CE 548
I
Suspended Growth Biological treatment
Process
Activated Sludge Principles
Activated Sludge Principles
• Wastewater is aerated in a tank
• Bacteria are encouraged to grow by providing
 Oxygen
 Food (BOD)
 Suitable temperature
 Time
• As bacteria consume BOD, they grow and multiply
• Treated wastewater flows into secondary clarifier
• Bacterial cells settle, removed from clarifier as sludge
• Part of sludge is recycled back to activated sludge tank, to maintain
bacteria population
• Remainder of sludge is wasted
Applications of activated sludge
processes
Process
Application
Conventional
Low-strength domestic waste, susceptible to shock loads
Complete-mix
General application, resistant to shock loads, surface aerators
Step-aeration
General application to wide range of wastes
Modified-aeration
Intermediate degree of treatment where cell tissue in the effluent is not objectionable
Contact-stabilization
Expansion of existing systems, package plants, flexible
Extended-aeration
Small communities, package plants, flexible, surface aerators
Kraus process
Low-nitrogen, high strength wastes
High-rate aeration
Use with turbine aerators to transfer oxygen and control the floc size, generals application
Pure-oxygen
General application, use where limited space is available, requires expensive oxygen source,
turbine or surface aerators
Conventional Activated Sludge
Completely-mixed Activated
Sludge
Step-aeration Activated Sludge
Contact Stabilization
Oxidation Ditch/Kraus Process
Design parameters for activated
sludge processes
Process
q c (d)
q (d)
F/M
Qr/Q
X (mg/L)
Conventional
5-15
4-8
0.2-0.4
0.25-5
1,500-3,000
Complete-mix
5-15
3-5
0.2-0.6
0.25-1
3,000-6,000
Step-aeration
5-15
3-5
0.2-0.4
0.25-0.75 2,000-3,500
Modified-aeration
0.2-0.5
1.5-3
1.5-5.0
0.05-0.15 200 – 500
0.5-1
0.2-0.6
0.25-1
Contact-stabilization 5-15
3-6
1,000-3,000
4,000-10,000
Extended-aeration
20-30
18-36
0.05-0.15 0.75-1.5
3,000-6,000
Kraus process
5-15
4-8
0.3-0.8
0.5-1
2,000-3,000
High-rate aeration
5-10
0.5-2
0.4-1.5
1-5
4,000-10,000
Pure-oxygen
8-20
1-3
0.25-1.0
0.25-0.5
6,000-8,000
Operational characteristics of
activated sludge processes
Process
Flow model
Conventional
Plug-flow
Diffused air, mechanical aerators
85-95
Complete-mix
Complete-mix
Diffused air, mechanical aerators
85-95
Step-aeration
Plug-flow
Diffused air
85-95
Modified-aeration Plug-flow
Diffused air
60-75
Contact-stabilization Plug-flow
Diffused air, mechanical aerators
80-90
Complete-mix Diffused air, mechanical aerators
75-95
Extended-aeration
Kraus process
Plug-flow
Aeration system
BOD5 removal efficiency (%)
Diffused air, mechanical aerators
85-95
High-rate aeration Complete-mix
Diffused air, mechanical aerators
75-90
Pure-oxygen
Mechanical aerators
85-95
Complete-mix
Wastewater Characterization
• AS design requires determining: 1.) aeration basin volume 2.) sludge
production 3.) oxygen needed and 4.) the effluent concentration of
important parameters.
• To design AS process, characterization of wastewater is required.
• Wastewater characteristics T8-1, p.666 can be grouped into the
following categories:
– carbonaceous substrates,
– nitrogen compounds,
– phosphorus compounds,
– total and volatile suspended solids,
– and alkalinity.
Wastewater Characterization
• Carbonaceous Constituents. Measured by BOD or COD.
• Unlike BOD, some portion of COD is nonbiodegradable. COD is
fractionalized in F8-4, p.668.
• Of interest is whether the COD is dissolved or soluble and how much
is particulate, comprised of colloidal and suspended solids.
• The nonbiodegradable soluble COD, nbsCOD, will be found in the
AS effluent and the nonbiodegradable particulates will contribute to
the sludge.
• Because the nonbiodegradable particulate COD, nbpCOD, is organic,
it will contribute to the VSS concentration of the wastewater and
mixed liquor in the AS and is referred to as the nonbiodegradable
volatile suspended solids, nbVSS.
Wastewater Characterization
• The influent wastewater will also contain nonvolatile influent
suspended inert solids, iTSS, that add to the MLSS.
• For biodegradable COD, understanding the fractions that are
measured as soluble, soluble readily biodegradable (rbCOD), and
particulate is important for AS process design.
• The rbCOD is quickly assimilated by the biomass, while the
particulate, must first be dissolved by extracellular enzymes and are
thus assimilated at much slower rates.
• The rbCOD is of particular interest, T8-3, p.669, and has a direct
effect on the AS biological kinetics and process performance.
•
•
•
•
A: Oxygen used for rbCOD
B: Oxygen used for nitrification
C: Oxygen used for particular COD
D: Oxygen used for endogenous decay
Wastewater Characterization
• COD and BOD may be correlated as the following:
bCOD consumed in the BOD test is equal to the oxygen consumed
(UBOD) plus the oxygen equivalent of the remaining cell debris:
bCOD = UBOD + 1.42 fd (YH) bCOD
bCOD
UBOD/BOD

BOD 1.0 - 1.42 f d (YH )
where;
f d  cell debris fraction
YH  synthesis yield coefficient
eq(8 - 1)
bCOD/BOD ratio varies between 1.6-1.7.
Wastewater Characterization
• Nitrogenous Compounds. F8-5, p.670
• Alkalinity: Adequate alkalinity is needed to achieve complete
nitrification, about 7.07 g CaCO3/gNH4-N.
• Additional alkalinity must be available to maintain the pH in the
range 6.8-7.4.
• Typically the amount of residual alkalinity required to maintain the
pH near neutral is between 70 and 80 mg/l as CaCO3.
Wastewater Characterization
• Summary Tabulation. P. 673.
COD = bCOD + nbCOD
bCOD = 1.6BOD
nbCOD = nbsCOD + nbpCOD
bCOD = sbCOD + rbCOD
TKN = NH4-N + ON
ON = bON + nbON
nbON = nbsON + nbpON
Where terms are defined in T8-2, p.667.
Study example 8-1 p 674
Fundamentals of Process Analysis
and Control
• Process design considerations:
–
–
–
–
–
–
Reactor type
Kinetics
SRT
Sludge production
Oxygen requirements
Others
• Reactor type selection considerations. T8-4, p. 678.
• Kinetics, summary of equations. T8-5, p.679.
• SRT: The SRT in effect represents the average period of time during
which the sludge has remained in the system and used to be called the
mean cell residence time. In AS sludge design it is the MOST critical
parameter as it affects just about every element of design. The SRT is
typically 3-5 days, T8-6, p.680.
Fundamentals of Process Analysis
and Control
• Sludge production: Excess solids are produced in the AS process and
must be properly disposed of or they will accumulate and exit in the
effluent.
• PX,VSS = YobsQ(S0-S)(1kg/103g)
eq. 8-14, p.681
The Yobs term is illustrated in F8-7, p.682.
• Oxygen Requirements: If all of the bCOD were oxidized, the oxygen
demand would equal the bCOD concentration. However, bacterial
oxidize a portion of the bCOD to provide energy and use the
remaining portion of the bCOD for cell growth. Oxygen is also used
for endogenous respiration which is a function of the SRT.
The total oxygen requirement including nitrification is:
R0 = Q(S0-S) – 1.42PX,bio + 4.33Q(NOx)
eq. 8-17, p.683
The last term deals with the effects of nitrogen.
Fundamentals of Process Analysis
and Control
• Nutrient requirements: Based on cell mass, 12.4% by weight of
nitrogen is required and phosphorus is usually assumed to be about
1/5 of the nitrogen. As a general rule, for SRT values > 7d, about 5g
of N and 1g of P will be required per 100g of BOD.
• ML Settling Characteristics: In the final clarifier, the MOs must be
separated. A commonly used measure of settling characteristics is the
SVI, the sludge volume index. The SVI is the volume of 1g of
sludge after 30 minutes of settling. The numerical value is calculated
from the test as follows:
settled volume of sludge ml/l (103mg/g)
SVI =
= ml/g
suspended solids mg/l
eq. 8-19
Fundamentals of Process Analysis
and Control
• Example:
• Given: A ML has a TSS of 3500mg/l and settles to a volume of 275
in 30 minutes in a 1L cylinder.
Find: SVI
settled volume of sludge ml/l (103mg/g) 275(103mg/g)
SVI =
=
= 78.6 ml/g
suspended solids mg/l
3500
SVI = 78.6 ml/g
A value of 100 mL/g is considered a good settling sludge and SVI
values below 100 are desired. SVI values above 150 are typically
associated with a problem, filamentous growth.
Fundamentals of Process Analysis
and Control
• Secondary Clarification: The design is typically based on the surface
overflow rate and solids loading rate, T8-7, p.687.
Overflow rates are based on wastewater flow rates instead of ML
flowrates.
Overflow rate 
influent flowrate
Q

clarifier surface area A
Solids loading rate:
(
Q  QR ( X 
Solids Loading Rate (SLR) 
A
Fundamentals of Process Analysis
and Control
Fundamentals of Process Analysis
and Control
• Effluent Characteristics: The major parameters of interest are
– organic compounds, sBOD usually less than 3 mg/l
– suspended solids, 5-15 mg/l
– and nutrients.
• Process Control.
– Maintaining DO in the aeration tanks.
– Regulating RAS
– Controlling WAS
• The most commonly used parameter for controlling the AS process is
SRT. The waste AS flow from the recycle line is usually used to
maintain the desired SRT. The MLSS is also used as a control.
Fundamentals of Process Analysis
and Control
• The DO should be 1.5-2 mg/l in all areas of the aeration tank. Values
above 2 mg/l may improve nitrification (when BOD is high). Values
above 4 mg/l do not improve operations but significantly increase
aeration costs.
• RAS Control:
– The RAS is returned from the final clarifier to the inlet of the
aeration tank.
– The solids form a sludge blanket in the bottom of the clarifier.
– Return sludge pumping rates of 50-75% of the average design
wastewater flowrates are typical. However, the design average
capacity is typically 100-150% of the average design flowrate.
– Return AS concentrations from the secondary clarifier range
typically from 4000-12,000 mg/l.
Fundamentals of Process Analysis
and Control
• Settleability: To calculate return-sludge flowrate, several techniques are used:
Settleability test:
In a 1000 ml graduated cylinder the volume of settleable solids after 30 minutes
is divided by the volume of clarified liquid (supernatant).
% ratio 
VS
100
VL
flowrate  Q 
VS
 100
VL
SVI (Sludge Volume Index) test:
QR / Q  100 /(100 / Pw SVI   1
where;
Pw  MLSS expressed as percentage
Fundamentals of Process Analysis
and Control
• Sludge Wasting: To maintain a given SRT, the excess AS produced
each day must be wasted, WAS.
– The sludge can be wasted from the RAS line or the aeration tank.
– The RAS is more concentrated thereby requiring smaller pumps.
– The WAS is discharged to the primary sedimentation tanks for cothickening or to sludge thickening facilities prior to digestion.
– If wasting is from the RAS line:
Qw 
VX
( X R SRT
eq(8 - 34)
– If wasting is done from the aeration tank:
Qw 
V
SRT
eq(8 - 36)
Fundamentals of Process Analysis
and Control
• Operational Problems:
– Bulking sludge: The MLSS floc does not compact or settle well and
floc is discharged in the clarifier effluent. The principal cause is
filamentous bacteria which are very competitive at low substrate,
nutrient or DO conditions.
– Rising sludge:
» The sludge has good settling characteristics but rises to the surface.
» The most common cause is denitrification in which nitrites and
nitrates are converted to nitrogen gas, N2 which makes the mass
buoyant.
» Rising sludge is differentiated from bulking sludge by the presence
of small gas bubbles and floating sludge in the secondary clarifiers.
» Rising sludge problems may be overcome by reducing the
detention time in the clarifier by increasing the RAS rate.
Fundamentals of Process Analysis
and Control
• Operational Problems:
– Foaming:
» Nocardia can be responsible for excessive foaming.
» The bacteria have hydrophobic cell surfaces and attach to air
bubbles where they stabilize the bubbles to cause foam.
» Usually found above the ML.
» Nocardia can by controlled by avoiding trapping foam in the
secondary treatment process and using chlorine spray.
Processes for BOD Removal and
Nitrification
• Three Activated-Sludge process design examples are provided in this
section (8-4) to demonstrate the application of the fundamental
principles to BOD removal and nitrification.
• The examples are:
1.
A single sludge complete-mix activated-sludge process without
and with nitrification. Example 8-2
2.
A sequencing batch reactor (SBR) with nitrification. Example 8-3
3.
A staged nitrification process. Example 8-4
Processes for BOD Removal and
Nitrification
Sequencing Batch Reactor
• (SBR) is a fill-and-draw activated-sludge treatment system. In SBR
aeration and sedimentation are carried out sequentially in the same
tank. The process takes place in five steps:
1. fill :
– addition of wastewater to reactor
– liquid level rises from 25% to 100%
– normally lasts 25% of full cycle time
2. react:
– complete the reaction
– Lasts 35% of cycle time.
Processes for BOD Removal and
Nitrification
Sequencing Batch Reactor
3.
settle:
– to allow solid separation to occur
– more efficient than continuous flow systems.
– Lasts 20%
4. draw:
– to remove clarified treated waste lasts from
– 5 - 30% of cycle time, typically 45 minutes
5. idle:
– to provide time for one reactor to complete
its fill cycle before switching to another unit.
– Sometimes omitted.
Processes for BOD Removal and
Nitrification
Sequencing Batch Reactor
• sludge wasting usually occurs during settle or idle phases.
• no need for recycling; both aeration and settling occur in the same
chamber
• Process kinetics:
Accumulation = inflow – outflow + reaction
ds
V  QS o  QS  rsuV
dt
 m XS
ds
 rsu  
dt
Y (K s  S 
So
 m 
K s ln
 X
t
St  (So  St 
 Y 
Processes for BOD Removal and
Nitrification
Staged activated-sludge process
• Consists of a series of complete-mix reactors.
• For the same reactor volume, rectors in series can provide greater
treatment efficiency than a single complete-mix reactor, or provide a
greater treatment capacity.
• The oxygen uptake is higher in the first stage and decreases gradually.
Processes for BOD Removal and
Nitrification
Overview of biological nitrogen removal processes
• All biological nitrogen removal processes include aerobic zone
(nitrification) and anoxic zone (denitrification).
• Categories of suspended growth biological nitrogen removal processes
include (1) single-sludge or (2) two-sludge.
• Single-stage processes: (three types)
– preanoxic: initial contact of influent and return activated sludge is
in the anoxic zone. (commonly used)
– Postanoxic: anoxic zone follows the aerobic zone.
– Simultaneous nitrification-denitrification (SNdN): both zones
exisis in a single reactor. Requires DO control.
• Two-sludge processes: consists of two separate stages for nitrification
followed by denitrification. (not commonly used)
Preanoxic
Postanoxic
Simultaneous
Two-sludge
Processes for Phosphorous Removal
Process for biological phosphorous removal
• Three biological phosphorous removal (BPR) configuration are
commonly used:
– Phoredox (A\O): represent any process with an anaerobic/aerobic
sequence to promote BPR. Nitrification does not take occur.
– A2O: process sequence, anaerobic/anoxic/aerobic. Nitrification
takes place.
– UCT (University of Cape Town): used for weak wastewater
where the addition of nitrate would have significant effect on the
BPR performance.
• The PhoStrip process: combines biological and chemical processes
for phosphorous removal.
Design of Physical Facilities for AS Process
Design of Aeration Tanks
After selecting the activated sludge process and the aeration system, the
next step is to design the aeration tanks and support facilities.
Aeration Tanks:
• constructed of reinforced concrete
• capacity is determined from process design
• for plants in a capacity range of
0.5 – 10 Mgal/d
minimum two tanks
10 – 15 Mgal/d
4 tanks
>50
Mgal/d
> 6 tanks
Some large plans have 30 to 40 tanks
Design of Physical Facilities for AS Process
•
•
•
•
•
•
•
Aeration Tanks:
wastewater depth in the tank should be 15 – 25 ft for diffusers to work
efficiently.
free board from 1 – 2 ft above waterline should be provided
width to depth ratio 1:1 – 2.2:1 (1.5:1 is common)
for large plants channel length can exceed 500 ft per tank
tanks may consist of one to four channels
length-to-width ratio of each channel should be at least 5:1
for mechanical aeration system, one aerator per tank is commonly used
with a free board 3.5 – 5 ft
Suspended Growth Aerated Lagoons
 Consists of shallow earthen basins varying in depth from 2-5m
provided with mechanical aerators.
 mechanical aerators provide oxygen and mixing
 Suspended growth aerated lagoons are operated on a flow-through basis
or with recycle.
 Lagoons with solid recycle are essentially the same as the activated
sludge process.
 Types of Suspended growth aerated lagoons:
 Facultative partially mixed
 Aerobic flow-through with partial mixing
 Aerobic with solids recycle and nominal complete mixing
 The general characteristics of these lagoon systems are summarized in
Table 8-29
Suspended Growth Aerated Lagoons
 Facultative partially mixed
 The energy input is sufficient to meet oxygen requirement but not
sufficient to maintain all of the solids in suspension.
 A portion of incoming solids will settle a long with a portion of the
biological solids (AS)
 Settled solids will undergo anaerobic decomposition
 The term facultative is derived from the aerobic and anaerobic processes
that occur in the lagoon
 Facultative lagoons must be dewatered and the accumulated soilds
removed.
 Not commonly used.
Suspended Growth Aerated Lagoons
 Aerobic flow-through with partial mixing
 The energy input is sufficient to meet oxygen requirement but not
sufficient to maintain all of the solids in suspension.
 τ = SRT
 Effluent solids are removed in an external sedimentation facility
 Aerobic flow-through with partial mixing
 Same as extended aeration AS process, with the exception that an earthen
basin is used in place of reinforced concrete reactor.
 Hydraulic detention time (up to 2 days) is longer than conventional
extended aeration process.
 Higher aeration requirement than aerobic flow-through lagoons to maintain
solids in suspension.
Suspended Growth Aerated Lagoons
 Process design for flow-through lagoons:
BOD removal: the basis of design is SRT , typical values of SRT range
from 3 – 6 days. Once SRT is selected S can be calculated using
equations from Ch. 7.
An alternative approach is to assume that removal can be described by
first-order function. (rsu = -kS). The pertinent equation for a single
aerated lagoon is:
S
1

S o 1  k
eq (8 - 72)
k = first-order removal-rate const. d-1
(k varies from 0.5 – 1.5 d-1)
Suspended Growth Aerated Lagoons
 Process design for flow-through lagoons:
For lagoons in series, the following equation can be used:
Co
Co

1  (kV / nQ n 1  (k / n n
where;
Cn 
eq (8 - 72)
n  number of lagoons in series
Oxygen requirements:
Can be computed in the same way as for activated sludge process.
Oxygen requirements have been found to vary from
0.7 – 1.4 the amount of BOD5 removed.
Suspended Growth Aerated Lagoons
 Process design for flow-through lagoons:
Temperature:
Temperature effect include:
 reduced biological activity and treatment efficiency.
 formation of ice.
Temperature can be estimated using:
Ti  Tw
(T  T  fA
 w a
Q
eq (8 - 73)
where;
Ti  influent w aste temperatu re, oC
Tw  lagoon wat er temerat ure, oC
Ta  ambient air temera ture, oC
f  proportion ality factor
the proportionality factor incorporates:
A  surface area, m 2
Q  wastewater flowrate, m 3 / d
• heat transfer coefficients
• effect of surface area increase due to aeration
Study example 8-13
• effect of wind and effect of humidity
Biological Treatment with Membrane
Separation
 Overview of membrane bioreactor (MBR) technology
 The Membrane Bioreactor (MBR) process is an emerging advanced
wastewater treatment technology that has been successfully applied at an
ever increasing number of locations around the world.
 In addition to their steady increase in number, MBR installations are also
increasing in terms of scale. Over 1500 installation in more than 1000
cities world-wide for municipal and industrial application have been
reported to range in capacity from few hundreds of cubic meters per day to
over 50,000 cubic meters per day.
 New large plants under construction include the new Brightwater
municipal wastewater treatment plant in King County in the State of
Washington which will treat approximately 144,000 cubic meters of
municipal sewage with peak flows up to 204,000 cubic meters, serving
over 100,000 households.
Biological Treatment with Membrane
Separation
 MBR Process Description
 Membrane bioreactors (MBRs) combine the use of biological processes
and membrane technology to treat wastewater.
 As shown in figure 1, within one process unit, a high standard of treatment
is achieved, replacing the conventional arrangement of aeration tank,
settling tank and filtration that generally produces what is termed as a
tertiary standard effluent.
 The dependence on disinfection is also reduced, since the membranes with
pore openings, generally in the 0.01-0.5 µm range, trap a significant
proportion of pathogenic organisms (Figure 2).
 Operating at a mixed liquor suspended solids (MLSS) concentration of up
to 20,000 mg/L and a sludge age of 30-60 days, MBRs offer additional
advantages over conventional activated sludge plants, including a smaller
footprint.
a) Biological Extended Aeration Tertiary Treatment Process
Influent
Effluent
Preliminary
Treatment
Aeration
Basin
Secondary
Clarifier
b) MBR Treatment Process
Effluent
Influent
Preliminary
Treatment
Aeration
Basin
Sand Filter
Biological Treatment with Membrane
Separation
 MBR Process Advantages
The ability to eliminate secondary clarifier and operate at higher MLSS
concentrations provide the following advantages:
 Higher volumetric loading rate resulting in shorted hyd. detention time.
 Longer SRT resulting in less sludge production.
 Operate at lower DO concentration.
 High-quality effluent (TSS, BOD, bacteria, turbidity, etc.) Table 8-30
 Less space required for wastewater treatment.
 MBR Process disadvantages:
 High capital cost and energy cost.
 Limited data on membrane life, (high cost for membrane replacement)
 Membrane fouling