Bioresource Technology 100 (2009) 4632–4641

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Bioresource Technology

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Fully coupled activated sludge model (FCASM): Model development

Peide Sun *, Ruyi Wang, Zhiguo FangCollege of Environmental Science and Engineering, Zhejiang Gongshang University, 198, Jiaogong Road, Hangzhou 310012, China

a r t i c l e i n f o

Article history:Received 4 January 2009Received in revised form 28 April 2009Accepted 28 April 2009Available online 27 May 2009

Keywords:Activated sludge processAnaerobic maintenance processBiological nutrient removalSub-microscopic mechanism model

0960-8524/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.biortech.2009.04.065

* Corresponding author. Tel.: +86 571 88840581; faE-mail address: pdsun@126.com (P. Sun).

a b s t r a c t

A sub-microscopic mechanism model named Fully Coupled Activated Sludge Model (FCASM) about bio-logical nutrient removal in the wastewater treatment process was developed in the present study. Thefunctional organisms existing simultaneously in the activated sludge system were separated into eightgroups, including aerobic heterotrophic organisms, nitrite reducing organisms, nitrate reducing organ-isms, ammonium oxidizing autotrophs, nitrite oxidizing autotrophs, non-denitrifying phosphorus-accu-mulating organisms (PAOs), denitrifying phosphorus-accumulating bacteria (DPB), and glycogen-accumulating organisms (GAOs). In FCASM, the interaction relationships of the eight functional microor-ganisms were taken fully into account. FCASM could model biological nitrogen removal via nitrite bysplitting nitrification process and denitrification process into two-step reactions, and the autotrophsand denitrifying organisms were divided into two groups, respectively. What’s important, FCASMincluded the anaerobic maintenance processes of sequential utilization of polyphosphate followed byglycogen for PAOs and DPB and glycolysis of the intracellular stored glycogen for GAOs.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

The activated sludge process is a complex system includingmany biological conversion and transportation processes. In orderto control the biological processes effectively, mathematical mod-els, which have been regarded as one of the most useful and pow-erful tools for development and optimization of wastewatertreatment processes, are inevitably needed for the quantitativeevaluation of the activated sludge processes (Salem et al., 2002).Since the ASMs were proposed by International Water Association(IWA) task group, mathematical models about the biological pro-cesses in activated sludge systems have been paid much moreattention by the researchers in the world (Henze et al., 1987,1995, 1999; Gujer et al., 1999). ASMs, which can provide us witha standard set of basic models, are of great importance in biologicalwastewater treatment processes. However, great disadvantage isthat most of the model assumption neglected nitrite as the inerme-diate product, and the nitrification and denitrification process wasassumed as a single-step process in the previous ASMs. Therefore,ASMs have been considered not to be appropriate for these situa-tions in wastewater treatment process when considerable nitriteaccumulates in the system because of the poor nitrification capac-ity, or the specific effluent nitrite limitation of the wastewatertreatment plant (WWTP). There is, therefore, an indispensableneed for the construction of separate models for the two-step nitri-fication and denitrification process. Great interestings on two-step

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nitrification models in wastewater treatment process have beenincreasing since 1990s (Andreottola et al., 1997; Marsili Libelliet al., 2001; Wett and Rauch, 2003). ASM1 and ASM3 proposedby IWA was enhanced respectively by Dosta et al. (2007) and Iac-opozzi et al. (2007) through introducing two-step model for nitri-fication process and considering both nitrite and nitratedenitrification processes. In the enhanced models, denitrificationwas taken by facultative heterotrophic biomass through anoxicrespiration either on nitrite or nitrate, and thus organic carbonwas removed. But actually, the facultative heterotrophic biomassincluded nitrite reducing organisms and nitrate reducing organ-isms. Thus, these models could not describe denitrification detail-edly and accurately.

Enhanced biological phosphorus removal (EBPR), which resultsfrom a group of organisms named phosphorus-accumulatingorganisms, has been widely accepted and regarded as one of themost economical and sustainable processes to remove phosphorusfrom wastewater. In 1980s, two extensively accepted mechanismmodes for biological phosphorus removal were proposed in waste-water treatment process. One was Comeau and Wentzel mode,which was represented by the models of ASM2(d) (Henze et al.,1995, 1999) and ASM3-bio-P model (Rieger et al., 2001). The otherwas Mino mode represented by the TUDP model (Meijer et al., 2001,2002). Based on the interaction mechanism between differentmicrobial community, ASM3-bio-P model and TUDP modelwere improved by Sun (2006 and 2007). In these models,phosphorus-accumulating organisms, including non-denitrifyingphosphorus-accumulating organisms (PAOs) and denitrifying phos-phorus-accumulating bacteria (DPB), actually were just assumed as

P. Sun et al. / Bioresource Technology 100 (2009) 4632–4641 4633

one functional group. However, the phosphorus removal mecha-nism of the two kinds of functional organisms is different. However,the phosphorus removal mechanism of the two kinds of functionalorganisms is different. For PAOs, acetate and propionate are takenup and stored in the cell as polyhydroxyalkanoates (PHA) underboth anaerobic and anoxic conditions, and then PHAs are oxidizedunder aerobic condition. For DPB, acetate and propionate are ab-sorbed only under anaerobic conditions, and then PHAs can be oxi-dized under both anoxic and aerobic conditions (Henze et al., 2002).As we can see, different behavior of PAOs and DPB exists under an-oxic condition. Obviously, it will not be so suitable when thesemodels are applied to describe phosphorus removal under anoxiccondition.

In recent years, an increasing number of studies showed thatglycogen-accumulating organisms (GAOs) could survive and prolif-erate under the alternating anaerobic/aerobic conditions in EBPRsystems. The growth and proliferation of GAOs in EBPR systemmight make a nutrient competition with PAOs and DPB (Minoet al., 1995; Liu et al., 1996). Because of the existence and adversefunction of GAOs in EBPR system, GAOs were then introduced intoactivated sludge models by some research fellows (Mino et al.,1995; Manga et al., 2001; Yagci et al., 2004; Whang et al., 2007).At the present time, most of the revised models were enhanced justby adding some biological processes of GAOs into ASM2(d). How-ever, the theories of growth and decay for heterotrophic andautotrophic organisms are different from that for phosphorus-accumulating organisms and GAOs. In order to evaluate the signif-icance of different endogenous processes in EBPR systems, manybatch starvation experiments were carried out by Lopez et al.(2006). Results showed that anaerobic starvation was best de-scribed by maintenance processes. A sequential utilization of poly-phosphate was followed by glycogen to generate maintenanceenergy while no significant decay of phosphorus-accumulatingorganisms was observed. Since polyphosphate cannot be stored inthe cell of GAOs, the energy required for anaerobic maintenanceis totally provided by the glycolysis of the intracellular stored gly-cogen (Filipe et al., 2001; Zeng et al., 2003a; Lopez-Vazquez et al.,2007). However, the endogenous utilization of glycogen for mainte-nance purpose was currently not represented in the available EBPRmodels. Therefore, the present study is to establish a sub-micro-scopic mechanism model for biological nutrient removal, namedFully Coupled Activated Sludge Model (FCASM), which describesthe biological processes between macrocosmic and microcosmicfield in wastewater treatments. The organisms that might existsimultaneously in activated sludge system were reclassified sys-tematically and detailedly, and the interaction relationships amongthem were fully taken into account in FCASM. FCASM splited nitri-fication process and denitrification process into two-step reactions.The autotrophs and the denitrifiying organisms were also dividedinto two groups respectively, capable of modeling short-cut nitrifi-cation–denitrification. Moreover, PAOs and DPB were introducedinto this model and were assumed as two functional groups to de-scribe biological phosphorus removal. The most important is thatthe anaerobic maintenance processes that a sequential utilizationof polyphosphate followed by glycogen for phosphorus-accumulat-ing organisms (PAOs and DPB) and glycolysis of the intracellularstored glycogen for GAOs were included in FCASM.

Fig. 1. Interaction relationships of microorganisms.

2. Model description

2.1. Mechanism

Activated sludge system is a complex biological system includ-ing many different functional organisms and biological processes.The major eight known groups existed simultaneously in activated

sludge system are aerobic heterotrophic organisms, nitrite reduc-ing organisms, nitrate reducing organisms, ammonium oxidizingautotrophs, nitrite oxidizing autotrophs, PAOs, DPB and GAOs.Based on FCASM, the interaction relationships among the eightgroups of microorganisms in activated sludge system were shownin Fig. 1. During the aerobic phase, dissolved oxygen (DO) is themain promoting effect for aerobic growth of all organisms with dif-ferent oxygen affinity. It is much easier for heterotrophic organ-isms to obtain dissolved oxygen than that for the ammoniumoxidizing autotrophs and nitrite oxidizing autotrophs in activatedsludge system. Therefore, the heterotrophic organisms will growmuch better than the ammonium oxidizing autotrophs and nitriteoxidizing autotrophs when organic substance concentration is highenough for microbial growth in the aerobic system. Firstly, ammo-nium is oxidized to nitrite by ammonium oxidizing autotrophs,and subsequently nitrite is oxidized to nitrate by nitrite oxidizingautotrophs. Biological denitrification is a microbial reduction pro-cess from NO�3 to NO�2 by nitrate reducing organisms and furtherto N2 by nitrite reducing organisms. The nitrite and nitrate concen-tration is affected by the activities of the four groups of organisms.The nitrite will be accumulated when the activities of nitrite oxi-dizing autotrophs are inhibited, which will result in short-cut nitri-fication–denitrification. Firstly, oxygen concentration in aerobicsystem is one of the most important factors for nitrite accumula-tion. High oxygen concentration will promote the consumptionof nitrite, while low oxygen concentration is an obvious disadvan-tage for nitrite production (Garrido et al., 1997; Pollice et al., 2002).

4634 P. Sun et al. / Bioresource Technology 100 (2009) 4632–4641

Secondly, the concentration of free ammonia (FA) in relatively lowrange is sufficient for the inhibition of nitrite oxidizing autotrophs.Low FA level such as 0.6 mg L�1 could inhibit the nitrite oxidizingautotrophs, and results in the accumulation of nitrite (Fux et al.,2002). Thirdly, since the range of optimal temperature for ammo-nium oxidizing autotrophs is from 22 �C to 27 �C, short-cut nitrifi-cation–denitrification will be accomplished when the temperaturerange from 22 �C to 27 �C. However, high activities of nitrite oxidiz-ing autotrophs were found in the temperature below 15 �C(Hyungseok, 1999). In the anaerobic phase, PAOs, DPB and GAOsmake a competition for nutrient substances, especially for organicmatters. If GAOs dominate in the activated sludge, the deteriora-tion of biological phosphorus removal might occure in the system.

2.2. Assumptions and features

Basically, the preliminary assumptions of FCASM were the sameto the assumptions of ASM1 with some modification as following(Henze et al., 1987). (i) The activated sludge system did not needto be operated at constant temperature. As a variable, temperaturewas coupled into kinetic rate equations directly. (ii) The effects ofnitrogen and phosphorus on cell growth were taken into account,but the effects of inorganic nutrients were not included in FCASM.(iii) Hydrolysis was associated with the total heterotrophic organ-isms, and the hydrolysis rate was the function of XOH, XDNS, XDNB,XPAO, XDPB, XGAO (Meijer et al., 2001). (iv) All readily biodegradablesubstrates were taken up and stored into the internal cell as com-ponent prior to growth. Subsequently, the internal component wasused for microbial biomass growth. (v) Endogenous respirationwas applied for the loss of organisms. The type of electron accep-

Table 1Simple definition of FCASM components.

Character No. Symbol Definition

Soluble 1 SO2 Dissolved oxygen2 SS Readily biodegradable organic substrates3 SI Inert soluble organic material4 SNH4 Ammonium plus ammonia nitrogen, assumed to NHþ4 onl5 SNO3 Nitrate nitrogen6 SNO2 Nitrite nitrogen7 SN2 Dinitrogen8 SPO4 Inorganic soluble phosphorus, primarily orthophosphates

HPO2�4

9 SALK Bicarbonate alkalinity

10 XI Inert particulate organic materialParticulate11 XS Slowly biodegradable substrates12 XOH Aerobic heterotrophic organisms13 XSTO,OH A cell internal storage product of aerobic heterotrophic o14 XDNS Nitrite reducing organisms15 XSTO,DNS A cell internal storage product of nitrite reducing organis16 XDNB Nitrate reducing organisms17 XSTO,DNB A cell internal storage product of nitrate reducing organis18 XNS Ammonium oxidizing autotrophs19 XNB Nitrite oxidizing autotrophs20 XPAO Non-denitrifying phosphorus-accumulating organisms21 XPP,PAO Polyphosphate of non-denitrifying phosphorus-accumula22 XPHA,PAO A cell internal storage product of non-denitrifying phosph

hydroxyalkanoates (PHA)23 XGLY,PAO A cell internal storage product of non-denitrifying phosph

stored glycogen24 XDPB Denitrifying phosphorus-accumulating bacteria25 XPP,DPB Polyphosphate of denitrifying phosphorus-accumulating b26 XPHA,DPB A cell internal storage product of denitrifying phosphorus

(PHA)27 XGLY,DPB A cell internal storage product of denitrifying phosphorus

glycogen28 XGAO Glycogen-accumulating organisms29 XPHA,GAO A cell internal storage product of glycogen-accumulating30 XGLY,GAO A cell internal storage product of glycogen-accumulating31 XTSS Total suspended solids

tors present in different conditions did affect the loss of all frac-tions of the biomass.

The same model construction method proposed in ASMs wasused in FCASM. The main features of FCASM were shown as follow-ing. (i) FCASM is a sub-microscopic mechanism model. The majoreight different functional groups of organisms were included, andthe interaction relationships were fully taken into account in themodel. (ii) Short-cut nitrification–denitrification could be per-formed in the model by controlling the oxygen concentration inthe aerobic system, since two-step nitrification–denitrificationprocess was included in FCASM. (iii) Inhibition of GAOs and glyco-gen storage in PAOs was taken into account in FCASM. Anaerobicmaintenance processes of PAOs, DPB and GAOs were included inthe model. (iv) As a variable, temperature was combined into ki-netic rate equations directly, and the temperature effects on bio-logical processes could be modeled by coupling temperaturemodels.

3. Components in the model

Thirty one components including nine soluble and 22 sus-pended components were involved in FCASM. All these compo-nents were defined in Table 1.

4. Kinetic processes

Seventy two kinetic bioprocesses described in Table 2 were in-cluded in FCASM.

Aerobic heterotrophic organisms. Dissolved oxygen can be onlyused as electron acceptor by the aerobic heterotrophic organisms.

Dimension

M (O2) � L�3

M (COD) � L�3

M (COD) � L�3

y M (N) � L�3

M (N) � L�3

M (N) � L�3

M (N) � L�3

. Assumed that it consists of 50 percent H2PO�4 and 50 percent M (P) � L�3

mol ðHCO�3 Þ � L�3

M (COD) � L�3

M (COD) � L�3

M (COD) � L�3

rganisms M (COD) � L�3

M (COD) � L�3

ms M (COD) � L�3

M (COD) � L�3

ms M (COD) � L�3

M (COD) � L�3

M (COD) � L�3

M (COD) � L�3

ting organisms M (P) � L�3

orus-accumulating organisms. It includes poly- M (COD) � L�3

orus-accumulating organisms. It denotes the intracellularly M (COD) � L�3

M (COD) � L�3

acteria M (P) � L�3

-accumulating bacteria. It includes poly-hydroxyalkanoates M (COD) � L�3

-accumulating bacteria. It denotes the intracellularly stored M (COD) � L�3

M (COD) � L�3

organisms. It includes poly-hydroxyalkanoates (PHA) M (COD) � L�3

organisms. It denotes the intracellularly stored glycogen M (COD) � L�3

M (TSS) � L�3

Table 2Kinetic rate equation for FCASM (the sub-processes shaded are innovative).

P. Sun et al. / Bioresource Technology 100 (2009) 4632–4641 4635

4636 P. Sun et al. / Bioresource Technology 100 (2009) 4632–4641

P. Sun et al. / Bioresource Technology 100 (2009) 4632–4641 4637

able 3toichiometric matrix and composition matrix of FCASM.

rocess 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 31SO2 SS SI SNH4 SNO3 SNO2 SN2 SPO4 SALK XI XS XOH XSTO,OH XDNS XSTO,DNS XTSS

Hydrolysis 1 � fSI fSI y1 m1 z1 �1 t1

erobic heterotrophic organisms XOH

Aerobic storage of COD x2 �1 y2 m2 z2 YSTO;O2 t2Aerobic growth x3 y3 m3 z3 1 �1=YOH;O2 t3Aerobic endog. resp. x4 y4 m4 z4 fXI �1 t4Aerobic resp. of XSTO,OH x5 �1 t5Anoxic endog. resp. y6 x6 �x6 m6 z6 fXI �1 t6Anoxic resp. of XSTO,OH x7 �x7 z7 �1 t7itrite reducing organisms XDNS

Aerobic storage of COD x8 �1 y8 m8 z8 YSTO;O2 t8Aerobic growth x9 y9 m9 z9 1 �1=YDNS;O2 t9

0 Aerobic endog. resp. x10 y10 m10 z10 fXI �1 t101 Aerobic resp. of XSTO,DNS x11 �1 t112 Anoxic storage of COD �1 y12 x12 �x12 m12 z12 YSTO;NO2 t123 Anoxic growth y13 x13 �x13 m13 z13 1 �1=YDNS;NO2 t134 Anoxic endog. resp. y14 x14 �x14 m14 z14 fXI �1 t145 Anoxic resp. of XSTO,DNS x15 �x15 z15 �1 t15

omposition matrixThOD [g(ThOD)] �1 1 1 �64/14 �48/14 �24/14 1 1 1 1 1 1Nitrogen [g(N)] iN,SS iN,SI 1 1 1 1 iN,XI iN,XS iN,BM iN,BM

Phosphorus [g(P)] iP,SS iP,SI 1 iP,XI iP,XS iP,BM iP,BM

Ionic charge (mole+) 1/14 �1/14 �1/14 �1.5/31 �1observable TSS [g(TSS)] iTSS,XI iTSS,XS iTSS,BM iTSS,STO iTSS,BM iTSS,STO �1

rocess 1 2 3 4 5 6 7 8 9 10 16 17 18 19 31SO2 SS SI SNH4 SNO3 SNO2 SN2 SPO4 SALK XI XDNB XSTO,DNB XNS XNB XTSS

itrate reducing organism XDNB

6 Aerobic storage of COD x16 �1 y16 m16 z16 YSTO;O2 t167 Aerobic growth x17 y17 m17 z17 1 �1=YDNB;O2 t178 Aerobic endog. resp. x18 y18 m18 z18 fXI �1 t189 Aerobic resp. of XSTO,DNB x19 �1 t190 Anoxic storage of COD �1 y20 X20 �x20 m20 z20 YSTO;NO3 t201 Anoxic growth y21 x21 �x21 m21 z21 1 �1=YDNB;NO3 t212 Anoxic endog. resp. y22 x22 �x22 m22 z22 fXI �1 t223 Anoxic resp. of XSTO,DNB y23 x23 �x23 z23 �1 t23

mmonium oxidizing autotrophs XNS

4 Aerobic growth x24 y24 1/YNS m24 z24 1 t245 Aerobic endog. resp. x25 y25 m25 z25 �1 t256 Anoxic endog. resp. y26 x26 �x26 m26 z26 �1 t26

itrite oxidizing autotrophs XNB

7 Aerobic growth x27 y27 1/YNB �1/YNB m27 z27 1 t278 Aerobic endog. resp. x28 y28 m28 z28 �1 t289 Anoxic endog. resp. y29 x29 �x29 m29 z29 �1 t29

omposition matrixThOD [g(ThOD)] �1 1 1 �64/14 �48/14 �24/14 1 1 1 1 1Nitrogen [g(N)] iN,SS iN,SI 1 1 1 1 iN,XI iN,BM iN,BM iN,BM

Phosphorus [g(P)] iP,SS iP,SI 1 iP,XI iP,BM iP,BM iP,BM

Ionic charge (mole+) 1/14 �1/14 �1/14 �1.5/31 �1observable TSS [g(TSS)] iTSS,XI iTSS,BM iTSS,STO iTSS,BM iTSS,BM �1

4638P.Sun

etal./Bioresource

Technology100

(2009)4632–

4641

TS

P

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A234567N89111111

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Table 3 (continued)

Process 1 2 4 5 6 7 8 9 10 20 21 22 23 31SO2 SS SNH4 SNO3 SNO2 SN2 SPO4 SALK XI XPAO XPP,PAO XPHA,PAO XGLY,PAO XTSS

Non-denitrifying phosphorus-accumulating organisms XPAO

30 Storage of XPHA,PAO �YSS�P y30 m30 z30 �YPO4�P 1 �(1 � YSS-P) t3031 Anaerobic lysis of XPP,PAO 1 z31 �1 t3132 Anaerobic lysis of XGLY,PAO 1 �1 t3233 Aerobic storage of XPP,PAO x33 y33 �1 z33 1 �YPHA;O2�P t3334 Aerobic storage of XGLY,PAO x34 y34 m34 z34 �1=YGLY;O2�P 1 t3435 Aerobic growth x35 y35 m35 z35 1 �1=YPAO;O2

t3536 Aerobic endog. resp. x36 y36 m36 z36 fXI �1 t3637 Aerobic resp. of XPHA,PAO x37 �1 t3738 Aerobic lysis of XPP,PAO 1 z38 �1 t3839 Aerobic resp. of XGLY,PAO x39 �1 t3940 Anoxic endog. resp. y40 x40 �x40 m40 z40 fXI �1 t4041 Anoxic resp. of XPHA,PAO x41 �x41 z41 �1 t4142 Anoxic lysis of XPP,PAO 1 z42 �1 t4243 Anoxic resp. of XGLY,PAO x43 �x43 z43 �1 t43

Composition matrix1 ThOD [g(ThOD)] �1 1 �64/14 �48/14 �24/14 1 1 1 12 Nitrogen [g(N)] iN,SS 1 1 1 1 iN,XI iN,BM

3 Phosphorus [g(P)] iP,SS 1 iP,XI iP,BM 14 Ionic charge (mole+) 1/14 �1/14 �1/14 �1.5/31 �1 �1/315 observable TSS [g(TSS)] iTSS,XI iTSS,BM 3.23 iTSS,STO iTSS,GLY �1

Process 1 2 4 5 6 7 8 9 10 24 25 26 27 31SO2 SS SNH4 SNO3 SNO2 SN2 SPO4 SALK XI XDPB XPP,DPB XPHA,DPB XGLY,,DPB XTSS

Denitrifying phosphorus-accumulating bacteria XDPB

44 Anaerobic storage of XPHA,DPB �YSS-D y44 m44 z44 �YPO4�D 1 �(1 � YSS-D) t4445 Anaerobic lysis of XPP,DPB 1 z45 �1 t4546 Anaerobic lysis of XGLY,DPB 1 �1 t4647 Aerobic storage of XPP,DPB x47 y47 �1 z47 1 �YPHA;O2�D t4748 Aerobic storage of XGLY,DPB x48 y48 m48 z48 �1=YGLY;O2�D 1 t4849 Aerobic growth x49 y49 m49 z49 1 �1=YDPB;O2 t4950 Aerobic endog. resp. x50 y50 m50 z50 fXI �1 t5051 Aerobic resp. of XPHA,DPB x51 �1 t5152 Aerobic lysis of XPP,DPB 1 z52 �1 t5253 Aerobic resp. of XGLY,DPB x53 z53 �1 t5354 Anoxic storage of XPP,DPB y54 x54 �x54 �1 z54 1 �YPHA,NOX-D t5455 Anoxic storage of XGLY,DPB y55 x55 �x55 m55 z55 �1/YGLY,NOX-D 1 t5556 Anoxic growth y56 x56 �x56 m56 z56 1 �1/YDPB,NOX

t5657 Anoxic endog. resp. y57 x57 �x57 m57 z57 fXI �1 t5758 Anoxic resp. of XPHA,DPB x58 �x58 z58 �1 t5859 Anoxic lysis of XPP,DPB 1 z59 �1 t5960 Anoxic resp. of XGLY,DPB x60 �x60 z60 �1 t60

Composition matrix1 ThOD [g(ThOD)] �1 1 �64/14 �48/14 �24/14 1 1 1 12 Nitrogen [g(N)] iN,SS 1 1 1 1 iN,XI iN,BM

3 Phosphorus [g(P)] iP,SS 1 iP,XI iP,BM 14 Ionic charge (mole+) 1/14 �1/14 �1/14 �1.5/31 �1 �1/315 observable TSS [g(TSS)] iTSS,XI iTSS,BM 3.23 iTSS,STO iTSS,GLY �1

(continued on next page)

P.Sunet

al./BioresourceTechnology

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4632–4641

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4640 P. Sun et al. / Bioresource Technology 100 (2009) 4632–4641

Soluble readily biodegradable organic substrates (RBCOD) are ab-sorbed and stored as cell internal storage by these organisms.Endogenous respiration process will take place under both aerobicand anoxic condition.

Nitrite reducing organisms and nitrate reducing organisms. Thesekinds of organisms are facultative heterotrophic organisms. Underanoxic condition, nitrite and nitrate can be converted into dinitro-gen and nitrite by nitrite reducing and nitrate reducing organisms,respectively. In the presence of oxygen, their kinetic processes aresimilar to those taking by aerobic heterotrophic organisms underaerobic condition (Dosta et al., 2007).

Ammonium oxidizing autotrophs and nitrite oxidizing autotrophs.Ammonium can be oxidized into nitrite by ammonium oxidizingautotrophs, and nitrite can be oxidized into nitrate by nitrite oxi-dizing autotrophs, with oxygen as the exclusive electron acceptor(Iacopozzi et al., 2007):

NHþ4 þ 3=2O2 ! NO�2 þ 2Hþ þH2Oðammonium oxidizing autotrophsÞ

NO�2 þ 1=2O2 ! NO�3 ðnitrite oxidizing autotrophsÞ4NO�3 þ C2H4O2 ! 4NO�2 þ 2CO2 þH2O

ðnitrate reducing organismsÞ8NO�2 þ 3C2H4O2 þH2O ! 4N2 þ 8OH� þ 6CO2

ðnitrite reducing organismsÞ

Non-denitrifying phosphorus-accumulating organisms and denitri-fying phosphorus-accumulating bacteria. Under anoxic condition,acetate and propionate are taken up and stored internally in themicrobial cell as polyhydroxyalkanoates (PHA) by PAOs, whilePHA is oxidized and stored in the cell by DPB (Henze et al.,2002). A sequential utilization of polyphosphate followed by glyco-gen will take place for maintenance purposes when they are ex-posed in anaerobic condition (Lopez et al., 2006).

Glycogen-accumulating organisms. PHA can be oxidized, and gly-cogen can be synthesized with nitrate or dissolved oxygen as elec-tron acceptors by GAOs. However, phosphorus in wastewater cannot be removed by GAOs (Zeng et al., 2003b). The energy requiredfor anaerobic maintenance is totally provided by the glycolysis ofthe intracellular stored glycogen (Filipe et al., 2001; Zeng et al.,2003a; Lopez-Vazquez et al., 2007).

5. Stoichiometry

Table 3 showed the stoichiometric matrix and composition ma-trix of FCASM. The composition matrix was based on five equationsfor COD, nitrogen, phosphorus, total suspended solids and ioniccharge. These equations were used to estimate the stoichiometriccoefficients of SO2 (SNO3 ; SNO2 and SN2 in denitrification) from COD,SNH4 from nitrogen, SPO4 from phosphorus, XTSS from total sus-pended solids, and SALK from ionic charge.

6. Conclusions

In this study, FCASM was established and conducted, which wasbased on a new concept of sub-microscopic mechanism analysis. InFCASM, the microbial communities were divided into eight differ-ent functional groups in the activated sludge system for nutrientremoval. Two-step nitrification–denitrification process was in-cluded, and the autotrophs and the denitrifying organisms were di-vided into two functional groups, respectively. Temperature effectson biological processes could be modeled if temperature modelwas coupled in FCASM. FCASM included the anaerobic mainte-nance processes. For PAOs and DPB, a sequential utilization of poly-phosphate followed by glycogen was involved for anaerobicmaintenance. And glycolysis of the intracellular stored glycogen

P. Sun et al. / Bioresource Technology 100 (2009) 4632–4641 4641

for anaerobic maintenance by GAOs was included. However, fur-ther research should be carried out in order to fully understandthe mechanism of biological nutrient removal in activated sludgesystem, and other biological processes, which have not yet takeninto account in this model, should be involved in future FCASMs.

Acknowledgements

This study was financially supported by the Key Project of Nat-ural Science Foundation of Zhejiang Province (No. Z507721), theProject of Hangzhou Science and Technology Development (No.20061133B23) and the Project Supported by Scientific ResearchFund of Zhejiang Provincial Education Department (No.Y200804075). The authors would like to gratefully acknowledge-ment Professor Maoxin Guo and Associate Prof. Juqing Lou fromCollege of Environmental Science and Engineering, Zhejiang Gong-shang University. The authors also wish to thank the other gradu-ate students and teachers who provided their valuable suggestionsin our research group and in the College of Environmental Scienceand Engineering, Zhejiang Gongshang University.

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