanks,except that a grit
hopper about 0.9 m deep,
Influent and effluent baffles are used frequently
for hydraulic control,
Wastewater should be introduced in the direction
of the roll,To determine the required headloss
through the chamber,the expansion in volume
caused by the air must be considered,
Vortex-Type Grit Chambers
Effluent exits the center of the top of the
unit from a rotating cylinder,or "eye" of the
fluid,Centrifugal and gravitational forces
within this cylinder minimize the release of
particles with densities greater than water,
Grit settles by gravity to the bottom of the
unit,while organics,including those
separated from grit particles by centrifugal
forces,exit principally with the effluent,
Grit Characteristics,Quantities,Processing,and
Disposal
In addition to these materials,grit includes eggshells,bone
chips,seeds,coffee grounds,and large organic particles,
Characteristics of Grit
Generally,what is removed as grit is predominantly inert
and relatively dry material,However,grit composition can
be highly variable,with moisture content ranging from 13
to 65 percent,and volatile content from 1 to 56 percent,A
bulk density of 1600 kg/m3 is commonly used for grit,
Unwashed grit may contain 50 percent or more of organic
material,
Quantities of Grit
The quantities of grit will vary greatly from one location to
another,depending on the type of sewer system,the
characteristics of the drainage area,the condition of the
sewers,the frequency of street sanding to counteract icing
conditions,the types of industrial wastes,the number of
household garbage grinders served,and in areas with
sandy soils,
Disposal of Grit
The most common method of grit disposal is transport to a
landfill,In some large plants,grit is incinerated with solids,
As with screenings,some states require grit to be lime
stabilized before disposal in a landfill,Disposal in all cases
should be done in conformance with the appropriate
environmental regulations,
5-7 Primary Sedimentation
Efficiently designed and operated primary sedimentation
tanks should remove from 50 to 70 percent of the
suspended solids and from 25 to 40 percent of the BOD,
Description
The selection of the type of sedimentation unit for a given
application is governed by the size of the installation,by
rules and regulations of local control authorities,by local
site conditions,and by the experience and judgment of the
engineer,Two or more tanks should be provided so that
the process may remain in operation while one tank is out
of service for maintenance and repair work,At large plants,
the number of tanks is determined largely by size
limitations,
Rectangular Tanks
Rectangular sedimentation tanks may use either chain-and-
flight solids collectors or traveling-bridge-type collectors,
Equipment for settled solids removal generally consists of
a pair of endless conveyor chains,manufactured of alloy
steel,cast iron,or thermoplastic,Attached to the chains at
approximately 3-m intervals are scraper flights made of
wood or fiberglass,extending the full width of the tank or
bay,The solids settling in the tank are scraped to solids
hoppers in small tanks and to transverse troughs in large
tanks,
Rectangular tanks may also be cleaned by a
bridge-type mechanism that travels up and
down the tank on rubber wheels or on rails
supported on the sidewalls,One or more
scraper blades are suspended from the
bridge,Some of the bridge mechanisms are
designed so that the scraper blades can be
lifted clear of the solids blanket on the
return travel,
Because flow distribution in rectangular tanks is
critical,one of the following inlet designs is used,
(1) full-width inlet channels with inlet weirs,(2)
inlet channels with submerged ports or orifices,(3)
or inlet channels with wide gates and slotted
baffles,Inlet weirs are effective in spreading flow
across the tank width,Inlet ports can provide good
distribution across the tank width if the velocities
are maintained in the 3 to 9 m/min range,inlet
baffles are effective in reducing the high initial
velocities and distribute flow over the widest
possible cross-sectional area,Where full-width
baffles are used,they should extend from 150 mm
below the surface to 300 mm below the entrance
opening,
For installations of multiple rectangular tanks,
below-grade pipe and equipment galleries can be
constructed integrally with the tank structure and
along the influent end,The galleries are used to
house the sludge pumps and sludge drawoff piping,
Galleries can also be connected to service tunnels
for access to other plant units,
Scum is usually collected at the effluent end of
rectangular tanks with the flights returning at the
liquid surface,Water sprays can also move the
scum,
Multiple rectangular tanks require less land
area than multiple circular tanks,
Rectangular tanks also lend themselves to
nesting with preaeration tanks and aeration
tanks in activated-sludge plants,thus
permitting common wall construction and
reducing construction costs,
Fig,5-26 Typical circular primary sedimentation tanks,
(a)center feed; (b)peripheral feed
In circular tanks the flow pattern is radial,To achieve a
radial flow pattern,the wastewater to be settled can be
introduced in the center or around the periphery of the tank,
The wastewater is transported to the center of the tank in a
pipe suspended from the bridge,or encased in concrete
beneath the tank floor,
The center well has a diameter typically between 15 and 20
percent often total tank diameter and ranges from 1 to 2.5
m in depth and should have a tangential energy-dissipating
inlet within the feedwell,
The energy-dissipating device functions to collect influent
from the center column and discharge it tangentially into
the upper 0.5 to 0.7 m of the feedwell,The discharge ports
are sized to produce a velocity of ≤ 0.75 m/s at maximum
flow and 0.30 to 0.45 m/s at average flow,
The depth of the feedwell should extend about 1 meter
below the energy-dissipating inlet ports,
The clarified liquid is skimmed off over weirs on both
sides of a centrally located weir trough,
Circular tanks 3.6 to 9 m in diameter have the
solids-removal equipment supported on beams
spanning the tank,Tanks 10.5 m in diameter and
larger have a central pier that supports the
mechanism and is reached by a walkway or bridge,
The bottom of the tank is sloped at about 1 in 12
(vertical,horizontal) to form an inverted cone,and
the solids are scraped to a relatively small hopper
located near the center of the tank,
Multiple tanks are customarily arranged in groups
of two or four,The flow is divided among the
tanks by a flow-split structure,commonly located
between the tanks,
Stacked (Multilevel) Clarifiers
Stacked clarifiers originated in Japan in the 1960s
where limited land area is available for the
construction of wastewater-treatment facilities,
In addition to saving space,advantages claimed
for stacked clarifiers include less piping and
pumping requirements,Because the facilities are
more compact and have less exposed surface area,
better control of odors and volatile organic
compound emissions is possible,Disadvantages
include higher construction cost than conventional
clarifiers and more complex structural design,
Sedimentation Tank Performance
The efficiency of sedimentation basins with respect to the
removal of BOD and TSS is reduced by
(1) eddy currents formed by the inertia of the incoming
fluid,
(2) wind-induced circulation cells formed in uncovered
tanks,
(3) thermal convection currents,
(4) cold or warm water causing the formation of density
currents that move along the bottom of the basin and warm
water rising and flowing across the top of the tank,
(5) thermal stratification in hot arid climates,
Temperature Effects
Temperature effects can be significant in
sedimentation basins,It has been shown that
a 1。 Celsius temperature differential
between the incoming wastewater and the
wastewater in the sedimentation tank will
cause a density current to form,
Design Considerations
If all solids in wastewater were discrete particles
of uniform size,uniform density,uniform specific
gravity,and uniform shape,the removal efficiency
of these solids would be dependent on the surface
area of the tank and time of detention,
Detention Time
Normally,primary sedimentation tanks are
designed to provide 1.5 to 2.5 h of detention based
on the average rate of wastewater flow,In cold
climates,increases in water viscosity at lower
temperatures retard particle settling in clarifiers
and reduce performance at wastewater
temperatures below 20oC,
Surface Loading Rates
Sedimentation tanks are normally designed on the basis of
a surface loading rate,
Scour Velocity
To avoid the resuspension (scouring) of settled particles,
horizontal velocities through the tank should be kept
sufficiently low,
Characteristics and Quantities of Solids (Sludge)
and Scum
The solids volume will depend on (1) the
characteristics of the untreated wastewater,
including strength and freshness; (2) the period of
sedimentation and the degree of purification to be
effected in the tanks; (3) the condition of the
deposited solids,including specific gravity,water
content,and changes in volume under the influence
of tank depth or mechanical solids-removal devices;
and (4) the period between solids-removal
operations,
5-8 High-rate Clarification
High-rate clarification employs physical/chemical
treatment and utilizes special flocculation and
sedimentation systems to achieve rapid settling,
Advantages of high-rate clarification are
(1) units are compact and thus reduce space requirements,
(2) start-up times are rapid (usually less than 30 min) to
achieve peak efficiency,
(3) a highly clarified effluent is produced,
Enhanced Particle Flocculation
Enhanced particle flocculation involves the addition of an
inert ballasting agent (usually silica sand or recycled
chemically conditioned sludge),
The polymer appears to coat the ballasting particles and
forms the "glue' that binds the chemical floc to the
ballasted particles
The particles grow as the larger,faster-settling particles
overtake and collide with slower-settling particles,The
velocity gradient G for flocculation is important as a high
gradient will cause a breakdown in the floc particles,and
insufficient agitation will inhibit floc formation,
Applications for high-rate clarification
include
(1) providing advanced primary treatment,
(2) treating wet-weather flows and
combined sewer overflows,
(3) treating waste filter backwash water,
(4) treating return flows from solids-
processing facilities,
5-9 Large-scale Swirl and Vortex Separators for
Combined Wastewater and Stormwater
These devices are compact solids-separation units with no
moving parts,Concentrated foul matter is intercepted for
treatment while the cleaner,treated flow discharges to
receiving waters,
5-10 Flotation
Separation is brought about by introducing fine gas
(usually air) bubbles into the liquid phase,The bubbles
attach to the particulate matter,and the buoyant force of
the combined particle and gas bubbles is great enough to
cause the particle to rise to the surface,Particles that have
a higher density than the liquid can thus be made to rise,
The rising of particles with lower density than the liquid
can also be facilitated,
In wastewater treatment,flotation is used principally to
remove suspended matter and to concentrate biosolids,
The principal advantages of flotation over sedimentation
are that very small or light particles that settle slowly can
be removed more completely and in a shorter time,
Description
Air bubbles are added or caused to form by (1) injection of
air while the liquid is under pressure,followed by release
of the pressure (dissolved-air flotation),and (2) aeration at
atmospheric pressure (dispersed-air flotation),
Dissolved-Air Flotation
In dissolved-air flotation (DAF) systems,air is dissolved in
the wastewater under a pressure of several atmospheres,
followed by release of the pressure to the atmospheric level,
Dispersed-Air Flotation
It is used in industrial applications for the
removal of emulsified oil and suspended
solids,air bubbles are formed by
introducing the gas phase directly into the
liquid phase through a revolving impeller,
The advantages of a dispersed-air flotation
system are (1) compact size,(2) lower
capital cost,and (3) capacity to remove
relatively free oil and suspended solids,
The quantities of float skimmings are
significantly higher than the pressurized
unit,3 to 7 percent of the incoming flow as
compared to less that 1 percent for
dissolved-air systems,
Chemical Additives
Chemicals are commonly used to aid the flotation
process,These chemicals,for the most part,
function to create a surface or a structure that can
easily absorb or entrap air bubbles,
Various organic polymers can be used to change
the nature of either the air-liquid interface or the
solid-liquid interface,or both,
Design Considerations for Dissolved-Air Flotation
Systems
Factors that must be considered in the design of flotation
traits include the concentration of particulate matter,
quantity of air used,the particle-rise velocity,and the
solids loading rate,
The performance of a dissolved-air flotation system
depends primarily on the ratio of the volume of air to the
mass of solids (A/S) required to achieve a given degree of
clarification,
5-11 Oxygen Transfer
The functioning of aerobic processes,such as
activated sludge,biological filtration,and aerobic
digestion,depends on the availability of sufficient
quantities of oxygen,
Description
The low solubility of oxygen and the consequent
low rate of oxygen transfer,
Either air or oxygen can be introduced into the
liquid,or the liquid in the form of droplets can be
exposed to the atmosphere,
In wastewater-treatment plants,submerged-bubble
aeration is most frequently accomplished by
dispersing air bubbles in the liquid at depths up to
10 m; depths up to 30 m have been used in some
European designs,
Turbine mixers may be used to disperse air
bubbles,they are designed both to mix the liquid
in the basin and to expose it to the atmosphere in
the form of small liquid droplets,
Evaluation of Oxygen Transfer Coefficient
Oxygen Transfer in Clean Water,
The accepted test method involves the
removal of dissolved oxygen (DO) from a
known volume of water by the addition of
sodium sulfite followed by re-oxygenation
to near the saturation level,The DO of the
water volume is monitored during the
reaeration period by measuring DO
concentrations at several different points,
Oxygen Transfer in Wastewater
Typically,oxygen is maintained at a level of 1 to 3 mg/L,
The mass transfer coefficient KLa is also a function of
temperature,intensity of mixing and hence of the type of
aeration device used and the geometry of the mixing
chamber,and constituents in the water,
Effects of Mixing Intensity and Tank Geometry
In most cases an aeration device is rated for a range of
operating conditions using tap water having a low TDS
concentration,
Effects of Wastewater Characteristics
The correction factor is used to correct the test system
oxygen transfer rate for differences in oxygen solubility
due to constituents in the water such as salts,particulates,
and surface-active substances,Values ofβ vary from about
0.7 to 0.98,
Application of Correction Factors
The actual amount of oxygen required must be obtained by
applying factors to a standard oxygen requirement that
reflect the effects of salinity-surface tension (beta factor),
temperature,elevation,diffused depth (for diffused
aeration systems),the desired oxygen operating level,and
the effects of mixing intensity and basin configuration,
The fouling factor F is used to account for both
internal and external fouling of air diffusers,
Internal fouling is caused by impurities in the
compressed air,whereas external fouling is caused
by the formation of biological slimes and
inorganic precipitants,
5-12 Aeration Systems
The systems used depend on the function to be performed,
type and geometry of the reactor,and cost to install and
operate the system,
Diffused-Air Aeration
A diffused-air system consists of diffusers that are
submerged in the wastewater,header pipes,air mains,and
the blowers and appurtenances through which the air
passes,
Porous Diffusers
Domes,disks,or tube diffusers are mounted on or screwed
into air manifolds,
Dome and disk diffusers may also be installed in a grid
pattern on the bottom of the aeration tank to provide
uniform aeration throughout the tank,
These materials generally fall into the categories of rigid
ceramic and plastic materials and flexible plastic,rubber,
or cloth sheaths,
When the air is turned on,the sheath expands and each slot
acts as a variable aperture opening; the higher the air
flowrate,the greater the opening,
Advantages cited for aeration panels are (1) ultra-fine
bubbles are produced that significantly improve oxygen
transfer and system energy efficiency,(2) large areas of the
tank floor can be covered,which facilitates mixing and
oxygen transfer,and (3) foulants can be dislodged by
"bumping," i.e.,increasing the airflow to flex the
membrane,
Disadvantages are (1) the panel is a proprietary design and
thus lacks competitive bidding,(2) the membrane has a
higher headloss,which may affect blower performance in
retrofit applications,and (3) increased blower air filtration
is required to prevent internal fouling,
Nonporous Diffusers
The advantages of lower cost,less maintenance,
and the absence of stringent air-purity
requirements may offset the lower oxygen transfer
efficiency and energy cost,
Diffuser Performance
Aeration devices are conventionally evaluated in
clean water and the results adjusted to process
operating conditions through widely used
conversion factors,
Factors commonly used to convert the oxygen
transfer required for clean water to wastewater are
the alpha,beta,and theta factors,
The presence of constituents such as detergents,
dissolved solids,and suspended solids can affect
the bubble shape and size and result in diminished
oxygen transfer capability,
Blowers
There are three types of blowers commonly used for
aeration,centrifugal,rotary lobe positive displacement,
and inlet guide vane-variable diffuser,Centrifugal blowers
are almost universally used where the unit capacity is
greater than 425 m3/min of free air,Rated discharge
pressures range normally from 48 to 62 kN/m2,
The operating point of the blower is determined,similar to
a centrifugal pump,by the intersection of the head-capacity
curve and the system curve,
Because it is necessary to meet a wide range
of airflows and pressures at a wastewater-
treatment plant,provisions have to be
included in the blower system design to
regulate or turn down the blowers,Methods
to achieve regulation or turndown are (1)
flow blowoff or bypassing,(2) inlet
throttling,(3) adjustable discharge diffuser,
(4) variable-speed driver,and (5) parallel
operation of multiple units,
For higher discharge pressure applications
(> 55 kN/m2) and for capacities smaller than
425 m3/min of free air per unit,rotary-lobe
positive-displacement blowers are
commonly used,The positive-displacement
blower is a machine of constant capacity
with variable pressure,
The performance curve typically is a falling-
head curve where the pressure decreases as
the inlet volume increases,Blowers are
rated at standard air conditions,defined as a
temperature of 20oC,a pressure of 760 mm
Hg,and a relative humidity of 36 percent,
Standard air has a specific weight of 1.20
kg/m3,
Air Piping
Air piping consists of mains,valves,meters,and other
fittings that transport compressed air from the blowers to
the air diffusers,Because the pressures are low(less than
70 kN/m2),lightweight piping can be used,
The piping should be sized so that losses in air headers and
diffuser manifolds are small in comparison to the losses in
the diffusers,Typically,if headlosses in the air piping
between the last flow-split device and the farthest diffuser
are less than 10 percent of the headloss across the diffusers,
good air distribution through the aeration basin can be
maintained,Valves and control orifices are an important
consideration in piping design
The discharge pressure at the blowers will be the sum of
the above losses,the depth of water over the air diffusers,
and the loss through the diffusers,
The high temperature of the air discharged by blowers 60
to 80oC,It is essential,however,that provisions be made
for pipe expansion and contraction,
Pipe materials are often stainless steel,fiberglass,or
plastics suitable for higher temperatures,Other materials
used include mild steel or cast iron with external coatings
(e.g.,coal tar or vinyl),Interior surfaces include cement
lining or coal tar or vinyl coatings,
Mechanical Aerators
Aerators with vertical axis and aerators with horizontal axis,
Both groups are further subdivided into surface and
submerged aerators,
In submerged aerators,oxygen is entrained from the
atmosphere and,for some types,from air or pure oxygen
introduced in the tank bottom,In either case,the pumping
or agitating action of the aerators helps to keep the
contents of the aeration tank or basin mixed,
Surface Mechanical Aerators with Vertical Axis
Surface aerators consist of submerged or partially
submerged impellers that are attached to motors mounted
on floats or on fixed structures,The impellers are
fabricated from steel,cast iron,noncorrosive alloys,and
fiberglass-reinforced plastic and causing a rapid change in
the air-water interface to facilitate solution of the air,
High-speed aerators are almost always mounted on floats,
These units were originally developed for use in ponds or
lagoons where the water surface elevation fluctuates,or
where a rigid support would be impractical,Surface
aerators may be obtained in sizes from 0.75 to 100 kW (1
to 150 hp),
Mechanical Aerators with Horizontal Axis
The surface aerator is patterned after the original Kessener
brush aerator,a device used to provide both aeration and
circulation in oxidation ditches,Angle steel,steel of other
shapes,or plastic bars or blades are now used instead of
bristles,
The disks are submerged in the wastewater for
approximately one-eighth to three-eighths of the diameter,
Standard conditions exist when the temperature is 20oC,
the dissolved oxygen is 0.0 mg/L,and the test liquid is tap
water,
6-1 Role Of Chemical Unit Processes In
Wastewater Treatment
Application of Chemical Unit Processes
Currently the most important applications of
chemical unit processes in wastewater treatment
are for (1) the disinfection of wastewater,(2) the
precipitation of phosphorus,(3) the coagulation of
particulate matter,(4)oxidation and reduction of
industrial pollutants,
6 Chemical Unit Processes
Tab,6-1 Applications of chemical unit process in wastewater treatment
Considerations in the Use of Chemical Unit
Processes
One of the inherent disadvantages associated with
most chemical unit processes,as compared with
the physical unit operations,is that they are
additive processes (i.e.,something is added to the
wastewater to achieve the removal of something
else),As a result,there is usually a net increase in
the dissolved constituents in the wastewater,High
operation cost and secondary pollution are also
their disadvantages,
Similarly,when chlorine is added to
wastewater,the TDS of the effluent is
increased,If the treated wastewater is to be
reused,the increase in dissolved
constituents can be a significant factor,This
additive aspect is in contrast to the physical
unit operations and the biological unit
processes,which may be described as being
subtractive,in that wastewater constituents
are removed from the wastewater,
6-2 Fundamentals Of Chemical Coagulation
Colloidal particles found in wastewater
typically have a net negative surface charge,
The size of colloids (about 0.01 to 1μ m
and is such that the attractive body forces
between particles are considerably less than
the repelling forces of the electrical charge,
Under these stable conditions,Brownian
motion keeps the particles in suspension,
Coagulation is the process of destabilizing
colloidal particles so that particle growth
can occur as a result of particle collisions,
Basic Definitions
Typical coagulants and flocculants include
natural and synthetic organic polymers,
metal salts such as alum or ferric sulfate,
and prehydrolized metal salts such as poly-
aluminum chloride (PACl,PAC) and poly-
iron chloride (PIC1,PFC),
Flocculants,especially organic polymers,are also
used to enhance the performance of granular
medium filters and in the dewatering of digested
biosolids,In these applications,the flocculant
chemicals are often identified as filter aids,
The purpose of flocculation is to produce particles,
by means of aggregation,that can be removed by
inexpensive particle-separation procedures such as
gravity sedimentation and filtration,
Nature of Particles in Wastewater
Because colloidal particles cannot be removed by
sedimentation in a reasonable period of time,
chemical methods (i.e.,the use of chemical
coagulants and flocculant aids) must be used to
help bring about the removal of these particles,
Particle Shape and Flexibility,
Particle shapes found in wastewater can be
described as spherical,semispherical,ellipsoids of
various shapes (e.g.,prolate and oblate),rods of
various length and diameter (e.g.,E,coli),disk
and disklike,strings of various lengths,and
random coils,
The shape of the particles will affect the electrical
properties,the particle-particle interactions,and
particle-solvent interactions,
Particle-Solvent Interactions
There are three general types of colloidal particles
in liquids,hydrophobic or "water-hating,"
hydrophilic or "water-loving," and association
colloids,
The third type of colloid is known as an
association colloidal,typically made up of
surface-active agents such as soaps,synthetic
detergents,and dyestuffs which form organized
aggregates,
Development and Measurement of Surface Charge
Surface charge develops most commonly through
(1) isomorphous replacement,
(2) structural imperfections,
(3) preferential adsorption,
(4) ionization
Isomorphous Replacement
Charge development through isomorphous replacement
occurs in clay and other soil particles,in which ions in the
lattice structure are replaced with ions from solution (e.g.,
the replacement of Si4+ with Al3+),
Structural Imperfections
In clay and similar particles,charge development can occur
because of broken bonds on the crystal edge and
imperfections in the formation of the crystal,
Preferential Adsorption
When oil droplets,gas bubbles,or other chemically inert
substances are dispersed in water,they will acquire a
negative charge through the preferential adsorption of
anions (particularly hydroxyl ions),
Ionization
In the case of substances such as proteins or
microorganisms,surface charge is acquired through the
ionization of carboxyl and amino groups,
Measurement of Surface Potential
The potential at the surface of the cloud (called the surface
of shear) is sometimes measured in wastewater-treatment
operations,The measured value is often called the zeta
potential,
Particle-Particle Interactions
It should be noted that the van der Waals
forces of attraction do not come into play
until the two plates are brought together in
close proximity to each other,
Particle Destabilization with Potential-Determining
Ions and Electrolytes
The effect of the charge can be overcome by (1) the
addition of potential-determining ions,which will be taken
up by or will react with the colloid surface to lessen the
surface charge and (2) the addition of electrolytes,which
have the effect of reducing the thickness of the diffuse
electric layer and,thereby,reduce the zeta potential,
Use of Potential-Determining Ions
The magnitude of the effect will depend on
the concentration of potential-determining
ions added,It is interesting to note that
depending on the concentration and nature
of the counter-ions added,it is possible to
reverse the charge of the double layer and
develop a new stable particle,
Use of Electrolytes
Increased concentration of a given electrolyte will cause a
decrease in zeta potential and a corresponding decrease in
repulsive forces,The concentration of an electrolyte that is
needed to destabilize a colloidal suspension is known as the
critical coagulation concentration (CCC),
Particle Destabilization and Aggregation with Polyelectrolytes
Important natural poly-electrolytes include polymers of
biological origin and those derived from starch products
such as cellulose derivatives and alginates,
Depending on whether their charge,when placed in water,is
negative,positive,or neutral,these poly-electrolytes are
classified as anionic,cationic,and nonionic,respectively,
Charge Neutralization
Because wastewater particles normally are charged
negatively,cationic poly-electrolytes are used for
this purpose,
Because of the large number of particles found in
wastewater,the mixing intensity must be sufficient
to bring about the adsorption of the polymer onto
the colloidal particles,With inadequate mixing,the
polymer will eventually fold back on itself and its
effectiveness in reducing the surface charge will be
diminished,Further,if the number of colloidal
particles is limited,it will be difficult to remove
them with low poly-electrolyte dosages,
Polymer Bridge Formation
A bridge is formed when two or more particles become
adsorbed along the length of the polymer,Bridged particles
become intertwined with other bridged particles during the
flocculation process,The size of the resulting three-
dimensional particles grows until they can be removed
easily by sedimentation,Where particle removal is to be
achieved by the formation of particle-polymer bridges,the
initial mixing of the polymer and the wastewater
containing the particles to be removed must be
accomplished in a matter of seconds,
Charge Neutralization and Polymer Bridge Formation
The third type of poly-electrolyte action may be classified
as a charge neutralization and bridging phenomenon,
which results from using cationic poly-electrolytes of
extremely high molecular weight,
Formation of Hydrolysis Products
In the past,it was thought that free A13+ and Fe3+ were
responsible for the effects observed during particle
aggregation; it is now known,however,that their hydrolysis
products are responsible,
It should be noted that the complex compounds are known
as coordination compounds,which are defined as a central
metal ion (or atom) attached to a group of surrounding
molecules or ions by coordinate covalent bonds,The
surrounding molecules or ions are known as ligands,and the
atoms attached directly to the metal ion are called ligand
donor atoms,Ligand compounds of interest in wastewater
treatment include carbonate (CO32-),chloride (C1-),
hydroxide (OH),ammonia (NH3),and water (H2O),
Over the past 50 years,it has been observed that the
intermediate hydrolysis reactions of Al(III) are much more
complex than would be predicted on the basis of a model
in which a base is added to the solution,
Further,because the hydrolysis reactions follow a stepwise
process,the effectiveness of aluminum and iron will vary
with time,For example,an alum slurry that has been
prepared and stored will behave differently from a freshly
prepared solution when it is added to a wastewater,
Action of Hydrolyzed Metal Ions
1,Adsorption and charge neutralization
2,Adsorption and interparticle bridging
3,Enmeshment in sweep floc
If a sufficient concentration of metal salt is added,large
amounts of metal hydroxide floc will form,In turn,as
these floc particles settle,they sweep through the water
containing colloidal particles,The colloidal particles that
become enmeshed in the floc will thus be removed from
the wastewater,
Solubility of Metal Salts
The operating region for alum precipitation is from a pH
range of 5 to about 7,with minimum solubility occurring at
a pH of 6.0,and from about 7 to 9 for iron precipitation,
with minimum solubility occurring at a pH of 8.0,
Operating Regions for Action of Metal Salts
Fig,6-4 Typical operating ranges for alum coagulation
For example,optimum particle removal by sweep
floc occurs in the pH range of 7 to 8 with an alum
dose of 20 to 60 mg/L,
Generally,for many wastewater effluents that
have high pH values (e.g.,7.3 to 8.5),low alum
dosages in the range of 5 to 10 mg/L will not be
effective,With proper pH control it is possible to
operate with extremely low alum dosages,
Because the characteristics of wastewater will
vary from treatment plant to treatment plant,
bench-scale and pilot-plant tests must be
conducted to establish the appropriate chemical
dosages,
6-3 Chemical Precipitation For Improved Plant
Performance
Chemical precipitation,as noted previously,involves the
addition of chemicals to alter the physical state of dissolved
and suspended solids and facilitate their removal by
sedimentation,
Since about 1970,the need to provide more complete
removal of the organic compounds and nutrients (nitrogen
and phosphorus) contained in wastewater has brought about
renewed interest in chemical precipitation,In current
practice,chemical precipitation is used (1) as a means of
improving the performance of primary settling facilities,(2)
as a basic step in the independent physical-chemical
treatment of wastewater,(3) for the removal of phosphorus,
and (4) for the removal of heavy metals,
Alum,
The insoluble aluminum hydroxide is a gelatinous floc that
settles slowly through the wastewater,sweeping out
suspended material and producing other changes,The
reaction is exactly analogous when magnesium bicarbonate
is substituted for the calcium salt,
If less than this amount of alkalinity is available,it must be
added,Lime is commonly used for this purpose when
necessary,but it is seldom required in the treatment of
wastewater,
Lime,
Much more lime is generally required when it is used alone
than when sulfate of iron is also used where industrial
wastes introduce mineral acids or acid salts into the
wastewater,
Enhanced Removal of Suspended Solids in Primary
Sedimentation
With chemical precipitation,it is possible to remove 80 to
90 percent of the total suspended solids (TSS) including
some colloidal particles,50 to 80 percent of the BOD,and
80 to 90 percent of the bacteria,Comparable removal
values for well-designed and well-operated primary
sedimentation tanks without the addition of chemicals are
50 to 70 percent of the TSS,25 to 40 percent of the BOD,
and 25 to 75 percent of the bacteria,
Independent Physical-Chemical Treatment
In some localities,industrial wastes have rendered
municipal wastewater difficult to treat by biological means,
In such situations,physical-chemical treatment may be an
alternative approach,This method of treatment has met
with limited success because of its lack of consistency in
meeting discharge requirements,high costs for chemicals,
handling and disposal of the great volumes of sludge
resulting from the addition of chemicals,and numerous
operating problems,
Because of these reasons,new applications of physical-
chemical treatment for municipal wastewater are rare,
Physical-chemical treatment is used more extensively for
the treatment of industrial wastewater,
The filter is shown as optional,but its use is recommended
to reduce the blinding and headloss buildup in the carbon
columns,
The handling and disposal of the sludge resulting from
chemical precipitation is one of the greatest difficulties
associated with chemical treatment,Sludge is produced in
great volume from most chemical precipitation operations,
often reaching 0.5 percent of the volume of wastewater
treated when lime is used,
Fig,6-5 Typical flow diagram of an independent physical-
chemical treatment plant
6-4 Chemical Precipitation For Phosphorus Removal
The removal of phosphorus from wastewater involves the
incorporation of phosphate into TSS and the subsequent
removal of those solids,Phosphorus can be incorporated
into either biological solids (e.g.,microorganisms) or
chemical precipitates,
The topics to be considered include
(1) the chemistry of phosphate precipitation,
(2) strategies for phosphorous removal,
(3) phosphorus removal using metal salts and polymers
(4) phosphorus removal using lime,
Chemistry of Phosphate Precipitation
The chemical precipitation of phosphorus is brought about
by the addition of the salts of multivalent metal ions that
form precipitates of sparingly soluble phosphates,The
multivalent metal ions used most commonly are calcium
[Ca(II)],aluminum [Al(III)],and iron [Fe(III)],
Because the chemistry of phosphate precipitation with
calcium is quite different than with aluminum and iron,
the two different types of precipitation are considered
separately in the following discussion,
Phosphate Precipitation with Calcium,
When lime is added to water it reacts with the natural
bicarbonate alkalinity to precipitate CaCO3,As the pH
value of the wastewater increases beyond about 10,excess
calcium ions will then react with the phosphate to
precipitate hydroxylapatite Ca10(PO4)6(OH)2,
Because of the reaction of lime with the alkalinity of the
wastewater,the quantity of lime required will,in general,
be independent of the amount of phosphate present and
will depend primarily on the alkalinity of the wastewater,
The quantity of lime required to precipitate the phosphorus
in wastewater is typically about 1.4 to 1.5 times the total
alkalinity expressed as CaCO3,
Because a high pH value is required to precipitate
phosphate,coprecipitation is usually not feasible,When
lime is added to raw wastewater or to secondary effluent,
pH adjustment is usually required before subsequent
treatment or disposal,Recarbonation with carbon dioxide
(CO2) is used to lower the pH value
Phosphate Precipitation with Aluminum and Iron,
These reactions are deceptively simple and must be
considered in light of the many competing reactions and
their associated equilibrium constants,and the effects of
alkalinity,pH,trace elements,and ligands found in
wastewater,Therefore,dosages are generally established
on the basis of bench-scale tests and occasionally by full-
scale tests,
Strategies for Phosphorus Removal
The general locations where phosphorus can be removed may
be classified as (1) pre-precipitation,(2) coprecipitation,and
(3) postprecipitation,
Pre-precipitation,
Factors affecting the choice of chemical for phosphorus removal
1,Influent phosphorus level
2,Wastewater suspended solids
3,Alkalinity
4,Chemical cost(including transportation)
5,Reliability of chemical supply
6,Sludge handling facilities
7,Ultimate disposal methods
8,Compatibility with other treatment processes
Coprecipitation,
The addition of chemicals to form precipitates that are
removed along with waste biological sludge is defined as
"coprecipitation." Chemicals can be added to (1) the
effluent from primary sedimentation facilities,(2) the
mixed liquor (in the activated-sludge process),or (3) the
effluent from a biological treatment process before
secondary sedimentation,
Phosphorus Removal Using Metal Salts and
Polymers
Because polyphosphates and organic phosphorus are less
easily removed than orthophosphorus,adding aluminum or
iron salts after secondary treatment (where organic
phosphorus and polyphosphorus are transformed into
orthophosphorus) usually results in the best removal,Some
additional nitrogen removal occurs because of better
settling,but essentially no ammonia is removed unless
chemical additions to primary treatment reduce BOD
loadings to the point where nitrification can occur,A
number of the important features of adding metal salts and
polymers at different points in the treatment process are
discussed in this section,
Metal Salt Addition to Primary Sedimentation Tanks,
Organic phosphorus and polyphosphate are removed by
more complex reactions and by adsorption onto floc
particles,Adequate initial mixing and flocculation are
necessary upstream of primary facilities,whether separate
basins are provided or existing facilities are modified to
provide these functions,
In low-alkalinity waters,the addition of a base is
sometimes necessary to keep pH in the 5 to 7 range,Alum
generally is applied in a molar ratio in the range of a 1.4 to
2.5 mole Al/mole P,
Metal Salt Addition to Secondary Treatment,
In trickling filter systems,the salts are added to the
untreated wastewater or to the filter effluent,Phosphorus is
removed from the liquid phase through a combination of
precipitation,adsorption,exchange,and agglomeration,
and removed from the process with either the primary or
secondary sludges,or both,
Theoretically,the minimum solubility of AlPO4 occurs at
about pH 6.3,and that of FePO4 occurs at about pH 5.3;
however,practical applications have yielded good
phosphorus removal anywhere in the range of pH 6.5 to
7.0,which is compatible with most biological treatment
processes,
The use of ferrous salts is limited because they produce
low phosphorus levels only at high pH values,In low-
alkalinity waters,either sodium aluminate and alum or
ferric plus lime,or both,can be used to maintain the pH
higher than 5.5,
Dosages generally fall in the range of a 1 to 3 metal ion-
phosphorus molar ratio,
Metal Salt and Polymer Addition to Secondary Clarifiers,
In certain cases,such as trickling filtration and extended
aeration activated-sludge processes,solids may not
flocculate and settle well in the secondary clarifier,This
settling problem may become acute in plants that are
overloaded,
Aluminum and iron salts,along with certain organic
polymers,can also be used to coagulate colloidal particles
and to improve removals on filters,
Dosages of aluminum and iron salts usually fall in the
range of 1 to 3 metal ion/phosphorus on a molar ratio basis
if the residual phosphorus in the secondary effluent is
greater than 0.5 mg/L,To achieve phosphorus levels below
0.5 mg/L,significantly higher metal salt dosages and
filtration will be required,
Polymers may be added (1) to the mixing zone of a highly
mixed or internally recirculated clarifier,(2) preceding a
static or dynamic mixer,or (3) to an aerated channel,
Although mixing times of 10 to 30 seconds have been used
for polymers,shorter mixing times are favored,
Phosphorus Removal Using Lime
The use of lime for phosphorus removal is declining
because of (1) the substantial increase in the mass of
sludge to be handled compared to metal salts and (2) the
operation and maintenance problems associated with the
handling,storage,and feeding of lime,
Although lime recalcination lowers chemical costs,it is a
feasible alternative only for large plants,
The carbon dioxide from this process or other onsite stack
gas (containing 10 to 15 percent carbon dioxide) is
generally used as the source of recarbonation for pH
adjustment of the wastewater,
Lime Addition to Primary Sedimentation Tanks,
In the trickling filter process,the carbon dioxide generated
during treatment is usually sufficient to lower the pH
without recarbonation,
The dosage for low lime treatment is usually in the range of
75 to 250 mg/L as Ca(OH)2 at pH values of 8.5 to 9.5,In
low lime systems,however,the conditions required for
precipitation are more specialized; the Ca2+/Mg2+ mole
ratio is ≤5/1,
Lime Addition Following Secondary Treatment,
Generally,there is a second injection of carbon dioxide to
the second-stage effluent to reduce the formation of scale,
To remove the residual levels of TSS and phosphorus,the
secondary clarifier effluent is passed through a multimedia
filter or a membrane filter,Care should be taken to limit
excess calcium in the filter feed to ensure cementing of the
filter media will not occur,
Phosphorus Removal with Effluent Filtration
The removal of phosphorus by chemical addition to the
contact filtration process is used in many parts of the
country to remove phosphorus from wastewater treatment
plant effluents which are discharged to sensitive water
bodies,A two-stage filtration process has proved to be
very effective for the removal of phosphorus,Based on the
performance data from full-scale installations,phosphorus
levels equal to or less than 0.02 mg/L have been achieved
in the filtered effluent,
Comparison of Chemical Phosphorus Removal
Processes
Tab,6-4 Advantages and disadvantages of chemical addition in
various sections of a treatment plant for phosphorus removal
Estimation of Sludge Quantities from Phosphorus
Precipitation
The additional BOD and TSS removals afforded by
chemical addition to primary treatment may also solve
overloading problems on downstream biological systems,
or may allow seasonal or year-round nitrification,
depending on biological system designs,The BOD
removal in the primary sedimentation operation is on the
order of 50 to 60 percent at a pH of 9.5,The amount of
primary sludge will also increase significantly,
6-5 Chemical Precipitation For Removal Of Heavy
Metals And Dissolved Inorganic Substances
The technologies available for the removal of heavy metals
from wastewater include chemical precipitation,carbon
adsorption,ion exchange,and reverse osmosis,Of these
technologies,chemical precipitation is most commonly
employed for most of the metals,
Common precipitants include hydroxide (OH) and sulfide
(S2-),Carbonate (CO32-) has also been used in some special
cases,Metal may be removed separately or coprecipitated
with phosphorus,
Precipitation Reactions
Metals of interest include arsenic (As),barium (Ba),
cadmium (Cd),copper (Cu),mercury (Hg),nickel (Ni),
selenium (Se),and zinc (Zn),
In wastewater treatment facilities,metals are precipitated
most commonly as metal hydroxides through the addition
of lime or caustic to a pH of minimum solubility,
In practice,the minimum achievable residual metal
concentrations will also depend on the nature and
concentration of the organic matter in the wastewater as
well as the temperature,
Tab,6-5 Solubility products for free metal ion
concentrations in equilibrium with hydroxides and sulfides
Tab,6-6 Practical effluent concentration levels achievable
in heavy metals removal by precipitation
Coprecipitation with Phosphorus
When chemical precipitation is used,anaerobic digestion
for sludge stabilization may not be possible because of the
toxicity of the precipitated heavy metals,As noted
previously,one of the disadvantages of chemical
precipitation is that it usually results in a net increase in the
total dissolved solids of the wastewater that is being treated,
6-6 Chemical Oxidation
Chemical oxidation in wastewater treatment typically
involves the use of oxidizing agents such as ozone (O3),
hydrogen peroxide (H202),permanganate (MnO4),chloride
dioxide (ClO2),chlorine (C12) or (HOC1),and oxygen (O2)
Advanced oxidation process (AOPs) in which the free
hydroxyl radical (HO.) is used as a strong oxidant to
destroy specific organic constituents and compounds that
cannot be oxidized by conventional oxidants such as ozone
and chlorine are discussed in later chapters,
Oxidation-Reduction Reactions,
While an oxidizing agent causes the oxidation to occur,it is
reduced in the process,
Half-Reaction Potentials,
Of the many properties that can be used to characterize
oxidation-reduction reactions,the electrical potential (i.e.,
voltage) or emf of the half reaction is used most commonly,
The half-reaction potential is a measure of the tendency of
a reaction to proceed to the right,Half reactions with large
positive potential,E。,tend to proceed to the right as
written,Conversely,half reactions with large negative
potential,E。,tend to proceed to the left,
Tab,6-7 Standard electrode potentials for oxidation half
reactions for chemical disinfection
Tab,6-8 Typical applications of chemical oxidation in
wastewater collection,treatment,and disposal
Chemical Oxidation of Ammonia
The chemical process in which chlorine is used to oxidize
the ammonia nitrogen in solution to nitrogen gas and other
stable compounds is known as breakpoint chlorination,
Perhaps the most important advantage of this process is
that,with proper control,all the ammonia nitrogen in the
wastewater can be oxidized,
However,because the process has a number of
disadvantages including the buildup of acid (HCl) which
will react with the alkalinity,the buildup of total dissolved
solids,and the formation of unwanted chloro-organic
compounds,ammonia oxidation is seldom used today,
6-7 Chemical Neutralization,Scale Control,And
Stabilization
Scaling control is required for nanofiltration and reverse
osmosis treatment to control the formation of scale,
pH Adjustment
Lime can be purchased as quicklime or slaked hydrated
lime,high-calcium or dolomitic lime,and in several
physical forms,Limestone and dolomitic limestone are
cheaper but less convenient to use and slower in reaction
rate,Because they can become coated in certain waste-
treatment applications,their use is limited,Depending on
the sensitivity of the environment,two-stage neutralization
may be required,The reagent chemicals can be fed
automatically,in the form of solutions,slurries,or dry
materials,
Analysis of Scaling Potential
With the increasing use that is being made of nanofiltration,
reverse osmosis,and electrodialysis in wastewater reuse
applications,adjustment of the scaling characteristics of
the effluent to be treated is important to avoid calcium
carbonate and sulfate scale formation,Depending on the
recovery rate,the concentration of salts can increase by a
factor of up to 10 within the treatment module,When such
a salt concentration increase occurs,it is often possible to
exceed the solubility product of calcium carbonate and
other scale-forming compounds,The formation of scale
within the treatment module will cause a deterioration in
the performance,ultimately leading to the failure of the
membrane module,
Scaling Control
Usually,CaCO3 scale control can be achieved using one or
more of the following methods,
,Acidifying to reduce pH and alkalinity
,Reducing calcium concentration by ion exchange or lime
softening
,Adding a scale inhibitor chemical (antiscalant) to increase
the apparent solubility of CaCO3 in the concentrate stream
,Lowering the product recovery rate
Stabilization
Wastewater effluent that is demineralized with reverse
osmosis will generally require pH and calcium carbonate
adjustment (stabilization) to prevent metallic corrosion,
due to the contact of the demineralized water with metallic
pipes and equipment,
Corrosion occurs because material from the solid is
removed (solubilized) to satisfy the various solubility
products,Demineralized water typically is stabilized by
adding lime to adjust the LSI,using the procedure outlined
above,
6-8 Chemical Storage,Feeding,Piping,And Control
Systems
Coagulants in the dry solid form generally are converted to
solution or slurry form prior to introduction into the
wastewater,Coagulants in the liquid form are usually
delivered to the plant in a concentrated form and have to be
diluted prior to introduction into the wastewater,
Chemicals in the gas form (generally stored as a liquid),
typically used for disinfection purposes,are either
dissolved in water before injection or are injected directly
into the wastewater,
Chemical feeders are generally designed to be (1)
proportioning,feeding chemical in proportion to the
influent wastewater flowrate,and (2) constant feed,
designed to deliver chemical at a fixed rate regardless of
the influent flowrate,
Dry Chemical-Feed Systems
The units are sized according to the volume of wastewater,
treatment rate,and optimum length of time for chemical
feeding and dissolving,Hoppers used with powdered
chemicals that are compressible and can form an arch such
as lime are equipped with positive agitators and a dust-
collection system,
Liquid Chemical-Feed Systems
The storage tank is sized based upon the stability of the
chemical,feed rate requirements,delivery constraints (cost,
size of tank truck,etc.),and availability of the supply,
7 Suspended Growth Biological Treatment
Processes
7-1 Introduction to the Activated-Sludge
Process
Historical Development
Experiments conducted at the Lawrence
Experiment Station during 1912 and 1913 by
Clark and Gage
With air input into wastewater,growths of
organisms could be cultivated in bottles and in
tanks partially filled with roofing slate spaced
about 25 mm (1 in) apart and would greatly
increase the degree of purification,
Lockett found that the sludge played an important
part,
The basic activated-sludge treatment process,as
illustrated on Fig,7-la and b,consists of the
following three basic components,(1) a reactor in
which the microorganisms responsible for
treatment are kept in suspension and aerated; (2)
liquid-solids separation,usually in a sedimentation
tank; and (3) a recycle system for returning solids
removed from the liquid-solids separation unit
back to the reactor,
Fig,7-1Description of Basic Process
An important feature of the activated-sludge
process is the formation of flocculent settleable
solids,
For these applications,various modifications of
conventional activated-sludge processes are used,
including sequencing batch reactors,oxidation
ditch systems,aerated lagoons,or stabilization
ponds,
Evolution of the Activated-Sludge Process
A number of activated-sludge processes and design
configurations have evolved since its early conception as a
result of (1) engineering innovation in response to the need
for higher-quality effluents from wastewater treatment
plants; (2) technological advances in equipment,
electronics,and process control; (3) increased
understanding of microbial processes and fundamentals;
and (4) the continual need to reduce capital and operating
costs,
The use of a plug-flow process became
problematic when industrial wastes were
introduced because of the toxic effects of some of
the discharges,
With the development of simple inexpensive
program logic controllers (PLCs) and the
availability of level sensors and automatically
operated valves,the sequencing batch reactor
(SBR) process (see Fig,7-1c) became more
widely used by the late 1970s,especially for
smaller communities, Some of the stages are not
aerated (anaerobic or anoxic stages) and internal
recycle flows may be used,
Recent Process Developments
As noted above,numerous modifications of
the activated-sludge process have evolved
in the last 10 to 20 years,aimed principally
at effective and efficient removal of
nitrogen and phosphorus,
7-2 Wastewater Characterization
Activated-sludge process design requires
determining (1) the aeration basin volume,
(2) the amount of sludge production,(3) the
amount of oxygen needed,and (4) the
effluent concentration of important
parameters,
7-3 Fundamentals of Process Analysis and Control
Important factors that must be considered in
the selection of reactor types for the
activated-sludge process include (1) the
effects of reaction kinetics,(2) oxygen
transfer requirements,(3) nature of the
wastewater,(4) local environmental
conditions,(5) presence of toxic or
inhibitory substances in the influent
wastewater,(6) costs,and (7) expansion to
meet future treatment needs,
Selection of Reactor Type
Selection of Solids Retention Time and Loading
Criteria
The common parameters used are the solids retention time
(SRT),the food to biomass (F/M) ratio (also known as
food to microorganism ratio),and the volumetric organic
loading rate(Lv),
Solids Retention Time
The SRT,in effect,represents the average period of time
during which the sludge has remained in the system,SRT
is the most critical parameter for activated-sludge design as
SRT affects the treatment process performance,aeration
tank volume,sludge production,and oxygen requirements,
Sludge Production
Sludge will accumulate in the activated-sludge
process if it cannot be processed fast enough by an
undersized sludge-handling facility,
The observed yield decreases as the SRT is
increased due to biomass loss by more
endogenous respiration,The yield is higher when
no primary treatment is used,as more nbVSS
remains in the influent wastewater(A-B),
Oxygen Requirements
As an approximation,for BOD removal only,the oxygen
requirement will vary from 0.90 to 1.3 kg O2/kg BOD
removed for SRTs of 5 to 20 d,
Nutrient Requirements
Using the formula C5H7NO2,for the composition of cell
biomass,about 12.4 percent by weight of nitrogen will be
required,The phosphorus requirement is usually assumed
to be about one-fifth of the nitrogen value,As a general
role,for SRT values greater than 7 d,about 5 g nitrogen
and 1 g phosphorus will be required per 100 g of BOD to
provide an excess of nutrients,
Other Chemical Requirements
The amount of alkalinity required for nitrification,taking
into account cell growth,is about 7.07 g CaCO3/g NH4-N,
In addition to the alkalinity required for nitrification,
additional alkalinity must be available to maintain the pH
in the range from 6.8 to 7.4,Typically the amount of
residual alkalinity required to maintain pH near a neutral
point (i.e.,pH ≈ 7) is between 70 and 80 mg/L as CaCO3,
Mixed-Liquor Settling Characteristics
Clarifier design must provide adequate clarification of the
effluent and solids thickening for the activated-sludge
solids,
Two commonly used measures developed to quantify the
settling characteristics of activated sludge are the sludge
volume index (SVI) and the zone settling rate,
The SVI is determined by placing a mixed-liquor sample in
a 1- to 2-L cylinder and measuring the settled volume after
30 min and the corresponding sample MLSS concentration,
For example,a mixed-liquor sample with a 3000 mg/L
TSS concentration that settles to a volume of 300 mL in 30
min in a 1-L cylinder would have an SVI of 100 mL/g,A
value of 100 mL/g is considered a good settling sludge
(SVI values below 100 are desired),SVI values above 150
are typically associated with filamentous growth,
Use of Selectors
Because solids separation is one of the most
important aspects of biological wastewater
treatment,a biological selector (a small
contact tank) is often incorporated in the
design to limit the growth of organisms that
do not settle well,An appropriate selector
design can be added before the activated-
sludge aeration basin,
Process Control
To maintain high levels of treatment performance
The principal approaches to process control are (1)
maintaining dissolved oxygen levels in the
aeration tanks,(2) regulating the amount of return
activated sludge (RAS),and (3) controlling the
waste-activated sludge (WAS),
The parameter used most commonly for
controlling the activated-sludge process is SRT,
Dissolved Oxygen Control,
When oxygen limits the growth of microorganisms,
filamentous organisms may predominate and the
settleability and quality of the activated sludge
may be poor,
In general,the dissolved oxygen concentration in
the aeration tank should be maintained at about
1.5 to 2 mg/L in all areas of the aeration tank,
Return Activated-Sludge Control
Return sludge concentrations from secondary clarifiers
range typically from 4000 to 12,000 mg/L,
Settleability
If the settleable solids occupied a volume of 275 mL after
30 min of settling,the percentage volume would be equal
to 38 percent [(275 mL / 725 mL) * 100],If the plant flow
were 2 m3/s,the return sludge rate should be 0.38 × 2
m3/s = 0.76 m3/s,
Sludge Blanket Level
The optimum level is determined by experience
and is a balance between settling depth and sludge
storage,The optimum depth of the sludge blanket
usually ranges between 0.3 and 0.9 m,
Considerations,
(1) diurnal flow variations
(2) sludge production variations
(3)changes in the settling characteristics of the
sludge,
Sludge Wasting
An alternative method of wasting
sometimes used is withdrawing mixed
liquor directly from the aeration tank or the
aeration tank effluent pipe,
Oxygen Uptake Rates
OUR or respiration rate used to assess the
presence of toxic or inhibitory substances in
the influent wastewater,
Microscopic Observations
Specific information gathered includes
? changes in floc size and density
? status of filamentous organism growth in the floc
? presence of Nocardia bacteria,
? type and abundance of higher life-forms such as
protozoans and rotifers,
A decrease in the protozoan population may be indicative
of DO limitations,operation at a lower SRT inhibitory
substances in the wastewater,
Operational Problems
The most common problems encountered in the
operation of an activated-sludge plant are bulking
sludge,rising sludge,and Nocardia foam,
Bulking Sludge
In a bulking sludge condition,the MLSS floc does
not compact or settle well,and floc particles are
discharged in the clarifier effluent,
The other type of bulking,viscous
bulking,is caused by an excessive amount
of extracellular biopolymer,Viscous
bulking is usually found with nutrient-
limited systems or in a very high loading
condition with wastewater having a high
amount of rbCOD,
This structure,in contrast to the preferred
dense floc with good settling properties,has
an increased surface area to mass ratio,
which results in poor settling,
The classification system is based on
morphology (size and shape of cells,length
and shape of filaments),staining responses,
and cell inclusions,
One of the kinetic features of filamentous
organisms that relates to these conditions is
that they are very competitive at low substrate
concentrations whether it be organic substrates,
DO,or nutrients,Thus,lightly loaded
complete-mix activated-sludge systems or low
DO (<0.5 mg/L) operating conditions provide
an environment more favorable to filamentous
bacteria than to the desired floc-forming
bacteria,
When the influent wastewater contains
fermentation products such as volatile fatty
acids(VFA) and reduced sulfur compounds
(sulfides and thiosulfate),Thiothrix can
proliferate,
View the mixed liquor under the microscope,
A reasonable quality phase-contrast
microscope with magnification up to 1000
times (oil immersion) is necessary to view
the filamentous bacteria or fungi structure
and size,
Process Loading/Reactor Configuration
In many cases,complete-mix systems with long
SRTs and subsequent low F/M ratios experience
filamentous growths,In such systems,the
filamentous organisms are more competitive for
substrate,
We can use selector processes to solve these
problems because they provide conditions that
cause selection of floc-forming bacteria in lieu of
filamentous organisms as the dominant population,
Temporary Control Measures
In an emergency situation or while the aforementioned
factors are being investigated,chlorine and hydrogen
peroxide may be used to provide temporary help,
Chlorination of return sludge has been practiced quite
extensively as a means of controlling bulking,
Rising Sludge
The most common cause of this phenomenon is
denitrification,in which nitrites and nitrates in the
wastewater are converted to nitrogen gas,If enough gas is
formed,the sludge mass becomes buoyant and rises or
floats to the surface,
Nocardia Foam,
Two bacteria genera,Nocardia and Microthrix parvicella,
are associated with extensive foaming in activated-sludge
processes,These organisms have hydrophobic cell surfaces
and attach to air bubbles,where they stabilize the bubbles
to cause foam,
Nocardia has a filamentous structure,and
the filaments are very short and are
contained within the floc particles,
Microthrix parvicella has thin filaments
extending from the floc particles,
The foam is thick,has a brown color,and
can build up in thickness of 0.5 to 1 m,but
is more pronounced with diffused aeration
and with higher air flowrates,
Methods that can be used to control Nocardia include (1)
avoiding trapping foam in the secondary treatment process,
(2) avoiding the recycle of skimmings into the secondary
treatment process,and (3) using chlorine spray on the
surface of the Nocardia foam,
The addition of a small concentration of cationic polymer
has been used with some success for controlling Nocardia
foaming, Reducing the oil and grease content from
discharges to the collection system from restaurants,truck
stops,and meatpacking facilities by effective degreasing
processes can help control potential Nocardia problems,
Activated-Sludge Selector Processes
The high substrate concentration in the selector
favors the growth of nonfilamentous organisms,
A selector is a small tank (20 to 60 min contact
time) or a series of tanks in which the incoming
wastewater is mixed with return sludge under
aerobic,anoxic,and anaerobic conditions,
The goal in the selector is to have most of the
rbCOD consumed by the floc-forming bacteria,
With biological nutrient-removal
processes(脱氮除磷 ),improved sludge-
settling characteristics,and,in many cases,
minimal filamentous bacteria growth has
been observed,The anoxic or anaerobic
metabolic conditions used in these
processes favor growth of the floc-forming
bacteria,The filamentous bacteria cannot
use nitrate or nitrite for an electron acceptor,
Sequencing Batch Reactor Process
For continuous-flow applications,at
least two SBR tanks must be
provided so that one tank receives
flow while the other completes its
treatment cycle,
Sludge Wasting in SBRs
A unique feature of the SBR system is that
there is no need for a return activated-
sludge (RAS) system,
Because of the substrate concentration
changes with time,the substrate utilization
and oxygen demand rates change,
progressing from high to low levels,
Tab,7-6 Computation approach for the design of a SBR
7-5 Process for Biological Nitrogen Removal
Nitrogen removal is needed to prevent
eutrophication),or for groundwater recharge or
other reuse applications,
Nitrogen removal can be either an integral part of
the biological treatment system or an add-on
process to an existing treatment plant,
Following the discussion of design issues,design
examples are provided for (1) the anoxic/aerobic
process,(2) step-feed anoxic/aerobic process,(3)
intermittent aeration,(4) a sequencing batch
reactor,and (5) postanoxic denitrification with
methanol addition,
First,regions of low DO or zero DO
concentration may be present within the
basin as a function of the mixing regime,
Second,activated-sludge floc can contain
both aerobic and anoxic zones,
Single-Sludge Simultaneous Nitrification
Denitrificatlon (SNdN) Processes
The nitrification and denitrification rates
are a function of the reaction kinetics,floc
size,floc density,floc structure,rbCOD
loading,and bulk liquid DO concentration,
Fig,7-16 Operation of a Nitrox oxidation ditch process using
intermittent aeration,(a)aerobic conditions; (b)anoxic conditions;
(c)variations in ORP,DO,ammonia and nitrate
7-6 Processes for Biological Phosphorus Removal
Barnard (1974) was the first to clarify the need for
anaerobic contacting between activated sludge and
influent wastewater before aerobic degradation to
accomplish biological phosphorus removal,
Biological Phosphorus-Removal Processes
The main difference between the Phoredox (A/O)
process and the A2O processes shown on Fig,7-
17a and b is that nitrification does not occur in the
Phoredox (A/O) process,Low operating SRTs are
used to prevent the initiation of nitrification,
Fig,7-17 Typical biological phosphorus removal process
Process Design Considerations
The process design considerations for
BPR(biological phosphorus removal)
processes include (1) wastewater
characteristics,(2) anaerobic contact time,
(3) SRT,(4) waste sludge processing
method,and (5) chemical addition
capability,
The conversion of rbCOD to VFAs (including
acetate)occurs quickly through fermentation in the
anaerobic zone and 7 to 10 mg of acetate results in
about 1.0 mg P removal,The more acetate,the
more cell growth,and,thus,more phosphorus
removal,
During wet-weather conditions,especially in the
winter,BPR may be difficult to achieve due to
cold,low strength wastewater that does not readily
become anaerobic,
With continuous VFA addition,the effluent
soluble phosphorus concentration decreased from
2.5 to 0.3 mg/L,
Anaerobic Contact Time
Detention times of 0.25 to 1.0 h are
adequate for fermentation of rbCOD,
Polyhydroxybutyrate(PHB) provides energy
for phosphorus uptake and storage,
Solids Retention Time
First,the final amount of phosphorus removed is
proportional to the amount of biological
phosphorus-storing bacteria wasted,
Second,at long SRTs the biological phosphorus
bacteria are in a more extended endogenous phase,
which will deplete more of their intracellular
storage products,If the intracellular glycogen(糖
元 ) is depleted,less efficient acetate uptake and
PHB storage will occur in the anaerobic contact
zone,thus making the overall BPR process less
efficient,
Waste Sludge Processing
Phosphorus is released when the bacteria
that contain stored phosphorus are subject
to anaerobic conditions,
Anaerobic conditions in thickening and/or
digestion can thus result in the release of
significant amounts of phosphorus,
Thickening of waste sludge by dissolved air
flotation,gravity belt thickeners,or rotary-
drum thickeners is preferred over gravity
thickening of waste sludge to minimize
phosphorus release,
Chemical Addition Capability
Phosphorus can be removed biologically to dissolved
concentrations as low as 0.20 to 0.30 mg/L,provided
sufficient rbCOD is available,
Where there are insufficient amounts of rbCOD in the
influent wastewater,chemical addition is necessary to
provide enough phosphorus removal to meet the effluent
discharge concentration needed,Alum or iron salts may be
used and may be applied at a number of locations in the
liquid stream treatment process,
Iron salts may be preferred in some cases over alum salts
for primary treatment applications,because they have the
additional advantage of removing sulfide to help reduce
odors,
Process Control
Filter backwash recycle flows should be
sent to the aerobic zone instead of the
anaerobic or anoxic zones,Recycle streams
with significant concentrations of DO and
nitrate can have an adverse impact on
process performance,
Effluent Suspended Solids
The phosphorus content in the mixed-liquor
solids is greater than that from the
conventional activated-sludge process due
to the biological phosphorus storage,
At 3 to 6 percent phosphorus in the solids,
the phosphorus contribution in an effluent
having a TSS concentration of 10 mg/L
would be 0.3 to 0.6 mg/L,values that are
significant if the effluent standard is less
than 1.0 mg P/L,To provide very low
effluent phosphorus concentrations,effluent
filtration may be required,
Methods to Improve Phosphorus-Removal
Efficiency in BPR Systems
1,Provide supplemental acetate by direct
purchase or by primary sludge fermentation,
2,Reduce the process SRT,
3,Add alum or iron salts in primary
treatment or for effluent polishing,
4,Reduce the amount of nitrate and/or
oxygen entering the anaerobic zone,
Fig,7-18 Examples of fermentation reactors for producing
volatile fatty acids(VFAs) used for puosphorus removal
A deeper depth primary clarifier design has also been proposed
to provide sufficient holding time for the settled primary sludge
for hydrolysis and acid fermentation,
The additional VFA supply to the BPR
process decreased the average effluent
phosphorus concentration from about 1.5 to
0.5 mg/L,Alum was later added before the
secondary clarifiers at a dosage of about 8
mg/L (as alum) to further reduce the
effluent phosphorus concentration,
With both prefermentation and alum
addition,the effluent phosphorus
concentration averaged less than 0.20 mg/L,
Fig,7-19 Typical non-Nocardia froth on activated-
sludge aeration tank sludge return channels
Wastewater normally contains soap,detergents,and other
surfactants that produce foam when the wastewater is
aerated,
The wind may lift the froth off the tank surface
and blow it about,contaminating whatever it
touches,The froth,besides being unsightly,is a
hazard to those working with it because it is very
slippery,even after it collapses,In addition,once
the froth has dried,it is difficult to remove,
A commonly used system for spiral-roll tanks
consists of a series of spray nozzles mounted
above the surface in areas where the froth collects,
Screened effluent or clear water is sprayed
through these nozzles and physically breaks down
the froth as it forms,Another approach is to meter
a small quantity of antifoaming chemical additive
into the spray water,
Fig,7-20 Typical circular sludge collection mechanisms,
(a)suction type; (b)spiral type scrapper
Fig,7-22 Alternative methods of flow splitting,(a)hydraulic
symmetry; (b)flow measurement and feedback control;
(c)hydraulic split with Weirs; (d)inlet feed gate control
Fig,7-23 Energy-dissipating inlet devices used in circular clarifiers,
(a)schematic of a center column energy-dissipating inlet and flocculation
feed well; (b)view of an energy-dissipating feed well
Tank Inlet Design
Poor distribution or jetting of the tank
influent can increase the formation of
density currents and scouring of settled
sludge,etc,
Tank inlets should dissipate influent energy,
distribute the flow evenly in horizontal and
vertical directions,mitigate density currents,
minimize sludge blanket disturbance,and
promote flocculation,
Fig,7-24 Alternative peripheral baffle arrangements,
(a)Stamford; (b)unnamed; (c)Mckinney; (d)interior trough
Scum Removal
Typical scum-removal equipment includes beach
and scraper type,rotating pipe-through skimmer,
and slotted pipes,
Scum should not be returned to the plant
headworks because microorganisms responsible
for foaming (typically Nocardia) will be recycled,
causing foaming problems to persist because of
continuous seeding of the unwanted
microorganisms,
7-8 Suspended Growth Aerated Lagoons
Types of Suspended Growth Aerated Lagoons
Classified based on the manner in which the
solids are handled are,
1,Facultative partially mixed
2,Aerobic flow through with partial mixing
3,Aerobic with solids recycle and nominal
complete mixing
Facultative Partially Mixed Lagoon
The energy input is only sufficient to
transfer the amount of oxygen required for
biological treatment,but is not sufficient to
maintain the solids in suspension,
Because the energy input will not maintain
the solids in suspension,a portion of the
incoming solids will settle along with a
portion of the biological solids produced
from the conversion of the soluble organic
substrate,
Temperature
Because suspended growth aerobic flow-through
lagoons are often installed and operated in
locations with widely varying climatic conditions,
the effects of temperature change must be
considered in their design,The two most
important effects of temperature are (1) reduced
biological activity and treatment efficiency and (2)
the formation of ice,
Surface aerators tend to further cool lagoons in
cold weather,but submerged diffused air systems
add heat to some extent,
In multiple lagoon systems,cold-weather
effects can be mitigated by seasonal
changes in the method of operation,During
the warmer months,the lagoons would be
operated in parallel; in the winter,they
would be operated in series,In the winter
operating mode,the downstream aerators
could be turned off and removed,and the
lagoon surface is allowed to freeze,
With this method of operation it is
possible to achieve a 60 to 70 percent
removal of BOD even during the
coldest winter months,Still another
method that can be used to improve
performance during the winter months
is to recycle a portion of the solids
removed by settling,
Mixing Requirements
The threshold energy input value for the
suspension of biosolids is about 1.5 to 1.75
kW/l03 m3,For depths greater than 3.7m,
aerators with draft tubes may be considered
to prevent solids deposition,
The hydraulic detention time for the
complete-mix lagoon will typically vary
from 1.5 to 3 d,The total retention time for
the facultative lagoons is on the order of 3d,
7-9 Biological Treatment with Membrane Separation
Membrane biological reactors (MBRs),
consisting of a biological reactor (bioreactor
with suspended biomass and solids
separation by microfiltration membranes
with nominal pore sizes ranging from 0.1 to
0.4 μm,are finding many applications in
wastewater treatment,
The ability of MBR to eliminate secondary
clarification and operate at higher MLSS
concentration provides the following advantages,
( 1 ) higher volumetric loading rates and thus
shorter reactor hydraulic retention times; (2)
longer SRTs resulting in less sludge production; (3)
operation at low DO concentrations with potential
for simultaneous nitrification-denitrification in
long SRT designs; (4) high-quality effluent in
terms of low turbidity bacteria,TSS,and BOD;
and;(5) less space required for wastewater
treatment,
Disadvantages of MBRs include high capital costs,
limited data on membrane life,potential high cost
of periodic membrane replacement or scouring,
higher energy costs,and the need to control
membrane fouling,
Membrane bioreactor systems have two basic
configurations,(1) the integrated bioreactor that
uses membranes immersed in the bioreactor and (2)
the recirculated MBR in which the mixed liquor
circulates through a membrane module situated
outside the bioreactor,
The membranes are mounted in modules
(sometimes called cassettes) that can be lowered
into the bioreactor,The modules are comprised of
the membranes,support structure for the
membranes,feed inlet and outlet connections,and
an overall support structure,
The membranes are subjected to a vacuum
(less than 50 kPa) that draws water
(permeate) through the membrane while
retaining solids in the reactor,To maintain
TSS within the bioreactor and to clean the
exterior of the membranes,compressed air
is introduced through a distribution
manifold at the base of the membrane
module,As the air bubbles rise to the
surface,scouring of the membrane surface
occurs; the air also provides oxygen to
maintain aerobic conditions,
Fig,7-25 Typical membrane bioreactors,(a)schematic of placement of
membrane bundles in an activated Sludge reactor ; (b)membrane bundle
in position to be placed in a membrane bioreactor
In the MBR system,MLSS concentrations in the
range of 8000 to 10,000 mg/L appear to be most
cost-effective when all factors are considered,
The membrane flux rate,defined as the mass or
volume rate of transfer through the membrane
surface [in terms of L/m2.h] is an important design
and operating parameter that affects the process
economics,
Membrane Fouling Control
In the activated-sludge reactor,biomass coats the
outer layer of the membranes used in an integrated
MBR during effluent withdrawal,Finer particles
may penetrate the inner pores of the membrane,
causing an increase in pressure loss,
First,coarse bubble aeration is provided at the
bottom of the membrane tank directly below the
membrane fibers,The air bubbles flow upward
between the vertically oriented fibers,causing the
fibers to agitate against one another to provide
mechanical scouring,
Second,filtration is interrupted about every 15 to
30 min and the membrane fibers are backwashed
for 30 to 45 s,
Typically,a low concentration of chlorine (<5
mg/L) is maintained in the backflush water to
inactivate and remove microbes,Third,about
three times per week a strong sodium hypochlorite
solution (about 100 mg/L) or citric acid is used in
the backflush mode for 45 min,
The cassettes can be removed easily from the
aeration basin by an overhead hoist system for a
periodic chemical-bath cleaning,
When removed for cleaning,the cassettes are
submerged in a high-concentration chlorine
solution bath in a separate small tank or basin,
External cleaning occurs about every 3 to 6
months,
The combination of air scour,backflushing,and
maintenance cleaning is not completely effective
in controlling membrane fouling,and the pressure
drop across the membrane increases with time,At
a maximum operating pressure drop of ≈60 kPa,
the membranes are removed from the aeration
basin for a recovery cleaning,
8 Attached Growth Biological Treatment Processes
8-1 Background
Nonsubmerged Attached Growth Processes
A trickling filter is a nonsubmerged
fixed-film biological reactor using rock
or plastic packing over which
wastewater is distributed continously,
The limitations of the trickling filter
included a relatively high incidence of
clogging,the long rest period required,
headloss,and the relatively low
loading that could be used,
In the 1950s,plastic packing began to
replace rock,which allowed the use of
higher loading rates and taller filters (also
known as biotowers) with less land area,
improved process efficiency,and reduced
clogging,
In the 1960s,practical designs were
developed for rotating biological contactors
(RBCs),where the packing is rotated in the
wastewater treatment tank,
Both trickling filters and RBCs have been
used as aerobic attached growth processes
for BOD removal only,combined BOD
removal and nitrification,
In comparison to the activated-sludge process,
disadvantages encountered for trickling filters
are a poorer effluent quality in terms of BOD
and TSS concentrations,greater sensitivity to
lower temperatures,odor production,and
uncontrolled solids sloughing events,
Trickling filters and RBCs have also been used
in combined processes with activated sludge to
utilize the benefits of both processes,in terms
of energy savings and effluent quality,
8-2 Trickling Filters
The depth of the rock packing ranges from
0.9 to 2.5 m and averages 1.8 m,Rock filter
beds are usually circular,and the liquid
wastewater is distributed over the top of the
bed by a rotary distributor,
Trickling filters that use plastic packing
have been built in round,square,and other
shapes with depths varying from 4 to 12 m,
In addition to the packing,other
components of the trickling filter include a
wastewater dosing or application system,an
underdrain,and a structure to contain the
packing,The underdrain system is
important both for collecting the trickling
filter effluent liquid and as a porous
structure through which air can circulate,
In practice,a portion of the liquid collected
in the underdrain system or the settled
effluent is recycled to the trickling filter
feed flow,usually to dilute the strength of
the incoming wastewater and to maintain
enough wetting to keep the biological slime
layer moist,
Distributor arms extend across the trickling
filter inner diameter and have variable
openings to provide a uniform application
rate per unit area,The distributor arms are
rotated by the force of the water exiting
through their opening or by the use of
electric drives,
In some cases,especially for square or
rectangular filters,fixed flat-spray nozzles
have been used,
Primary clarification is necessary before
rock trickling filters,
In some installations,a wire-mesh screen is
placed over the top of plastic packing to
collect debris that can be vacuumed off
periodically,
The biological community in the filter
includes aerobic and facultative bacteria,
fungi,algae,and protozoans,Higher
animals,such as worms,insect larvae,and
snails,are also present,
Pseudomonas,and Alcaligenes are among the
bacterial species commonly associated with the
trickling filter,Within the slime layer,where
adverse conditions prevail with respect to growth,
the filamentous forms Sphaerotilus natans and
Beggiatoa will be found,In the lower reaches of
the filter,the nitrifying bacteria will be present,
The fungi present are also responsible for waste
stabilization,but their role is usually important
only under low-pH conditions,At times,fungi
growth can be so rapid that the filter clogs and
ventilation becomes restricted,Among the fungi
species that have been identified are Fusarium,
Mucor,Penicillium,Geotrichum,Sporatichum,
and various yeasts,
Algae can grow only in the upper reaches of the
filter where sunlight is available,Generally,algae do
not take a direct part in waste degradation,but
during the daylight hours they add oxygen to the
percolating wastewater,
The function of protozoa is to feed on the
biological films and,as a result,effluent turbidity
decreases and the biofilm is maintained in a higher
growth state,The higher animals,such as worms,
snails,and insects,feed on the biological film,
Bacteria in the slime layer enter an
endogenous respiration state and lose their
ability to cling to the packing surface,
The liquid then washes the slime off the
packing,and a new slime layer starts to
grow,The phenomenon of losing the slime
layer is called sloughing and is primarily a
function of the organic and hydraulic
loading on the filter,The hydraulic loading
accounts for shear velocities,and the
organic loading accounts for the rate of
metabolism in the slime layer,
Trickling Filter Classification and Applications
Rock filter designs have been classified as
low- or standard-rate,intermediate-rate,and
high-rate,Plastic packing is used typically
for high-rate designs,Much higher organic
loadings have been used for rock or plastic
packing designs in "roughing" applications
where only partial BOD removal occurs,
Low-Rate Filters
Dosing tanks are small,usually with only a 2-min
detention time based on twice the average design
flow,so that intermittent dosing is minimized,
Even so,at small plants,low nighttime flows may
result in intermittent dosing and recirculation may
be necessary to keep the packing moist,If the
interval between dosing is longer than 1 or 2 h,the
efficiency of the process deteriorates because the
character of the biological slime is altered by a
lack of moisture,
If the nitrifying population is sufficiently
well established,and if climatic conditions
and wastewater characteristics are favorable,
a well-operated low rate filter can provide
good BOD removal and a highly nitrified
effluent,
Odors are a common problem,especially if
the wastewater is stale or septic,or if the
weather is warm,Filter flies (Psychoda)
may breed in the filters unless effective
control measures are used,
Intermediate- and High-Rate Filters
Recirculation of the filter effluent or final effluent
permits higher organic loadings,provides higher
dosing rates on the filter to improve the liquid
distribution and better control of the slime layer
thickness,provides more oxygen in the influent
wastewater flow,and returns viable organisms,
Recirculation also helps to prevent ponding in the
filter and to reduce the nuisance from odors and
flies,
Roughing Filters
Roughing filters are high-rate-type filters that treat
an organic load of more than 1.6 kg/m3·d and
hydraulic loadings up to 190 m3/m2·d,
Two-Stage Filters
Two-stage systems are also used where
nitrification is required,The first-stage filter and
intermediate clarifier reduce carbonaceous BOD,
and nitrification takes place in the second stage,
Design of Physical Facilities
(1) type and physical characteristics of filter
packing to be used; (2) dosing rate; (3) type and
dosing characteristics of the distribution system;
(4) configuration of the underdrain system; (5)
provision for adequate airflow (i.e.,ventilation),
either natural or forced air; and (6) sealing tank
design,
Distribution Systems
Clearance of 150 to 225 mm should be allowed between
the bottom of the distributor arm and the top of the bed,
The clearance permits the wastewater streams from the
nozzles to spread out and cover the bed uniformly,and it
prevents ice accumulations from interfering with the
distributor motion during freezing weather,
Nozzles are spaced unevenly so that greater flow per unit
of length is achieved near the periphery of the filter than at
the center,Headloss through the distributor is in the range
of 0.6 to 1.5 m,
Underdrains
The underdrain system for a rock filter usually has
precast blocks of vitrified clay or fiberglass
grating laid on a reinforced-concrete subfloor (see
Fig,8-5),The floor and underdrains must have
sufficient strength to support the packing,slime
growth,and the wastewater,The floor and
underdrain block slope to a central or peripheral
drainage channel at a 1 to 5 percent grade,The
effluent channels are sized to produce a minimum
velocity of 0.6 m/s at the average daily flowrate,
Underdrains may be open at both ends,so that
they may be inspected easily and flushed out if
they become plugged,
Fig,8-6 Typical under-drain system for Tower filter
Airflow
Natural ventilation has historically been the
primary means of providing airflow,but it is not
always adequate and forced ventilation using low-
pressure fans provides more reliable and
controlled airflow,
In the case of natural ventilation,the driving force
for airflow is the temperature difference between
the ambient air and the air inside the pores,If the
wastewater is colder than the ambient air,the pore
air will be cold and the direction of flow will be
downward,If the ambient air is colder than the
wastewater,the flow will be upward,
8-3 Rotating Biological Contactors
An RBC consists of a series of closely spaced
circular disks of polystyrene or polyvinyl chloride
that are submerged in wastewater and rotated
through it (see Fig,8-7),
The cylindrical plastic disks are attached to a
horizontal shaft and are provided at standard unit
sizes of approximately 3.5 m in diameter and 7.5
m in length,
The RBC unit is partially submerged (typically 40
percent) in a tank containing the wastewater,and
the disks rotate slowly at about 1.0 to 1.6
revolutions per minute,Mechanical drives are
normally used to rotate the units,but air-driven
units have also been installed,
A submerged RBC design was also introduced in
the early 1980s but has seen limited applications,
The submergence is 70 to 90 percent and air-drive
units are used to provide oxygen and rotation,The
advantages claimed for the submerged unit are
reduced loadings on the shaft and bearings,
improved biomass control by air agitation,the
ability to use larger bundles of disks,and ease of
retrofit into existing aeration tanks,
To prevent algae growth,protect the plastic disks
from the effects of ultraviolet exposure,and to
prevent excessive heat loss in cold weather,RBC
units are covered (see Fig,8-7b),
Part II Wastegas Engineering
9 Control of Primary Particles
9.1 Wall Collection Devices
The first three types of control devices we
consider--gravity settlers,cyclone separators,and
electrostatic precipitators--all function by driving
the particles to a solid wall,where they adhere to
each other to form agglomerates that can be
removed from the collection device and disposed
of,
9.1.1 Gravity Settlers
It is an old,unsophisticated device that must
be cleaned manually at regular intervals,
But it is simple to construct,requires little
maintenance,and has some use in industries
treating very dirty gases,e.g.,some smelters
and metallurgical processes,
Gravity settlers have little practical
industrial use because they are ineffective
for small particles,
At even modest velocities and common radii,
the centrifugal forces acting on particles can
be two orders of magnitude larger than the
gravity forces,For this reason centrifugal
particle separators are much more useful
than gravity settlers,
There are many other variants on the centrifugal
collector idea,but none approaches the cyclone in
breadth of application,These devices are simple
and almost maintenance-free,Because any
medium-sized welding shop can make one,the big
suppliers of pollution control equipment,who
have test data on the effects of small changes in
the internal geometry,have been unwilling to
make these data public,
When it is used to separate solids from liquids it is
generally called a hydroclone,
If gravity settlers and centrifugal separators
are devices that drive particles against a
solid wail,and if neither can function
effectively (at an industrial scale) for
particles below about 5 μm in diameter,
then for wall collection devices to work on
smaller particles,they must exert forces that
are more powerful than gravity or
centrifugal force,The electrostatic
precipitator (ESP) is like a gravity settler or
centrifugal separator,but electrostatic force
drives the particles to the wall,
The basic idea of all ESPs is to give the particles
an electrostatic charge and then put them in an
electrostatic field that drives them to a collecting
wall,This is an inherently two-step process,In one
type of ESP,called a two-stage precipitator,
charging and collecting are carried out in separate
parts of the ESR,This type,widely used in
building air conditioners,is sometimes called an
electronic air filter,However,for most industrial
applications the two separate steps are carried out
simultaneously in the same part of the ESP,
Figure 9.2 shows in simplified form a wire-and-
plate ESP with two plates,The gas passes between
the plates,which are electrically grounded (i.e.,
voltage = 0),Between the plates are rows of wires,
held at a voltage of typically -40 000 volts,The
power is obtained by transforming ordinary
alternating current to a high voltage and then
rectifying it through some kind of solid-state
rectifier,
On the plates the particles lose their charge and
adhere to each other and the plate,forming a
"cake."
Solid cakes are removed by rapping the
plates at regular time intervals with a
mechanical or electromagnetic rapper
that strikes a vertical or horizontal
blow on the edge of the plate,
Some of the cake is always re-entrained,
thereby lowering the wastegas flowrate
whereas enhancing the removal
efficiency of the system,
If the collected particles are liquid,
e.g.,sulfuric acid mist,they run down
the plate and drip off,
9.2 DIVIDING COLLECTION DEVICES
Filters and scrubbers divide the flow into
smaller parts where they can collect the
particles,
Two types of filters,surface filters and
depth filters,are commonly used in air
pollution control,
9.2.1 Surface Filters
The filter is a membrane (sheet steel,cloth,
wire mesh,or filter paper) with holes
smaller than the dimensions of the particles
to be retained,
Although industrial air filters rarely have holes
smaller than the smallest particles captured,they
often act as if they did,The reason is that,as fine
particles are caught on the sides of the holes of a
filter,they tend to bridge over the holes and make
them smaller,Thus as the amount of collected
particles increases,the cake of collected material
becomes the filter,and the filter medium (usually
a cloth) that originally served as a filter to collect
the cake now serves only to support the cake,and
no longer as a filter,
This cake of collected particles will have
average pore sizes smaller than the diameter
of the particles in the oncoming gas stream,
and thus will act as a sieve for them,The
particles collect on the front surface of the
growing cake,For that reason this is called
a surface filter,
One may visualize this situation with a
screen having holes 0.75 in,(1,91 cm) in
diameter,We could collect a layer of Ping-
Pong balls easily on this screen,Once we
had such a layer,we could then collect
cherries,which,by themselves,could pass
through the holes in the screen but cannot
pass through the spaces between the Ping-
Pong balls,Once we have a layer of cherries,
we could put on a layer of peas,then of rice,
then of sand,
The two most widely used designs of
industrial surface filters are shown in Figs,
9.13 and 9.14,Because the enclosing sheet
metal structure in both figures is normally
the size and roughly the shape of a house,
this type of gas filter is generally called a
baghouse,The design in Fig,9.13,most
often called a shake-deflate filter,consists
of a large number of cylindrical cloth bags
that are closed at the top like a giant
stocking,toe upward,
? Fig,9-4 Typical industrial baghouse of the shake-deflate design
These are hung from a support,Their lower ends
slip over and are clamped onto cylindrical sleeves
that project upward from a plate at the bottom,
The dirty gas flows into the space below this plate
and up inside the bags,The gas flows outward
through the bags,leaving its solids behind,The
clean gas then flows into the space outside the
bags and is ducted to the exhaust stack or to some
further processing,
For the baghouse in Fig,9.13 there must be
some way of removing the cake of particles
that accumulates on the filters,Normally
this is not done during gas-cleaning
operations,instead the baghouse is taken
out of the gas stream for cleaning,When the
gas flow has been switched off,the bags are
shaken by the support to loosen the
collected cake,
A weak flow of gas in the reverse direction
may also be added to help dislodge the cake,
thus deflating the bags,
Often metal rings are sewn into filter bags at
regular intervals so that the bag will only
partly collapse when the flow is reversed,
Typically,for a major continuous source
like a power plant,about five baghouses
will be used in parallel,with four operating
as gas cleaners during the time that the
other one is being shaken and cleaned,Each
baghouse might operate for two hours and
then be cleaned for 10 minutes; at all times
one baghouse would be out of service for
cleaning or waiting to be put back into
service,
Fig,9-14 Typical industrial baghouse of
the pulse-jet design
The other widely used baghouse design,
called a pulse-jet filter,is shown in Fig,
9.14,In it the flow during filtration is
inward through the bags,which are similar
to the bags in Fig,9.13 except their ends
open at the top,The bags are supported by
internal wire cages to prevent their collapse,
Just after the cleaning the control efficiency
will be less than just before the next
cleaning,but the average efficiency meets
the legal control requirements,
9,2.2 Depth Filters
Another class of filters,widely used for air
pollution control,does not form a coherent
cake on the surface,but instead collects
particles throughout the entire filter body,
The examples are the filters on filter-tipped
cigarettes and the lint filters on many home
furnaces,
Such filters are often used where the
particles to be caught are fine drops of
liquids that are only moderately viscous,
The most widespread air pollution control
use of depth filters is in the collection of
very fine liquid drops,sulfuric acid mist,
produced in sulfuric acid plants,
One brand uses the trade name Demister,
This kind of device is also used for cleaning
the air of industrial clean rooms or hospital
surgical suites and in personal protection
dust masks,The filters are thrown away
when they have collected enough particles
that their pressure drop begins to increase,
9.2.4 Scrubbers for Particulate Control
Scrubbers effectively divide the flow of
particle-laden gas by sending many small
drops through it,
A complete scrubber has several parts,as
sketched in Fig,9.20,
? Fig,Component parts of a scrubber installation
If possible,the engineer should try to save
money by finding a place where the
contaminated water stream can be recycled
inside the plant without first removing the
solids,
Obviously,if there is no good way to deal
with the contaminated water stream,then
the scrubber has merely changed an air
pollution problem into a water pollution
problem,
9.3 Choosing the Collectors
In choosing a primary particle collection
device one must consider the size of the
particles to be collected,the required
collection efficiency,the size of the gas
flow,the allowed time between cleanings,
and details of the nature of the particles,
The following rules of thumb may be
helpful,
1,Small or occasional flows can be treated by
throwaway devices,e.g.,cigarette and
motor oil filters,in which the collected
particles remain in the device,Large and
steady flows require collection devices that
operate continuously or semicontinously,
and from which the collected particles can
be removed continuously or
semicontinuously,
2,Sticky particles (e.g.,tars) must be collected either
on throwaway devices or into a liquid,as in a
scrubber or cyclone,filter,or wet ESP whose
collecting surfaces are continually coated with a
film of flowing liquid,
3,Particles that adhere well to each other but not to
solid surfaces are easy to collect,Those that do the
reverse often need special surfaces,e.g.,Teflon-
coated fibers in filters that release collected
particles well during cleaning,
4,Electrical properties of the particles are of
paramount importance in ESPs,and they are often
significant in other control devices where friction-
induced electro-static charges on the particles can
aid or hinder collection,
5,For nonsticky particles larger than about 5 μm,a
cyclone separator is probably the only device to
consider,
6,For particles much smaller than 5 μm one
normally considers ESPs,filters,and scrubbers,
Each of these can collect particles as small as a
fraction of a micron,
7,For large flows the pumping cost makes scrubbers
very expensive; other devices are chosen if
possible,
8,Corrosion resistance and acid dew point must
always be considered,
9.4 SUMMARY
1,Gravity settling chambers,cyclones,and
ESPs work by driving the particles to a
solid wall where they form agglomerates
that can be collected,These three devices
have similar design equations,
2,Filters and scrubbers divide the flow,They
have different design equations from
wall collection devices and from each other,
3,Both surface and depth filters are used for
particle collection,Surface filters are used
to collect most of the particles in a heavily
laden gas stream,Depth filters are mostly
used for the final cleanup of air or gas that
must be very clean or for fine liquid drops,
which coalesce on them and then drop off,
4,To collect small particles,a scrubber must
have a very large relative velocity between
the gas being cleaned and the liquid drops,
For this reason co-flow scrubbers are most
often used,The venturi scrubber is the most
widely used type of co-flow scrubber,
10 Control of Volatile Organic Compounds
10.1 Introduction
Volatile organic compounds (VOCs) are liquids or
solids that contain organic carbon (carbon bonded
to carbon,hydrogen,nitrogen,or sulfur,but not
carbonate carbon as in CaCO3 nor carbide carbon
as in CaC2 or CO or CO2),which vaporize at
significant rates,VOCs are probably the second-
most widespread and diverse class of emissions
after particulates,
VOCs are a large family of compounds
some (e.g.,benzene) are toxic and
carcinogenic,The principal concern with
VOCs is that they participate in the "smog"
reaction and also in the formation of
secondary particles in the atmosphere,
These latter are mostly in the fine particle
size range,Some VOCs are powerful
infrared absorbers and thus contribute to the
problem of global warning,
10.2 VOCs
VOCs are those organic liquids or solids whose
room temperature vapor pressures are greater than
about 0.01 psia (0.0007 atm) and whose
atmospheric boiling points are up to about 500。 F
(260。 C),which means most of these organic
compounds with less than about 12 carbon atoms,
A lighted cigarette produces a gaseous mixture of
high-boiling organic compounds; when this
mixture is cooled on leaving the cigarette it forms
a smoke of fine particulate droplets,
In common usage it would often be grouped
with the hydrocarbons,
Hydrocarbons are only slightly soluble in
water,so we can normally separate liquid
HCs from liquid water by simple phase
separation and decantation,However,the
water left behind often contains enough
dissolved hydrocarbon that it cannot be
discharged to the sewer or natural body of
water without additional treatment,
Polar VOCs,which almost all contain an
oxygen or nitrogen atom in addition to
carbons and hydrogens (alcohols,ethers,
aldehydes and ketones,carboxylic acids,
esters,amines,nitriles) are much more
soluble in water,This difference in
solubilities makes the polar VOCs easier to
remove from a gas stream by scrubbing
with water,but harder to remove from water
once they dissolve in it,
10.3 CONTROL BY CONCENTRATION AND
RECOVERY
10.3.1 Adsorption
Adsorption means the attachment of molecules to
the surface of a solid,In contrast,absorption
means the dissolution of molecules within a
collecting medium,which may be liquid or solid,
Generally,absorbed materials are dissolved into
the absorbent,like sugar dissolved in water,
whereas adsorbed materials are attached onto the
surface of a material,like dust on a wall,
Absorption mostly occurs into liquids,adsorption
mostly onto solids,
Industrial face masks of activated carbon
are worn by workers exposed to solvents,as
in paint spraying or solvent cleaning,The
worker's lungs suck the air in through thin
beds of activated carbon,contained in
replaceable cartridges on the face mask,
When the activated carbon is loaded (i.e.,
the solvent begins to come through into the
worker's breathing space) the cartridge of
activated carbon is discarded and a fresh
one installed,
10.3.2 Absorption (Scrubbing)
If we can find a liquid solvent in which the VOC
is soluble and in which the remainder of the
contaminated gas stream is insoluble,then we can
use absorption to remove and concentrate the
VOC for recovery and re-use,or destruction,The
standard chemical engineering method of
removing any component from a gas stream--
absorption and stripping--is sketched in Fig,10.15,
If we can find a liquid solvent in which the
gaseous component we wish to selectively remove
is much more soluble than are the other
components in the gas stream,the procedure is
quite straightforward,
The feed gas enters the absorber,which is a
vertical column in which the gas passes upward
and the liquid solvent passes downward,Normally,
bubble caps,sieve trays,or packing is used in the
interior of the column to promote good
countercurrent contact between the solvent and the
gas,The stripped solvent enters the top of the
column and flows countercurrent to the gas,
The loaded solvent,which now contains most of
the component we are removing from the gas,
passes to the stripper,which normally is operated
at a higher temperature and/or a lower pressure
than the absorber,
? Fig,Three plausible arrangements for scrubbing a
gas with a liquid,(a)bubbler;(b)spray
chamber;(c)packed
In Fig,10.15 the separated component is
shown leaving as a gas for use,sale,or
destruction,In some cases it is condensed
and leaves as a liquid,The stripped or lean
solvent is sent back to the absorber column,
Very large absorption-stripping systems
often use tray columns,but the small ones
used in most air pollution control
applications use internal packings,
1,It must afford reasonable solubility for the material to be
removed,and,if this material is to be recovered at
reasonable purity,it must not dissolve and thus carry along
any of the other components of the gas stream,
2,In the absorber,the gas being treated will come to
equilibrium with the stripped solvent,The vapor pressure
of the solvent,at absorber temperature,must be low
enough that if the cleaned gas is to be discharged to the
atmosphere,the emission of solvent is small enough to be
permissible,Some solvent is lost this way; the cost of
replacing it must be acceptable,If the solvent is water this
is not a problem (unless we need the gas to be dry for its
next use),but for other solvents this can be a problem,
3,At the higher temperature (or lower pressure) of the
stripping column,the absorbed material must come out of
solution easily,and the vapor pressure of the solvent must
be low enough that it does not contaminate the recovered
VOC,If the solvent vapor pressure in the stripper is too
large,one may replace the stripper by a standard
distillation column (combination stripper and rectifier) to
recover the transferred material at adequate purity,
4,The solvent must be stable at the conditions in the
absorber and stripper,and be usable for a considerable
time before replacement,
5,The solvent molecular weight should be as low as possible,
to maximize its ability to absorb,This requirement
conflicts with the low solvent vapor pressure requirement,
so that a compromise must be made,
10.4 Biological Oxidation (Biofiltration)
As discussed above,the ultimate fate of
VOCs is to be oxidized to CO2 and H20,
either in our engines or furnaces,or
incinerators,or in the environment,
Microorganisms can also oxidize the VOCs
contained in gas or air streams,
The typical biofilter consists of the
equivalent of a swimming pool,with a set
of gas distributor pipes at the bottom,
covered with several feet of soil or compost
or loam or other carriers in which the
microorganisms live,The contaminated gas
enters through the distributor pipes and
flows slowly up through soil,allowing time
for the VOC to dissolve in the water
contained in the soil,and then to be
oxidized by the microorganisms that live
there,
Typically these devices have soil depths of 3 to 4 ft,
void volumes of 50%,upward gas velocities of
0.005 to 0.5 ft/s,and gas residence times of 15 to
60 s,They work much better with polar VOCs,
which are fairly soluble in water (see Sec,10.2)
than with HCs whose solubility is much less,The
microorganisms must be kept moist,protected
from conditions that could injure them,and in
some cases given nutrients,Because of the long
time the gases must spend in them,these devices
are much larger
In spite of these drawbacks,there are some
applications for which they are economical,and
for which they are used industrially,
10.5 SUMMARY
1,VOCs are emitted from a wide variety of sources and have
a wide variety of individual components,each with its own
properties,We use VOCs mostly as petroleum-based fuels
and solvents,The majority of our VOC emissions come
from fuel and solvent usage,transportation,and storage,
2,The control alternatives are prevention,concentration and
recovery,or oxidation,
3,Some of these control options can also be used for non-
VOC emissions,e.g.,incineration for odor control of H2S,
adsorption for SO2 or mercury vapor,and leakage control
for any process source,
11 Control of Sulfur Oxides
The control of particulates and VOCs is
mostly accomplished by physical processes
(cyclones,ESPs,filters,leakage control,
vapor capture,condensation),
Some particles and VOCs are chemically
changed into harmless materials by
combustion,
Control of sulfur oxides and nitrogen
oxides is largely chemical rather than
physical,
Sulfur and nitrogen oxides are ubiquitous
pollutants,which have many sources,SO2,
SO3,and NO2 are strong respiratory irritants
that can cause health damage at high
concentrations,
These gases also form secondary particles in
the atmosphere,contributing to our PM10
and PM2.5 problems and impairing visibility,
They are the principal causes of acid rain,
11.1 The Elementary Oxidation-Reduction
Chemistry Of Sulfur And Nitrogen
Both sulfur and nitrogen in the elemental
state are relatively inert and harmless to
humans,Both are needed for life; all
animals require some N and S in their
bodies,However,the oxides of sulfur and
nitrogen are widely recognized air
pollutants,The reduced products also are,in
some cases,air pollutants,
Both hydrogen sulfide and ammonia are
very strong-smelling substances,gaseous at
room temperature (-60。 C and -33。 C
boiling points,respectively),and toxic in
high concentrations,(High concentrations
due to accidental releases often cause
fatalities,These occur in the production and
use of ammonia as a fertilizer and
refrigerant and in the production and
processing of "sour" gas and oil,which
contain hydrogen sulfide.),
In the atmosphere NO2 and SO3 react with water
to form nitric and sulfuric acids,which then react
with ammonia or any other available cation to
form particles of ammonium nitrate or sulfate or
some other nitrate or sulfate,These particles,
generally in the 0.1 to 1-μm size range,are very
efficient light-scatterers,They are significant
contributors to urban PMl0 and PM2.5 problems,
They are the principal causes of acid deposition
and of visibility impairment,NO and NO2 also
play a significant role in the formation of O3,
11.1 Dry systems
The solids handling and wet sludge
handling and disposal difficulties that are
integral to wet throwaway processes
induced engineers to develop dry
throwaway processes that would have fewer
corrosion and scaling difficulties and would
produce a waste product much easier to
handle and dispose of,All of these systems
inject dry alkaline particles into the gas
stream,where they react with the gas to
remove SO2,
The SO2-containing particles are then captured in
the particle collection device that the plant must
have to collect fly ash (most often a baghouse,
sometimes an ESP),If successful,this approach
eliminates the problems with disposal of wet
scrubber sludge and all the difficulties involved
with the wet limestone process,It increases the
volume of dry solids to be disposed of,but that is
considered a less difficult problem,The flow
diagrams for such systems are sketched in Fig,
11.7,
The first two call for the injection of powdered limestone or
lime into the boiler,In the high-temperature part of the
furnace the limestone would convert to lime,so that either
way the active reagent would be CaO,The desired reaction
is
CaO + SO2 ---> CaSO3
CaSO3 would then oxidize to CaSO4,In principle this
should work,but most tests have shown that to get high
SO2 collection efficiencies one must put a large excess of
lime or limestone into the system,thus increasing reagent
costs,increasing the load on the particle collector,and
increasing the volume of solid wastes to be disposed of,
However,if one uses more reactive (and much more
expensive) NaHCO3 or Na2CO3,the collection efficiency
is much better,mostly because of the much higher
chemical reactivity of these sodium salts,
Mass transfer between gases and solids is
much less well understood than that
between gases and liquids,so that the
design of these devices is much more
heavily dependent on test and empiricism
than is the design of systems like that in Fig,
10.15,
11.2 Regenerative systems
Table 11.4 shows an entirely different category of
systems,In these some kind of absorbent or
adsorbent is used to capture SO2 from the flue gas,
Then in some separate device or set of devices the
adsorbent or absorbent is regenerated to produce a
flow of relatively pure SO2 or H2SO4,
Recently,work has begun on regenerative
processes that will simultaneously capture both
SO2 and NOx,These systems have not yet
advanced to commercial scale,but they may have
a major role in future air pollution control,
11.3 SUMMARY
1,SO2 emissions from human activities are
mostly due to the combustion of sulfur-
containing fossil fuels and the smelting of
metal sulfide ores,
2,The overall control strategy for SO2
emissions is to convert the sulfur to
CaSO4.2H2O
and return it to the ground in some kind of
landfill,or use it to make wallboard,
3,For liquid or gaseous fuels containing reduced sulfur,the
most common approach
is to use catalytic processes to convert the contained sulfur
to H2S,remove that by
scrubbing the gas with a weakly alkaline solution,convert
the H2S to elemental
sulfur by the Claus process,and either sell that sulfur for
sulfuric acid production or place it in a landfill,
4,For metal sulfide ore smelting,which produces waste gases
with 4 percent or more SO2,the common approach is to
convert that SO2 to sulfuric acid,
5,For coal (or high-sulfur oil) used in a large power
plant,the most common approach is to burn the
coal and then treat the plant's exhaust gas
(typically containing about 0.1 percent SO2) with
limestone or lime in a forced-oxidation wet
scrubber or a spray dryer,to convert SO2 to
CaSO4, 2H20,which will then go to a landfill or a
wallboard plant,
6,Other alternatives are being explored,some in
large-scale demonstrations,They may replace
those just listed in the future,
Part III Solid Waste Engineering
12 Evolution of Solid Waste Management
12.1 Solid Waste— a Consequence of Life
Problems with the disposal of wastes can
be traced from the time when humans first
began to congregate in tribes,villages,and
communities and the accumulation of
wastes became a consequence of life,
12.2 Waste Generation In a
Technological Society
Materials Flow and Waste Generation
The Effects of Technological Advances
Of particular significance are the increasing use
of plastics and the use of frozen foods,which
reduce the quantities of food wastes in the
home but increase the quantities at agricultural
processing plants,
Thus,the engineers responsible for the design of
solid waste facilities must be aware of trends,even
though they cannot,of course,predict all the
changes in technology that will affect the
characteristics of solid wastes,
But important questions arise,Which elements of
society generate the greatest quantities of solid
waste and what is the nature of these wastes? How
can the quantities be minimized? What is the role
of resource recovery? Can disposal and recovery
technology keep up with consumer product
technology?
12.3 The Development of Solid Waste
Management
Solid waste management may be defined as
the discipline associated with the control of
generation,storage,collection,transfer and
transport,processing,and disposal of solid
wastes in a manner that is in accord with the
best principles of public health,economics,
engineering,conservation,aesthetics,and
other environmental considerations,and that
is also responsive to public attitudes,
In its scope,solid waste management includes all
administrative,financial,legal,planning,and
engineering functions involved in solutions to all
problems of solid wastes,The solutions may
involve complex interdisciplinary relationships
among such fields as political science,city and
regional planning,geography,economics,public
health,sociology,demography,communications,
and conservation,as well as engineering and
materials science,
Functional Elements of a Waste
Management System
The activities associated with the management
of solid wastes,from the point of generation to
final disposal have been grouped into the six
functional elements,(1) waste generation; (2)
waste handling and separation,storage,and
processing at the source; (3) collection; (4)
separation and processing and transformation
of solid wastes; (5) transfer and transport; and
(6) disposal,
Waste Generation
Waste Handling and Separation,Storage,
and Processing at the Source
Collection
Separation,Processing,and
Transformation of Solid Waste,
Transfer and Transport
Disposal
12.4 Integrated Solid Water Management
Hierarchy of Integrated Solid Waste management
Source Reduction
Recycling
Waste Transformation
Landfilling
Planning for Integrated Waste Management
Proper Mix of Alternatives and Technologies
Flexibility in Meeting Future Changes
Monitoring and Evaluation
12.5 Operation of Solid Waste Management
Systems
Management Issues
Setting Workable but Protective Regulatory
Standards,
Improving Scientific Methods for
Interpretation of Data
Identification of Hazardous and Toxic
Consumer Products Requiring Special Waste
Management Units
Paying for Improved Waste Management Units,
Designating Land Disposal Units at or near
Large Urban Centers
Establishing and Maintaining More
Qualified Managers to Develop and
Operate Waste Management Units
Future Challenges and Opportunities
Changing Consumption Habits in Society
Reducing the Volume of Waste at the
Source
Making Landfills Safer
Development of New Technologies
13 Sources,Types,and Composition of
Municipal Solid Wastes
13-1 Sources of Solid Wastes
(1) residential,
(2) commercial,
(3) institutional,
(4) construction and demolition,
(5) municipal services,
(6) treatment plant sites,
(7) industrial,
(8) agricultural,
13-2 Types of Solid Wastes
Residential and Commercial
Special Wastes
Hazardous Wastes
Construction and Demolition
Municipal Services
Treatment Plant Wastes and Other Residues
Industrial Solid Waste
Agricultural Wastes
13.3 Composition of Solid Wastes
Composition of MSW
Distribution of Individual Waste
Components
Effect of Waste Diversions on
Distribution of Components in
Residential MSW
13-4 Determination of the Composition of
MSW in the Field
Residential MSW
Commercial and Industrial MSW
The field procedure for component
identification for commercial and non-
process industrial solid wastes involves the
analysis of representative waste samples
taken directly from the source,not from a
mixed waste load in a collection vehicle,
13-5 Types of Materials Recovered from
MSW
Materials Commonly Separated from
MSW
Specifications for Recovered Materials
13-6 Future Changes in Waste Composition
Impacts of Waste Diversion Programs
Future Changes in Waste Components
14 Physical,Chemical,and Biological
Properties of Municipal Solid Waste
14- 1 Physical Properties of MSW
Specific Weight
Moisture Content
Particle Size and Size Distribution
Field Capacity
14- 2 Chemical properties of MSW
Proximate Analysis
Fusing Point of Ash
Ultimate Analysis of Solid Waste
Components
Typical proximate analysis and data for
materials found in residendtial,
commercial,and industrial solid
Essential Nutrients and Other Elements
14- 3 Biological Properties of MSW
Biodegradability of Organic Waste
Components
Production of Odors
Breeding of Flies
14- 4 Physical,Chemical,and Biological
Transformations of Solid Waste
Physical Transformations
Biological Transformations
15 Disposal of Solid Wastes and Residual
Matter
15-1 The Landfill Method of Solid Waste
Disposal
The Landfilling Process
Preparation of the site for landfilling,
Reactions Occurring in Landfills,
Concerns with the Landfilling of Solid
Wastes
15-2 Composition and Characteristics,
Generation and Control of Landfill Gases
Composition and Characteristics of
Landfill Gas
Generation of Landfill Gases
Management of Landfill Gas
15-3 Composition,formation and control of
leachate in landfills
Composition of Leachate
Water Balance and Leachate Generation
In Landfills
Fate of Constituents in Leachate in
Subsurface Migration
Control of Leachate in Landfills
Leachate Collection Systems
Leachate Management Options
15-4 Environmental Quality Monitoring at
Landfills
Vadose Zone Monitoring
Groundwater Monitoring
Landfill Air Quality Monitoring