Unit Process
Unit Process in
Biological Treatment
Modeling Suspended Growth
Treatment Processes
Description of treatment process:
All biological treatment reactor designs are based on using
mass balances across a defined volume for each constituent of
interest (i.e., biomass, substrate, etc.)
Biomass mass balance:
Accumulation = inflow – outflow + net growth
Q
Q
X
Q
X
r
V
eq
(7
-
32)
QX
V
dt
dX
g r
w e
w
o
Modeling Suspended Growth
Treatment Processes
Assuming stead-state and
X
o= 0, equation 7-32 can be
simplified:
Q
Q
w
X
e
Q
wX
r
r
gV
Eq
(7
-
33)
21)
-(7
Eq
X
k
Yr
r
g
su
d
Eq
(7
-
34)
d su
r w e
w
k
X
r
Y
VX
X
Q
X
Q
Q
day
each
system
the
form
removed
organisms
of
mass
reactor
the
in
organisms
of
mass
Modeling Suspended Growth
Treatment Processes
Equation 7-34 can be written as:
The term 1/SRT is related to
µ
, the specific biomass growth rate:
Q
Q
w
X
eQ
wX
rEq
(7
-
35)
VX
SRT
36)
-(7
Eq
d su
k
X
r
Y
SRT
1
37)
-(7
Eq
Modeling Suspended Growth
Treatment Processes
In Eq. (7-36) the term
(-r
su/
X
) is known as the
specific substrate
utilization rate U
and can be calculated as the following:
Substituting Eq. (7-12) into Eq. (7-36) yields:
Solving Eq. (7-39) for
S
yields:
38)
-(7
Eq
X
S
S
VX
S
S
Q
X
r
U
su o o
(
)
39)
-(7
Eq
d s
k
S
K
YkS
SRT
1
S
K
kXS
r
s su
Modeling Suspended Growth
Treatment Processes
Substrate mass balance:
Accumulation = inflow – outflow + generation
Substituting for
r
suand assuming steady-state, Eq. (7-41) can be
written as:
41)
-(7
eq
V
r
S
Q
QS
QS
V
dt
dS
su wo
42)
-(7
eq
S
K
kXS
Q
V
S
S
s o
eq
(7
-
43)
Modeling Suspended Growth
Treatment Processes
Mixed liquor solids concentration and solids production:
The solids production from a biological reactor represents the
mass of material that must be removed each day to maintain
the process:
Eq. (7-45) can be used to calculate the amount of solids
wasted for any of the mixed liquor components. For the
amount of biomass wasted (
P
X), the biomass concentration
X
can be used in place of
X
in Eq. (7-45).
3
VSS/m
g
tank,
areation
in
MLVSS
total
VSS/d
g
daily,
wasted
solids
total
where;
45)
-(7
eq
T VSS X
T VSS
X
X
P
SRT
V
X
P
T T
Modeling Suspended Growth
Treatment Processes
Mixed liquor solids concentration:
The total MLVSS equals the biomass concentration
X
plus the
nbVSS concentration
X
i:
A mass balance is needed to determine the nbVSS conc.:
Accumulation = inflow – outflow + generation
46)
-(7
eq
i
T
X
X
X
Modeling Suspended Growth
Treatment Processes
Mixed liquor solids concentration:
At steady-state and substituting Eq. (7-25)
for in Eq. (7-47) yields:
Combining Eq. (7-43) and Eq. (7-49) for X and Xi produces the
following equation that can be used to determine XT :
49)
-(7
eq
)
(
)
)(
(
/
)
(
,
SRT
f
k
X
SRT
X
X
i
o i
d d
dX
i/
dt
0
X
k
f
r
Xd
d(
d)
r
X,i
(C) (B) (A) influent in VSS adable Nonbiodegr debris Cell biomass ic Hetrotroph50)
-(7
eq
SRT
X
SRT
X
k
f
SRT
k
S
S
Y
SRT
X
d d o id o T
,1
Modeling Suspended Growth
Treatment Processes
Solids production:
The amount of VSS produced and wasted daily is as follows:
Eq. (7-43) is substituted for biomass concentration (X) in Eq. (7-51) to show VSS production rate in terms of the substrate removal,
influent VSS, and kinetic coefficients as follows:
k
SRT
f
k
X
V
QX
eq
(7
-
51)
S
S
QY
P
d d o id o VSS
X, ,
1
(C) (B) (A) influent in VSS adable Nonbiodegr debris Cell biomass ic Hetrotroph52)
-(7
eq
QX
SRT
k
SRT
S
S
QY
k
f
SRT
k
S
S
QY
P
o id o d d d o VSS
X, ,
1
1
Modeling Suspended Growth
Treatment Processes
Solids production:
The effect of SRT on the performance of an activated sludge system for soluble substrate removal is shown in figure 7-13
The total suspended solids (TSS) production can be calculated by modifying Eq. (7-52) assuming that a typical biomass VSS/TSS ratio of 0.85 as follows:
solids inorganic
influent
53)
-(7
eq
VSS
TSS
Q
C
B
A
P
X TSS(
o o)
85
.
0
85
.
0
Modeling Suspended Growth
Treatment Processes
The observed yield:
The observed yield for VSS can be calculated by dividing Eq. (7-52) by the substrate removal rate Q(So-S):
Oxygen requirements:
Oxygen used = bCOD removed – COD of waste sludge
56)
-(7
eq
S
S
X
SRT
k
SRT
Y
k
f
SRT
k
Y
Y
o i o d d d d obs
,)
(
1
)
)(
)(
(
)
(
1
cells g / O g tissues, cell of COD 1.42 kg/d day, per wasted VSS as biomass kg/d required, Oxygen where 59) -(7 eq 2
bio X o bio X o oP
R
P
S
S
Q
R
, ,42
.
1
)
(
Modeling Suspended Growth
Treatment Processes
Design and operating parameters:
Following are the design and operating parameters that are fundamentals to treatment and performance of the process:
-
SRT
-
Food to microorganisms (F/M) ratio
The SRT can be related to F/M by the following equation:
60)
-(7
eq
biomass microbial
total
rate substrate applied
total
X
S
VX
QS
M
F
o o
/
66)
-(7
eq
k
E
M
F
Y
Modeling Suspended Growth
Treatment Processes
Design and operating parameters:
-
Organic volumetric loading rate.
Defined as the amount of BOD or COD applied to the
aeration tank volume per day:
Modeling Suspended Growth
Treatment Processes
Modeling plug-flow reactors:
Developing a kinetic model for the plug-flow reactor is
mathematically difficult (X vary along the reactor). Two assumptions are made to simplify the modeling:
-
The concentration of microorganisms is uniform along the reactorThis assumption applies only when SRT/
5.-
The rate of substrate utilization is given by:
X
72)
-(7
eq
S
K
X
kS
r
s su
Modeling Suspended Growth
Treatment Processes
Modeling plug-flow reactors:
Integrating Eq. (7-72) over the retention time in the tank gives:
X
Biological Nitrification
Nitrification is the conversion (by oxidation) of Ammonia (NH
4-N) to nitrite (NO
2-N) and then to nitrate (NO
3-N).
The need for nitrification arises from water quality concerns:
-
Effect of ammonia on receiving water; DO demand, toxicity.-
Need to provide nitrogen removal for eutrophication control.-
Need to provide nitrogen removal for reuse applications.
The current drinking water MCL for nitrate is 45 mg/l as
nitrate or 10 mg/l as nitrogen.
Biological Nitrification
Process description:
-
Nitrification is commonly achieved with BOD removal in the samesingle-sludge process.
-
In case of the presence of toxic substances in the wastewater, atwo-sludge system is considered.
Biological Nitrification
Process description:
-
The oxygen required for complete oxidation of ammonia is 4.57 g O2/ g N oxidized.-
The alkalinity (alk) requirement is 7.14 g alk as CaCO3 for each g of ammonia nitrogen (as N).O
H
CO
NO
O
HCO
NH
4
2
3
2
2
2
3
2
2
3
2Biological Denitrification
Process description:
-
Denitrification is the biological reduction of nitrate (NO3) to nitric oxide (NO), nitrous oxide (N2O), and nitrogen (N).-
The purpose is to remove Nitrogen from wastewater.-
Compared to alternatives of ammonia stripping, breakpointchlorination, and ion exchange, biological nitrogen removal is more cost-effective and used more often.
-
Concerns over eutrophication and protection of groundwater againstBiological Denitrification
Stoichiometry:
-
In denitrification, nitrate is used as the electron acceptor instead ofoxygen and the COD or BOD as the carbon source (electron donor).
-
The carbon source can be the influent wastewater COD or externalsource (Methanol).
-
One equivalent of alkalinity is produced per equivalent of nitratereduced. (3.57 g alk per g nitrate)
Biological Phosphorus Removal
Process description:
-
Phosphorous removal is done to control eutrophication.-
Chemical treatment using alum or iron salts is the most commonlyused technology for phosphorous removal.
-
The principle advantages of biological phosphorous removal arereduced chemical costs and less sludge production.
-
In the biological removal of phosphorous, the phosphorous in theinfluent is incorporated into cell biomass which is removed by sludge wasting.
-
Phosphorous accumulating organisms (PAOs) are encouraged toAnaerobic Fermentation and Oxidation
Process description:
-
Used primarily for the treatment of waste sludge and high strengthorganic waste.
-
Advantages include low biomass yield and recovery of energy in theform of methane.
-
Conversion of organic matter occurs in three steps:– Step1 (Hydrolysis): involves the hydrolysis of higher-molecular-mass compounds into compounds suitable for use as a source of energy and carbon.
– Step2 (Acidogenesis): conversion of compounds from step1 into lower-molecular-mass intermediate compounds.
(nonmethanogenic bacteria)
Anaerobic Fermentation and Oxidation
Process description:
-
For efficient anaerobic treatment, the reactor content should be:– void of O2
– free of inhibiting conc. of heavy metals and sulfides – pH ~ 6.6 – 7.6
– sufficient alkalinity to ensure pH is not <6.2 (methane bacteria will not function below 6.2).
-
Methanogenic bacteria has slow growth rate, therefore:– require long detention time for waste stabilization
– yield is low: less sludge production and most organic matter is converted to CH4 gas.