AGSM 337/BAEN 465
Biological Treatment Processes
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The following are excellent sources of info on biological treatment processes
 Shuler, M. L. and F. Kargi, 2002. Bioprocess Engineering: Basic Concepts (2 edition).
Prentice Hall.
 Metcalf and Eddy, Inc., 2003, Wastewater Engineering: Treatment and Reuse, revised by
Tchobanoglous, George, and Burton, Franklin L., Fourth Ed., McGraw-Hill, New York.
 Viessman, Warren, Jr. and Hammer, Mark J., 2005, Water Supply and Pollution Control,
7th Ed., Prentice Hall.
Treatment Goal
Reduce organic material (BOD) in wastewater streams by converting to settleable solids and
gaseous compounds (CO2)
Microbial Characteristics
Three basic types based on oxygen utilization:
 aerobic – air (oxygen) is required to support microbial activity
 anaerobic – microbial activity proceeds in the absence of oxygen
 facultative – microorganisms can grow either with or without oxygen using different
metabolic processes
Two types based on temperature preference
 mesophilic – organisms prefer temperatures in the range of 20 to 45 °C
 thermophilic – organisms prefer temperatures in the range of 45 to 60 °C
Aerobic Growth
Microbial biomass utilizes organic matter along with oxygen to create more biomass and
oxidized metabolic byproducts:
Organic matter + microbes + oxygen → More microbes + Metabolic byproducts + Energy
(C, H, O, N, P, S) + biomass
→ more biomass
+ CO2, NO3, SO4, PO4, H2O + energy
Anaerobic Growth
Anaerobic growth takes place in the absence of oxygen. In this case, microbes must obtain
oxygen from the organic molecules used as food. Other food molecules are rearranged as the
microbes obtain energy from them. This leads to the formation of methane (CH4), CO2, and
other metabolic byproducts. Anaerobic conversion of biomass takes place in stages with
different types of organisms carrying out the processes for each stage.
First anaerobic phase
In the first anaerobic phase, the complex molecules in organic matter such as polysaccharides
and proteins are hydrolyzed to simple sugars and amino acids. These are then converted to new
biomass and organic acids by the microbes.
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Biological Treatment Processes
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Hydrolysis reactions:
Polysaccharides (e.g., starch) → glucose and other simple sugars
Proteins → amino acids
Microbial growth:
Organic matter
+ microbes → more microbes + organic acids
(glucose, amino acids, etc) + biomass → more biomass + acetic and other organic acids
Second anaerobic phase
In the second anaerobic phase, the organic acids, hydrogen and CO2 produced in the first phase
are converted to more biomass, CO2, CH4 and other reduced metabolic byproducts. Amino acids
also are broken down, releasing ammonia.
Organic acids, amino acids, fats + microbes → more microbes + CO2 + CH4 + other byproducts
Many of the organic acids (e.g., propionic and butyric acids) along with H2S produced during
anaerobic growth have unpleasant odors. In addition, the H2S is toxic so care must be taken
when working with anaerobic processes.
Microbial Growth Curve
In batch processes, microbial growth follows a sequence of phases as shown in the figure below:
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Biological Treatment Processes
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Lag Phase
There is a lag phase following inoculation of the medium during which the cells adapt to their
new environment. The cells may have to make important constituents because of dilution when
placed in the new medium and/or prepare to grow with different nutrients by making new
Exponential Growth Phase
This is the most important phase for most microbial processes during which cells are growing
and dividing on a regular time cycle. For waste treatment processes, this is the phase during
which most of the organic matter is consumed. Because there are excess quantities of nutrients
available during this phase, the microorganisms grow and divide on a periodic basis causing the
amount of microbial biomass to double each period. This leads to exponential growth behavior
which is shown in equation 1:
x  x0 e t
where x is the biomass concentration (mass basis) at time t,
x0 is the biomass concentration at the start of the exponential phase,
µ is specific growth rate of the organisms, and
t is the time since the start of the exponential phase.
The specific growth rate is a function of the environment of the microorganisms including
factors such as pH, temperature and nutrients available. For given conditions with excess
nutrients available, there is a maximum rate at which microorganisms can grow and divide. This
is the maximum specific growth rate, µmax. As nutrients become limited, growth slows down.
This behavior is modeled by the Monod equation:
 max s
Ks  s
where s is the substrate (nutrient) concentration and
Ks is the saturation constant.
Stationary Phase
During the stationary phase, the supply of nutrients becomes limiting and growth slows. In
addition, some cells begin to die. The growth and death processes are approximately in
equilibrium so there is no net growth or death.
Death Phase
During the death phase, cell death exceeds cell growth so there is a decline in the population of
viable cells.
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Biological Treatment Processes
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CSTR Bioreactor Systems
The typical bioreactor used for the activated sludge waste treatment process is a continuous
stirred tank reactor or CSTR. In this type of reactor, the tank is fed continuously with a constant
flow rate, Q, and effluent is removed at the same rate. The reactor is thoroughly mixed so the
contents are uniform throughout, and the effluent has the same composition as the contents of the
tank. The reactor operates at steady state so flow rates and compositions remain constant over
time. A schematic of a CSTR is shown in the figure below:
Q is volumetric flow rate
VR is the liquid volume in the reactor
x0 is microbial biomass concentration in the feed
x is microbial biomass concentration in the reactor
s0 is limiting nutrient concentration in feed
s is limiting nutrient concentration in reactor
A mass balance on microbial biomass gives the following equation:
Qx0  Qx  VR  x  0
The microbial biomass concentration in the feed, x0, normally is so small it can be considered
negligible so the mass balance reduces to
Qx  VR  x
  Q / VR  D  1 
where D is the dilution rate and
θ is the mean residence time in the reactor.
If we assume Monod growth behavior, we can combine equations 2 and 5 to get
 max s
Ks  s
Solving for s, we get
 max  D
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Determine the volume of a CSTR needed to treat waste water flowing at a rate of 150 m3/h with
a BOD (substrate) concentration of 250 mg/L to achieve a final concentration of 50 mg/L if µmax
= 0.01 h-1 and Ks = 10 mg/L.
If we assume the amount of microorganisms in the feed to the CSTR is negligible and that
growth follows the Monod equation, then the dilution rate is given by equation 6. Note that the
substrate concentration in the reactor is the same as the effluent concentration of 50 mg/L
(because the tank is completely mixed) and that is the concentration to use in equation 6.
 max s
Ks  s
0.01 50 mg
 0.0083 h -1
10  50 mg
Using equation 5, the CSTR volume needed is
Q 150 m 3
 18,000 m 3
Washout from CSTR
Since there is a limit to the rate at which cells can divide under the best conditions (µmax), the
dilution rate must also be limited. If the dilution rate is greater than approximately µmax, the
steady state solution will be that all cells wash out of the reactor; i.e., the mean residence time in
the reactor is too short for the cells to divide, so eventually (when the reactor reaches steady
state) all cells will have been removed in the effluent from the reactor.
CSTR with Recycle
One way to avoid washout at high dilution rates is to concentrate cells from the effluent and
recycle them back into the reactor creating a CSTR with recycle system:
Q, x, x0, s, s0, and Vr as defined for CSTR above
Qr is the volumetric flow rate of the recycle stream
Qw is the volumetric flow rate of the cell waste stream
xr is the concentration of biomass in the recycle stream (and also waste stream)
xe is the concentration of biomass in the clarified effluent stream (often negligible)
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Biological Treatment Processes
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In the CSTR with recycle system, the liquid has a hydraulic residence time, θ, given by
Since cells are separated in the clarifier and recycled, they will have a residence time in the
system different than the hydraulic residence. The cell residence time, θc, is
c 
Vr x
Qw xw
The conversion of substrate (limiting nutrient, BOD for wastewater streams) to cell biomass is
generally consistent in aerobic systems and is described by the yield coefficient, Y:
mass cells produced
s0  s mass substrateconsumed
With recycle, some cells may remain in the system a very long time and eventually die.
Therefore, there is a death (or decay) rate, kd, associated with cells in a recycle system.
If you assume there is no biomass in the feed (x0 = 0) and there are no cells in the liquid effluent
stream (xe =0), the concentration of biomass in the reactor is given by
 cY s0  s 
 1  k d c 
Note this is the same equation as equation 7 in the WWTP notes (Notes 5).
For activated sludge systems, cell concentration (x or X) is often measured as mixed liquor
volatile suspended solids (MLVSS). Mixed liquor is the mixture of wastewater and sludge in the
Values of growth parameters for domestic wastewater treatment
mg BOD5/L
mg VSS/mg BOD5
Source: Davis, M.L., and Masten, S.J., 2009, Principles of Environmental Engineering and
Science, 2nd Ed., McGraw-Hill, Boston.