环境工程概论（英语版）：2

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,