Seasonal characteristics of supernatant organics and its effect onmembrane fouling in a full-scale membrane bioreactor

Jianyu Sun, Kang Xiao, Yinghui Mo, Peng Liang n, Yuexiao Shen, Ningwei Zhu, Xia Huang nn

State Key Joint Laboratory of Environment Simulation and Pollution Control, THU-Beijing Origin Water Joint Research Center for Environmental MembraneTechnology, School of Environment, Tsinghua University, Beijing 100084, China

a r t i c l e i n f o

Article history:Received 29 August 2013Received in revised form20 October 2013Accepted 2 November 2013Available online 9 November 2013

Keywords:Full-scale membrane bioreactorTemperatureFouling behaviorSupernatant organicsStatistical correlation analysis

a b s t r a c t

Despite the potentially important effect of temperature on membrane fouling, very few investigationshave been conducted on full-scale membrane bioreactors (MBRs) regarding seasonal variation of fouling.In this study, fouling behavior in a full-scale MBR (capacity 60,000 m3/d) in northern China wasmonitored for a whole year. As the mixed liquor temperature dropped from 27 to 13 1C, the filtrationresistance increased from 0.6�1013 to 2.6�1013 m�1, which was attributable to higher concentration ofsupernatant organics at lower temperatures. Humic substances were the predominant supernatantorganics (10–25 mg/L) in comparison with polysaccharides and proteins (both 5–15 mg/L). The depen-dence of seasonal supernatant fouling potential on hydrophilic/hydrophobic composition and molecularweight distribution of the organics were analyzed based on statistical correlation. Humic substances in allhydrophobicity and molecular weight ranges correlated closely with the fouling potential. Hydrophilicpolysaccharides and large-molecular-weight proteins were also found to contribute to the foulingpotential.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Although membrane bioreactors (MBRs) have been rapidlydeveloped and increasingly applied in the area of wastewatertreatment and reclamation worldwide, membrane fouling remainsan inevitable and stubborn issue that deteriorates the long-termperformance of MBRs and hinders the broadening application inpractice [1–4]. Existing researches and engineering operationalexperience in the MBR field demonstrate that the temperature ofthe liquid under filtration plays an important role in membranefouling [5–15]. The performance of membrane filtration during thewinter season is of particular concern. Therefore, it is important tostudy the impact of temperature on membrane fouling and therelated mechanisms.

It has been reported by several researchers that severe membranefouling can occur under low temperature conditions in MBRs[5,6,8,9,11,13,14]. This phenomenon was mostly attributed to thedependence of the compositions and concentrations of foulants inthe mixed liquor upon temperature [5,6,8,9]. As has been mentionedextensively, MBR supernatant containing soluble microbial products(SMPs) is the major source of membrane foulants [2,4,16–20]. Tem-perature can alter the microbial community structure andmetabolism,

and hence the compositions and concentrations of SMPs [5,21]. As aresult, fouling behavior differs in the MBRs at varied temperatures.

However, most existing studies on the impact of temperatureon membrane fouling were conducted in lab/pilot-scale MBRs.Full-scale MBRs differ significantly from lab/pilot-scale MBRs inprocess configuration and operating conditions [16,22], but theeffect of temperature on fouling has been rarely investigated infull-scale MBRs. Lyko et al. [23] and Krzeminski et al. [6] studiedthe seasonal variation of fouling behavior in two full-scale MBRs inGermany and Netherlands, with capacities of 45,000 m3/d and2400 m3/d respectively. Deterioration of membrane filtration per-formance was observed at low temperatures. Polysaccharides andproteins in the MBR supernatant were investigated to explain thedeteriorated filtration performance of the full-scale MBR [23].Nevertheless, humic substances, also ubiquitous in MBR super-natant [3,16], were beyond the vision of these studies. In addition,it should be noted that besides the compositions and concentra-tions, the physicochemical properties of the supernatant organicsincluding hydrophilicity/hydrophobicity and molecular weightalso deserve attention, because they may govern the interactionsbetween membranes and foulants [17,24–27]. However, their rolesin the influence of temperature on membrane fouling have beenscarcely investigated in full-scale MBRs.

The objective of this study is to demonstrate the effect oftemperature on membrane fouling in a full-scale MBR treatingmunicipal wastewater with a capacity of 60,000 m3/d and elucidatethe influential mechanisms of temperature. Monthly monitoring

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0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.memsci.2013.11.003

n Corresponding author. Tel.: þ86 10 62796790.nn Corresponding author. Tel.: þ86 10 62772324.E-mail addresses: liangpeng@tsinghua.edu.cn (P. Liang),

xhuang@tsinghua.edu.cn (X. Huang).

Journal of Membrane Science 453 (2014) 168–174

was conducted for a whole year in the full-scale MBR, regardingthe seasonal change of its filtration performance, the foulingpotential of its supernatant, and the supernatant organic composi-tions including polysaccharides, proteins and humic substances.The hydrophilicity/hydrophobicity and molecular weight distribu-tions of the supernatant organics were also investigated. Statisticalcorrelation analysis was performed between the fouling potentialand characteristics of the supernatant organics to identify thedominant foulants in the full-scale MBR.

2. Materials and methods

2.1. The full-scale MBR

The full-scale MBR investigated in this study was the key processof a wastewater treatment plant, which was located in Beijing,north of China (40100′N, 116125′E). This plant collected and treatedthe municipal wastewater from the residential nearby with acapacity of 60,000 m3/d. Raw wastewater was treated by 8-mmscreen, aerated grit basin and 1-mm screen successively to removemost suspended solids, and thenwas introduced into an anaerobic–anoxic–aerobic process and then the membrane tank (Fig. S1 inSupplementary data). The effluent from the MBR reached the levelI-A national standard (GB 18918-2002); 10,000 m3/d of the effluentwas further purified by a reverse osmosis (RO) process and suppliedto the Olympic Parks nearby, while the other was reclaimed forurban miscellaneous consumption.

The MBR consisted of two parallel membrane tanks and wasoperated at a solid retention time of 20 d and a hydraulic retentiontime of 17 h. The trans-membrane pressure (TMP) and the filtra-tion flux of the MBR were daily monitored and recorded by anautomatic control system. The hollow fiber membrane in use(Siemens, Germany) was of hydrophilic polyvinylidene fluoride(PVDF) material with a nominal pore size of 0.04 μm. The intrinsicresistance of the clean membrane was 1.2�1012 m�1 [1]. Duringthe sampling and monitoring of this study, there was no back-washing process in the full-scale MBR operation. The membranewas weekly cleaned in place using sodium hypochlorite solution(500 ppm), and was also off-line cleaned once using sodiumhypochlorite solution (2000–3000 ppm) at the beginning of 2012.

2.2. Sampling of mixed liquor from the full-scale MBR

Mixed liquor was taken out from the membrane tank, andimmediately filtered by a filter paper and then a glass-fibermembrane (0.7 μm, GF/F, Whatman, UK) in order to remove thesuspended solids. The resultant permeate was termed as the MBRsupernatant. All the samples were temporarily conserved in a 4 1Ccontainer and transported to the laboratory within 24 h forsubsequent analyses. All the analyses were conducted immedi-ately in order that the results could best represent the condition inthe full-scale MBR. The sampling was conducted monthly for awhole year. To ensure the representativeness of each sampling,duplicate samples were obtained by taking one sample from eachmembrane tank.

2.3. Assessment of fouling potential of MBR supernatant

The fouling potential of the MBR supernatant was evaluated bystirred dead-end filtration experiments performed in the labora-tory. The MBR supernatant was filtered by a 0.22 μm hydrophilicPVDF membrane (GVWP, Millipore, USA) with an effective filtra-tion area of 41.8 cm2, which served as the reference membrane toassess the fouling potential of the MBR supernatant. The filtrationwas conducted at 20 1C in a stirred dead-end filtration cell

(Amicon 8400, Millipore, USA) at a constant pressure of 7 kPaand a stirring rate of 170 rpm. The slope of the flux decline curveswith filtration volume was used as an indicator for the foulingpotential of the MBR supernatant, defined as ‘fouling index’ [17].

2.4. Characterization of MBR supernatant organics

Hydrophilicity/hydrophobicity fractionation of the MBR super-natant was conducted according to a column chromatographicprocedure [17], yielding two fractions comprising hydrophilicsubstances (HIS) and hydrophobic substances (HOS). Molecular-weight fractionation of the MBR supernatant was performedthrough successive ultrafiltration using regenerated cellulosemembranes (PLHK&PLAC, Millipore, USA) with two molecularweight cutoffs of 100 kDa and 1 kDa, hence yielding three fractionshaving different molecular weight intervals.

The MBR supernatant and each of its fractions were characterizedin terms of total organic carbon (TOC) (TOC-V CPH, Shimadzu, Japan),polysaccharides [28], proteins and humic substances [29]. Glucose(Beijing Chemical Works, China), bovine serum albumin (Sigma, USA)and humic substances (Fluka, Switzerland) were used as the standardsubstances for the measurements of polysaccharides, proteins andhumic substances, respectively. Before measurement of proteins andhumic substances using the modified Lowry method, divalent cationswere removed using gel-type cation exchange resins (AmberliteIR-120 Na, Acros Organics, Belgium) in order to exclude the inter-ference of calcium and magnesium ions [29].

2.5. Statistical analysis

Experimental results were analyzed from the statistical view-point to identify whether there were significant correlationsamong temperature, fouling parameters, and supernatant charac-teristics. Pearson correlation analysis is a common measure toevaluate the linear correlation between two variables. The analysisgives two fundamental values, i.e. r and p, to evaluate thecorrelation. r Represents the linearity of the correlation betweentwo variables, ranging from �1 to 1. An absolute value of r close to1 indicates a correlation close to linear. The correlation is positivewhen r is positive and vice versa. p Represents the confidence levelof the correlation. In this study, correlation was considered highlysignificant when po0.01, and moderately significant when po0.1.All the correlation analyses in this study were carried out using theIBM SPSS 20 software (IBM, USA).

3. Results and discussion

3.1. Seasonal change of fouling behavior in the full-scale MBR

Beijing is a northern city that has four distinct seasons. Duringthe whole year sampling of this study, the temperature of the mixedliquor in the full-scale MBR varied from 13 1C to 27 1C (Fig. 1(a))with the air temperature of Beijing. This variation of temperatureacross a whole year was less significant than that reported in aprevious research in a pilot-scale MBR in Beijing [5].

Although the flux of the full-scale MBR was fixed at approximate10 L/m2 h, the TMP varied significantly with months, hoveringaround 25 kPa in summer but rising sharply up to about 60 kPa inwinter (Fig. 1(a)). The variation of TMP in the full-scale MBR withmonths might result from change in solution viscosity (which isa function of temperature) and/or the fouling of the membrane(i.e. accumulation of foulants at the membrane surface or into thepores). In order to elucidate the influence of temperature on thereal fouling behavior in the full-scale MBR, filtration resistance was

J. Sun et al. / Journal of Membrane Science 453 (2014) 168–174 169

calculated according to the Darcy's law

R¼ PμJ

ð1Þ

where R (m�1) is filtration resistance, P (Pa) is TMP, J (m3/m2 s) isflux and μ (Pa s) is the viscosity of the permeate. In this study, dueto the low solute concentration in the permeate, the permeateviscosity at any given temperature was basically equal to that ofwater, which could be calculated according to a handbook ofchemistry and physics using the water viscosity at 20 1C [30].

The results presented in Fig. 1(b) clearly show that the filtrationresistance also varied significantly with months, indicating thatthe temperature of the mixed liquor substantially affected thefouling behavior, with more severe fouling at lower temperatures.Similar results were found in several lab/pilot-scale MBRs asreported previously [5,9,12].

3.2. Dependence of fouling behavior in the full-scale MBR onsupernatant characteristics

Membrane fouling in the full-scale MBR was probably inducedby the soluble organics in the supernatant and/or the suspendedsludge in the mixed liquor. In order to identify the key foulantsthat affected membrane fouling, Pearson correlation analysis wasconducted to correlate the fouling behavior with the concentrationof TOC in the supernatant and with the concentration of MLSS(Fig. 2(a)). It was the TOC concentration of supernatant, ratherthan the MLSS concentration, that showed positive correlationwith filtration resistance (po0.1). Moreover, when membranemodules were taken out from the membrane tank, there wasscarcely sludge or biofilm deposited on the membrane surface,which was possibly due to the maintenance chemical cleaning

once a week. Therefore, the supernatant was identified as themajor factor causing membrane fouling in the full-scale MBR.

To further corroborate the idea that the supernatant organicsdominated membrane fouling in the full-scale MBR, the filtrationresistance of the full-scale MBR was correlated with the foulingpotential of the supernatant. The fouling potential of the super-natant was evaluated by stirred dead-end filtration tests using the‘fouling index’ as the indicator (see Section 2.3 for detail). Anapproximately positive linear correlation (r¼0.795) was foundbetween the fouling index and the filtration resistance (Fig. 2(b)).This significant correlation (po0.01) suggested that the foulingindex measured in the laboratory could well represent the foulingin actual membrane tanks in this study. Since the fouling indexindicated the fouling potential in the initial pore blocking process,it was suggested that pore blocking process strongly correlatedwith the actual membrane fouling behavior. Furthermore, it wasthe MBR supernatant that impacted the actual membrane foulingin the full-scale MBR.

3.3. Seasonal change of supernatant organic compositions

The major membrane foulants in the MBR supernatant arereported to be the soluble microbial products (SMPs) mainlyincluding polysaccharides, proteins and humic substances [22].

Apr2011

MayJun Jul AugSep OctNovDec Jan2012

FebMar0

20

40

60

80

TMP Flux Temperature

TMP

(kPa

)

0

10

20

30

40

10

15

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25

30

35

Tem

pera

ture

(°C

)

Apr2011

May Jun Jul Aug Sep Oct NovDec Jan2012

Feb Mar0.0

0.5

1.0

1.5

2.0

2.5

3.0

Filtr

atio

n re

sist

ance

(1013

m-1

)

Flux

(L/m

h).

Fig. 1. Variation of filtration performance of the full-scale MBR in 1 year. TMP,filtration flux and mixed liquor temperature are shown in (a) and mixed liquorfiltration resistance is shown in (b).

0.0 0.5 1.0 1.5 2.0 2.5 3.00

10

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TOC vs. Resistancer = 0.402p = 0.071 < 0.1

)L/gm(

noitartnecnocC

OTtnatanrepuS

Filtration resistance in the full-scale MBR(1013 m-1)

0

5

10

15

20

25

30

MLSS vs. Resistancer = 0.068p = 0.769

MLS

S co

ncen

tratio

n (g

/L)

0.0 0.5 1.0 1.5 2.0 2.5 3.00

10

20

30

40

50

r = 0.795p = 0.000 < 0.01 m(tset

elacs-baleht

nixedni

gniluoF-1

)

Filtration resistance in the full-scale MBR (1013 m-1)

Fig. 2. Correlation of mixed liquor filtration resistance in the full-scale MBR with(a) supernatant TOC concentration, MLSS concentration and (b) supernatant foulingindex obtained from the lab-scale filtration test.

J. Sun et al. / Journal of Membrane Science 453 (2014) 168–174170

In view of this, seasonal change of these organics was investigatedin this study to better understand the fouling behavior in the full-scale MBR (Fig. 3). The concentration of polysaccharides decreasedfrom about 15 mg/L to lower than 5 mg/L from spring to summer,and rose up again in winter. Humic substances showed a similarvariation tendency to that of polysaccharides, ranging from about10 mg/L to 25 mg/L. The concentration of proteins decreased fromabout 15 mg/L to lower than 5 mg/L from spring to summer, andfluctuated up to about 8 mg/L in winter. It is clear that humicsubstances were predominant among the three types of super-natant organic foulants, while polysaccharides and proteins exhib-ited similar concentration ranges.

Despite the possible contribution to fouling, humic substancesreceived less attention than polysaccharides and proteins inprevious studies regarding the organic compositions and theirinfluence on fouling in full-scale MBRs. There is one studyconducted by Shen et al. [16] quantifying the concentration ofhumic substances in MBR supernatant taken from 10 full-scaleMBRs in China. Compared with their results that demonstrated aconcentration range from 3 mg/L to 10 mg/L, a high concentrationlevel of humic substances was found in this study. Full-scale MBRsthat follow a conventional activated sludge process are supposedto possess abundant microorganisms growing at the endogenousstage, which benefits the production of biomass-associated pro-ducts (BAP) containing a large proportion of humic substances[31]. Therefore, the higher concentration of humic substances inthe full-scale MBR investigated in this study suggested that therewere probably more microorganisms staying at the endogenousstage than the MBRs investigated by Shen et al. [16].

With respect to polysaccharides and proteins, Drews reportedthat the typical concentration of polysaccharides in lab/pilot-scaleMBRs ranged from 20 mg/L to 100 mg/L [22], which was higherthan that in this study. Similar conclusion of proteins can be drawnas compared with the investigations of Wu et al. and Ma et al. onpilot-scale MBRs [5,18]. These comparisons indicate that differentconditions exist between lab/pilot-scale and full-scale MBRs and itis considerably necessary for one to perform studies directly onthe full-scale MBRs when aiming at optimizing the practicalapplication of MBR.

Pearson correlation analysis was conducted to examine theeffect of temperature on the concentration of supernatant organics(Fig. 4). All the three types of organics investigated in this studycorrelated significantly with temperature with po0.05. Specifically,

polysaccharides showed the most significant negative correlationwith temperature, followed by humic substances and then proteins.These negative correlations indicated that more organics wereproduced at lower temperatures. Similar effect of temperature onsupernatant organic was found by Ma et al. [5] and van den Brinket al. [8], whose researches were both carried out in pilot-scaleMBRs. A low temperature could be an environmental stress factor tothe sludge, promoting the production of extracellular substances(EPS) and the release of SMP to the liquid phase, and thus elevatingtheir aqueous concentrations [32]. Another factor that may influ-ence the production of EPS has been reported to be feed waterquality [33]; however, no significant seasonal variation of feedwater quality was observed in this study (see Fig. S2 in Supple-mentary data). Moreover, characteristics of suspended solids werealso concerned. Pearson correlation analysis showed that organiccharacteristic of EPS hardly correlated with temperature, filtrationresistance in the full-scale MBR or the fouling index obtained fromthe lab-scale filtration test (see Table S1 in Supplementary data). Itwas indicated that sludge properties was not the key factor betweentemperature and membrane fouling in this case. Therefore, thetemperature effect on the concentration of supernatant organicswas considered to be of prime importance. As a consequence of theincreased concentration of supernatant organics, membrane foulingin the full-scale MBR was aggravated at low temperatures. Giventhat polysaccharides, proteins and humic substances all exhibited

Apr2011

May Jun Jul Aug Sep Oct Nov Dec Jan2012

Feb Mar0

5

10

15

20

25 Polysaccharides Proteins Humic substances

Con

cent

ratio

n (m

g/L)

Fig. 3. Concentration variation of polysaccharides, proteins and humic substancesin MBR supernatant in 1 year.

Fig. 4. Correlation of supernatant organics concentration with temperature((a) polysaccharides; (b) proteins; (c) humic substances).

J. Sun et al. / Journal of Membrane Science 453 (2014) 168–174 171

significant correlations with the temperature, more detailed inves-tigation into different fractions having varied physicochemicalproperties is necessary to identify the dominant foulants in thefull-scale MBR.

3.4. Statistical correlation between supernatant fouling potentialand organic compositions

Supernatant organics can induce membrane fouling by severalmechanisms: being adsorbed by membranes within their pores orat their surfaces, blocking membrane pores mechanically, orforming gel layers at membrane surfaces, etc. [2,4,17]. Hydrophi-licity/hydrophobicity and molecular weight distributions are twoof the most important physicochemical properties involved in themechanisms as mentioned above [16,25]. Hence, the hydrophili-city/hydrophobicity and molecular weight distributions of thesupernatant organics were investigated to identify the dominantfoulants and give an insight into the fouling mechanisms in thefull-scale MBR.

Box charts were plotted in Fig. 5 to provide annual informationabout the concentration ranges of polysaccharides, proteins andhumic substances in different hydrophilic/hydrophobic andmolecular-weight fractions of the MBR supernatant. In terms ofthe hydrophilicity/hydrophobicity distribution, polysaccharidesand proteins mostly concentrated in HIS, while humic substanceswere found most in HOS. With respect to molecular weightdistribution, the supernatant organics were fractionated into threemolecular weight intervals. The two boundaries of 100 kDa and

1 kDa corresponded to molecular diameters of roughly 10 nm and1 nm, respectively. Polysaccharides distributed evenly among thethree molecular weight fractions. Most proteins were large inmolecular size, concentrating in the fractions of larger than 1 kDa.On the contrary, most humic substances were small molecules andconcentrated in the fractions with molecular weights smaller than100 kDa.

Pearson correlation analysis was conducted between the over-all fouling potential of the MBR supernatant and the organicconcentration of each fraction to identify the primary foulantsgoverning the fouling behavior of the MBR supernatant (Table 1(a) and (b)). All the fractions of humic substances correlatedsignificantly with the overall fouling index of the MBR super-natant, regardless of their hydrophilicity/hydrophobicity andmolecular weights. Moreover, statistical analysis between filtra-tion resistance in the full-scale MBR and supernatant componentsalso showed that actual membrane fouling only correlated withhumic substances (r¼0.424, p¼0.056o0.1) rather than polysac-charides (r¼0.280, p¼0.21940.1) and proteins (r¼0.330,p¼0.14440.1). This also gained credence that humic substancesplayed a significant role in actual membrane fouling behavior. Thisexplained why humic substances were the only organics thatconsiderably affected the fouling behavior of the MBR supernatantwhen correlations between the fouling index and the total con-centration of polysaccharides, proteins and humic substances wereevaluated by Pearson analysis (Table 1(c)). Except the polysacchar-ides that were hydrophilic and the proteins that had a molecularweight larger than 100 kDa, other fractions of polysaccharides andproteins presented insignificant correlations with the overallfouling index of the MBR supernatant. The results shown inTable 1 suggest that humic substances, hydrophilic polysacchar-ides and large-molecular-weight proteins were the dominantfoulants in the supernatant that induced fouling in the full-scale MBR.

Some suppositions about the membrane fouling mechanisms inthe MBR can be made from the aforementioned results. Humicsubstances, the most predominant foulants in the full-scale MBR,might participate in several processes that could cause membranefouling, including adsorption by the membrane (hydrophobicsubstances with small molecular sizes), pore blocking (largemolecules) and gel layer formation (hydrophilic substances).Polysaccharides impacted membrane fouling mainly by gel layerformation (hydrophilic substances), while pore blocking wasprobably the major influence of proteins (large molecules) onmembrane fouling. Distinct from previous researches carried outin full-scale MBRs [16,23], this study demonstrates that humicsubstances dominated membrane fouling instead of polysacchar-ides and proteins. As discussed before, the production of humicsubstances was a result of the growing of microorganisms atendogenous stage in the MBR, which was fed with the outflowof a conventional activated sludge process and thus having a loworganic load.

3.5. Implications for practical operation of MBR

Seasonal change of filtration performance in a full-scale MBRwas investigated in this study. Serious membrane fouling wasfound when the temperature was low, which resulted from theincreased production of supernatant organics. It was suggestedthat the temperature of mixed liquor be controlled (by e.g. locatingthe reactor tanks indoor or underground) to achieve a stable lowconcentration of supernatant organics. Other strategies could beapplied to adjust the characteristics of supernatant organics if thetemperature control is not easy to implement. For exampledecreasing supernatant organics via improving the flocculationability of the mixed liquor (by e.g. dosing ozone and coagulants)

Polysaccharides Proteins Humic substances0

4

8

12

16

20

Con

cent

ratio

n (m

g/L)

HIS HOS

Polysaccharides Proteins Humic substances0

3

6

9

12

15

> 100 kDa 1 ~ 100 kDa < 1 kDa

Con

cent

ratio

n (m

g/L)

Fig. 5. Concentrations of polysaccharides, proteins and humic substances indifferent hydrophilic/hydrophobic fractions (a) and different molecular-weightfractions (b). HIS and HOS denote hydrophilic and hydrophobic substancesrespectively.

J. Sun et al. / Journal of Membrane Science 453 (2014) 168–174172

are strategies worth considering [3]. The secretion of SMPs mightbe suppressed by adjusting the respiration state of microorgan-isms. For MBRs connected with a conventional activated sludgeprocess like in this study, adjusting the sludge recirculationbetween MBR and the conventional process is an option to changethe food to microorganism ratio and hence the respiration state.Also notable are the predominance of humic substances in theMBR supernatant and their significant role in membrane fouling asfound in this study. Operational parameters (e.g. SRT) could beadjusted to affect the balance between utilization-associatedproducts (UAP) and biomass-associated products (BAP) in SMP[34,35], thereby altering the concentration and composition ofMBR supernatant organics.

4. Conclusions

Monthly monitoring of a full-scale MBR was conducted for 1year. Temperature showed significant correlation with membranefouling. Serious membrane fouling was observed at lower tem-peratures. Temperature also showed significant negative correla-tion with the concentration of supernatant organics. Statisticalanalysis indicated that humic substances, regardless of theirhydrophilicity/hydrophobicity and molecular weight, correlatedstrongly with the fouling potential of supernatant. In addition,hydrophilic polysaccharides and large-molecular-weight proteinsalso contributed to membrane fouling. This study demonstratesthat seasonal change of membrane fouling in the full-scale MBRcan be caused by the effect of temperature on the characteristics ofsupernatant organics.

Acknowledgment

This work was supported by the Major Science and TechnologyProgram for Water Pollution Control and Treatment (No.2011ZX07317-002), the International Program of MOST (No.S2011ZR0434) and Program for Changjiang Scholars and Innova-tive Research Team in University. Support provided to Dr. Xiao for

this work by China Postdoctoral Science Foundation fundedproject (No. 2012M510033) is also gratefully acknowledged.

Appendix A. Supplementary material

Supplementary data associated with this article can be found in theonline version at http://dx.doi.org/10.1016/j.memsci.2013.11.003.

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Table 1Pearson correlation analysis between the fouling index and the concentrations of organic components in the supernatanta.

(a) Fouling index vs. hydrophilic/hydrophobic fractionsb

Polysaccharides Proteins Humic substances

HIS HOS HIS HOS HIS HOS

r 0.543nn 0.298 �0.140 0.328 0.554nn 0.886nn

p 0.009 0.178 0.535 0.137 0.007 0.000

(b) Fouling index vs. molecular-weight fractions

Polysaccharides Proteins Humic substances

4100 kDa 1–100 kDa o1 kDa 4100 kDa 1–100 kDa o1 kDa 4100 kDa 1–100 kDa o1 kDa

r 0.345 0.328 0.336 0.463n �0.148 �0.314 0.538nn 0.480n 0.570nn

p 0.116 0.136 0.126 0.030 0.512 0.155 0.010 0.024 0.006

(c) Fouling index vs. entirety of supernatant

Polysaccharides Proteins Humic substances

r 0.099 0.099 0.651nn

p 0.532 0.532 0.000

a The significance of Pearson correlation was denoted by asterisk marks: single asterisk (n) denotes moderate significance at a confidence level of 90% (po0.1); doubleasterisks (nn) denote high significance at a confidence level of 99% (po0.01).

b HIS: hydrophilic substances; HOS: hydrophobic substances.

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