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Wastewater Treatment Design
1. Introduction
The wastewater treatment plant "Sydney Clean Water" provides currently for primary
wastewater treatment only. Due to its necessity of achieving higher effluent quality requirements
related to BOD, Ammonia-N, Total P, SS, and Total N parameters, Sydney Clean Water has
requested to Guilcapi and Kamanzi Company to provide a design appraisal for the inclusion of
secondary wastewater treatment process at the existing plant. After the analysis of different
phosphorus removal processes, the A2/O (anaerobic/anoxic/aerobic) process was selected for the
proposal design due to its simplicity and several advantages. The upgraded treatment works
include a description of the proposal process, a layout plan with all necessary upgrades for the
inclusion of the secondary wastewater treatment, a draft design procedure for a proposed
capacity of 120,000 ep, and a critical comment on all the aspects for the proposed modification
including sludge management.
2. A2/O process, upgrades and layout plan
The biological phosphorus removal configuration, A2/O (anaerobic/anoxic/aerobic) process was
selected for the upgrade treatment at the wastewater treatment plant "Sydney Clean Water" after
the analysis of advantages and disadvantages of six phosphorus removal processes (Table 1).
The A2/O process was selected due to its good removal of nitrogen and phosphorus, good
generation of settling sludge, addition of alkalinity to the system, savings in energy, and easy
operation.
Table 1. Evaluation of advantages and disadvantages of phosphorus removal processes (Metcalf
& Eddy., 2003)
Process
Parameters of evaluation
P removal N removal Simple
operation
Good settling sludge
production Cost (save energy)
A/O √ x √ √ √
A2/O √ √ √ √ √
UCT √ √ x √ -
VIP √ √ x √ -
Bardenpho (5- stage) √ √ x √ -
SBR √ √ x - -
PhoStrip √ √ x - -
√: Advantage; x: Limitation; -: No information
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The A2/O process also known as 3 Stage Phoredox (EPA, 2009), is based on the A/O
(anaerobic/aerobic) process. The stages in the A2/O process are anaerobic, anoxic and aerobic.
Phosphorus removal occurs in the anaerobic stage whereas BOD removal with nitrification
ocurrs in the aerobic stage. In the anoxic zone, nitrate is removed through the denitrification
process. (Metcalf & Eddy, 2003). A2/O process has some limitations, such as reduction of the
activity of polyphosphate accumulating organisms (PAOs), reduction in the efficiency of
phosphorus removal due to the recycle of RAS with nitrate content to the anaerobic stage and
limitation of nitrogen removal by internal recycle ratio (Metcalf & Eddy, 2003).
The proposed secondary treatment will be located after the current Sydney Clean Water's
primary wastewater treatment as it is shown in the Figure 1.
Fig. 1 Proposed secondary treatment for Sydney Clean Water. Adapted from (Stuetz, 2012)
3. Basic design data and assumed parameters
The required upgraded treatment works have been designed on the basis of provided design
data, kinetic data and some other important assumptions considered. However, it is important to
note that some given kinetic data were erroneous, hence corrections were applied to bring them
into recommended ranges as proposed by Metcalf & Eddy ( 2003). A summary of major
assumptions is shown below and in Table 2. The rest of the assumptions are shown in Appendix A
Design flow Q: in order to obtain the daily flow rate from the proposed 120,000 equivalent
population (ep.), an average daily water consumption of 212 L/ep. was considered
(DEPARTMENT OF ENVIRONMENT AND RESOURCE MANAGEMENT, 2010) along
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with an 80% recovery factor. A peaking factor equal to 1.93 was selected (Harmon
coefficient) based on the given population. The flow rate was therefore given by
.
Process temperature: an average process temperature of 20º C was set in order to ease the
calculations; therefore, corrections on kinetic coefficients for temperature were not required.
Influent TKN: the entire TKN influent has been considered to be in the form of NH4-N.
Readily biodegradable chemical oxygen demand (rbCOD): a critical design parameter
such as rbCOD was not given, so following Metcalf & Eddy ( 2003) suggestion, 15-25 % of
bCOD range was considered to that end. In order to account for rbCOD consumed in
anaerobic phase, 25% was attributed to phosphorus removal and 10 % to denitrification.
Table 2. Design data and assumed parameters
Parameter Units Value Parameter Units Value CommentsQ ep. 120,000 BOD mg/L 10 Data given
BOD mg/L 280 NH4-N mg/L 2 Data givenTSS mg/L 320 Total P mg/L 0.3 Data givenTKN mg/L 60 SS mg/L 30 Data given
Total P mg/L 12 Total N mg/L 10 Data givenNO3-N mg/L 8 NO3-N = Total N - NH4-N
Parameter Value Comments
g MLVSS/g BOD removed 0.61 Data giveng MLVSS/g bCOD 0.38 Converted, bCOD=1.6 BOD
kg BOD utilized / kg VSS.d 20 Data giveng VSS/g VSS.d 0.12 Typical value (Metcalf & Eddy, 2003, p704)
g bCOD/m3 20 Typical value (Metcalf & Eddy, 2003, p704)
d-1 at 20 °C 0.4 Data given
Kg VSS/Kg NH4+ - N nitrified 0.12 Typical value (Metcalf & Eddy, 2003, p705)
mg NH4+ - N /L 0.74 Typical value (Metcalf & Eddy, 2003, p705)
mg/L 3,000 Data given
L/ep . day 212
% 80
1.93
Unitless 0.25 (Metcalf & Eddy, 2003, p757)
Unitless 1.6 (Metcalf & Eddy, 2003, p669)
Unitless 1.5 (Metcalf & Eddy, 2003, p708)
mg/L 2 (Metcalf & Eddy, 2003, p706)
° C 20
g/m3 0.5 (Metcalf & Eddy, 2003, p706)
g VSS/g VSS.d 0.08 (Metcalf & Eddy, 2003, p705)Unitless 0.15 (Metcalf & Eddy, 2003, p704)
Unitless 0.78 (Metcalf & Eddy, 2003, p714)Unitless 0.8 (Metcalf & Eddy, 2003 ,p666)
No value of nbVSSg/g 6.6 (Metcalf & Eddy, 2003, p 804)
g/g 8 (Metcalf & Eddy, 2003, p 802)
h 1 (Metcalf & Eddy, 2003, pp 756,763)
(DEPARTMENT OF ENVIRONMENT AND
RESOURCE MANAGEMENT, 2010)
Y
Assumptions
Units
Kinetic Data
Kinetic Constant for Nitrifying Bacteria
Y
Detention time in the anoxic tank
kdKs
KN
YN
µNMax
Ko
FS (TKN peak /TKN average)
DO
rbCOD/bCOD
bCOD/BOD
Recovery factor
Peaking factor (Harmon coefficient)
Effluent quality requirementsInfluent quality parameters
MLSS
Water consumption/per capita
K1
Average process temperature
rbCOD/P
rbCOD/NO3-N
Px,bio = Px,vssVSS/TSSNox/TKN
kdn
fd
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4. Draft design procedure
The process design considerations for the wastewater secondary treatment included influent
wastewater characteristics, effluent quality requirements, kinetic data, kinetic constant for
nitrifying bacteria, among other assumptions shown previously in Table 2. The kinetic data such
as microbial decay coefficient and saturation coefficient as well as the kinetic constants, such as
nitrified yield coefficient and half saturation constant for NH3 oxidation where replaced by
bibliography typical values due to the data given was out of range (Metcalf & Eddy, 2003).
The design of A2/O process was carried out in three steps: BOD and nitrification, phosphorus
removal, and denitrification. The system configuration selected was single-sludge biological
nitrogen-removal process and complete-mix activated sludge process, which consider
computation approaches for the activated-sludge process for BOD removal and nitrification and
for the anoxic/aerobic process design (Metcalf & Eddy, 2003). The nomenclature, equations,
and summary of calculations and calculations in detail are shown is appendices C, D, E, and F
respectively.
BOD and nitrification
The computation approach for the design of the activated-sludge process for BOD removal and
nitrification was carried out following the next steps:
1. It was selected a minimum DO concentration of 2.0 mg/L for nitrification to calculate the
specific growth rate for nitrification using eq. 1. It was selected a nitrification safety factor of
1.5 and computed the design SRT using eq. 3 (Metcalf & Eddy, 2003).
2. The maximum specific growth rate (µm) was computed using eq. 4 based on Y and K1 and
the effluent substrate concentration (S) was calculated using eq. 5.
3. The biomass production was computed using eq. 9 based on the values A, B and C
calculated using eqs. 6, 7 and 8, respectively. NOx was determined under the assumption that
NOx 0.8 TKN (Metcalf & Eddy, 2003). However, the real value of NOx was computed by
performing a nitrogen balance using eq. 10. This value was stated as correct due to its similarity
to the previously NOx concentration assumed.
4. In order to calculate the mass of VSS and TSS using eqs. 12 and 13, respectively, the
concentration of VSS and TSS was considered. The concentration of VSS was assumed to be
equal to the due to there was not found information to assume a value of nbVSS. The
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concentration of TSS was computed using eq. 11 assuming that VSS = 80% TSS (Metcalf &
Eddy, 2003) due to the lack of information of VSS concentration in the influent.
5. The aeration tank volume was computed using eq. 14 and the MLSS given. For the design of
each basin, there were considered 4 tanks, a depth of 6.5 m (included freeboard) and a width to
depth ration of 1.5:1 (Metcalf & Eddy, 2003).
Phosphorus removal
Phosphorus removal will be achieved in the anaerobic stage of the A2/O process designed
through the following steps:
1. In order to determine rbCOD available for P removal using eq. 25, it was performed the
nitrate mass balance at the influent in the reactor using eq. 22 considering no NO3 concentration
in the influent, NO3-N RAS = NO3 effluent, and a RAS recycle ratio of 0.6 (Metcalf & Eddy,
2003). It was also required the calculation of rbCOD equivalent using eq. 23 considering a
rbCOD/nitrate ratio = 6.6, bCOD/BOD=1.6 and rbCOD/bCOD = 0.25 due to this value was no
given (Metcalf & Eddy, 2003). In the anaerobic zone, rbCOD is taken by the phosphorus-storing
bacteria after its rapidly conversion to acetate via fermentation (Metcalf & Eddy, 2003).
2. The phosphorus removed by BPR mechanism was calculated using eq. 26 considering that 8 g
rbCOD/g P is removed by biological phosphorus removal (Metcalf & Eddy, 2003).
3. The phosphorus used for heterotrophic biomass synthesis was computed using eq. 28
considering a phosphorus content of heterotrophic biomass of 0.015 g P/ g biomass (Metcalf &
Eddy, 2003).
4. The concentration of P removed was computed using eq. 29 considering the concentration of
biological P removal and the concentration of P used for biomass growth. The target value of
0.3 mg/L was completely achieved.
Denitrification
The anoxic zone design to account for denitrification of nitrate considers a recycle stream from
aerobic zone with oxygen from nitrification being recovered. The followed procedure in
designing this stage is as follows.
1. The active biomass concentration is determined by eq. 32 whereas the amount of nitrate fed to
the anoxic tank is calculated using eq.33, eq.34 and eq.35. The volume required for anoxic zone
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is calculated using eq.36 by selecting an appropriate detention time. However, consideration is
given to the recycled nitrate depletion.
2. The amount of nitrate that can be reduced was calculated using eq.38. It is function of the
specific denitrification rate (SDNR) which was obtained from proposed empirical relationships
(Metcalf & Eddy, 2003) between F/Mb ratio (eq. 37) and the fraction of rbCOD/bCOD. When
the nitrate fed is not similar to the nitrate reduced, different detention times have to be chosen
and calculations have to be repeated. Comparison of SDNR as a function of MLSS with
conventional observed values were then done. SDNR was obtained using eq. 39.
3. As nitrate has the ability to act as electron acceptor, oxygen demand may be reduced. The
oxygen credit as well as the net oxygen required were calculated using eq.40 and eq. 41,
respectively. Fine bubble aeration design was done by computing the air flow rate through eq.
42 and eq. 43. Eq. 43 converted the mass flowrate into volumetric flowrate using air density and
diffusers efficiency factor. In this last context, ideal design conditions in terms of temperature
and altitude were assumed in order to ease the calculations.
4. As a final design step, required alkalinity as concentration of CaCO3 was calculated using
eq.46 and eq. 47. This is an important factor because some reactions have tendency to modify
the pH impacting on the system performance. The resulting alkalinity was then compared to the
alkalinity needed for nitrification only in order to assess the amount that can be saved.
Secondary clarifier design
As part of the proposed upgrade, a secondary clarifier which is necessary for settling and
removing suspended solids that encompass the nutrients was designed. This design included 3
steps namely, definition of the return sludge recycle ratio, determination of clarifiers’ sizes and
solids loading. The clarifier area was calculated using eq. 54 as function of the design flow rate
and an the assumed hydraulic application rate. Once the number of clarifiers was fixed, the
diameter of each basin was determined (circular clarifiers). The solids loading was calculated
using eq. 58. This is benchmarked to a typical accepted range as suggested by Metcalf & Eddy (
2003). When the solids loading is not within that range, the selected number of clarifiers needs
to be changed.
A summary of the main results is shown in Appendix .
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6. Critical comment on the proposed modification and sludge management
The design of the proposed secondary wastewater treatment was based on a number of
assumptions and considerations in order to simplify the calculations. Therefore, some aspects of
the proposed modification required due attention as part of the main assumptions were rooted on
ideal conditions as opposed to field dynamic conditions.
Process temperature: A 20ºC average process temperature was assumed. This condition was
suitable for most microorganisms metabolisms, such as nitrifiers, PAOs and heterotrophs;
however, temperature fluctuations are to be mostly expected in nitrification stage given that
autotrophs's activity is sensibly reduced below 20ºC impacting on the sludge retention time. In
addition, temperature influences gas transfer rate as well as settling factors (Metcalf & Eddy,
2003), so temperature changes need to be taken into account in biological treatment design.
Accurate wastewater characterization data: In order to provide quality design to meet the
required biological nutrient removal, sufficient or accurate data were required. Some
assumptions can be drawn from a wide range of possibilities which may impact on the
performance of the process. Therefore, a better influent characterization is necessary for the
system performance analysis and for the limitation of the overall capital and operational
expenditures resulting from an overdesign. On the other hand, trace metals testing and quality
data record may be beneficial when is required the prediction of bulking.
Readily biodegradable chemical oxygen demand (rbCOD): given that influent rbCOD
fraction was not provided, a 25 % rbCOD/bCOD ratio was assumed from recommended values
by Metcalf & Eddy ( 2003). However, for this same ratio, other authors such as Henze (1992)
suggests even increased values, specifically 56% of total COD. Information from COD
fractionation is necessary to depict limitations of rbCOD in phosphorus removal or in anoxic
denitrification as this may need an external carbon source (for example methanol) which in turn
entails additional operating costs. For the same reason, 10% rbCOD consumption for
denitrification with no other external carbon source was assumed considering prior consumption
in aerobic zone.
Bulking Sludge: Complete-mix operation, low dissolved oxygen and low food to microbe ratio
influence the bulking sludge phenomena attributed to outstanding filamentous growth (Martins
et al., 2004) in activated sludge reactors. The possibility of bulking events needs to be given due
attention as our proposed design lies in relatively low F/M ratio; therefore, a lower sludge
retention time was proposed for that end. By involving a denitrification stage, bulking sludge
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may not be expected. However, as no specific quality data were provided, it may even be better
to monitor potential industrial discharges given that nutrient decline can cause unexpected
bulking events.
Clarifier performance: high clarifier performance is anticipated by the inclusion of a
biological phosphorus removal unit at the basin head that delivers compact and easily settleable
flocs. Therefore, total suspended solids met effluent quality requirements.
Sludge Management & Phosphorus: sludge handling is an important aspect that needs to be
taken into consideration when designing biological treatment units, particularly anaerobic BPR.
The last unit generates sludge with considerable phosphorus concentration. This is enhanced
with recycle flows from the secondary clarifier with high phosphorus concentration as well.
Phosphorus is released throughout sludge thickening and digestion. On the other hand, struvite
deposition is another factor that needs to be closely monitored particularly in turbulence areas
(valves, pipes, pumps, etc) where precipitation of struvite is likely to occur (Doyle and Parsons,
2002). New technologies to prevent precipitation exist among which ultrasonic vibration that
restrain struvite crystal formation. Nevertheless, struvite crystallization for recovering
phosphorus is among the technologies that are utilized to treat liquors from sludge digesters. The
crystallized struvite is a promising slow-release agriculture fertilizer which is economically
profitable (Marti et al., 2008). In that same context, Metcalf & Eddy ( 2003) suggests opting for
methods to recover phosphorus capable of producing bio solids than can later be used for
agricultural purposes.
7. Conclusion
In order to achieve acceptable effluent quality parameters, the secondary wastewater treatment
process proposed for the wastewater treatment plant "Sydney Clean Water" was A2/O process
which was developed in 3 stages, namely BOD removal and nitrification, phosphorus removal
and denitrification, along with the clarifier design for settling and removing suspended solids.
Due to the limited data and some erroneous values given, a number of assumptions for the
design were taken based on practical information from the literature. The proposed A2/O process
will achieve satisfactorily the effluent quality targets for the parameters BOD, NH4-N, Total P,
SS, and Total N, BOD will be removed at 99.8%, P at 100% and NH4-N at 97%. Some aspects
of the proposed modification that require attention given the main assumptions rooted on ideal
conditions are the process temperature, need for accurate wastewater characterization data,
rbCOD, bulking sludge, and sludge management.
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References
DEPARTMENT OF ENVIRONMENT AND RESOURCE MANAGEMENT 2010. Planning Guidelines for Water Supply and Sewerage. Brisbane: Department of Environment and Resource Management.
DOYLE, J. D. & PARSONS, S. A. 2002. Struvite formation, control and recovery. Water Research, 36, 3925-3940.
EPA 2009. Nutrient Control Design Manual, State of Technology Review Report. Ohio. HENZE, M. 1992. Characterization of wastewater for modelling of activated sludge processes. Water
Science & Technology, 25, 1-16. MARTI, N., FERRER, J., SECO, A. & BOUZAS, A. 2008. Optimisation of sludge line management to
enhance phosphorus recovery in WWTP. Water Research, 42, 4609-4618. MARTINS, A. M. P., PAGILLA, K., HEIJNEN, J. J. & VAN LOOSDRECHT, M. C. M. 2004. Filamentous bulking
sludge—a critical review. Water Research, 38, 793-817. METCALF & EDDY, T., G., BURTON, F. L. & STENSEL, H. D. 2003. Wastewater engineering : treatment and
reuse, Boston, McGraw-Hill. STUETZ, R. 2012. CVEN9857 Wastewater Treatment, Biological Phosphorous Removal. University of
New South Wales, Sydney.
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APPENDIX A
Table 2 contin. Design data and assumed parameters
m 6.5 (Metcalf & Eddy, 2003, p817)
1.5 : 1 (Metcalf & Eddy, 2003, p817)
mg/L 9.08 (Metcalf & Eddy, 2003, p1747)
Unitless 0.65 (Metcalf & Eddy, 2003, pp429-430)
Unitless 0.95 (Metcalf & Eddy, 2003, pp 429-430)
Unitless 0.9 (Metcalf & Eddy, 2003, pp 429-430)
(Metcalf & Eddy, 2003, p429)
35%(Metcalf & Eddy, 2003 pp 707)
g CaCO3/g NH4-N 7.14
g/m3 80(Metcalf & Eddy, 2003, p 718)
g/m3 as CaCO3 140 (Metcalf & Eddy, 2003, p 717)
Return sludge mass concentration g/m3 8
g/m3 3
m3/m2.d 22 (Metcalf & Eddy, 2003, p802)
3
g P/g 0.015 (Metcalf & Eddy, 2003, p 807)
Alkalinity as CaCO3 produced per NO3-N Oxidized g/g 3.57 (Metcalf & Eddy, 2003, p790)
Mixing energy 10 kW/1000 m3
Assumptions
CS,20
α
β
F
CS,TH=CS,TH=CS,20
Depth of basin
Width to depth ratio
Phosphorus content of heterotrophic
Fine bubble ceramic diffusers with an aeration clean water
O2 transfer efficiency
Required alkalinity to transform
ammonium to nitrate
Residual alkalinity concentration to
maintain pH in range 6.8-7
Influent alkalinity
Design MLSS XTSS concentration
Hydraulic application rate
Number of clarifiers
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APPENDIX B
SUMMARY OF THE MAIN RESULTS
Parameter Unit Value Comment
Average wastewater flow m3/d 39,279 1ep =212 L/day
Average BOD load kg/d 10,998.12
Average TKN load kg/d 2,359.74
Aerobic SRT d 9.8 >7, (Metcalf & Eddy,p.614)
Aeration tanks Number 4 4 tanks (Metcalf & Eddy,p.817)
Aeration tank volume, ea m3 5,626
Aeration Tank Length m 90approx. 150m per tank, (Metcalf &
Eddy,p.817)
Aeration Tank Width m 10W:D ratio 1.5, (Metcalf &
Eddy,p817)
Aeration Tank Depth m 6.5 4-7.5, (Metcalf & Eddy,p817)
Hydraulic detention time h 14
MLSS g/m3 3,000
MLVSS g/m3 1,623
F/M g/g.d 0.30 0.04-1, (Metcalf & Eddy,p680)
BOD loading kg BOD/m3.d 0.49 0.04-1, (Metcalf & Eddy,p680)
Observed yield kg TSS/kg bCOD 0.39
kg VSS/kg BOD 0.50
Oxygen required kg/h 841
Air flowrate at average wastewater
flowm3/min 148,3
RAS ratio Unitless 0.60
Clarifier hydraulic application rate m3/m2.d 22 16-24, (Metcalf & Eddy,Table 8.7)
Clarifiers Number 3
Diameter,m 27
Alkalinity addition as Na(HCO3) kg/d 17,986
P used for biomass growth g/m3 1.22
P removed mg/L 12.75
P content of waste sludge % 7.2
DENITRIFICATION
Effluent NO3-N g/m3 8
Internal recycle ratio Unitless 4.2
RAS recycle ratio Unitless 0.6
Anoxic volume m3 2,291
MLSS g/m3 3,000
Overall SDNR g NO3-N/g MLSS.d 0.11
Detention time h 1.4
Mixing power kW 23
Alkalinity required kg/d as CaCO3 5,297
Effluent BOD mg/L 0.603 Target value achieved
Effluent NH4-N mg/L 2 Target value achieved
Effluent NO3 mg/L 8 Target value achieved
Effluent P mg/L 0 Target value achieved
Effluent TSS mg/L 30 Target value achieved
BIOLOGICAL PHOSPHOROUS REMOVAL
EFFLUENT QUALITY PARAMETERS
BOD REMOVAL AND NITRIFICATION
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APPENDIX C
NOMENCLATURE
Q: influent wastewater flowrate (m3/d) Ko: oxygen inhibition coefficient, g/m3 µm: Maximum specific growth rate (d)
BOD: Biological oxygen demand (mg/L) DO: Dissolved oxygen, mg/L Fd: cell debris fraction (unitless)
TSS: Total suspended solids (mg/L) µNmax: Maximum specific growth rate of nitrifying bacteria,
g new cells/g cells . d MLSS: mixed-liquor suspended solids, mg/L
TKN: influent TKN concentration (mg/L) µm: Maximum specific growth rate, (d) PX,bio: Biomass production (kg VSS/d)
Ne: effluent NH4-N concentration, mg/L µn: Specific growth rate for nitrification (d-1) PX,VSS: Solid production as VSS (kg/d)
Total P: Total phosphorus (mg/L) µ: Specific growth rate (d-1) PX,TSS: Solid production as TSS (kg/d)
NH4-N: Ammonia as Nitrogen (mg/L) SRT: Solid retention time (d) Fraction VSS: fraction of VSS over TSS, unitless
Y: Heterotrofic yield coefficient (kg VSS produced/Kg BOD FS: Safety Factor XVSS * V: Mass of VSS (kg)
K1: Maximum specific susbtrate utilisation rate (Kg BOD/Kg
VSS.d) So: Influent substrate concentration (mg/L) XTSS * V: Mass of TSS (kg)
Kd: Microbial decay coefficient (d-1) S: Effluent substrate concentration (mg/L) V: Total volume of aeration tanks (m3)
Ks: Saturation coefficient (mg/L) A: heterotrophic biomass, kg/day Ƭ: Detention time (h)
YN: Nitrifier yield coefficient (Kg VSS produced/Kg NH4+ - N
nitrified) B: cell debris, kg/d Lorg: Volumetric BOD (kg/m3.d)
Kdn: Endogenous decay coefficient for nitrifying organisms (g
VSS/g VSS∙d) C: nitrifying bacteria biomass, kg/day Yobs,TSS: Observed yield base on TSS (g TSS/gBOD)
KN: Half-velocity constant, substrate concentration at one-half the
maximun specific substrate utilization rate (g/m3) D: Nonbiodegradable VSS in influent, kg/day Yobs,VSS: Observed yield base on VSS (g VSS/gBOD)
rbCOD: Readily biodegradable chemical oxygen demand (mg/L) TSSo: influent wastewater TSS concentration (mg/L) NOx influent: Concentration of NH4-N in the influent flow
that is nitrified (mg/L)
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N: nitrogen concentration, g/m3 VSSo: influent wastewater VSS concentration (mg/L) β: Salinity-surface tension correction factor (unitless)
Xb : active biomass concentration (mg/L)
F/Mb: BOD F/M ratio based on activated biomass
concentration (gBOD/g biomass .d) F: fouling factor (unitless)
kd: endogenous decay coeff. (1/day) NOr: nitrate removed (g/d) : Oxygen saturation concentration in clean water at
temperature t and altitude h (mg/L)
NOx effluent: nitrogen oxides in the effluent (mg/L) SDNR (MLSS): specific denitrification rate referred based
on MLSS (g NO3-N/g MLVSS. d)
CS,20: Dissolved oxygen saturation concentration in clean
water at 20C and 1 atm or 760 mmHg (mg/L)
IR : internal recycle ratio (unitless) R1: Oxygen credit (kg/h) CL : operating oxygen concentration (mg/L)
Qanoxic : flow rate to anoxic tank (m3/d) Ro: Net oxygen required (kg/h) E: diffusers oxygen transfer efficiency (unitless)
NOx feed: NO3 - N fed to the anoxic tank (kg/d) AOTR: actual oxygen transfer rate under field conditions
(kg O2/h)
Alk produced-denitrification: alkalinity produced in denitrification (
g/m3)
Vnox: volume anoxic tank (m3) SOTR: Standard Oxygen Transfer Rate in Tap Water at
20°C and zero dissolved oxygen (kg O2/h)
NOx RAS: nitrogen oxides in the effluent in the return
activated sludge (mg/L)
SDNR: Specific denitrification rate (g NO3-N /g MLVSS.d) α: Oxygen transfer correction factor for waste (unitless)
D: diameter (m)
Qr: RAS flowrate (m3/d) R: return activated sludge (RAS) recycle ratio (unitless) A: total area of clarifier (m2)
Xr: Return sludge mass concentration (g/m3) X: Mixed-liquor suspended solids (mg/L)
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APPENDIX D
EQUATIONS
Equation Reference Equation Reference
BOD REMOVAL AND NITRIFICATION
1 12
2 13
3
14
4
15
5
16
6 MLVSS = Fraction VSS * MLVSS
17
7
18
8
19
9
20
10 21
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11
PHOSPHORUS REMOVAL
22 27
rbCOD equivalent = NO3 Nreact * 6.6 23 P used for biomass growth = phosphorus
content of heterotrophic biomass * Px,bio 28
rbCOD available for P removal = rbCOD influent - rbCOD
equivalent
24 P removed = Biological P removal + P used for
biomass growth 29
25 Total P in sludge = P removed * Q /1000 30
Biological P removal = rbCOD available for P removal /
ratio rbCOD/P 26
Phosphorus % = (Total P in sludge / PX, TSS)
* 100 31
DENITRIFICATION
32
41
33
42
34
43
35
44
36 45
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37
46
38
47
SDNR (MLSS) = SDNR * (Xb/MLSS)
39
48
40 49
SECONDARY CLARIFIER
50
55
51
56
52
57
53
58
54
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APPENDIX E
CALCULATIONS
DESIGN CALCULATIONS SUMMARY
Parameter Value Unit Recommended values/range Comment
BOD REMOVAL AND NITRIFICATION
Specific growth rate for nitrification 0.15 d-1
Theoretical sludge retention time 6.5 d
Design sludge retention time 9.8 d >7 (Metcalf & Eddy, p.614) In range
Maximum specific growth rate 7.6 d-1
Effluent Substrate Concentration
(BOD effluent)
0.603 mg/L
Readily biodegradable chemical oxygen demand 112 mg/L
Heterotrophic Biomass 3,074.5 kg/d
Cell Debris 540.5 kg/d
Nitrifying bacteria biomass 123.8 kg/d
Biomass production 3,738.8 kg VSS/d
Amount of ammonia oxidized to nitrate 46.6 g/m3
Solid production as VSS 3,738.8 kg/d
Solid production as TSS 6,912.5 kg/d
Mass of MLVSS 36,518 kg
Mass of MLSS 67,515 kg
Aeration tank volume 22,505 m3
Number of aeration basins 4 unitless 4 for this flow rate range, (Metcalf &
Eddy, p.817)
As recommended
Volume of each basin 5,626 m3
Depth of each basin 6.5 m 4 - 7.5 (Metcalf & Eddy, p.817) In range
Width of each basin 10 m W:D = 1.5
(Metcalf & Eddy, p.817)
Ratio respected
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Length of each basin 90 m Approximately 150m per tank (Metcalf
& Eddy, p.817)
As recommended
Detention time 14 h
Fraction VSS 0.54 g/m3
MLVSS 1,623 g/m3
F/M 0.30 g/g.d 0.04 –1(Metcalf & Eddy, p.680) In range
Volumetric BOD 0.49 kg/ m3.d
Observed yield based on TSS 0.63 g TSS/
g BOD
Observed yield based on VSS 0.50 g VSS/
g BOD
Alkalinity used for nitrification 332.6 g/m3
Mass of alkalinity needed for nitrification (as CaCO3) 10,706 kg/d
Alkalinity needed as sodium bicarbonate 17,986 kg/d
PHOSPHORUS REMOVAL
Nitrate fed to reactor 3 mg/L
rbCOD available for Phosphorus removal 92.2 mg/L
Phosphorus removed by BPR mechanism 11.5 mg/L
Phosphorus used for heterotrophic biomass synthesis in addition to
phosphorus storage due to BPR
3,198 kg/d
Phosphorus used for biomass growth 1.22 mg/L
Phosphorus removed 12.75 mg/L 100% phosphorus removal is
achieved.
Phosphorus content of waste sludge 500.7 kg/d
Phosphorus content of waste sludge 7.2 %
DENITRIFICATION
Active biomass concentration 1,334 mg/L
Internal recycle ratio 4.2 unitless
Flow rate to anoxic tank 189,414 m3/d
Nitrate fed to anoxic tank 1,515 kg/d NO3-N from aeration basin
effluent
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Volume of anoxic tank 2,291 m3
Detention time in anoxic tank 1.4 h
F/Mb 3.60 g/g.d
Specific denitrification rate (SDNR) 0.42 g NO3-N /g
biomass.d
Nitrate removed 1,562 kg/d
SDNR (MLSS) 0.11 g/g.d
Oxygen demand (nitrification) 841 kg/h
Oxygen credit 181 kg/h
Net O2 required 661 kg/h
Oxygen transfer rate (SOTR) 1,547 kg/h
Air flow rate 272.9 m3/min
Alkalinity produced in denitrification 137.7 g/ m3
Net alkalinity needed in the process 134.8 g/ m3
Mass of alkalinity needed 5,297 kg/d as CaCO3
Alkalinity savings 5,410 kg/d
Anoxic zone mixing energy 23 kW
SECONDARY CLARIFIER
Return sludge recycle ratio 0.6 unitless
Total clarifier area 1,718 m2
Number of clarifiers 3 unitless
Area per clarifier 573 m2
Diameter of each clarifier 27 m 10-40 (Metcalf & Eddy, p.833) In range
Solid loading 5 kg MLSS/ m2.h 4-6 (Metcalf & Eddy, table 8.7) In range
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APPENDIX F
CALCULATIONS IN DETAIL
Assignment No. 2
Wastewater Treatment Design
BOD REMOVAL AND NITRIFICATION Specific growth rate for nitrification
(1)
Theoritical SRT and Design SRT (2)
(3)
Maximum specific growth rate (4)
Effluent Substrate Concentration (BOD effluent)
(5)
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Assignment No. 2
Wastewater Treatment Design
Biomass production
A (Heterotrophic Biomass) (6)
B (Cell Debris) (7)
C (Nitifying bacteria biomass)
(8)
(9)
Amount of ammonia oxidized to nitrate
(10)
Concentration and mass of VSS and TSS in the aeration basin
Concentration of VSS and TSS
PX,VSS = Px,bio
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Assignment No. 2
Wastewater Treatment Design
(11)
Mass of VSS and TSS
(12)
(13)
Design of aeration tank volume
(14)
Assumed number of basins = 4
Vbasin = V/4
Vbasin =22,505/4 = 5,626 m3
Assumed depth = 6.5 m Width to depth ratio = 1.5 : 1
Width = 6.5 *1.5 = 9.75 ~10 m
Length = Vbasin / Depth / Width
Length = 5626 / 6.5 / 10 = 89 ~ 90m
Detention time
(15)
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Assignment No. 2
Wastewater Treatment Design
(16)
(17)
F/M (18)
Volumetric BOD (19)
Observed yield base on TSS (20)
Observed yield base on VSS
(21)
Oxygen Demand
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Assignment No. 2
Wastewater Treatment Design
Ro = (39,279/1,000 * (448 - 4.723) - 1.42 * 5,131.874 + 4.33 * 39,279 * 42.32/1000)/24 = 722 kg/h
Mass of alkalinity needed as CaCO3 for nitrification
PHOSPHOROUS REMOVAL
P design requirement
NO3 effluent = 8 mg/L
NO3-N in RAS
NO3-N RAS = NO3-N effluent = 8 mg/L
Nitrate mass balance at influent in the reactor
(22)
rbCOD available for P removal
(23) rbCOD equivalent = NO3 Nreact * 6.6
rbCOD equivalent = 3*6.6 = 19.8 mg/L
rCOD available for P removal
(24) rbCOD available for P removal = rbCOD influent - rbCOD equivalent
rbCOD available for P removal= 112 - 19.8 = 92.2 mg/L
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Assignment No. 2
Wastewater Treatment Design
Readily biodegradable chemical oxygen demand
(25)
Phosphorus removed by BPR mechanism (26)
Biological P removal = rbCOD available for P removal / ratio rbCOD/P
Biological P removal = 92.2/8 = 11.5 mg P/L
Phosphorus used for heterotrophic biomass synthesis in addition to phosphorus storage due to BPR
(27)
Px, bio = 3074,5 + 123,8 = 3,198.3 kg/d
(28) P used for biomass growth = phosphorus content of heterotrophic biomass * Px,bio
P used for biomass growth = 0.015 * 3,198.3 = 1.22 mg/L
P removed
(29) P removed = Biological P removal + P used for biomass growth
P removed = 11.5+ 1.22 = 12.75mg/L
P content of waste sludge
(30) Total P in sludge = P removed * Q /1000
Total P in sludge = 12.75 * 39,279 / 1000 = 500.7 kg/d (31)
Phosphorus % = (Total P in sludge / PX, TSS) * 100
Phosphorus % = (500.7/ 6,912.5)*100 = 7.2
DENITRIFICATION
Active biomass concentration
(32)
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Assignment No. 2
Wastewater Treatment Design
Internal recycle ratio (33)
Amount of NO3-N fed to the anoxic tank
(34) Flowrate to anoxic tank = IR*Q + R*Q
Flowrate to anoxic tank = 4.2 * 39,279 + 0.6 * 39,279 = 189,414 m3/d (35)
Nox feed = 189,414 * 8 /1,000= 1,515 kg/d
Volume of anoxic tank
(36)
F/Mb in the anoxic tank
(37)
SDNR
SDNR = 0.42
Amount of NO3-N that can be reduced
(38)
Capacity ratio
Capacity ratio = NOr/Nox feed
Capacity ratio = 1,562 / 1,515 ~ 1 => Ƭ= 1.4 is therefore acceptable
Comparison of the computed value to conventional observed SDNR values based on MLSS
(39)
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Assignment No. 2
Wastewater Treatment Design
SDNR (MLSS) = 0.25 * (1,334 / 3,000) = 0.11 g/g.d
Oxygen supplied by nitrate reduction
(40)
(41) Ro = Ro (nitrification) - R1
Ro = 841 - 181 = 661 kg/h
Air Flow Rate
(42)
(43)
Check alkalinity (44)
(45)
(46)
Mass of alkalinity needed
(47)
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Assignment No. 2
Wastewater Treatment Design
Mass of alkalinity needed = 134.8 * 39,279/1,000 = 5,297 kg/d as CaCO3
Alkalinity savings
(48)
Alkalinity savings = 10,706 - 5,297 = 5,410 kg/d
Anoxic zone mixing energy
(49)
Anoxic zone mixing energy = 2,291 * 10 /1000 = 23 kW total
Secondary Clarifier Design
Return sludge recycle ratio
(50)
(51)
(52)
(53)
Size of clarifier
(54)
(55)
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Assignment No. 2
Wastewater Treatment Design
(56)
(57)
Solid loading
(58)
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