Wastewater Treatment Design 1. Introductionrepositorio.educacionsuperior.gob.ec/bitstream/28000/1562/1/T... · Wastewater Treatment Design 1. ... The wastewater treatment plant "Sydney

Solange Kamanzi z3402599 Carla Guilcapi z3402968

CVEN9857 – Wastewater Treatment 1

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



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



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)



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



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



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



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 .



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



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.



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.



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



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



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)



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)



APPENDIX D

EQUATIONS

Equation Reference Equation Reference


1 12

2 13

3

14

4

15

5

16

6 MLVSS = Fraction VSS * MLVSS

17

7

18

8

19

9

20

10 21



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



37

46

38

47

SDNR (MLSS) = SDNR * (Xb/MLSS)

39

48

40 49

SECONDARY CLARIFIER

50

55

51

56

52

57

53

58

54



APPENDIX E

CALCULATIONS

DESIGN CALCULATIONS SUMMARY

Parameter Value Unit Recommended values/range Comment


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



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



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



APPENDIX F

CALCULATIONS IN DETAIL

Assignment No. 2


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)

Calcs By: Solange Kamanzi Date: 02/10/2012 Carla Guilcapi Page: 1



Assignment No. 2


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




Assignment No. 2


(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)




Assignment No. 2


(16)

(17)

F/M (18)

Volumetric BOD (19)

Observed yield base on TSS (20)

Observed yield base on VSS

(21)

Oxygen Demand




Assignment No. 2


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




Assignment No. 2


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)




Assignment No. 2


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)




Assignment No. 2


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)




Assignment No. 2


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)




Assignment No. 2


(56)

(57)

Solid loading

(58)


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