7 Suspended Growth Biological Treatment
Processes
7-1 Introduction to the Activated-Sludge
Process
Historical Development
Experiments conducted at the Lawrence
Experiment Station during 1912 and 1913 by
Clark and Gage
With air input into wastewater,growths of
organisms could be cultivated in bottles and in
tanks partially filled with roofing slate spaced
about 25 mm (1 in) apart and would greatly
increase the degree of purification,
Lockett found that the sludge played an important
part,
The basic activated-sludge treatment process,as
illustrated on Fig,7-la and b,consists of the
following three basic components,(1) a reactor in
which the microorganisms responsible for
treatment are kept in suspension and aerated; (2)
liquid-solids separation,usually in a sedimentation
tank; and (3) a recycle system for returning solids
removed from the liquid-solids separation unit
back to the reactor,
Fig,7-1Description of Basic Process
An important feature of the activated-sludge
process is the formation of flocculent settleable
solids,
For these applications,various modifications of
conventional activated-sludge processes are used,
including sequencing batch reactors,oxidation
ditch systems,aerated lagoons,or stabilization
ponds,
Evolution of the Activated-Sludge Process
A number of activated-sludge processes and design
configurations have evolved since its early conception as a
result of (1) engineering innovation in response to the need
for higher-quality effluents from wastewater treatment
plants; (2) technological advances in equipment,
electronics,and process control; (3) increased
understanding of microbial processes and fundamentals;
and (4) the continual need to reduce capital and operating
costs,
The use of a plug-flow process became
problematic when industrial wastes were
introduced because of the toxic effects of some of
the discharges,
With the development of simple inexpensive
program logic controllers (PLCs) and the
availability of level sensors and automatically
operated valves,the sequencing batch reactor
(SBR) process (see Fig,7-1c) became more
widely used by the late 1970s,especially for
smaller communities, Some of the stages are not
aerated (anaerobic or anoxic stages) and internal
recycle flows may be used,
Recent Process Developments
As noted above,numerous modifications of
the activated-sludge process have evolved
in the last 10 to 20 years,aimed principally
at effective and efficient removal of
nitrogen and phosphorus,
7-2 Wastewater Characterization
Activated-sludge process design requires
determining (1) the aeration basin volume,
(2) the amount of sludge production,(3) the
amount of oxygen needed,and (4) the
effluent concentration of important
parameters,
7-3 Fundamentals of Process Analysis and Control
Important factors that must be considered in
the selection of reactor types for the
activated-sludge process include (1) the
effects of reaction kinetics,(2) oxygen
transfer requirements,(3) nature of the
wastewater,(4) local environmental
conditions,(5) presence of toxic or
inhibitory substances in the influent
wastewater,(6) costs,and (7) expansion to
meet future treatment needs,
Selection of Reactor Type
Selection of Solids Retention Time and Loading
Criteria
The common parameters used are the solids retention time
(SRT),the food to biomass (F/M) ratio (also known as
food to microorganism ratio),and the volumetric organic
loading rate(Lv),
Solids Retention Time
The SRT,in effect,represents the average period of time
during which the sludge has remained in the system,SRT
is the most critical parameter for activated-sludge design as
SRT affects the treatment process performance,aeration
tank volume,sludge production,and oxygen requirements,
Sludge Production
Sludge will accumulate in the activated-sludge
process if it cannot be processed fast enough by an
undersized sludge-handling facility,
The observed yield decreases as the SRT is
increased due to biomass loss by more
endogenous respiration,The yield is higher when
no primary treatment is used,as more nbVSS
remains in the influent wastewater(A-B),
Oxygen Requirements
As an approximation,for BOD removal only,the oxygen
requirement will vary from 0.90 to 1.3 kg O2/kg BOD
removed for SRTs of 5 to 20 d,
Nutrient Requirements
Using the formula C5H7NO2,for the composition of cell
biomass,about 12.4 percent by weight of nitrogen will be
required,The phosphorus requirement is usually assumed
to be about one-fifth of the nitrogen value,As a general
role,for SRT values greater than 7 d,about 5 g nitrogen
and 1 g phosphorus will be required per 100 g of BOD to
provide an excess of nutrients,
Other Chemical Requirements
The amount of alkalinity required for nitrification,taking
into account cell growth,is about 7.07 g CaCO3/g NH4-N,
In addition to the alkalinity required for nitrification,
additional alkalinity must be available to maintain the pH
in the range from 6.8 to 7.4,Typically the amount of
residual alkalinity required to maintain pH near a neutral
point (i.e.,pH ≈ 7) is between 70 and 80 mg/L as CaCO3,
Mixed-Liquor Settling Characteristics
Clarifier design must provide adequate clarification of the
effluent and solids thickening for the activated-sludge
solids,
Two commonly used measures developed to quantify the
settling characteristics of activated sludge are the sludge
volume index (SVI) and the zone settling rate,
The SVI is determined by placing a mixed-liquor sample in
a 1- to 2-L cylinder and measuring the settled volume after
30 min and the corresponding sample MLSS concentration,
For example,a mixed-liquor sample with a 3000 mg/L
TSS concentration that settles to a volume of 300 mL in 30
min in a 1-L cylinder would have an SVI of 100 mL/g,A
value of 100 mL/g is considered a good settling sludge
(SVI values below 100 are desired),SVI values above 150
are typically associated with filamentous growth,
Use of Selectors
Because solids separation is one of the most
important aspects of biological wastewater
treatment,a biological selector (a small
contact tank) is often incorporated in the
design to limit the growth of organisms that
do not settle well,An appropriate selector
design can be added before the activated-
sludge aeration basin,
Process Control
To maintain high levels of treatment performance
The principal approaches to process control are (1)
maintaining dissolved oxygen levels in the
aeration tanks,(2) regulating the amount of return
activated sludge (RAS),and (3) controlling the
waste-activated sludge (WAS),
The parameter used most commonly for
controlling the activated-sludge process is SRT,
Dissolved Oxygen Control,
When oxygen limits the growth of microorganisms,
filamentous organisms may predominate and the
settleability and quality of the activated sludge
may be poor,
In general,the dissolved oxygen concentration in
the aeration tank should be maintained at about
1.5 to 2 mg/L in all areas of the aeration tank,
Return Activated-Sludge Control
Return sludge concentrations from secondary clarifiers
range typically from 4000 to 12,000 mg/L,
Settleability
If the settleable solids occupied a volume of 275 mL after
30 min of settling,the percentage volume would be equal
to 38 percent [(275 mL / 725 mL) * 100],If the plant flow
were 2 m3/s,the return sludge rate should be 0.38 × 2
m3/s = 0.76 m3/s,
Sludge Blanket Level
The optimum level is determined by experience
and is a balance between settling depth and sludge
storage,The optimum depth of the sludge blanket
usually ranges between 0.3 and 0.9 m,
Considerations,
(1) diurnal flow variations
(2) sludge production variations
(3)changes in the settling characteristics of the
sludge,
Sludge Wasting
An alternative method of wasting
sometimes used is withdrawing mixed
liquor directly from the aeration tank or the
aeration tank effluent pipe,
Oxygen Uptake Rates
OUR or respiration rate used to assess the
presence of toxic or inhibitory substances in
the influent wastewater,
Microscopic Observations
Specific information gathered includes
? changes in floc size and density
? status of filamentous organism growth in the floc
? presence of Nocardia bacteria,
? type and abundance of higher life-forms such as
protozoans and rotifers,
A decrease in the protozoan population may be indicative
of DO limitations,operation at a lower SRT inhibitory
substances in the wastewater,
Operational Problems
The most common problems encountered in the
operation of an activated-sludge plant are bulking
sludge,rising sludge,and Nocardia foam,
Bulking Sludge
In a bulking sludge condition,the MLSS floc does
not compact or settle well,and floc particles are
discharged in the clarifier effluent,
The other type of bulking,viscous
bulking,is caused by an excessive amount
of extracellular biopolymer,Viscous
bulking is usually found with nutrient-
limited systems or in a very high loading
condition with wastewater having a high
amount of rbCOD,
This structure,in contrast to the preferred
dense floc with good settling properties,has
an increased surface area to mass ratio,
which results in poor settling,
The classification system is based on
morphology (size and shape of cells,length
and shape of filaments),staining responses,
and cell inclusions,
One of the kinetic features of filamentous
organisms that relates to these conditions is
that they are very competitive at low substrate
concentrations whether it be organic substrates,
DO,or nutrients,Thus,lightly loaded
complete-mix activated-sludge systems or low
DO (<0.5 mg/L) operating conditions provide
an environment more favorable to filamentous
bacteria than to the desired floc-forming
bacteria,
When the influent wastewater contains
fermentation products such as volatile fatty
acids(VFA) and reduced sulfur compounds
(sulfides and thiosulfate),Thiothrix can
proliferate,
View the mixed liquor under the microscope,
A reasonable quality phase-contrast
microscope with magnification up to 1000
times (oil immersion) is necessary to view
the filamentous bacteria or fungi structure
and size,
Process Loading/Reactor Configuration
In many cases,complete-mix systems with long
SRTs and subsequent low F/M ratios experience
filamentous growths,In such systems,the
filamentous organisms are more competitive for
substrate,
We can use selector processes to solve these
problems because they provide conditions that
cause selection of floc-forming bacteria in lieu of
filamentous organisms as the dominant population,
Temporary Control Measures
In an emergency situation or while the aforementioned
factors are being investigated,chlorine and hydrogen
peroxide may be used to provide temporary help,
Chlorination of return sludge has been practiced quite
extensively as a means of controlling bulking,
Rising Sludge
The most common cause of this phenomenon is
denitrification,in which nitrites and nitrates in the
wastewater are converted to nitrogen gas,If enough gas is
formed,the sludge mass becomes buoyant and rises or
floats to the surface,
Nocardia Foam,
Two bacteria genera,Nocardia and Microthrix parvicella,
are associated with extensive foaming in activated-sludge
processes,These organisms have hydrophobic cell surfaces
and attach to air bubbles,where they stabilize the bubbles
to cause foam,
Nocardia has a filamentous structure,and
the filaments are very short and are
contained within the floc particles,
Microthrix parvicella has thin filaments
extending from the floc particles,
The foam is thick,has a brown color,and
can build up in thickness of 0.5 to 1 m,but
is more pronounced with diffused aeration
and with higher air flowrates,
Methods that can be used to control Nocardia include (1)
avoiding trapping foam in the secondary treatment process,
(2) avoiding the recycle of skimmings into the secondary
treatment process,and (3) using chlorine spray on the
surface of the Nocardia foam,
The addition of a small concentration of cationic polymer
has been used with some success for controlling Nocardia
foaming, Reducing the oil and grease content from
discharges to the collection system from restaurants,truck
stops,and meatpacking facilities by effective degreasing
processes can help control potential Nocardia problems,
Activated-Sludge Selector Processes
The high substrate concentration in the selector
favors the growth of nonfilamentous organisms,
A selector is a small tank (20 to 60 min contact
time) or a series of tanks in which the incoming
wastewater is mixed with return sludge under
aerobic,anoxic,and anaerobic conditions,
The goal in the selector is to have most of the
rbCOD consumed by the floc-forming bacteria,
With biological nutrient-removal
processes(脱氮除磷 ),improved sludge-
settling characteristics,and,in many cases,
minimal filamentous bacteria growth has
been observed,The anoxic or anaerobic
metabolic conditions used in these
processes favor growth of the floc-forming
bacteria,The filamentous bacteria cannot
use nitrate or nitrite for an electron acceptor,
Sequencing Batch Reactor Process
For continuous-flow applications,at
least two SBR tanks must be
provided so that one tank receives
flow while the other completes its
treatment cycle,
Sludge Wasting in SBRs
A unique feature of the SBR system is that
there is no need for a return activated-
sludge (RAS) system,
Because of the substrate concentration
changes with time,the substrate utilization
and oxygen demand rates change,
progressing from high to low levels,
Tab,7-6 Computation approach for the design of a SBR
7-5 Process for Biological Nitrogen Removal
Nitrogen removal is needed to prevent
eutrophication),or for groundwater recharge or
other reuse applications,
Nitrogen removal can be either an integral part of
the biological treatment system or an add-on
process to an existing treatment plant,
Following the discussion of design issues,design
examples are provided for (1) the anoxic/aerobic
process,(2) step-feed anoxic/aerobic process,(3)
intermittent aeration,(4) a sequencing batch
reactor,and (5) postanoxic denitrification with
methanol addition,
First,regions of low DO or zero DO
concentration may be present within the
basin as a function of the mixing regime,
Second,activated-sludge floc can contain
both aerobic and anoxic zones,
Single-Sludge Simultaneous Nitrification
Denitrificatlon (SNdN) Processes
The nitrification and denitrification rates
are a function of the reaction kinetics,floc
size,floc density,floc structure,rbCOD
loading,and bulk liquid DO concentration,
Fig,7-16 Operation of a Nitrox oxidation ditch process using
intermittent aeration,(a)aerobic conditions; (b)anoxic conditions;
(c)variations in ORP,DO,ammonia and nitrate
7-6 Processes for Biological Phosphorus Removal
Barnard (1974) was the first to clarify the need for
anaerobic contacting between activated sludge and
influent wastewater before aerobic degradation to
accomplish biological phosphorus removal,
Biological Phosphorus-Removal Processes
The main difference between the Phoredox (A/O)
process and the A2O processes shown on Fig,7-
17a and b is that nitrification does not occur in the
Phoredox (A/O) process,Low operating SRTs are
used to prevent the initiation of nitrification,
Fig,7-17 Typical biological phosphorus removal process
Process Design Considerations
The process design considerations for
BPR(biological phosphorus removal)
processes include (1) wastewater
characteristics,(2) anaerobic contact time,
(3) SRT,(4) waste sludge processing
method,and (5) chemical addition
capability,
The conversion of rbCOD to VFAs (including
acetate)occurs quickly through fermentation in the
anaerobic zone and 7 to 10 mg of acetate results in
about 1.0 mg P removal,The more acetate,the
more cell growth,and,thus,more phosphorus
removal,
During wet-weather conditions,especially in the
winter,BPR may be difficult to achieve due to
cold,low strength wastewater that does not readily
become anaerobic,
With continuous VFA addition,the effluent
soluble phosphorus concentration decreased from
2.5 to 0.3 mg/L,
Anaerobic Contact Time
Detention times of 0.25 to 1.0 h are
adequate for fermentation of rbCOD,
Polyhydroxybutyrate(PHB) provides energy
for phosphorus uptake and storage,
Solids Retention Time
First,the final amount of phosphorus removed is
proportional to the amount of biological
phosphorus-storing bacteria wasted,
Second,at long SRTs the biological phosphorus
bacteria are in a more extended endogenous phase,
which will deplete more of their intracellular
storage products,If the intracellular glycogen(糖
元 ) is depleted,less efficient acetate uptake and
PHB storage will occur in the anaerobic contact
zone,thus making the overall BPR process less
efficient,
Waste Sludge Processing
Phosphorus is released when the bacteria
that contain stored phosphorus are subject
to anaerobic conditions,
Anaerobic conditions in thickening and/or
digestion can thus result in the release of
significant amounts of phosphorus,
Thickening of waste sludge by dissolved air
flotation,gravity belt thickeners,or rotary-
drum thickeners is preferred over gravity
thickening of waste sludge to minimize
phosphorus release,
Chemical Addition Capability
Phosphorus can be removed biologically to dissolved
concentrations as low as 0.20 to 0.30 mg/L,provided
sufficient rbCOD is available,
Where there are insufficient amounts of rbCOD in the
influent wastewater,chemical addition is necessary to
provide enough phosphorus removal to meet the effluent
discharge concentration needed,Alum or iron salts may be
used and may be applied at a number of locations in the
liquid stream treatment process,
Iron salts may be preferred in some cases over alum salts
for primary treatment applications,because they have the
additional advantage of removing sulfide to help reduce
odors,
Process Control
Filter backwash recycle flows should be
sent to the aerobic zone instead of the
anaerobic or anoxic zones,Recycle streams
with significant concentrations of DO and
nitrate can have an adverse impact on
process performance,
Effluent Suspended Solids
The phosphorus content in the mixed-liquor
solids is greater than that from the
conventional activated-sludge process due
to the biological phosphorus storage,
At 3 to 6 percent phosphorus in the solids,
the phosphorus contribution in an effluent
having a TSS concentration of 10 mg/L
would be 0.3 to 0.6 mg/L,values that are
significant if the effluent standard is less
than 1.0 mg P/L,To provide very low
effluent phosphorus concentrations,effluent
filtration may be required,
Methods to Improve Phosphorus-Removal
Efficiency in BPR Systems
1,Provide supplemental acetate by direct
purchase or by primary sludge fermentation,
2,Reduce the process SRT,
3,Add alum or iron salts in primary
treatment or for effluent polishing,
4,Reduce the amount of nitrate and/or
oxygen entering the anaerobic zone,
Fig,7-18 Examples of fermentation reactors for producing
volatile fatty acids(VFAs) used for puosphorus removal
A deeper depth primary clarifier design has also been proposed
to provide sufficient holding time for the settled primary sludge
for hydrolysis and acid fermentation,
The additional VFA supply to the BPR
process decreased the average effluent
phosphorus concentration from about 1.5 to
0.5 mg/L,Alum was later added before the
secondary clarifiers at a dosage of about 8
mg/L (as alum) to further reduce the
effluent phosphorus concentration,
With both prefermentation and alum
addition,the effluent phosphorus
concentration averaged less than 0.20 mg/L,
Fig,7-19 Typical non-Nocardia froth on activated-
sludge aeration tank sludge return channels
Wastewater normally contains soap,detergents,and other
surfactants that produce foam when the wastewater is
aerated,
The wind may lift the froth off the tank surface
and blow it about,contaminating whatever it
touches,The froth,besides being unsightly,is a
hazard to those working with it because it is very
slippery,even after it collapses,In addition,once
the froth has dried,it is difficult to remove,
A commonly used system for spiral-roll tanks
consists of a series of spray nozzles mounted
above the surface in areas where the froth collects,
Screened effluent or clear water is sprayed
through these nozzles and physically breaks down
the froth as it forms,Another approach is to meter
a small quantity of antifoaming chemical additive
into the spray water,
Fig,7-20 Typical circular sludge collection mechanisms,
(a)suction type; (b)spiral type scrapper
Fig,7-22 Alternative methods of flow splitting,(a)hydraulic
symmetry; (b)flow measurement and feedback control;
(c)hydraulic split with Weirs; (d)inlet feed gate control
Fig,7-23 Energy-dissipating inlet devices used in circular clarifiers,
(a)schematic of a center column energy-dissipating inlet and flocculation
feed well; (b)view of an energy-dissipating feed well
Tank Inlet Design
Poor distribution or jetting of the tank
influent can increase the formation of
density currents and scouring of settled
sludge,etc,
Tank inlets should dissipate influent energy,
distribute the flow evenly in horizontal and
vertical directions,mitigate density currents,
minimize sludge blanket disturbance,and
promote flocculation,
Fig,7-24 Alternative peripheral baffle arrangements,
(a)Stamford; (b)unnamed; (c)Mckinney; (d)interior trough
Scum Removal
Typical scum-removal equipment includes beach
and scraper type,rotating pipe-through skimmer,
and slotted pipes,
Scum should not be returned to the plant
headworks because microorganisms responsible
for foaming (typically Nocardia) will be recycled,
causing foaming problems to persist because of
continuous seeding of the unwanted
microorganisms,
7-8 Suspended Growth Aerated Lagoons
Types of Suspended Growth Aerated Lagoons
Classified based on the manner in which the
solids are handled are,
1,Facultative partially mixed
2,Aerobic flow through with partial mixing
3,Aerobic with solids recycle and nominal
complete mixing
Facultative Partially Mixed Lagoon
The energy input is only sufficient to
transfer the amount of oxygen required for
biological treatment,but is not sufficient to
maintain the solids in suspension,
Because the energy input will not maintain
the solids in suspension,a portion of the
incoming solids will settle along with a
portion of the biological solids produced
from the conversion of the soluble organic
substrate,
Temperature
Because suspended growth aerobic flow-through
lagoons are often installed and operated in
locations with widely varying climatic conditions,
the effects of temperature change must be
considered in their design,The two most
important effects of temperature are (1) reduced
biological activity and treatment efficiency and (2)
the formation of ice,
Surface aerators tend to further cool lagoons in
cold weather,but submerged diffused air systems
add heat to some extent,
In multiple lagoon systems,cold-weather
effects can be mitigated by seasonal
changes in the method of operation,During
the warmer months,the lagoons would be
operated in parallel; in the winter,they
would be operated in series,In the winter
operating mode,the downstream aerators
could be turned off and removed,and the
lagoon surface is allowed to freeze,
With this method of operation it is
possible to achieve a 60 to 70 percent
removal of BOD even during the
coldest winter months,Still another
method that can be used to improve
performance during the winter months
is to recycle a portion of the solids
removed by settling,
Mixing Requirements
The threshold energy input value for the
suspension of biosolids is about 1.5 to 1.75
kW/l03 m3,For depths greater than 3.7m,
aerators with draft tubes may be considered
to prevent solids deposition,
The hydraulic detention time for the
complete-mix lagoon will typically vary
from 1.5 to 3 d,The total retention time for
the facultative lagoons is on the order of 3d,
7-9 Biological Treatment with Membrane Separation
Membrane biological reactors (MBRs),
consisting of a biological reactor (bioreactor
with suspended biomass and solids
separation by microfiltration membranes
with nominal pore sizes ranging from 0.1 to
0.4 μm,are finding many applications in
wastewater treatment,
The ability of MBR to eliminate secondary
clarification and operate at higher MLSS
concentration provides the following advantages,
( 1 ) higher volumetric loading rates and thus
shorter reactor hydraulic retention times; (2)
longer SRTs resulting in less sludge production; (3)
operation at low DO concentrations with potential
for simultaneous nitrification-denitrification in
long SRT designs; (4) high-quality effluent in
terms of low turbidity bacteria,TSS,and BOD;
and;(5) less space required for wastewater
treatment,
Disadvantages of MBRs include high capital costs,
limited data on membrane life,potential high cost
of periodic membrane replacement or scouring,
higher energy costs,and the need to control
membrane fouling,
Membrane bioreactor systems have two basic
configurations,(1) the integrated bioreactor that
uses membranes immersed in the bioreactor and (2)
the recirculated MBR in which the mixed liquor
circulates through a membrane module situated
outside the bioreactor,
The membranes are mounted in modules
(sometimes called cassettes) that can be lowered
into the bioreactor,The modules are comprised of
the membranes,support structure for the
membranes,feed inlet and outlet connections,and
an overall support structure,
The membranes are subjected to a vacuum
(less than 50 kPa) that draws water
(permeate) through the membrane while
retaining solids in the reactor,To maintain
TSS within the bioreactor and to clean the
exterior of the membranes,compressed air
is introduced through a distribution
manifold at the base of the membrane
module,As the air bubbles rise to the
surface,scouring of the membrane surface
occurs; the air also provides oxygen to
maintain aerobic conditions,
Fig,7-25 Typical membrane bioreactors,(a)schematic of placement of
membrane bundles in an activated Sludge reactor ; (b)membrane bundle
in position to be placed in a membrane bioreactor
In the MBR system,MLSS concentrations in the
range of 8000 to 10,000 mg/L appear to be most
cost-effective when all factors are considered,
The membrane flux rate,defined as the mass or
volume rate of transfer through the membrane
surface [in terms of L/m2.h] is an important design
and operating parameter that affects the process
economics,
Membrane Fouling Control
In the activated-sludge reactor,biomass coats the
outer layer of the membranes used in an integrated
MBR during effluent withdrawal,Finer particles
may penetrate the inner pores of the membrane,
causing an increase in pressure loss,
First,coarse bubble aeration is provided at the
bottom of the membrane tank directly below the
membrane fibers,The air bubbles flow upward
between the vertically oriented fibers,causing the
fibers to agitate against one another to provide
mechanical scouring,
Second,filtration is interrupted about every 15 to
30 min and the membrane fibers are backwashed
for 30 to 45 s,
Typically,a low concentration of chlorine (<5
mg/L) is maintained in the backflush water to
inactivate and remove microbes,Third,about
three times per week a strong sodium hypochlorite
solution (about 100 mg/L) or citric acid is used in
the backflush mode for 45 min,
The cassettes can be removed easily from the
aeration basin by an overhead hoist system for a
periodic chemical-bath cleaning,
When removed for cleaning,the cassettes are
submerged in a high-concentration chlorine
solution bath in a separate small tank or basin,
External cleaning occurs about every 3 to 6
months,
The combination of air scour,backflushing,and
maintenance cleaning is not completely effective
in controlling membrane fouling,and the pressure
drop across the membrane increases with time,At
a maximum operating pressure drop of ≈60 kPa,
the membranes are removed from the aeration
basin for a recovery cleaning,
Processes
7-1 Introduction to the Activated-Sludge
Process
Historical Development
Experiments conducted at the Lawrence
Experiment Station during 1912 and 1913 by
Clark and Gage
With air input into wastewater,growths of
organisms could be cultivated in bottles and in
tanks partially filled with roofing slate spaced
about 25 mm (1 in) apart and would greatly
increase the degree of purification,
Lockett found that the sludge played an important
part,
The basic activated-sludge treatment process,as
illustrated on Fig,7-la and b,consists of the
following three basic components,(1) a reactor in
which the microorganisms responsible for
treatment are kept in suspension and aerated; (2)
liquid-solids separation,usually in a sedimentation
tank; and (3) a recycle system for returning solids
removed from the liquid-solids separation unit
back to the reactor,
Fig,7-1Description of Basic Process
An important feature of the activated-sludge
process is the formation of flocculent settleable
solids,
For these applications,various modifications of
conventional activated-sludge processes are used,
including sequencing batch reactors,oxidation
ditch systems,aerated lagoons,or stabilization
ponds,
Evolution of the Activated-Sludge Process
A number of activated-sludge processes and design
configurations have evolved since its early conception as a
result of (1) engineering innovation in response to the need
for higher-quality effluents from wastewater treatment
plants; (2) technological advances in equipment,
electronics,and process control; (3) increased
understanding of microbial processes and fundamentals;
and (4) the continual need to reduce capital and operating
costs,
The use of a plug-flow process became
problematic when industrial wastes were
introduced because of the toxic effects of some of
the discharges,
With the development of simple inexpensive
program logic controllers (PLCs) and the
availability of level sensors and automatically
operated valves,the sequencing batch reactor
(SBR) process (see Fig,7-1c) became more
widely used by the late 1970s,especially for
smaller communities, Some of the stages are not
aerated (anaerobic or anoxic stages) and internal
recycle flows may be used,
Recent Process Developments
As noted above,numerous modifications of
the activated-sludge process have evolved
in the last 10 to 20 years,aimed principally
at effective and efficient removal of
nitrogen and phosphorus,
7-2 Wastewater Characterization
Activated-sludge process design requires
determining (1) the aeration basin volume,
(2) the amount of sludge production,(3) the
amount of oxygen needed,and (4) the
effluent concentration of important
parameters,
7-3 Fundamentals of Process Analysis and Control
Important factors that must be considered in
the selection of reactor types for the
activated-sludge process include (1) the
effects of reaction kinetics,(2) oxygen
transfer requirements,(3) nature of the
wastewater,(4) local environmental
conditions,(5) presence of toxic or
inhibitory substances in the influent
wastewater,(6) costs,and (7) expansion to
meet future treatment needs,
Selection of Reactor Type
Selection of Solids Retention Time and Loading
Criteria
The common parameters used are the solids retention time
(SRT),the food to biomass (F/M) ratio (also known as
food to microorganism ratio),and the volumetric organic
loading rate(Lv),
Solids Retention Time
The SRT,in effect,represents the average period of time
during which the sludge has remained in the system,SRT
is the most critical parameter for activated-sludge design as
SRT affects the treatment process performance,aeration
tank volume,sludge production,and oxygen requirements,
Sludge Production
Sludge will accumulate in the activated-sludge
process if it cannot be processed fast enough by an
undersized sludge-handling facility,
The observed yield decreases as the SRT is
increased due to biomass loss by more
endogenous respiration,The yield is higher when
no primary treatment is used,as more nbVSS
remains in the influent wastewater(A-B),
Oxygen Requirements
As an approximation,for BOD removal only,the oxygen
requirement will vary from 0.90 to 1.3 kg O2/kg BOD
removed for SRTs of 5 to 20 d,
Nutrient Requirements
Using the formula C5H7NO2,for the composition of cell
biomass,about 12.4 percent by weight of nitrogen will be
required,The phosphorus requirement is usually assumed
to be about one-fifth of the nitrogen value,As a general
role,for SRT values greater than 7 d,about 5 g nitrogen
and 1 g phosphorus will be required per 100 g of BOD to
provide an excess of nutrients,
Other Chemical Requirements
The amount of alkalinity required for nitrification,taking
into account cell growth,is about 7.07 g CaCO3/g NH4-N,
In addition to the alkalinity required for nitrification,
additional alkalinity must be available to maintain the pH
in the range from 6.8 to 7.4,Typically the amount of
residual alkalinity required to maintain pH near a neutral
point (i.e.,pH ≈ 7) is between 70 and 80 mg/L as CaCO3,
Mixed-Liquor Settling Characteristics
Clarifier design must provide adequate clarification of the
effluent and solids thickening for the activated-sludge
solids,
Two commonly used measures developed to quantify the
settling characteristics of activated sludge are the sludge
volume index (SVI) and the zone settling rate,
The SVI is determined by placing a mixed-liquor sample in
a 1- to 2-L cylinder and measuring the settled volume after
30 min and the corresponding sample MLSS concentration,
For example,a mixed-liquor sample with a 3000 mg/L
TSS concentration that settles to a volume of 300 mL in 30
min in a 1-L cylinder would have an SVI of 100 mL/g,A
value of 100 mL/g is considered a good settling sludge
(SVI values below 100 are desired),SVI values above 150
are typically associated with filamentous growth,
Use of Selectors
Because solids separation is one of the most
important aspects of biological wastewater
treatment,a biological selector (a small
contact tank) is often incorporated in the
design to limit the growth of organisms that
do not settle well,An appropriate selector
design can be added before the activated-
sludge aeration basin,
Process Control
To maintain high levels of treatment performance
The principal approaches to process control are (1)
maintaining dissolved oxygen levels in the
aeration tanks,(2) regulating the amount of return
activated sludge (RAS),and (3) controlling the
waste-activated sludge (WAS),
The parameter used most commonly for
controlling the activated-sludge process is SRT,
Dissolved Oxygen Control,
When oxygen limits the growth of microorganisms,
filamentous organisms may predominate and the
settleability and quality of the activated sludge
may be poor,
In general,the dissolved oxygen concentration in
the aeration tank should be maintained at about
1.5 to 2 mg/L in all areas of the aeration tank,
Return Activated-Sludge Control
Return sludge concentrations from secondary clarifiers
range typically from 4000 to 12,000 mg/L,
Settleability
If the settleable solids occupied a volume of 275 mL after
30 min of settling,the percentage volume would be equal
to 38 percent [(275 mL / 725 mL) * 100],If the plant flow
were 2 m3/s,the return sludge rate should be 0.38 × 2
m3/s = 0.76 m3/s,
Sludge Blanket Level
The optimum level is determined by experience
and is a balance between settling depth and sludge
storage,The optimum depth of the sludge blanket
usually ranges between 0.3 and 0.9 m,
Considerations,
(1) diurnal flow variations
(2) sludge production variations
(3)changes in the settling characteristics of the
sludge,
Sludge Wasting
An alternative method of wasting
sometimes used is withdrawing mixed
liquor directly from the aeration tank or the
aeration tank effluent pipe,
Oxygen Uptake Rates
OUR or respiration rate used to assess the
presence of toxic or inhibitory substances in
the influent wastewater,
Microscopic Observations
Specific information gathered includes
? changes in floc size and density
? status of filamentous organism growth in the floc
? presence of Nocardia bacteria,
? type and abundance of higher life-forms such as
protozoans and rotifers,
A decrease in the protozoan population may be indicative
of DO limitations,operation at a lower SRT inhibitory
substances in the wastewater,
Operational Problems
The most common problems encountered in the
operation of an activated-sludge plant are bulking
sludge,rising sludge,and Nocardia foam,
Bulking Sludge
In a bulking sludge condition,the MLSS floc does
not compact or settle well,and floc particles are
discharged in the clarifier effluent,
The other type of bulking,viscous
bulking,is caused by an excessive amount
of extracellular biopolymer,Viscous
bulking is usually found with nutrient-
limited systems or in a very high loading
condition with wastewater having a high
amount of rbCOD,
This structure,in contrast to the preferred
dense floc with good settling properties,has
an increased surface area to mass ratio,
which results in poor settling,
The classification system is based on
morphology (size and shape of cells,length
and shape of filaments),staining responses,
and cell inclusions,
One of the kinetic features of filamentous
organisms that relates to these conditions is
that they are very competitive at low substrate
concentrations whether it be organic substrates,
DO,or nutrients,Thus,lightly loaded
complete-mix activated-sludge systems or low
DO (<0.5 mg/L) operating conditions provide
an environment more favorable to filamentous
bacteria than to the desired floc-forming
bacteria,
When the influent wastewater contains
fermentation products such as volatile fatty
acids(VFA) and reduced sulfur compounds
(sulfides and thiosulfate),Thiothrix can
proliferate,
View the mixed liquor under the microscope,
A reasonable quality phase-contrast
microscope with magnification up to 1000
times (oil immersion) is necessary to view
the filamentous bacteria or fungi structure
and size,
Process Loading/Reactor Configuration
In many cases,complete-mix systems with long
SRTs and subsequent low F/M ratios experience
filamentous growths,In such systems,the
filamentous organisms are more competitive for
substrate,
We can use selector processes to solve these
problems because they provide conditions that
cause selection of floc-forming bacteria in lieu of
filamentous organisms as the dominant population,
Temporary Control Measures
In an emergency situation or while the aforementioned
factors are being investigated,chlorine and hydrogen
peroxide may be used to provide temporary help,
Chlorination of return sludge has been practiced quite
extensively as a means of controlling bulking,
Rising Sludge
The most common cause of this phenomenon is
denitrification,in which nitrites and nitrates in the
wastewater are converted to nitrogen gas,If enough gas is
formed,the sludge mass becomes buoyant and rises or
floats to the surface,
Nocardia Foam,
Two bacteria genera,Nocardia and Microthrix parvicella,
are associated with extensive foaming in activated-sludge
processes,These organisms have hydrophobic cell surfaces
and attach to air bubbles,where they stabilize the bubbles
to cause foam,
Nocardia has a filamentous structure,and
the filaments are very short and are
contained within the floc particles,
Microthrix parvicella has thin filaments
extending from the floc particles,
The foam is thick,has a brown color,and
can build up in thickness of 0.5 to 1 m,but
is more pronounced with diffused aeration
and with higher air flowrates,
Methods that can be used to control Nocardia include (1)
avoiding trapping foam in the secondary treatment process,
(2) avoiding the recycle of skimmings into the secondary
treatment process,and (3) using chlorine spray on the
surface of the Nocardia foam,
The addition of a small concentration of cationic polymer
has been used with some success for controlling Nocardia
foaming, Reducing the oil and grease content from
discharges to the collection system from restaurants,truck
stops,and meatpacking facilities by effective degreasing
processes can help control potential Nocardia problems,
Activated-Sludge Selector Processes
The high substrate concentration in the selector
favors the growth of nonfilamentous organisms,
A selector is a small tank (20 to 60 min contact
time) or a series of tanks in which the incoming
wastewater is mixed with return sludge under
aerobic,anoxic,and anaerobic conditions,
The goal in the selector is to have most of the
rbCOD consumed by the floc-forming bacteria,
With biological nutrient-removal
processes(脱氮除磷 ),improved sludge-
settling characteristics,and,in many cases,
minimal filamentous bacteria growth has
been observed,The anoxic or anaerobic
metabolic conditions used in these
processes favor growth of the floc-forming
bacteria,The filamentous bacteria cannot
use nitrate or nitrite for an electron acceptor,
Sequencing Batch Reactor Process
For continuous-flow applications,at
least two SBR tanks must be
provided so that one tank receives
flow while the other completes its
treatment cycle,
Sludge Wasting in SBRs
A unique feature of the SBR system is that
there is no need for a return activated-
sludge (RAS) system,
Because of the substrate concentration
changes with time,the substrate utilization
and oxygen demand rates change,
progressing from high to low levels,
Tab,7-6 Computation approach for the design of a SBR
7-5 Process for Biological Nitrogen Removal
Nitrogen removal is needed to prevent
eutrophication),or for groundwater recharge or
other reuse applications,
Nitrogen removal can be either an integral part of
the biological treatment system or an add-on
process to an existing treatment plant,
Following the discussion of design issues,design
examples are provided for (1) the anoxic/aerobic
process,(2) step-feed anoxic/aerobic process,(3)
intermittent aeration,(4) a sequencing batch
reactor,and (5) postanoxic denitrification with
methanol addition,
First,regions of low DO or zero DO
concentration may be present within the
basin as a function of the mixing regime,
Second,activated-sludge floc can contain
both aerobic and anoxic zones,
Single-Sludge Simultaneous Nitrification
Denitrificatlon (SNdN) Processes
The nitrification and denitrification rates
are a function of the reaction kinetics,floc
size,floc density,floc structure,rbCOD
loading,and bulk liquid DO concentration,
Fig,7-16 Operation of a Nitrox oxidation ditch process using
intermittent aeration,(a)aerobic conditions; (b)anoxic conditions;
(c)variations in ORP,DO,ammonia and nitrate
7-6 Processes for Biological Phosphorus Removal
Barnard (1974) was the first to clarify the need for
anaerobic contacting between activated sludge and
influent wastewater before aerobic degradation to
accomplish biological phosphorus removal,
Biological Phosphorus-Removal Processes
The main difference between the Phoredox (A/O)
process and the A2O processes shown on Fig,7-
17a and b is that nitrification does not occur in the
Phoredox (A/O) process,Low operating SRTs are
used to prevent the initiation of nitrification,
Fig,7-17 Typical biological phosphorus removal process
Process Design Considerations
The process design considerations for
BPR(biological phosphorus removal)
processes include (1) wastewater
characteristics,(2) anaerobic contact time,
(3) SRT,(4) waste sludge processing
method,and (5) chemical addition
capability,
The conversion of rbCOD to VFAs (including
acetate)occurs quickly through fermentation in the
anaerobic zone and 7 to 10 mg of acetate results in
about 1.0 mg P removal,The more acetate,the
more cell growth,and,thus,more phosphorus
removal,
During wet-weather conditions,especially in the
winter,BPR may be difficult to achieve due to
cold,low strength wastewater that does not readily
become anaerobic,
With continuous VFA addition,the effluent
soluble phosphorus concentration decreased from
2.5 to 0.3 mg/L,
Anaerobic Contact Time
Detention times of 0.25 to 1.0 h are
adequate for fermentation of rbCOD,
Polyhydroxybutyrate(PHB) provides energy
for phosphorus uptake and storage,
Solids Retention Time
First,the final amount of phosphorus removed is
proportional to the amount of biological
phosphorus-storing bacteria wasted,
Second,at long SRTs the biological phosphorus
bacteria are in a more extended endogenous phase,
which will deplete more of their intracellular
storage products,If the intracellular glycogen(糖
元 ) is depleted,less efficient acetate uptake and
PHB storage will occur in the anaerobic contact
zone,thus making the overall BPR process less
efficient,
Waste Sludge Processing
Phosphorus is released when the bacteria
that contain stored phosphorus are subject
to anaerobic conditions,
Anaerobic conditions in thickening and/or
digestion can thus result in the release of
significant amounts of phosphorus,
Thickening of waste sludge by dissolved air
flotation,gravity belt thickeners,or rotary-
drum thickeners is preferred over gravity
thickening of waste sludge to minimize
phosphorus release,
Chemical Addition Capability
Phosphorus can be removed biologically to dissolved
concentrations as low as 0.20 to 0.30 mg/L,provided
sufficient rbCOD is available,
Where there are insufficient amounts of rbCOD in the
influent wastewater,chemical addition is necessary to
provide enough phosphorus removal to meet the effluent
discharge concentration needed,Alum or iron salts may be
used and may be applied at a number of locations in the
liquid stream treatment process,
Iron salts may be preferred in some cases over alum salts
for primary treatment applications,because they have the
additional advantage of removing sulfide to help reduce
odors,
Process Control
Filter backwash recycle flows should be
sent to the aerobic zone instead of the
anaerobic or anoxic zones,Recycle streams
with significant concentrations of DO and
nitrate can have an adverse impact on
process performance,
Effluent Suspended Solids
The phosphorus content in the mixed-liquor
solids is greater than that from the
conventional activated-sludge process due
to the biological phosphorus storage,
At 3 to 6 percent phosphorus in the solids,
the phosphorus contribution in an effluent
having a TSS concentration of 10 mg/L
would be 0.3 to 0.6 mg/L,values that are
significant if the effluent standard is less
than 1.0 mg P/L,To provide very low
effluent phosphorus concentrations,effluent
filtration may be required,
Methods to Improve Phosphorus-Removal
Efficiency in BPR Systems
1,Provide supplemental acetate by direct
purchase or by primary sludge fermentation,
2,Reduce the process SRT,
3,Add alum or iron salts in primary
treatment or for effluent polishing,
4,Reduce the amount of nitrate and/or
oxygen entering the anaerobic zone,
Fig,7-18 Examples of fermentation reactors for producing
volatile fatty acids(VFAs) used for puosphorus removal
A deeper depth primary clarifier design has also been proposed
to provide sufficient holding time for the settled primary sludge
for hydrolysis and acid fermentation,
The additional VFA supply to the BPR
process decreased the average effluent
phosphorus concentration from about 1.5 to
0.5 mg/L,Alum was later added before the
secondary clarifiers at a dosage of about 8
mg/L (as alum) to further reduce the
effluent phosphorus concentration,
With both prefermentation and alum
addition,the effluent phosphorus
concentration averaged less than 0.20 mg/L,
Fig,7-19 Typical non-Nocardia froth on activated-
sludge aeration tank sludge return channels
Wastewater normally contains soap,detergents,and other
surfactants that produce foam when the wastewater is
aerated,
The wind may lift the froth off the tank surface
and blow it about,contaminating whatever it
touches,The froth,besides being unsightly,is a
hazard to those working with it because it is very
slippery,even after it collapses,In addition,once
the froth has dried,it is difficult to remove,
A commonly used system for spiral-roll tanks
consists of a series of spray nozzles mounted
above the surface in areas where the froth collects,
Screened effluent or clear water is sprayed
through these nozzles and physically breaks down
the froth as it forms,Another approach is to meter
a small quantity of antifoaming chemical additive
into the spray water,
Fig,7-20 Typical circular sludge collection mechanisms,
(a)suction type; (b)spiral type scrapper
Fig,7-22 Alternative methods of flow splitting,(a)hydraulic
symmetry; (b)flow measurement and feedback control;
(c)hydraulic split with Weirs; (d)inlet feed gate control
Fig,7-23 Energy-dissipating inlet devices used in circular clarifiers,
(a)schematic of a center column energy-dissipating inlet and flocculation
feed well; (b)view of an energy-dissipating feed well
Tank Inlet Design
Poor distribution or jetting of the tank
influent can increase the formation of
density currents and scouring of settled
sludge,etc,
Tank inlets should dissipate influent energy,
distribute the flow evenly in horizontal and
vertical directions,mitigate density currents,
minimize sludge blanket disturbance,and
promote flocculation,
Fig,7-24 Alternative peripheral baffle arrangements,
(a)Stamford; (b)unnamed; (c)Mckinney; (d)interior trough
Scum Removal
Typical scum-removal equipment includes beach
and scraper type,rotating pipe-through skimmer,
and slotted pipes,
Scum should not be returned to the plant
headworks because microorganisms responsible
for foaming (typically Nocardia) will be recycled,
causing foaming problems to persist because of
continuous seeding of the unwanted
microorganisms,
7-8 Suspended Growth Aerated Lagoons
Types of Suspended Growth Aerated Lagoons
Classified based on the manner in which the
solids are handled are,
1,Facultative partially mixed
2,Aerobic flow through with partial mixing
3,Aerobic with solids recycle and nominal
complete mixing
Facultative Partially Mixed Lagoon
The energy input is only sufficient to
transfer the amount of oxygen required for
biological treatment,but is not sufficient to
maintain the solids in suspension,
Because the energy input will not maintain
the solids in suspension,a portion of the
incoming solids will settle along with a
portion of the biological solids produced
from the conversion of the soluble organic
substrate,
Temperature
Because suspended growth aerobic flow-through
lagoons are often installed and operated in
locations with widely varying climatic conditions,
the effects of temperature change must be
considered in their design,The two most
important effects of temperature are (1) reduced
biological activity and treatment efficiency and (2)
the formation of ice,
Surface aerators tend to further cool lagoons in
cold weather,but submerged diffused air systems
add heat to some extent,
In multiple lagoon systems,cold-weather
effects can be mitigated by seasonal
changes in the method of operation,During
the warmer months,the lagoons would be
operated in parallel; in the winter,they
would be operated in series,In the winter
operating mode,the downstream aerators
could be turned off and removed,and the
lagoon surface is allowed to freeze,
With this method of operation it is
possible to achieve a 60 to 70 percent
removal of BOD even during the
coldest winter months,Still another
method that can be used to improve
performance during the winter months
is to recycle a portion of the solids
removed by settling,
Mixing Requirements
The threshold energy input value for the
suspension of biosolids is about 1.5 to 1.75
kW/l03 m3,For depths greater than 3.7m,
aerators with draft tubes may be considered
to prevent solids deposition,
The hydraulic detention time for the
complete-mix lagoon will typically vary
from 1.5 to 3 d,The total retention time for
the facultative lagoons is on the order of 3d,
7-9 Biological Treatment with Membrane Separation
Membrane biological reactors (MBRs),
consisting of a biological reactor (bioreactor
with suspended biomass and solids
separation by microfiltration membranes
with nominal pore sizes ranging from 0.1 to
0.4 μm,are finding many applications in
wastewater treatment,
The ability of MBR to eliminate secondary
clarification and operate at higher MLSS
concentration provides the following advantages,
( 1 ) higher volumetric loading rates and thus
shorter reactor hydraulic retention times; (2)
longer SRTs resulting in less sludge production; (3)
operation at low DO concentrations with potential
for simultaneous nitrification-denitrification in
long SRT designs; (4) high-quality effluent in
terms of low turbidity bacteria,TSS,and BOD;
and;(5) less space required for wastewater
treatment,
Disadvantages of MBRs include high capital costs,
limited data on membrane life,potential high cost
of periodic membrane replacement or scouring,
higher energy costs,and the need to control
membrane fouling,
Membrane bioreactor systems have two basic
configurations,(1) the integrated bioreactor that
uses membranes immersed in the bioreactor and (2)
the recirculated MBR in which the mixed liquor
circulates through a membrane module situated
outside the bioreactor,
The membranes are mounted in modules
(sometimes called cassettes) that can be lowered
into the bioreactor,The modules are comprised of
the membranes,support structure for the
membranes,feed inlet and outlet connections,and
an overall support structure,
The membranes are subjected to a vacuum
(less than 50 kPa) that draws water
(permeate) through the membrane while
retaining solids in the reactor,To maintain
TSS within the bioreactor and to clean the
exterior of the membranes,compressed air
is introduced through a distribution
manifold at the base of the membrane
module,As the air bubbles rise to the
surface,scouring of the membrane surface
occurs; the air also provides oxygen to
maintain aerobic conditions,
Fig,7-25 Typical membrane bioreactors,(a)schematic of placement of
membrane bundles in an activated Sludge reactor ; (b)membrane bundle
in position to be placed in a membrane bioreactor
In the MBR system,MLSS concentrations in the
range of 8000 to 10,000 mg/L appear to be most
cost-effective when all factors are considered,
The membrane flux rate,defined as the mass or
volume rate of transfer through the membrane
surface [in terms of L/m2.h] is an important design
and operating parameter that affects the process
economics,
Membrane Fouling Control
In the activated-sludge reactor,biomass coats the
outer layer of the membranes used in an integrated
MBR during effluent withdrawal,Finer particles
may penetrate the inner pores of the membrane,
causing an increase in pressure loss,
First,coarse bubble aeration is provided at the
bottom of the membrane tank directly below the
membrane fibers,The air bubbles flow upward
between the vertically oriented fibers,causing the
fibers to agitate against one another to provide
mechanical scouring,
Second,filtration is interrupted about every 15 to
30 min and the membrane fibers are backwashed
for 30 to 45 s,
Typically,a low concentration of chlorine (<5
mg/L) is maintained in the backflush water to
inactivate and remove microbes,Third,about
three times per week a strong sodium hypochlorite
solution (about 100 mg/L) or citric acid is used in
the backflush mode for 45 min,
The cassettes can be removed easily from the
aeration basin by an overhead hoist system for a
periodic chemical-bath cleaning,
When removed for cleaning,the cassettes are
submerged in a high-concentration chlorine
solution bath in a separate small tank or basin,
External cleaning occurs about every 3 to 6
months,
The combination of air scour,backflushing,and
maintenance cleaning is not completely effective
in controlling membrane fouling,and the pressure
drop across the membrane increases with time,At
a maximum operating pressure drop of ≈60 kPa,
the membranes are removed from the aeration
basin for a recovery cleaning,
