Desalination 270 (2011) 227–232

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Influence of anoxic and anaerobic hydraulic retention time on biological nitrogen andphosphorus removal in a membrane bioreactor

Patrick Brown a, Say Kee Ong b,⁎, Yong-Woo Lee c

a HDR Engineering, Inc., 300 E Locust Street, Des Moines, IA 50309, USAb Department of Civil, Construction and Environmental Engineering, 486 Town Engineering Building, Iowa State University, Ames, IA 50011, USAc Department of Applied Chemistry, Hanyang University, 1271 Sa 1-dong, Sangrok-gu, Ansan-si, Gyeonggi-do 426-791, Republic of Korea

⁎ Corresponding author. Tel.: +1 515 294 3297; fax:E-mail address: skong@iastate.edu (S.K. Ong).

0011-9164/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.desal.2010.12.001

a b s t r a c t

a r t i c l e i n f o

Article history:Received 16 November 2009Received in revised form 30 November 2010Accepted 1 December 2010Available online 3 January 2011

Keywords:Hydraulic retention timeMembrane bioreactorPhosphorusNitrogenBNRAnoxicAnaerobic

Influence of anaerobic and anoxic hydraulic retention time (HRT), a commonly used parameter by treatmentplant design engineers and operators, on nitrogen and phosphorus removal was studied in an anaerobic/anoxic/oxic membrane bioreactor with fixed recycle rates. Anaerobic HRTs were varied between 0.5 and 3 h,anoxic HRTs were varied between 1 and 5 h while the aerobic HRT was fixed at 8 h. Total nitrogen removalsranged from 76% to 89% andwere found to increase with increasing anoxic HRTs but wereminimally impactedby increasing anaerobicHRTs exceptwhen the anoxicHRTwas at 5 h. Phosphorus removals ranged from40% to82% andwere found to increase with increasing anaerobic HRTs from 0.5 to 2 h but decreased for an anaerobicHRT of 3 h and with an increase in anoxic HRTs. The study shows that optimal removals for both nitrogen andphosphorus require balancing the conflicting needs of a longer anoxic HRT for increased nitrogen removal butdecreased phosphorus removal and an optimal anaerobic HRT for phosphorus removal without impacting thenitrogen removal. The most favorable HRTs for combined optimal nitrogen and phosphorus removal werefound to be 2-h anaerobic and 4-h anoxic.

+1 515 294 8216.

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, control of the discharge of nutrients into theenvironment has become the focus of increased attention and strictregulations. New treatment technologies are required in many casesto meet new nutrient discharge standards for municipal wastewatertreatment systems. Conventional activated sludge systems have beendeveloped, which allow for biological nutrient removal (BNR) throughthe combination of various anaerobic, anoxic, and aerobic reactors.These processes include the anaerobic/anoxic/oxic (A2O), UniversityCape Town (UCT), Virginia Initiative Plant (VIP), and Bardenphosystems and typically include 3–5 separate reactors or reaction zones(excluding clarifiers) [1]. Processes with multiple reactor basins andclarifiers, as well as multiple sludge recirculations, can be both largeand costly.

In the past 10 years, membrane bioreactors (MBRs) have beendemonstrated to be a viable alternative to the conventional activatedsludge system for carbonaceous and nutrient removal. The MBRoffers several advantages over conventional activated sludge systemsincluding excellent solids removal and complete retention of biomasswithin the system that allows high biomass concentrations of up to

20 g/L [2]. The MBR also allows for high solids retention time (SRT),which can be operated independently of the hydraulic retention time(HRT) [3]. Operation at high SRTs increases treatment performanceand reduces sludge production, which in turn reduces sludge disposalcosts. In multistage BNR systems, the SRT has been shown to influencephosphorus removal [4]. Traditionally it was thought that high sludgeages did not allow for biological phosphorus removal due to bacterialcell lysis and subsequent phosphorus release. However, several studieshave reported biological phosphorus removal at high sludge ages inBNR MBR systems [5–8].

The differing anaerobic and anoxic requirements for biologicalnitrogen and phosphorus removal will likely lead to a range ofanaerobic and anoxic HRTs that provide the best conditions for BNR.A review of the literature found no systematic studies focused onquantifying the influence of different anaerobic and anoxic HRTs onnutrient removal, although it has been proposed that the reactor HRTmay have a significant influence on biological nutrient removal [4].Conventional BNR systems typically have an anaerobic reactor(s)for selection of phosphorus accumulating organisms (PAOs) with atypical HRT of 0.5–2 h, an anoxic reactor(s) for denitrification witha typical HRT of 1–4 h, and an aerobic reactor for nitrification andenhanced phosphorus uptake by PAOs with a typical HRT of 4–12 h[1]. Data from a recent study of seven full-scale conventional A2Otreatment plants with anaerobic HRTs ranging from 1.5 to 3.3 h andanoxic HRTs ranging from 1.7 to 6.1 h showed differences in nutrientremoval for various HRTs [9].

228 P. Brown et al. / Desalination 270 (2011) 227–232

The anaerobic HRT is important for phosphorus removal; theproducts of fermentation are stored by PAOs as polyhydroxyalkanoate(PHA) energy reserves for future use. Depending on the character-istics of the influent, if the anaerobic HRT is too short (roughly 0.5 hor less), anoxic conditions may develop and the PAOs will notdevelop sufficient energy reserves to perform enhanced phospho-rus uptake in the aerobic zone. Increasing the anaerobic HRT willincrease the availability of fatty acids for PAOs, although with toomuch retention time, the energy supply for the PAOs will becomedepleted and lead to secondary release of phosphorus [10]. Thesecondary release of phosphorus can harm the enhanced biologicalphosphorous (EBPR) removal process [4]. The exact time for onsetof secondary phosphorus release depends on the characteristicsof the influent. In general, influent with high concentration ofVFAs would tend to require less anaerobic HRT. Excessive anaerobicHRT can also harm nitrogen removal by limiting the availablechemical oxygen demand (COD) for denitrification later in thetreatment process.

The HRT of the anoxic zone has implications for both nitrogenand phosphorus removal. If the anoxic HRT is short (typically lessthan 1 h depending on reactor conditions and influent), incompletedenitrification is possible. Increasing the anoxic HRT to 3–4 hwill improve nitrogen removal, although HRTs past the point ofcomplete denitrification will not further improve nitrogen removaland can negatively affect phosphorus removal. Maintaining asmall amount of nitrate to stimulate denitrifying PAOs has beensuggested to be beneficial for nutrient removal [4]. Several studieshave suggested that PAOs are more efficient in enhanced phospho-rus uptake in aerobic environments than anoxic, and phosphorusremoval is harmed by excessive anoxic HRT [11]. In addition, apotential conflict occurs between PAOs and denitrifiers, which maycompete for the same carbon source to carry out their metabolism[12].

The optimal HRT for biological nitrogen removal is not the same asfor biological phosphorus removal. Biological nitrogen removal excelsat low anaerobic HRT and high anoxic HRT, and the opposite is true forbiological phosphorus removal, which prefers high anaerobic HRT andlow anoxic HRT.

In light of the aforementioned issues, the goal of this studywas to determine the influence of different anaerobic and anoxicHRTs on biological removal of nitrogen and phosphorus in a MBR.Characterization of treatment performance was in terms of anoxicand anaerobic HRTs, which are commonly used by wastewatertreatment plant design engineers and operators. The results of this

Fig. 1. Biological nutrient removal MBR process diagram

study were then utilized to estimate the HRTs needed for optimalbiological removal of both nitrogen and phosphorus using MBRs.

2. Methods and materials

Laboratory experiments were conducted in a bench-scale mem-brane bioreactor system with an A2O arrangement consisting of threeseparate reactors: anaerobic, anoxic, and aerobic (Fig. 1). The reactordesign was based on Ersu et al., who found that an optimal recyclearrangement was to recycle the membrane permeate to the anoxicreactor and the returnmixed liquor to the anaerobic reactor [5]. For allexperiments, both recycles were at a flow rate equal to that of influentflow rate (100%). Recycling membrane permeate allowed for moreefficient biological phosphorus removal compared to recycling mixedliquor to the anoxic reactor but may be more expensive since energyhas been expended to obtain the permeate [5]. The anaerobic andanoxic reactors were both cylindrical shaped with a total volume of12 L each and employed magnetic stirrers to provide complete mixconditions. Flow from the anaerobic to anoxic and aerobic reactorswasbygravity. The aerobic reactorwas rectangular to accommodate themembrane filter with a maximum volume of 12 L. The HRT of eachreactorwas varied by adjusting the reactor volume. Themembranewasa plate-frame, double-sided cellulose membrane filter manufacturedby Kubota Co., Japan, with a nominal pore size of 0.2 μm and a totalfiltration area of 0.15 m2.

An air diffuser centered beneath the membrane provided airscouring of the membrane to reduce fouling and maintain a dissolvedoxygen concentration of at least 2 mg/L in themixed liquor. A syntheticwastewater designed to simulate medium strength municipal waste-water was used throughout the study (Table 1). Before the HRTexperiments started, baseline data were collected for the MBR withoutthe anaerobic and anoxic reactors for comparison purposes with thethree-stage biological nutrient removal MBR.

The statistical software package JMP™ version 6.0 (SAS Institute Inc.,Cary, NC)was used to create the experimental design. The experimentalruns are shown as part of Table 2. The aerobic HRT was fixed at 8 hso that the impact of the anaerobic and anoxic HRTs on BNR couldbe isolated or studied. The 8-h aerobic HRT also ensures completenitrification and meets the flux limitations of using a single membranefilter.

Dissolved oxygen (DO) concentration in the mixed liquor wasmonitored using a DO meter (Orion Model 830) while ammonia(NH3–N) was measured using an ammonia meter (Orion Model290A). Oxidation reduction potential (ORP) was measured with an

(gravity flow from anaerobic to anoxic to aerobic).

Table 1Synthetic wastewater composition and constituents.

Concentration, mg/L

IngredientCalcium sulfate 40Ferric chloride 3Isomil (Simulac™) 20 mL (1% by volume)Magnesium sulfate 4Nutrient broth (Difco™) 250Potassium chloride 5Sodium bicarbonate 63Sodium biphosphate monobasic 60Sodium citrate 500

CompositionChemical oxygen demand (COD) 494±4a

Total nitrogen (TN) 45.9±0.9Ammonia nitrogen (NH3–N) 22.7±0.8Nitrate nitrogen (NO3

−–N) 0.4±0.1Nitrite nitrogen (NO2

−–N) 0.17±0.03Total soluble phosphorus (TP) 14.4±0.3Suspended solids 27.3±3.5pH 7.2±0.1

a Statistical α=0.05, 95% CI.

229P. Brown et al. / Desalination 270 (2011) 227–232

ORP meter (Orion Model 260). Total nitrogen (TN), nitrate (NO3–N),nitrite (NO2

−–N), and total phosphorus (TP)were analyzed using Hachtest reagents listed as Product no. 26722-45, 26053-45, 26083-45, and27426-45, respectively. COD, soluble COD (sCOD), 5-day biochemicaloxygen demand (BOD5), total suspended solids (TSS), and volatilesuspended solids (VSS) were analyzed in accordance with theStandardMethods [13]. Chemical constituents and reactor parameterswere measured a minimum of twice per week and three to four timesper week during steady-state conditions.

The reactor was seeded using 20 L (about 5 gallons) of activatedsludge from the Boone wastewater treatment plant, Iowa, andthe system was initially operated in a 12-h aerobic batch modeto improve acclimation of the microorganisms to the syntheticwastewater. After 6 cycles (72 h), continuous mode was initiatedwith 1 L/h influent feed and 100% recycle of permeate (equal tothe influent flowrate) and 100% return mixed liquor. The HRTswere fixed at 2-h anaerobic, 3-h anoxic, and 8-h aerobic. No mixedliquor was wasted from the system for several days to increase thebiomass concentration after which the SRT of the aerobic reactorwas gradually increased to 50 days. From this point, the reactor wasoperated until steady-state conditions developed. For each run, the

Table 2Summary of steady-state reactor performance.

Hydraulic retention time (h) Run

Anaerobic Anoxic Aerobic

0 0 8 Baseline0.5 1 8 130.5 4 8 40.5 5 8 21 2 8 71 3 8 111 5 8 52 1 8 32 2 8 6*2 3 8 12 4 8 92 5 8 103 2 8 143 4 8 83 5 8 12

a Statistical α=0.05, 95% confidence interval; Run six data are from [5].

reactor was operated until sampling results were within a 10% rangefor a period of 5 days.

3. Results and discussion

3.1. Membrane and MBR performance

The membrane module performed well for all test runs, with anaverage flux of 13.0±0.44 L/(h m2) and average transmembranepressure of 0.47±0.06 bar (6.8±0.87 psi). Transmembrane pressureslowly rose until about day 45 of the study where it stabilized at about0.5 bar (7.25 psi). Flux was relatively constant, with occasionalincreases due to fouling of the membrane. The membrane wascleaned with a brush approximately every 10 days to maintain asufficient flux. This cleaning schedule is reasonable compared to a full-scale membrane application that requires regular chemical cleaningto maintain stable operation [3,14].

The average steady-state results for VSS, DO, pH, and ORP for eachrun are presented in Fig. 2. Over the study, TSS concentrations averaged4987±410 mg/L in the anaerobic reactor, 4423±556 mg/L in theanoxic reactor, and 7381±507 mg/L in the aerobic reactor. The VSSconcentrations averaged 3623±366 mg/L in the anaerobic reactor,3244±427 mg/L in the anoxic reactor, and 5224±391 mg/L in theaerobic reactor. DO was maintained below 0.1 mg/L in the anaerobicand anoxic reactors and above 2 mg/L in the aerobic reactor. ORPwasstable with averages of−250±10mV anaerobic,−160±8 mV anoxic,and 158±3 mV aerobic. ORP measurements confirmed distinct dif-ferences between anaerobic, anoxic, and aerobic reactors. Run six with2-h anaerobic HRT and 2-h anoxic HRT as shown in Fig. 2 was from astudy by Ersu et al. [5] using a similar experimental system and wasincluded in the figure for comparison purposes.

A summaryof the average steady-statepercentnutrient removals foreach run is presented in Table 2. Soluble COD of the effluent was fairlyconstant for all runs, with an average concentration of 27.4±2.8 mg/L.The high mixed liquor suspended solids (MLSS) concentrations in theMBR system (aerobic reactor) allowed for low mass and hydraulicloadings of approximately 0.15 kg COD/kg MLSS/day and 0.75 to1.2 kg COD/m3/day, respectively. The loading rates of the MBR werelower thanmost conventional systems, which typically operate at massloading rates of 0.3 to 0.6 kg COD/kg MLSS/day and hydraulic loadingrates of 0.8 to 2 kg COD/m3/day [1]. The low loading rates of the MBRallow it to operatemore efficiently and to handle higher COD loads thana conventional system. Total phosphorus and nitrogen removals

Removal (%)a

TN TP sCOD Ammonia

27.7±2.5 16.6±1.8 90.5±0.9 98.8±0.176.0±1.2 63.4±2.9 95.6±0.5 98.3±0.286.4±1.2 56.7±1.1 98.8±0.4 98.7±0.683.6±0.7 40.3±1.5 96.0±0.5 99.2±0.182.5±0.7 68.9±1.2 97.8±0.5 98.3±0.584.9±2.1 70.3±1.1 98.6±0.4 98.8±0.588.7±0.3 63.4±0.8 97.2±0.1 98.2±0.577.8±1.2 81.7±0.8 96.2±0.5 99.5±0.178.2±2.8 81.4±0.9 94.5±0.8 98.5±0.581.1±0.9 72.0±2.7 95.6±2.9 99.3±0.387.7±0.6 71.3±1.2 98.8±0.1 98.8±0.185.5±1.4 68.4±2.6 97.7±0.3 98.5±0.183.6±2.0 70.9±1.7 94.8±1.6 99.1±0.481.8±0.4 62.1±2.6 98.6±0.1 99.1±0.278.1±1.0 58.0±3.8 97.3±1.8 99.2±0.3

Fig. 2. Steady-state results for SS, DO, pH, and ORP (95% confidence level); Run six was from reference [5].

230 P. Brown et al. / Desalination 270 (2011) 227–232

indicated an influence of varied HRT, with removal ranging from 40% to82% and 76% to 89%, respectively.

3.2. Influence of anaerobic and anoxic HRT on biological phosphorus andnitrogen removal

Figs. 3 and 4 show the impact of anaerobic and anoxic HRTs ontotal phosphorus and nitrogen removal for the various experimental

runs. All runs demonstrated enhanced biological phosphorousremoval, which was confirmed by the increase in phosphorus contentof the aerobic sludge (Fig. 5). Enhanced phosphorus removal istypically indicated by the phosphorus content of the sludge beinggreater than approximately 2.5–3% dry weight [1,15]. EBPR wasalso indicated by observing the cyclic release of phosphorus in theanaerobic reactor and uptake of phosphorus in the aerobic reactor(data not provided).

Fig. 3. Total phosphorus removal for different anaerobic HRTs.

Fig. 4. Total nitrogen removal for different anoxic HRTs.

Fig. 6. Phosphorous release per COD consumed per gram VSS in anaerobic reactor forvarious anaerobic HRT.

231P. Brown et al. / Desalination 270 (2011) 227–232

As shown in Fig. 3, total phosphorus removal increased with anincrease in anaerobic HRT from 0.5 to 2 h. Increasing the anaerobicHRT promoted anaerobic phosphorus release and ultimately increasedphosphorus percent removal. However, lower total phosphorusremoval at 3-h anaerobic HRT may indicate secondary releaseof phosphorous. Secondary release by the PAOs occurs when theVFA supply has been depleted and phosphorus is released withoutformation of the PHA energy reserves needed for EBPR. The releasedphosphorous is not taken up by the PAOs in the later anoxic or aerobicconditions [16]. Merseth [17] demonstrated that low VFA availabilityin the anaerobic zone harms the EBPR process. A study by Wangand Park [18] showed that long anaerobic contact time decreased thePHA in the biomass, which in turn resulted in “less energy for thesubsequent uptake of the released phosphorus.” The above observa-tion is reflected in Fig. 6 where the total phosphorous release perCOD consumed per gram VSS in the anaerobic reactor increased for0.5- to 2-h anaerobic HRT but had a decreased release at 3-h HRT.

Fig. 5. Phosphorus content (%) in aerobic sludge.

Several researchers have suggested that the fermentation rate of thewastewater organic substrates, which is slower than the VFAs uptakein the anaerobic reactor, may be used in estimating the anaerobicreactor HRT rather than the release of phosphorus [19,20].

With respect to anoxic HRT, phosphorus removal decreasedmoderately with increasing anoxic HRT from 1 to 5 h (see Fig. 3).Increasing anoxic HRT increases the potential for anoxic phosphorusuptake by PAOs,which is less efficient than aerobic phosphorus uptake[21]. Evidence of anoxic phosphorus uptake was supported byincreased anoxic sludge phosphorus content with increases in anoxicHRT (Fig. 7). These data are consistent with the decreased phosphorusremoval with increasing anoxic HRTs.

Fig. 4 shows that total nitrogen removal ranged between 76% and89% for the different runs. Average steady-state concentrations of totalnitrogen in the effluent ranged from 5.0 to 10.9 mg/L as N. Effluentammonia concentration was stable throughout the study with anaverage concentration of 0.15±0.04 mg/L. The 8-h aerobic HRT and50 days SRT were sufficient for complete nitrification. Total nitrogenpercent removals increased with increasing anoxic HRT from 1 to 4 hand then a slight decrease from 4 to 5 h. Increasing the anoxic HRTfrom 1 to 4 h increased the time possible for denitrification, leadingto increased nitrogen removal percents. The slight decrease in totalnitrogen removal at 5-h anoxic HRTmay be caused by depletion of thereadily available electron donor to denitrifiers. Nitrogen removalappeared to be less sensitive to the anaerobic HRT, being relativelysteady from 0.5 to 2 h with a moderate decrease from 2 to 3 h. Fig. 8shows the total nitrogen removal per unit COD consumed per gramVSS in the anaerobic reactor. The general trend shows increased TNremoval from 1- to 5-h anoxic HRT. Influence from varied anaerobicHRT also likely influenced TN removal rates.

Fig. 7. Phosphorus content of anoxic sludge.

Fig. 8. Nitrogen removal per COD consumed per gram VSS in anoxic reactor for variousanoxic HRTs.

232 P. Brown et al. / Desalination 270 (2011) 227–232

3.3. Determination of optimum anaerobic and anoxic HRT

If there is a desire to focus on either nitrogenor phosphorus removal,then Figs. 3 and 4 can be used as a guide to adjust the anaerobic andanoxic HRTs. However, if there is a desire to simultaneously maximizenitrogen and phosphorus removal, care must be taken to balance thesometimes conflicting requirements of biological nitrogen and phos-phorus removal. For a system where both nitrogen and phosphorusremovals are to be optimized, theproductof total phosphorousand totalnitrogen percent removal (TN% removal×TP% removal) may be usedto assess the optimal HRTs for both anaerobic and anoxic HRTs for theBNR MBR system. Two runs that showed the highest combined TN andTP removal were 2-h anaerobic HRT–2-h anoxic HRT and 2-h anaerobicHRT–4-h anoxic HRT.

In general, phosphorus removal greatly benefited from increasedanaerobicHRTs andwas harmed by increased anoxic HRTs. In contrast,nitrogen removal improved with increased anoxic HRTs and showedslight decrease with increased anaerobic HRT. Increasing anaerobicHRT from 0.5 to 2 h greatly improved phosphorus removal and hadlittle effect upon nitrogen removal. Therefore, it is recommended thatanaerobic HRT be increased when possible to 2 h. The exact anaerobicHRT could shift slightly depending on the wastewater characteristics.Selecting the optimal anoxic HRT is more difficult than the anaerobicHRT, because both nitrogen and phosphorus removal are sensitive tothe anoxic HRT. Nitrogen removal rapidly increases with increasedanoxic HRT from 1 to about 4 h, while phosphorus removal is steadyfrom 1 to 2 h, and decreases from 2 to 5 h. The recommended anoxicHRT is approximately 4 h to optimize both nitrogen and phosphorusremoval. Note that the study was conducted with 100% recycle ofmixed liquor and 100% recycle of permeate, and therefore, the optimalremoval would change accordingly. However, with higher recycle ofthe activated sludge and permeate, higher nitrogen and phosphorusremoval were observed by others [5]. In addition, optimal conditionsmay be influenced by the influent wastewater characteristics.

4. Conclusions

Experiments investigating the influence of anaerobic and anoxicHRT on biological nitrogen and phosphorus removal were conductedin a laboratory-scale membrane bioreactor with an anaerobic andanoxic reactor. The anaerobic HRT varied from 0.5 to 3 h, anoxicHRT varied from 1 to 5 h while the aerobic HRT was fixed at 8 h.Recycle of the mixed liquor and permeate were kept constant at 100%of influent. Excellent average COD and ammonia removals of 95% and

99% respectively, were observed throughout the study while steady-state total nitrogen removal as high as 89% and total phosphorusremoval as high as 82% were obtained.

The results revealed a conflict between the anaerobic and anoxicHRT requirements for nitrogen and phosphorus removal. In general,increasing anaerobic HRT improved phosphorus removal and slightlydecreased nitrogen removal, while increasing anoxic HRT decreasedphosphorus removal and increased nitrogen removal. The trends inphosphorus removal were supported by observation of the biomassphosphorus concentrations. Nitrogen removal decreased at highanaerobic and anoxic HRTs, possibly due to decreased availabilityof COD for denitrification. Using the product of TP and TN percentremovals as an assessment tool, the anaerobic and anoxic HRTsthat gave optimal total phosphorus and total nitrogen removal were2-h anaerobic HRT and 4-h anoxic HRT.

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