11
CE 548 II
Fundamentals of Biological Treatment
22
Modeling Suspended Growth Modeling Suspended Growth Treatment ProcessesTreatment 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
32)-(7 eq VrXQXQQQXVdt
dXgrwewo
33
Modeling Suspended Growth Modeling Suspended Growth Treatment ProcessesTreatment Processes
44
Modeling Suspended Growth Modeling Suspended Growth Treatment ProcessesTreatment Processes
Assuming stead-state and Xo = 0, equation 7-32 can be simplified:
33)-(7 Eq VrXQXQQ grwew
21)-(7 Eq XkYrr dsug
34)-(7 Eq d
surwew kX
rY
VX
XQXQQ
day each system the form removed organisms of mass
reactor the in organisms of massSRT
55
Modeling Suspended Growth Modeling Suspended Growth Treatment ProcessesTreatment Processes
Equation 7-34 can be written as:
The term 1/SRT is related to µ, the specific biomass growth rate:
35)-(7 Eq rwew XQXQQ
VXSRT
36)-(7 Eq dsu kX
rY
SRT
1
37)-(7 Eq SRT
1
66
Modeling Suspended Growth Modeling Suspended Growth Treatment ProcessesTreatment Processes
In Eq. (7-36) the term (-rsu/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
SS
VX
SSQ
X
rU oosu
)(
39)-(7 Eq ds
kSK
YkS
SRT
1
SK
kXSr
ssu
40)-(7 Eq
1
1
d
ds
kYkSRT
SRTkKS
77
Modeling Suspended Growth Modeling Suspended Growth Treatment ProcessesTreatment Processes
Substrate mass balance:
Accumulation = inflow – outflow + generation
Substituting for rsu and assuming steady-state, Eq. (7-41) can be written as:
41)-(7 eq VrSQQSQSVdt
dSsuwo
42)-(7 eq
SK
kXS
Q
VSS
so
43)-(7 eq
SRTk
SSYSRTX
d
o
1
0
88
Modeling Suspended Growth Modeling Suspended Growth Treatment ProcessesTreatment Processes
99
Modeling Suspended Growth Modeling Suspended Growth Treatment ProcessesTreatment 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 (PX), the biomass concentration X can be used in place of XT in Eq. (7-45).
3 VSS/mg tank, areation in MLVSS total
VSS/dg daily, wastedsolids total where;
45)-(7 eq
T
VSSX
TVSSX
X
PSRT
VXP
T
T
,
,
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Modeling Suspended Growth Modeling Suspended Growth Treatment ProcessesTreatment Processes
Mixed liquor solids concentration:
The total MLVSS equals the biomass concentration X plus the nbVSS concentration Xi :
A mass balance is needed to determine the nbVSS conc.:
Accumulation = inflow – outflow + generation
46)-(7 eq iT XXX
diX
i
io
iXiioi
r
X
X
VrSRTVXQXVdtdX
3g/m debris, cell from production nbVSS of rate
3g/m tank, areation in ionconcentrat nbVSS
3g/m influent, in ionconcentrat nbVSS Where
47)-(7 eq
,
,
,, //
1111
Modeling Suspended Growth Modeling Suspended Growth Treatment ProcessesTreatment 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 )())((/)(, SRTXkfSRTXX ddioi
0/ dtdX i
Xkfr ddXd )( iXr ,
(C)(B)(A)
influent in VSS
adableNonbiodegrdebris Cell
biomass
icHetrotroph
50)-(7 eq SRTX
SRT XkfSRTk
SSYSRTX io
ddd
oT
,
1
1212
Modeling Suspended Growth Modeling Suspended Growth Treatment ProcessesTreatment 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:
51)-(7 eq QX VXkf
SRTk
SSQYP iodd
d
oVSSX ,, 1
(C)(B)(A)
influent in VSS
adableNonbiodegrdebris Cell
biomass
icHetrotroph
52)-(7 eq QX SRTk
SRTSSQYkf
SRTk
SSQYP io
d
odd
d
oVSSX ,, 11
1313
Modeling Suspended Growth Modeling Suspended Growth Treatment ProcessesTreatment 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:
solidsinorganic influent
53)-(7 eq VSSTSSQCBA
P ooTSSX )(85.085.0,
1414
1515
Modeling Suspended Growth Modeling Suspended Growth Treatment ProcessesTreatment 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 SS
X
SRTk
SRTYkf
SRTk
YY
o
io
d
dd
dobs
,
)(1
))()((
)(1
cells g/ O g tissues, cell of COD 1.42
kg/d day, per wasted VSSas biomass
kg/d required, Oxygen
where
59)-(7 eq
2
bioX
o
bioXoo
PR
PSSQR
,
,42.1)(
Study example 7-6
1616
Modeling Suspended Growth Modeling Suspended Growth Treatment ProcessesTreatment 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
QSMF oo
/
66)-(7 eq kE
MFYSRT d
100)/(
1
1717
Modeling Suspended Growth Modeling Suspended Growth Treatment ProcessesTreatment 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:
3
3
3
3
m volume, tank aeration
g/m ion,concentrat BOD influent
/dm flowrate, influent
dBOD/m kg loading,organic volumetric
Where
67)-(7 eq
VSQ
L
kggV
QSL
o
org
oorg )/10)(( 3
1818
Modeling Suspended Growth Modeling Suspended Growth Treatment ProcessesTreatment 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 reactor This assumption applies only when SRT/ 5.
The rate of substrate utilization is given by:
X
72)-(7 eq SK
XkSr
ssu
1919
Modeling Suspended Growth Modeling Suspended Growth Treatment ProcessesTreatment Processes
2020
Modeling Suspended Growth Modeling Suspended Growth Treatment ProcessesTreatment Processes
Modeling plug-flow reactors:
Integrating Eq. (7-72) over the retention time in the tank gives:
X
ratio recycle
flow recycle withdilution after reactor to ionconcentrat influent
ionconcentrat effluent
ionconcentrat influent
where;
73)-(7 eq
1
/ln1
1
SSSSS
kSSKSS
SSYk
SRT
o
i
o
diso
o
2121
Biological NitrificationBiological Nitrification
Nitrification is the conversion (by oxidation) of Ammonia (NH4-N) to nitrite (NO2-N) and then to nitrate (NO3-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.
The total concentration of organic and ammonia nitrogen in municipal wastewater is typically in the range of 25-45 mg/l as nitrogen.
2222
Biological NitrificationBiological Nitrification
Process description:
Nitrification is commonly achieved with BOD removal in the same single-sludge process.
In case of the presence of toxic substances in the wastewater, a two-sludge system is considered.
Stoichiometry:
OHCONOOHCONH
NOONO
OHHNOONH
rNitrobactebacteriaNitrate
asNitrosomonbacteriaNitrite
223234
3)(
22
22)(
24
32222
22
24232
:reaction Overall
2323
Biological NitrificationBiological 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).
OHCONOOHCONH 223234 32222 :reaction Overall
2424
2525
Biological DenitrificationBiological 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, breakpoint chlorination, and ion exchange, biological nitrogen removal is more cost-effective and used more often.
Concerns over eutrophication and protection of groundwater against elevated NO3-N concentration.
2626
2727
Biological DenitrificationBiological Denitrification
Stoichiometry:
In denitrification, nitrate is used as the electron acceptor instead of oxygen and the COD or BOD as the carbon source (electron donor).
The carbon source can be the influent wastewater COD or external source (Methanol).
One equivalent of alkalinity is produced per equivalent of nitrate reduced. (3.57 g alk per g nitrate)
OHOHCONNOOHCH
OHNHOHCONNONOHC
675265
10310510
22233
3222331910
Methanol
Wastewater
2828
Biological Phosphorus RemovalBiological Phosphorus Removal
Process description:
Phosphorous removal is done to control eutrophication.
Chemical treatment using alum or iron salts is the most commonly used technology for phosphorous removal.
The principle advantages of biological phosphorous removal are reduced chemical costs and less sludge production.
In the biological removal of phosphorous, the phosphorous in the influent is incorporated into cell biomass which is removed by sludge wasting.
Phosphorous accumulating organisms (PAOs) are encouraged to grow and consume phosphorous. Therefore, the system is designed so that the reactor configuration provides advantage for PAOs to grow over other bacteria.
2929
Biological Phosphorus RemovalBiological Phosphorus Removal
3030
Biological Phosphorus RemovalBiological Phosphorus Removal
3131
Anaerobic Fermentation and OxidationAnaerobic Fermentation and Oxidation
Process description:
Used primarily for the treatment of waste sludge and high strength organic waste.
Advantages include low biomass yield and recovery of energy in the form 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)
– Step3 (Methanogenesis): conversion of intermediates into simpler end products (CH4 & CO2).
3232
Anaerobic Fermentation and OxidationAnaerobic 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.
– sludge produced is stable: suitable for composting
– require relatively high temp for adequate treat.
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