Bioresource Technology 102 (2011) 5722–5727

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Bioresource Technology

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Effect of nitrate recycling ratio on simultaneous biological nutrient removalin a novel anaerobic/anoxic/oxic (A2/O)-biological aerated filter (BAF) system

Yongzhi Chen a, Chengyao Peng a, Jianhua Wang a, Liu Ye b, Liangchang Zhang a, Yongzhen Peng a,⇑a Key Laboratory of Beijing for Water Quality Science and Water Environmental Recovery Engineering, Beijing University of Technology, Beijing 100124, Chinab Advanced Wastewater Management Centre (AWMC), The University of Queensland, St. Lucia, Brisbane 4072, Australia

a r t i c l e i n f o a b s t r a c t

Article history:Received 6 December 2010Received in revised form 27 February 2011Accepted 28 February 2011Available online 5 March 2011

Keywords:A2/O-BAF systemNitrate recycling ratioSimultaneous biological nitrogen andphosphorus removalDenitrifying phosphorus removalLow C/N ratio domestic wastewater

0960-8524/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.biortech.2011.02.114

⇑ Corresponding author. Tel./fax: +86 10 67392627E-mail addresses: pyz@bjut.edu.cn, zengwei_1@2

Peng).

A novel system integrating anaerobic/anoxic/oxic (A2/O) and biological aerated filter (BAF), which couldsolve the sludge retention time (SRT) conflicting problem between nitrifiers and polyphosphate accumu-lating organisms (PAOs) by shortening SRT for PAOs in A2/O and lengthening SRT for nitrifiers in BAF, wasinvestigated in this study. Various nitrate recycling ratios (100%, 200%, 300% and 400%) were applied to alab-scaled A2/O-BAF system to detect the simultaneous biological nitrogen and phosphorus removal per-formance while treating real domestic wastewater with low carbon to nitrogen (C/N) ratio. The concen-trations of chemical oxygen demand (COD), NHþ4 –N and total phosphorus (TP) in the effluent were lessthan 50.0, 0.5 and 0.5 mg/L, respectively, throughout the experiments. The removal efficiencies of totalnitrogen (TN) were 64.9%, 77.0%, 82.0% and 87.0%, under respective nitrate recycling ratios (increasingfrom 100% to 400%). By contrast, nitrate recycling ratios had neglectable effect on the removal efficienciesof COD and NHþ4 –N.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Eutrophication, resulting in frequent outbreaks of algal bloomsand threatening the reliable supply of drinking water resources,has become one of the most urgent problems to the sustainableeconomic development of china (Le et al., 2010; Zhang et al.,2009) and has gained significant attention worldwide in recentyears. It is mainly due to the excessive discharged nutrients (par-ticularly nitrogen and phosphorus) from wastewater.

Biological nutrient removal (BNR) processes are generally ac-cepted and widely applied in existing wastewater treatment plants(WWTPs) for integrated removal of nitrogen and phosphorus dueto its economic advantages compared with chemical treatmentmethods (Fan et al., 2009). Furthermore, BNR processes have bothhydraulical and biological flexibility (Van Nieuwenhuijzen et al.,2008). A number of biological treatment processes have beendeveloped for simultaneous removal of nitrogen and phosphorus.Such as the five-stage Bardenpho process, the anaerobic/anoxic/oxic (A2/O) process, The University of Cape Town (UCT) process,etc. Among them, the most commonly used process is the A2/Oprocess (Wang et al., 2006), which is a single-sludge suspendedgrowth system incorporating anaerobic, anoxic and aerobic zonesin sequence. However, there are three main operational problems

ll rights reserved.

.63.net, gl-222@tom.com (Y.

existing in A2/O process (Zhang et al., 2003), which are summa-rized as follows.

Firstly, the confliction problem between SRT of nitrifiers (longSRT) and polyphosphate accumulating organisms (PAOs) (shortSRT) (Van Loosdrecht et al., 1998) can not be solved.

Secondly, NO�3 –N in return sludge is an inhibiting factor to thephosphorus release in anaerobic zone where denitrifiers will com-pete with PAOs for external carbon source, and net phosphorus re-lease won’t occur until denitrification is completed (Barker andDold, 1996).

Finally, shortage of organic carbon sources in low strengthwastewater is often a rate-limiting factor to simultaneous nitrogenand phosphorus removal, especially under unfavorable conditionsuch as low influent carbon to nitrogen(C/N) ratio (Peng et al.,2006; Ahn et al., 2002). However, domestic wastewater is alwayslow in C/N ratio, which makes single sludge operation processmore difficult to meet the strict discharge standard. Therefore, itis a challenge to obtain good nutrient removal performance with-out modification to existing A2/O process configuration (Ma et al.,2009).

The novel system integrating A2/O-BAF had substantial advan-tages to solve the above mentioned problems. In this system, theA2/O reactor was mainly used for phosphorus removal and denitri-fication, and the BAF reactor was used for nitrification. Short SRTwas applied in A2/O reactor and relatively longer SRT was appliedin BAF to enrich nitrifiers, which not only benefited PAOs, but alsonitrifiers (Lee et al., 2001; Kuba et al., 1996). In the same time,

Y. Chen et al. / Bioresource Technology 102 (2011) 5722–5727 5723

NO�3 –N was recycled from BAF to the A2/O reactor’s anoxic zonesrather than its anaerobic zone to provide an extremely strict anaer-obic environment for phosphate release (Lee et al., 2009; Wanget al., 2004a). In addition, influent with lower C/N ratio also stim-ulated the growth of DPAOs, which were capable to use NO�3 –N aselectron acceptors in simultaneous removal of nitrogen and phos-phorus from wastewater (Hu et al., 2003). Furthermore, largeamount of COD was consumed in the anaerobic zone of A2/O reac-tor and reduced the C/N ratio of supernatant that flowing into theBAF reactor, which was favorable to the growth of nitrifying organ-isms in the biofilm and enhanced nitrification (Wang et al., 2004b).However, the impact of the operational factors on the performanceof this novel system was not well understood.

In this study, the nitrate recycling stream from BAF to A2/O wasinvestigated by taking into consideration of its high economicalpotential in A2/O-BAF system. The effects of this nitrate recyclingratio on the system’s simultaneous nitrogen and phosphorus re-moval performance in the system were observed via continuousflow by treating real domestic wastewater. A series of batch exper-iments were carried out to identify the population of PAOs func-tional to the excellent phosphorus removal in A2/O-BAF system.The optimized operational strategies of this system were alsodiscussed.

2. Methods

2.1. Experimental system

A laboratory-scaled A2/O-BAF system, (shown in Fig. 1), wasconsisted of an influent tank, an A2/O reactor, a secondary settlerand a BAF reactor.

The transparent plexiglas A2/O reactor had nine compartments(with a working volume of 3.3 L each) in sequence, and was sepa-rated by baffles to create anaerobic/anoxic/oxic zones. The firstcompartment was typically operated as anaerobic zone, followedby six compartments operated as anoxic zones, and the remainingtwo compartments were controlled as separated aerobic zones. Se-ven overhead mechanical stirrers with rectangular paddles wereinstalled over the anaerobic and anoxic zones. The aerobic zoneswere aerated by blower via porous stone diffusers, which were in-stalled at the bottom of the compartments. The mixed liquor fromA2/O reactor was settled in a cylindrical settler with a working vol-ume of 20 L. The inner diameter and the height of the up-flow BAFreactor were 10 and 200 cm, respectively. BAF reactor was packedwith light weight ceramists as biofilm carriers with a diameter of

Tank

Ana

erob

ic1

nitrate recycling

Return sludge

Influent

Blower

Ano

xic1

Ano

xic2

Ano

xic3

Ano

xic4

Ano

xic5

Fig. 1. Schematic diagram of A

2–4 cm. The media’s depth was 167 cm and the working volumeof the BAF reactor was 13 L.

The activated sludge used in the A2/O reactor was obtainedfrom Beijing Gaobeidian Sewage Treatment Plant (Beijing, China).The SRT in its original reactor was 15 days. The total suspendedsolids (TSS) concentration was approximately 4.0 g/L and the vola-tile suspended solids (VSS) concentration was around 3.1 g/L. TheBAF was operated under aerobic condition for nitrification. Thesystem was operated for around 60 days to achieve a steady state.All experiments were conducted at ambient temperature(15–18 �C).

2.2. Wastewater source

The wastewater source was collected from the residential areaof Beijing University of Technology (Beijing, China). The main char-acteristics of influent wastewater were presented in Table 1.

2.3. Analytical methods

COD, NHþ4 –N, NO�2 –N, NO�3 –N, Norg–N, TP, TSS and VSS wereanalyzed in accordance with standard methods (APHA/AWWA/WEF, 2005). DO, pH and temperature were monitored online byusing pH/Oxi 340i meter (WTW, Germany). ORP was monitoredonline by level 2 ORP meter (WTW, Germany). TN was measuredoffline with Multi N/C 3100 (Jena, Germany). The influent flow,sludge return and nitrate recycling ratio were controlled by theperistaltic pumps.

2.4. Batch experiment for activated sludge characterization

In order to evaluate the possibility of anaerobic phosphorus re-lease and aerobic/anoxic phosphorus uptake tests for sludge char-acterization, a series of batch experiments were performed asfollows. The activated sludge taken from the aerobic 2 of A2/O reac-tor was washed twice by tap water (to ensure no external carbonsource existed) and then put into a 2-L laboratory fermentor. Atthe beginning of the anaerobic phase, sodium acetate was addedto give an initial COD concentration of 200 mg/L. After 2 h of anaer-obic reaction, the activated sludge was washed again and KH2PO4

was added to give an initial TP concentration of 60 mg/L. Subse-quently, the sludge was divided evenly into two portions, onewas aerated for 2 h, and the other was tested under anoxic condi-tion, where KNO3 was added at the beginning to give an initialNO�3 –N concentration of 50 mg/L. pH was strictly controlled man-ually at 7.5 ± 0.1 by adding HCl (1.0 mol/L) or NaHCO3 (1.0 mol/L).

BAF

Aer

obic

2

Aer

obic

1

Efluent

Wasted sludge

Peristaltic pump

Settler

Ano

xic6

2/O-BAF biological system.

Table 1Main characteristics of influent wastewater.

Parameter Range Average

T/�C 14–16 15pH 7.3–7.6 7.4COD/(mg/L) 211–466 343NHþ4 –N/(mg/L) 65.5–71.3 67.9NO�2 –N/(mg/L) 0–0.22 0.004NO�3 –N/(mg/L) 0–0.94 0.01Norg–N/(mg/L) 2.0–5.0 2.5TN/(mg/L) 66.8–74.1 70.4C/N 3.0–6.7 4.9TP/(mg/L) 4.1–13.0 5.8

5724 Y. Chen et al. / Bioresource Technology 102 (2011) 5722–5727

2.5. Operational conditions

The operational conditions applied to the experimental periodwere summarized in Table 2.

3. Results and discussion

At the end of the start-up period, the A2/O-BAF system wasoperated until COD, NHþ4 –N and TP removal efficiencies reachedto 80%, 98%, and 93%, respectively.

3.1. The effects of various nitrate recycling ratios on A2/O-BAF system’soverall performance

Table 3 summarized the concentrations of COD, NHþ4 –N, NO�2 –N,NO�3 –N, Norg–N, TN, TP in influent and effluent and their averageremoval efficiencies at respective nitrate recycling ratios. Through-out the whole experiment, the concentrations of effluent COD,NHþ4 –N, Norg–N, and TP concentrations from the A2/O-BAF systemwere lower than 50.0, 0.5, 0.5, and 0.5 mg/L, respectively, whichall met the class A discharge standards (Integrated wastewater dis-charge standard, GB18918-2002, China). When nitrate recycling ra-tios were increased from 100% to 400%, the effluent TNconcentrations decreased from 24.9 to 9.5 mg/L, which was muchlower than the GB standard discharging rate, and the correspondingremoval efficiencies of TN were 64.9%, 77.0%, 82.0%, and 87.0%,respectively. The contributions of NO�2 –N, and Norg–N in the

Table 2Operational conditions used in experiment.

Items Parameter Value

Qin(L/day) Influent flow 96QR(L/day) Nitrate recycling flow 96/192/288/

384R(%) Nitrate recycling 100/200/300/

400r(%) Return sludge 100HRTana(h) Hydraulic residence time in anaerobic

zone0.8

HRTano(h) Hydraulic residence time in anoxiczones

5.0

HRTae(h) Hydraulic residence time in aerobiczones

1.7

HRTBAF(h) Hydraulic residence time in BAF 0.5SRT(day) Sludge residention time in A2/O reactor 15pH 7.3–7.9DOana/ano(mg/L) Dissolved oxygen in anaerobic/anoxic

zones<0.1

DOae(mg/L) Dissolved oxygen in aerobic zones 1.5–2.0DOBAF(mg/L) Dissolved oxygen in BAF 5–6ORPana(mV) Oxidation reduction potential in

anaerobic zone�350–�330

ORPano(mV) Oxidation reduction potential in anoxiczones

�200–�150

ORPae(mV) Oxidation reduction potential inaerobic zones

20–25

effluent were not considered in the discussion, as they were neg-lectable compared to other components.

3.2. Evolution of COD in A2/O-BAF system at various nitrate recyclingratios

The evolution of COD in A2/O-BAF system at various nitraterecycling ratios was depicted in Fig. 2. It could be seen that COD re-moval efficiencies in this system were always kept as high as 85%despite of the variation in influent COD concentration. The CODconcentration decreased significantly in anaerobic zone but onlyslightly in anoxic zones, while it was almost constant along withthe aerobic zones and BAF reactor.

COD was primarily utilized by PAOs in the anaerobic zone.Approximately 67% of COD was consumed in the anaerobic zone.The higher COD removal efficiency in anaerobic zone, the morepoly-hydroxy-alkanoate (PHA, electron donor) was produced andstored by denitrifying polyphosphate accumulating organisms(DPAOs), which facilitated both nitrogen and phosphorus removalin the anoxic zones.

On the other hand, around 15% COD was consumed by denitri-fiers in the following anoxic zones. In the anoxic zones, NO�3 –Ncould be used as electron acceptor by the denitrifying microorgan-isms. A higher nitrate recycling ratio will result in a higher NO�3 –Nload in the anoxic zones. Therefore, along with the increasing of ni-trate recycling ratio, a slightly high percentage of COD removal inthe anoxic zones was due to denitrification COD uptake and theaerobic oxidization as a result of DO recirculation (Andalib et al.,2011) from the BAF reactor. It was interesting that when nitraterecycling ratio was increased from R = 300% to R = 400%, a slowerCOD consumption increase was observed in the anoxic zones. Itindicated that the system was being operated under the conditionsthat the denitrification rate reaching its maximum. Thus, high ni-trate recycling ratio (above 400%) should be avoided in theA2/O-BAF system operation from an economical point of view.

Due to large amount of COD was consumed in the anaerobic andanoxic zones, the C/N ratio of supernatant dropped a lot beforeflowing to the BAF reactor. It was considered to be advantageousfor the nitrification because of the non-inhibitory effect causedby the observed COD (Ge et al., 2010). Therefore, the growth ofnitrifying organisms in the biofilm was favored and the nitrifica-tion performance of the system was enhanced as well.

3.3. Effects of nitrate recycling ratios on nitrogen removal

The effects of four various nitrate recycling ratios (R = 100%,R = 200%, R = 300%, and R = 400%) on nitrogen removal of the A2/O-BAF system were shown in Fig.2.

TN in the influent was mainly composed of NHþ4 –N (above 95%),and influent NHþ4 –N concentration was about 65.5–71.3 mg/L dur-ing the entire experiment period (Table 1). NHþ4 –N concentrationdecreased significantly in the anaerobic and anoxic zones due tothe dilution of return sludge (r = 100%) and nitrate recyclingstream (increasing from 100% to 400%). However, in the subse-quent aerobic zones, there was still no NHþ4 –N oxidation observed.In contrast, NHþ4 –N concentration had a significant reduction in theBAF reactor and the average effluent NHþ4 –N concentration was al-most 0 mg/L at all four nitrate recycling ratios (met the class A dis-charge standard). While effluent NO�3 –N concentration increasedsignificantly to 24.4, 16.1, 12.2, and 9.0 mg/L, respectively (Table3). Thus, NO�3 –N was the prominent compounds of TN in theeffluent.

The results obtained indicated that the washing out of nitrifiersfrom A2/O reactor had been successfully achieved by applying theshortened SRT strategy. Therefore, nitrification did not take placein the aerobic zones. The lengthened SRT applied in BAF reactor

inf. an. ano. ae. BAF0

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400 NH4

+- N NO2- - N

NO3- - N TN

COD

(a) R=100%

CO

D/(m

g/L)

Nitr

ogen

/(mg/

L)

TN cl ass B l i mi t

0

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+- N NO2- - N

NO3- - N TN

COD

(b) R=200%

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D/(m

g/L)

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ogen

(mg/

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TN class B limit

inf. an. ano. ae. BAF0

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+- N NO2- - N

NO3- - N TN

COD

(c) R=300%

CO

D/(m

g/L)

Nitr

ogen

/(mg/

L)

TN class A limit

inf. an. ano. ae. BAF

inf. an. ano. ae. B0

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0

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400(d) R=400%

CO

D/(m

g/L)

Nitr

ogen

/(mg/

L)

NH4+- N NO2

- - N

NO3- - N TN

COD

TN class A limit

Fig. 2. Evolution of COD and nitrogen in A2/O-BAF system at various nitrate recycling ratios.

Table 3Pollutant removal efficiencies by A2/O-BAF system at various recycling ratios.

Parameter R = 100% R = 200% R = 300% R = 400%

Inf. Eff. Rem. Inf. Eff. Rem. Inf. Eff. Rem. Inf. Eff. Rem.(mg/L) (mg/L) (%) (mg/L) (mg/L) (%) (mg/L) (mg/L) (%) (mg/L) (mg/L) (%)

COD 359.1 44.6 87.6 357.2 46.2 87.1 374.1 39.4 89.5 387.2 39.3 86.8NHþ4 –N 67.0 0.3 99.6 68.1 0.3 99.6 67.2 0.3 99.7 69.9 0.3 99.6NO�2 –N 0.05 0 100 0.05 0 100 0.05 0 100 0.00 0 100NO�3 –N 0.1 24.4 � 0.2 16.1 � 0.0 12.2 � 0.05 9.0 �

Norg–N 3.8 0.2 94.7 3.8 0.2 94.7 3.4 0.2 94.1 3.4 0.2 94.1TN 70.9 24.9 64.9 72.1 16.6 77.0 70.6 12.7 82.0 73.3 9.5 87.0TP 5.7 0.3 94.7 5.9 0.2 96.6 5.9 0.1 98.3 5.9 0.1 98.3

Y. Chen et al. / Bioresource Technology 102 (2011) 5722–5727 5725

highly enriched nitrifiers, which makes the performance of BAFreactor very reliable in this integrated system. In short, nitrificationwas not the rate-determining step in this novel integratingA2/O-BAF system.

As DO in BAF reactor was controlled between 5–6 mg/L, NO�3 –Nwas the only production in the effluent, which prevented the ad-verse effect of NO�2 –N in the nitrate recycling stream on PAOs /DPAOs in the anaerobic, anoxic, and aerobic zones of the A2/Oreactor.

TN concentration decreased in the anaerobic zone due to dilu-tion. Most of the TN was removed in the anoxic zones, and theTN concentrations at the outlet of the anoxic zones were decreasedto 32.6, 21.4, 15.5, and 14.0 mg/L (Fig. 2a–d) at various nitrate recy-cling ratios. TN concentration almost kept constantly in the subse-quent aerobic zones in A2/O reactor throughout the experiment. Itwas noticed that TN concentration had a slight decrease in the BAFreactor. TN concentrations in the effluent were 24.9, 16.6 mg/L(met the class B discharge standards), 12.7 mg/L (met the class Adischarge standards) and 9.5 mg/L (met the class A discharge stan-dards) when nitrate recycling ratios were 100%, 200%, 300%, and400% (Table 3).

As expected, TN removal efficiencies exhibited an incrementaltrend with the increase of nitrate recycling ratio. The main reasonfor this result is that the increase of nitrate recycling ratio increasesthe nitrate load supplied to the anoxic zones, which is translated inan increased TN removal. With respect to conventional activatedsludge processes, nitrogen might be significantly removed bymeans of nitrification and denitrification process together withbiomass assimilation. The anoxic denitrification capabilities playprominent roles in TN removal. As only in anoxic zones, all or partsof NO�3 –N would be reduced to nitrogen gas or other volatile oxides(Ge et al., 2010). TN removal rate was generally positively relatedto the anoxic denitrification capabilities.

More specifically, the removal efficiency of TN was increased by22.1% when the nitrate recycling ratio was increased from R = 100%to R = 400%. Among which, 12.1% of TN removal efficiency increasewas observed when R was increased from R = 100% to R = 200%. Theresults obtained also showed that a better denitrification could beobtained in the system with a nitrate recycling ratio of R = 200%.

On the other hand, a higher nitrate recycling ratio was benefi-cial for TN removal, but it could be economically non-profitablein case of an influent with low ammonium load. The economical

5726 Y. Chen et al. / Bioresource Technology 102 (2011) 5722–5727

cost of nitrate recycling was directly related to its flow rate. Theenergy consumption at a nitrate recycling ratio of R = 400% wasapproximately four times higher than that of R = 100%.

Based on our study, both TN removal efficiency and economicpotential should be taken into consideration when applying a rel-atively higher nitrate recycling ratio in a real situation. It is sug-gested that application of various nitrate recycling ratios shoulddepend on the actual conditions in the WWTPs. Nitrate recyclingimproved the homogeneous distribution of microbial communitiesin the reactors, and increased the removal efficiency of TN. As rec-ommended, a nitrate recycling ratio around 300% could provide aproper compromise between removal efficiency and costs. A highernitrate recycling ratio could be adopted when much stricter dis-charging standard is imposed to the WWTPs.

Additionally, TN concentration decrease occurred in BAF reac-tor, and this was probably because simultaneous nitrification anddenitrification (SND) took place in BAF reactor with the presenceof COD and micro-anoxic environment in the biofilm. (Zhanget al. 2011; Wang et al., 2010).

3.4. Effect of nitrate recycling ratio on phosphorus removal

TP removal behaviors in the system at various nitrate recyclingratios were shown in Fig. 3. TP concentration increased to the max-imum in the anaerobic zone after PAOs released phosphate byuptaking external COD and then decreased in the anoxic zonesdue to P-uptake by DPAOs and the dilution by the nitrate recyclingstream. In the subsequent aerobic zones P was further uptaken byPAOs to achieve complete biological P removal.

0 40 80 1200

10

20

30

40

50

60

TP/(m

g/L)

time/min

Anaerobic

Fig. 4. Phosphorus release and uptake und

inf. an. ano. ae. BAF0

10

20

30

40

50

60

TP/(m

g/L)

R=100% R=200% R=300% R=400%

Fig. 3. Evolution of TP in A2/O-BAF system at various nitrate recycling ratios.

Under various nitrate recycling ratios (R = 100%, R = 200%,R = 300%, and R = 400%), the specific P-uptake rate (SPUR) mea-sured in the anoxic zones were 1.29, 1.48, 1.58, and 1.65 gTP/(gVSS�day). TP removal efficiencies in the anoxic zones were upto 76.8%, 87.4%, 90.6%, and 93.7%. Afterwards, the residual TPwas further removed under aerobic conditions in aerobic zonesand the effluent TP concentrations were decreased to 0.3, 0.2, 0.1,and 0.1, respectively.

It had been reported that the major factor influencing the occur-rence of DPAOs and associated anoxic P-uptake was the nitrateload in the anoxic zones (Hu et al. 2002), only if the nitrate loadwas high enough or exceeding the denitrification potential of or-dinary heterotrophic organisms (OHOs), i.e. non-PAO organismsin the anoxic zones, could it be possible to stimulate DPAOs inthe system because the specific denitrification rate of OHOs wassignificantly larger than that of DPAOs. Due to this, if the nitrateload recycled to the anoxic zones was less than the denitrificationpotential of OHOs, the OHOs would compete with DPAOs for thelimited nitrate. Only when the nitrate load recycled to the anoxiczones exceeded the denitrification potential of OHOs, DPAOswould have opportunities to utilize the excessive nitrate and grewin the system. Therefore, TP removal efficiency in anoxic zones hada slight increase when nitrate recycling ratio was increased. This,together with the following P-uptake in aerobic zones, supportedthe fact that the effluent TP concentration was much lower thanthe discharge standard (Ge et al. 2010). Additionally, Zeng et al.(2011) and Ge et al. (2010) reported that 0.5 and 0.42 mg/L effluentPO3�

4 –P were obtained respectively by denitrifying phosphorus re-moval in A2/O process and modified step feed UCT process. There-fore, denitrifying phosphorus removal played an important role inthe enhanced biological phosphorus removal (EBPR) system.

In order to further understand the EBPR performance of the A2/O-BAF system, sludge characteristics were investigated by batchtests (shown in Fig. 4). The maximal anoxic and aerobic phospho-rus uptake rates (PUR) of the sludge in the A2/O reactor were com-pared. The anoxic PURmax was 17.6 mgP/(gVSS�h), while theaerobic PURmax was 26.3 mgP/(gVSS�h), and the ratio of anoxicPURmax to aerobic PURmax was 67.0%. It could be derived that about67.0% of the PAOs population were able to use nitrate as electronacceptors, which was in accordance with the study of Wachtmeisteret al. (1997).

In addition, the stability and efficiency of TP removal in A2/O-BAFsystem could be attributed to three reasons: (1) Nitrifiers were suc-cessfully washed out by applying shortened SRT in A2/O reactor.Therefore, under ideal conditions, NO�3 –N in the returned sludgeflowing into the anaerobic zone was much less, which provided anextremely strict anaerobic environment and drastically weakened

160 200 240

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NO

3−-N

/(mg/

L)

CO

D/(m

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TP COD TP TP

anaerobic

anoxicaerobic

NO3−- N

Aerobic/Anoxic

er anaerobic–anoxic/oxic conditions.

Y. Chen et al. / Bioresource Technology 102 (2011) 5722–5727 5727

the negative effect of NO�3 –N on phosphate release; (2) ShortenedSRT was applied in the A2/O reactor which favored the growth ofPAOs (Brdjanovic et al., 1998); (3) The enhancement of denitrifyingphosphorus removal in the anoxic zones also improved phosphorusremoval.

4. Conclusions

The main findings from this study are summarized as follows:(1) The novel integrated A2/O-BAF system could solve the SRT con-flicting problem between nitrifiers and PAOs by applying short-ened SRT for PAOs in the A2/O and lengthened SRT for nitrifiersin the BAF. (2) The effluent concentrations of COD, NHþ4 –N andTP were less than 50.0, 0.5 and 0.5 mg/L, respectively, throughoutthe experiments. The removal efficiencies of TN were 64.9%,77.0%, 82.0%, and 87.0%, under various nitrate recycling ratios(100%, 200%, 300%, and 400%).

Acknowledgements

This work was supported jointly by the 11th National KeyScience and Technology Special Projects (China) (2008ZX07317-007-105, 2008ZX07209-003); State Key Laboratory of Urban WaterResource and Environment (HIT) (China) (QAK200802).

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