DSpace at VNU: Fouling characterization and nitrogen removal in a batch granulation membrane bioreactor

Introduction The aerobic granular sludge process has been known to have many advantages as compared to the conventional activated sludge operations for about a decade.. The organic and n

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Fouling characterization and nitrogen removal in a batch granulation

membrane bioreactor

Bui Xuan Thanha,*, Chettiyapan Visvanathanb, Roger Ben Aimc

a Faculty of Environment, Ho Chi Minh City University of Technology, Building B9, 268 Ly Thuong Kiet Str., District 10, Ho Chi Minh City 70000, Viet Nam

b Environmental Engineering and Management Program, School of Environment, Resources and Development, Asian Institute of Technology, P.O Box 4,

Klong Luang, Pathumthani 12120, Thailand

c Université de Toulouse, INSA, UPS, INP, LISBP, 135 Avenue de Rangueil, F-31077 Toulouse, France

a r t i c l e i n f o

Article history:

Received 16 November 2012

Received in revised form

19 February 2013

Accepted 21 February 2013

Available online 19 March 2013

Keywords:

Aerobic granules

Membrane bioreactor

Extracellular polymeric substances

Fouling

a b s t r a c t

A submerged membrane bioreactor (MBR) combined with aerobic granulation reactor was investigated for the simultaneous organic/nitrogen removal and membrane fouling control Total nitrogen (TN) removal was 59% (1.76 mg TN/g VSS h) in the aerobic granulation reactor Thefiltration of granulation effluent or low operating F/M condition of the MBR could extend the filtration period of up to 78 days without any need for physical cleaning The soluble fraction was the main contributor to fouling compared to colloids and solids The soluble polysaccharides (sPS) had more adverse effects than that of soluble protein (sPN) The deposition on a unit of the membrane’s surface area was 11 mg sPS/L m2and

8 mg sPS/L m2 As a result, the BG-MBR could be an alternative treatment process for simultaneous organic/nitrogen removal and fouling control

1 Introduction

The aerobic granular sludge process has been known to have

many advantages as compared to the conventional activated sludge

operations for about a decade The aerobic granule possesses a

compact spherical structure, excellent settling ability, dense

biomass structure, high biomass retention, ability for simultaneous

nitriﬁcation-denitriﬁcation and removal of toxic substance (Beun

et al., 2002;Carvalho et al., 2006;Thanh et al., 2008; Shi et al.,

2011) The sludge is more stable in batch reactors due to the

exis-tence of feast and famine conditions in each cycle (Beun et al.,

2002) The organic and nitrogen removal in the granulation

sys-tem is high compared to that of conventional activated sludge

process (Arrojo et al., 2004;Tay et al., 2007;Thanh et al., 2009;

Lotito et al., 2012) However, the single granular sludge reactor was

not able to meet the efﬂuent standards due to the high suspended

solids content in the efﬂuent The suspended solids (SS)

concen-trations in the efﬂuent of the granulation reactor were high,

ranging from 75 to 250 mgVSS/L (Beun et al., 2002) and 200 to

450 mgTSS/L (Arrojo et al., 2004) Thus, a post treatment such as membraneﬁltration could be an add-in polishing step for complete treatment and water reuse

Membrane technology has been proven to be the most effective wastewater treatment system in recent decades The advantages are less footprint requirements due to a high substrate loading rate, good treated water quality which can be reused for appropriate operations, less sludge production rate, high biomass retention, and microbial diversity, among others (Visvanathan et al., 2000) Membrane fouling could be due to the deposition of suspended solids/ﬂocs (cake/gel formation, pore blocking), colloids (Bouhabila

et al., 2001) and solutes (Shane Trussell et al., 2006;Jarusutthirak and Amy, 2006;Miyoshi et al., 2012) Recently, it has been found that the fouling mechanism of the submerged MBR is mainly caused

by the deposition/accumulation of soluble extracellular polymeric substances (sEPS) on the membrane if reversible fouling (cake for-mation) is well controlled The sEPS mainly comprises of soluble polysaccharide (sPS) and soluble protein (sPN) The fouling potential

of sPS, sPN or both of them is still unclear Both sPS and sPN were some of the factors which inﬂuenced membrane fouling (Shane Trussell et al., 2006;Liang et al., 2007;Miyoshi et al., 2012) where the sPS played a major role as membrane foulant (Rosenberger et al.,

2006;Jarusutthirak and Amy, 2006;Kim and DiGiano, 2006)

* Corresponding author Tel.: þ84 907866073.

E-mail addresses: bxthanh@hcmut.edu.vn (B.X Thanh), visu@ait.ac.th

(C Visvanathan).

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International Biodeterioration & Biodegradation

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / i b i o d

International Biodeterioration & Biodegradation 85 (2013) 491e498

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At the moment, there exists very limited published information

related to the fouling behavior of the aerobic granular reactor

efﬂuent Some researchers studied the ﬁlterability of MBR seeding

with pre-cultivated aerobic granules for a short operation period

The granules used in the MBR were taken from a batch reactor.Li

et al (2005) reported that the permeability of the MBR seeding

with pre-cultivated granules was 50% higher than that of the

con-ventional MBR during 16 days of operation The author proposed

that the compact and round shaped structure of the granule might

cause less fouling due to the contact of lessﬂoc particles with the

membrane’s surface Additionally,Tay et al (2007)also reported

that the ﬁlterability of pre-cultivated granules was much better

than that of conventional sludge ﬂocs Granular sludge had a

membrane permeability loss of 1.68-fold less than conventional

sludgeﬂocs during the constant pressure test

In this study, a hybrid system includes a submerged MBR

following a sequencing batch airlift reactor (SBAR) to ﬁlter the

efﬂuent This is named as a batch granulation membrane bioreactor

(or BG-MBR) This combination was selected instead of inserting

the membrane inside the granulation reactor because granular

sludge was not stable in the continuous operation mode It is clearly

proven that granules could be stable with the cyclic feast and

famine conditions in a batch reactor (Beun et al., 2002;Tay et al.,

2007) The advantages of this hybrid system include high organic

and nitrogen removal efﬁciencies and fouling control This paper

focuses on the investigation of high loading simultaneous organic

and nitrogen removal and the fouling characteristics of the BG-MBR

system Further, the fouling behavior of sludge fractions was also

investigated

2 Materials and methods

2.1 Experimental setup

Fig 1describes the BG-MBR system including a SBAR

(granu-lation reactor), a settler and a submerged MBR The SBAR which

operated in batch mode consisted of four cycles of operation Air

was supplied through a porous stone diffuser from the bottom of the reactor Each batch of operation consisted of four stages namely feeding (6 min), reaction (high aeration rate: 3 h; and low aeration rate: 48 min), settling (3 min) and withdrawal (3 min) The high aeration rate is to achieve oxidation of organic and nitrogen com-pounds and granule stability Further, it was followed by low aeration to reduce the aeration cost and to enhance the nitrogen removal through the denitriﬁcation process occurring inside the core of the granule The denitriﬁcation process might be enhanced

by limitation of oxygen diffusivity into the core of the granule The SRT was not controlled in this study because the suspended solids from SBAR effluent fluctuated according to time The second unit was the settler The settler is a dual purpose tank to function as both holding and settling tank (denoted as“settler”) The effluent of SBAR was transferred into the settler which was then fed into the MBR in a continuous mode of operation Settled sludge of 500 mL/

d (twice, each 250 mL) from the settler was removed periodically Theﬁnal unit, the submerged MBR was used for the separation of liquid and solid fractions The remaining substrate, unsettled col-loids and pinﬂocs could be further biologically degraded in the MBR All these systems were controlled automatically by pro-grammable logic controller.Table 1shows the operating conditions

of BG-MBR system

2.2 Wastewater and support media The feeding wastewater contained 260 mg TOC/L (700 mg COD/

L as glucose), 190 mg N/L of NH4Cl, 50 mg/L of KH2PO4, 30 mg/L of CaCl2$2H2O, 12 mg/L of MgSO4$7H2O, and 4 mg/L of FeCl3 throughout the experiment Trace elements were added at the rate

of 1 mL/L of wastewater as described byThanh et al (2008) The shell carrier produced from the shell of white rose cockle was added to act as a support for microbial granule formation The carrier was used to enhance the structure, round shape, and physico-chemical characteristics of the granules The shells were dried, ground and sifted with sieve Nos 70 and 100, to reach a fraction between 150 and 212 mm The powder obtained was

Effluent valve

Settler

MBR

membrane Level control

Air supply

Influent valve

Manometer

SBAR

Permeate pump

Pump

B.X Thanh et al / International Biodeterioration & Biodegradation 85 (2013) 491e498 492

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then washed and dried before used in the experiment The

car-rier had a bulk density of 1.45 g/cm3and a weight loss of 2% at

550C within 20 min Initially, the amount of the carrier added

was 200 g (20 g/L) to SBAR Ten grams were added every month

to compensate for the loss through sampling and efﬂuent

discharge

2.3 Analytical methods

The supernatant samples from the settler and the MBR mixed

liquor were prepared once a day by centrifugation at 4500 rpm for

15 min (Centrifuge M/c Universal 320R, Germany) The procedure

for getting supernatant sample was described byBouhabila et al

(2001) Dissolved organic carbon (DOC) was determined by a TOC

analyzer (TOC-VCSN, Shimadzu, Japan) Parameters of ammonia,

nitrite and nitrate were measured according to standard methods

(APHA, 1998) (Total Nitrogen or TN¼ NH3eN þ NO2eN þ NO3eN)

In addition, the PN and PS were analyzed by methods ofLowry et al

(1951)andDubois et al (1956), respectively (EPS¼ PS þ PN) The

UVA254 was measured by using 1-cm quartz cell by UV/Visible

Spectrophotometer (U-2001, Hitachi, Japan) where speciﬁc

ultra-violet absorbance (SUVA) was calculated from the ratio of UV254

and DOC

During the operation, sludge characterization was conducted

for samples of granular sludge, mixed efﬂuent from SBAR and MBR

sludge SVI and MLSS measurements were determined according

to standard methods MLVSS of shell granular biomass was not able

to measure accurately by gravitational method according to

stan-dard methods For this kind of shell granules, measurement of

MLVSS of gravitational method is not accurate due to the loss of

shell carriers when mixed biomass (cell and carrier) are burned at

temperature more than 450C Thus the authors selected the

in-direct measurement method which measures the total organic

carbon (TOC) of cell biomass and then converts into cell mass

based on the cell formulae To measure MLVSS, sludge samples

were ground for 1 min with the Ultra-Turrax machine (Ika-Werk,

Germany) before homogenous sonication at 100 hz for 4 min (Ultra

Sonic processor, CP130, USA) The sample was then diluted with

milli-Q water and stirred in a volumetricﬂask for 10e20 min at

500 rpm until homogenization occured Biomass in terms of MLVSS was determined by measuring TOC of the homogenized sample Then, the value of TOC was converted to MLVSS (multiplied by the factor 2.05) (Tijhuis et al., 1994) The particle size distribution of samples for the MBR sludge and settler were determined by the laser diffraction technique (MastersizerS, Mal-vern, UK, and detection range of 0.05e900 mm) The size of colloidal fraction was examined by the zetasizer nanoZS after centrifuging at 4500 rpm and 4 C for 1 min (detection range of 0.6e6000 nm) The size of the granules was measured by a digital camera with a transparent scale located under the beaker con-taining the granules

The bound EPS (bEPS) of granular sludge, the MBR sludge and fouling layer sample were extracted using the cation exchange resin technique (Dowex HCR-S/S, 16e50 mesh, sodium form, Dow Chemical Company) according toFrølund et al (1996) For granular sludge sample, it was ground by the Ultra-Turrax equipment for one minute before carrying out resin extraction The extraction was conducted with resin dosage (60 g/gVSS) and stirring speed of

600 rpm for 45 min Then, the bEPS solution was centrifuged at

4500 rpm and 4C for 15 min (twice)

The amount of PS deposition on the membrane was quanti-fied with the same method adopted byKim and DiGiano (2006) Two fibres (about 10e30 cm) were cut off from the fouled membrane and washed with tap water until the membranefibre became white/clean like cleaned (initial) membrane (removal of entire fouling layer attached on thefibre) The fibres were cut into small segments and immersed into a test tube containing

2 ml milli-Q water Then the color reagent (1 ml of phenol 5% and 5 ml of concentrated H2SO4) was added into the test tube This analytical procedure is similar to Dubois’ measurement method The PS deposition on the membrane was measured at a wavelength of 490 nm and converted to the unit ofmg PS/cm2of ﬁbre

Modiﬁed fouling index (MFI) and cake resistance was measured

by a stirred cell (AMICON 8400 USA, diameter 67 mm, area¼ 41.8 cm2) with a stirring speed of 500 rpm and aﬂat sheet membrane with pore size of 0.22 mm under a constant trans-membrane pressure of one bar The raw experimental data (V and t) were used to plot t/V versus V graph to get the slope (s/L2) which represents the MFI of the sample The MFI is deﬁned as the gradient

of the linear region found in the cakeﬁltration equation (Eq.(1)) Cake resistance (1/m2) was estimated as multiplying by speciﬁc cake resistancea(m/kg) and cake mass C (kg/m3) (Boerlage et al.,

2002)

t

V ¼ m$a$C 2$A2$TMPVþ

m$Rm

In addition, the fouling potential of sludge fractions in the MBR, including suspended solids (SS), colloids (CL) and solutes (SL), were quantiﬁed The separation of SS, CL and SL were prepared according to Bouhabila et al (2001) The mixed liquor MBR sludge sample con-tained SSþ CL þ SL The sample, containing CL þ SL, was achieved by centrifugation at 4500 rpm and 4C for one minute Finally, the sample containing only SL was centrifuged twice at 4500 rpm and 4C for 15 min Resistance of each fraction was calculated as follows:

Rt, Rm, Rf are total, clean membrane and fouling resistance, respectively

Table 1

Operating conditions of BG-MBR system.

Working

volume (L)

Size (length

diameter)

þ Down-comer:

120 cm 11.5 cm

þ Raiser: 90 cm 7 cm

e 53 cm 10 cm

OLR (kg TOC/m 3 d) 0.86 0.22 e e

NLR (kg N/m 3 d) 0.6 0.1 e e

Operating mode þ 6 batches/d;

4 h/batch:

Feeding: 6 min;

Reaction: high aeration rate (3 h) followed by low aeration rate (48 min);

Settling: 3 min

Withdrawal: 3 min

e Intermittent suction (7 min on/

3 min off)

Air velocity

(cm/s)

þ High aeration: 1.67

þ Low aeration: 0.08

Membrane

module

e e PE 0.1mm, 0.42 m 2 ,

Mitsubishi, Japan Membrane

ﬂux (net)

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3 Results and discussions

3.1 Organic and nitrogen removal in the SBAR

Aerobic granules were cultured from conventional activated

sludge and with shell media Biomass started covering the surface

of shell carriers during the ﬁrst 10 days Shell granules started

forming after 30 days of operation The granule size gradually

increased and varied from 0.5 mm to 9.0 mm during next 80 days

The average size of matured granule was 4.9 1.0 mm The color of

granule changed from light yellow (initial granules) to dark yellow

(mature granules) The average settling velocity was 260 124 m/h

which was much higher than that of activated sludge (1e2 m/h)

Granules were stable during the study period The study on

nitro-gen removal and fouling behavior below was conducted since

granules matured in the reactor (i.e., after 110 days)

At the stage of matured granule formed, organic matter was

quickly removed by an aerobic granulation system in each batch

Fig 2shows evolution of concentrations of organic matter and

ni-trogen species with time in a typical batch 91% of TOC removal was

achieved within theﬁrst 30 min and 94% removal reached during

the 90 min of aeration stage The dissolved oxygen (DO)

concen-tration during the high aeration stage was saturated duringﬁrst 3 h

of the stage of high aeration rate (w6.5 mg/L) and then reduced to

4 mg/L during the next 48 min due to lower aeration rate The pH of

SBAR slightlyﬂuctuated after 2 h of operation due to simultaneous

nitriﬁcation (alkalinity consumption) and denitriﬁcation (alkalinity

production) in the outer and the inner layer of granules,

respec-tively The inﬂuent ammonia was converted into nitrite and nitrate

where nitrite was dominant due to partial nitriﬁcation in the SBAR

The complete nitriﬁcation did not occur due to the free ammonia

concentration in the reactor The free ammonia inhibited the

nitratation (Anthonisen et al., 1976) Complete nitrogen removal

(converted to nitrogen gas) was observed to occur during ﬁrst

30 min in which the bulk liquid was rich in organic and nitrite This reveals that the denitrification process could be achieved in the granules when organic substrate is available The simultaneous nitrification-denitrification (SND) could take place in both low and high aeration stages depending on the availability of organic sub-strate due to the structure of the granule The DO in the outer layer

of the granule was almost as high as the bulk liquid while that of the inner core was very low due to the limitation of oxygen transfer from the bulk liquid to the core of granules This phenomenon allowed the denitriﬁcation process to occur inside the core of the granule As reported byTijhuis et al (1994), the anoxic/anaerobic condition could be achieved at the depth of 300 mm below the granule surface In this study, the average size of the granules was about (4.7 1.4 mm) whose radius was almost much greater than the diffusion depth of oxygen inside the core of granule This could lead to the anoxic condition in the core Generally, the special spherical structure of granules favors the SND phenomena to occur

in the single aerobic granulation reactor even with bulk DO con-centration higher than 4 mg/L

Fig 2 shows that the ammonia nitrogen was completely oxidized into nitrite nitrogen during theﬁrst three hours of oper-ation in which the removal rate was 0.015 mg N/gVSS h (0.18 mg N/

L h) The nitrite production rate was 0.013 mg N/g VSS h (0.16 mg N/

L h) where it was converted from free ammonia with time Con-version of nitrite nitrogen into nitrate nitrogen was not signiﬁcant

in SBAR due to the existence of free ammonia in the reactor Pre-vious research reported that free ammonia inhibition threshold was 0.1e4.0 mg/L for nitrobacter which plays a role in the oxidation

of nitrite (Yang et al., 2004) In this process, the dynamic balance of nitrogen species occurred between ammonia consumption and nitrite production The concentration of TN did not change drasti-cally during the last 3.5 h which could conclude that the SND reached its maximum efficiency at this operating condition The SND only occurs during thefirst duration where the organic sub-strate is available The removal efficiency of TN becomes insignifi-cant since available organic matter is limited in the SBAR Some other research also found that the nitrite-oxidizing bacteria (NOB) are not favorable in granular sludge.Li et al (2013)reported that compared to sludgeflocs sludge granulation with selective sludge discharge help halt ammonia oxidation to the level of partial nitrification rather than complete nitrification This is also confirmed by the molecular analysis that aerobic granulation resulted in ammonia-oxidizing bacteria (AOB) enrichment and reduction of nitrite-oxidizing bacteria (NOB) In addition,Shi et al (2011)also postulated that a fairly large proportion of AOB was close to the granule surface but NOB were rarely found The gran-ules had excellent partial nitrification ability due to inhibition of free ammonia (FA) and limited DO diffusion within granules The data set at steady state during 44 days was used for nitrogen balance The result shows that the TN removal is 59% which in-cludes 12% TN removal by biological assimilation and 47% by the SND process in the SBAR The overall denitrification rate is 22.2 mg N/L/h (1.76 mg N/g VSS h) under aerobic condition without external substrate addition in the reactor In practice, complete nitrogen removal could be fully achieved in the availability of electron donor or lower ammonia concentration in the feed The organic matter removal and SND in the SBAR indicates that there was co-existence of heterotrophic, nitrifying and denitrifying population in the structure of aerobic granules Nitrogen removal could take place even in aerobic granulation reactor (DO greater than 4 mg/L) The results indicate that the complicated anaerobic/ anoxic/aerobic system could be integrated in a single aerobic granulation reactor

Table 2 summarizes the treatment performance of BG-MBR system most of the organic matter was removed in the SBAR

0

50

100

150

200

250

300

350

Time (min)

0 1 2 3 4 5 6 7 8 9

0

40

80

120

160

200

Time (min)

High aeration rate Low aeration rate

Fig 2 TOC, DO, pH and nitrogen species proﬁle of SBAR in a batch.

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(DOC removal>97%) The VSS concentration in the SBAR efﬂuent

and in the settler supernatant was 239 42 mg/L and 35 15 mg/L,

respectively The concentration of suspended solids in granulation

efﬂuent was as high as other research results The SBAR efﬂuent

contained certain amount of pinﬂocs, which could not be settled by

conventional gravitational settling This made the supernatant

turbid This is one of disadvantages of the granular sludge

tech-nology Approximately, 85% of suspended solids settled in the

settler before fed into the MBR The remaining soluble substrate

(mainly nitrite and organic residue) and unsettled pinﬂocs were

further aerobically treated in the MBR The SBAR efﬂuent and the

MBR permeate were rich in nitrite (75 18 mg/L) and nitrate

(77 12 mg/L), respectively Hence, the MBR plays the roles of

post-treatment as polishing and complete nitriﬁcation (Fig 3)

The SRT of SBAR was approximately 24 days which was

calcu-lated based on the ratio of sludge in the reactor over the sludge

wastage Particularly for the granular sludge reactor, the actual SRT

was much higher than the calculated one The SRT of granular

sludge reactor was different from that of the conventional activated sludge reactor At the steady state, the washed-out sludge was just the newly grown cells or pin ﬂocs generated from the biological assimilation and granule detachment The granules, containing old biomass, were retained in the reactor until disintegration This made the actual SRT is longer than the calculated one in the granular sludge reactor Granules were retained in the reactor since they had excellent settling ability compared toﬂocs Therefore, the slow-growing microorganisms could exist to perform the SND process and to degrade the refractory

3.2 Particle size distribution of the MBR sludge There are two modes of measurement for particle size distri-bution (PSD), namely volume distridistri-bution and number distridistri-bution

In terms of volume distribution, the particle size of the settler and the MBR mixed liquor was 98mm and 158mm, respectively How-ever in terms of number distribution it was 0.53mm and 0.20mm (Fig 4) For the light scattering technique, the volume distribution did not provide the representative size of majority of bio-particles because there was a large distribution range in the sludge sam-ples The volume of all small particles only made up of small vol-ume percentage Therefore, the number distribution mode could

reﬂect more accurately the actual size of the measured samples The colloidal size measurement conﬁrms that the nanosize of the MBR sludge was 262 nm (0.26 mm), almost similar to the result achieved from the mixed sludge sample (0.20mm) (Fig 4) For the colloidal size measurement, the number and volume distribution were rather identical because the centrifugation step removed all the large particles and made the two distribution curves become narrow and comparable

The MBR sludge sample showed wider distribution and a smaller size than the settler sample (number distribution) Again, this indicates that the sludge ﬂocs were disintegrated and/or

deflocculated in the MBR due to the endogenous condition of the MBR The shear stress of aeration, again, could break the linkage of floc structure and produce pin flocs, debris and soluble EPS The destructuration was certainly due to erosion strengths or to rup-tures of the network of polysaccharides fibrils which was the support of the different compounds and particularly of the cells Wisniewski and Grasmick (1998)found thatflocs decrease in the settleable fraction and consequently, an increase in the non-settleable one This is in line with this result in which the particle size was reduced The deflocculation makes the particle size smaller which corresponds to the increase of smaller sludge par-ticles in the MBR compared to the settler

In the fouling sense, the particle size of the MBR sludge was larger than the pores of the membrane, thus the particles had less possibility to inﬁltrate into the membrane pores If the fouling is caused by the suspended solids, it is a reversible fouling which could be eliminated by a physical cleaning technique

-50

0

50

100

150

200

250

Influent settler MBR

supernatant

permeate

L) NH4-N NO3-N NO2-N TN

Fig 3 Nitrogen species change in the BG-MBR

Table 2

Treatment performance of BG-MBR.

Overall TN removal (%) 59 19 3.3

MLVSS (mg/L) 12,600 (240) 35 (15) 2200 (600)

Settling velocity (m/h) 260 (125) <10 <10

Average particle size

(% volume)

4.9 (0.2) mm 62.3 (1.3)mm 108.6 (1.5)mm

a Removal efﬁciency of settler is calculated during a batch (4 h).

b removal efﬁciency of MBR is calculated between settler and permeate.

0 5 10 15 20 25

0 10 20 30 40

nanosize (nm)

MBR (volume) Settler (volume) MBR (number) Settler (number)

(volume) (number)

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3.3 Fouling behavior in the MBR

The MBR was operated under endogenous conditions with a low

incoming substrate and a low biomass concentration The bPN

(bounded PN) is usually much higher than bPS (bounded PS) in the

MBR sludge The bPS/bPN ratio inMassé et al (2006)was 0.25e0.5

and inLe Clech et al (2006)was 1/3 In this study it is 0.7.Massé

et al (2006) noticed that protein compounds are more easily

degradable than polysaccharides The reason for the low sPN

con-centration in the bulk liquid was due to the quick sPN consumable

rate compared to the bounded one While the sPS was not readily

degradable as sPN, sPS was accumulated in the MBR with time In

addition, the deﬂocculation phenomena occurred in the MBR due

to its low operating F/M conditions This causes the production of

EPS brought about by the release of bridging polymers from the

ﬂocs structure (Wisniewski and Grasmick, 1998)

Fig 5illustrates the fouling behavior of three different sludge

fractions, namely suspended solids (as SS), colloids (as CL) and

solute (as SL) The fouling potential in term of MFI of SS, CL and SL

fractions occupied 12, 39 and 49% of the total fouling potential of

MBR sludge, respectively The resistance of SS, CL and SL fractions

was 0.01 1012m1, 0.33 1012m1and 2.38 1012m1(inferred

fromTable 3) which makes up 2, 12 and 86%, respectively This could support the notion that the suspended solids and colloidal fractions do not strongly inﬂuence ﬂux decline The soluble fraction

is the main fouling contributor among the sludge fractions in the case of granulation efﬂuent The comparison of fouling potential of sludge fractions with other studies is presented inTable 4

In addition, it was observed that the formation of the cake layer took a long time (70 days) to form on the membrane surface even without backwash application The membrane was fouled on day

78 with the fouling rate as low as 0.027 kPa/d There was no complete cake layer formation on the membrane surface during operation The white color (original) of membraneﬁbres could still

be seen at most area offibres This observation is very different from the case of conventional submerged MBR Sludge cake fully covers all the surface of membranefibres as reported byKhan and Visvanathan (2008) It appears that the low F/M operating condi-tions or low organic loading rate could prolong thefiltration period This study is in line with results of some researchers (Barker and Stuckey, 1999;Rosenberger et al., 2006;Shane Trussell et al., 2006) Fig 5shows an increment of sPS in the MBR supernatant and then a slight reduction in the permeate On the other hand, the trend of sPN shows a slight increment with sPS while sPN does not increase much in the MBR supernatant and is negligible in the permeate As observed, sPS is always much higher than sPN in the filtration system (the ratio of sPS/sEPS ¼ 0.72e0.98) The concen-trations of sPS in the settler, the MBR supernatant and permeate were 7.2 1.1 mg/L, 14.9 2.6 mg/L and 10.3 2.2 mg/L, respec-tively, during the membrane fouling cycle (78 days); while sPN concentrations were 2.9 2.1 mg/L, 3.5 1.3 mg/L and 0.2 0.2 mg/L

The sPS in the settler could be the byproduct of substrate metabolism generated from the granular sludge activity The in-crease of sPS in the MBR supernatant as compared to the settler could be caused by two reasons, namely rejection by membrane (Liang et al., 2007) and deﬂocculation.Barker and Stuckey (1999) postulated that the bound EPS is hydrolyzed to soluble EPS which

is called biomass associated product (BAP) In addition, the DOC of the settler, the MBR supernatant and permeate were 6.0 3.1 mg/L, 9.8 5.2 mg/L and 2.9 1.9 mg/L, respectively, during the duration

of the operation The concentrations of sPN in the settler and in the MBR supernatant were similar during operation This can be explained by the biodegradation of protein compounds in the MBR

0

5

10

15

20

0.6 0.7 0.8 0.9 1.0

0.00

0.05

0.10

0.15

0.20

0 2 4 6 8 10 12 14 16

Accumulation Deposition

Fig 5 Soluble characteristics (settler, MBR supernatant, permeate).

Table 3

Fouling potential of sludge fractions in MBR.

Fractions of sludge SS-CL-SL CL-SL SL

MFI 20 (10 3 s/L 2 ) 86.7 (0.986) a 76 (0.999) a 42.4 (0.996) a

a*C (1/m 2 ) 3.02*10 14 2.65*10 14 1.48*10 14

R t (m1) 2.83*10 12 2.82*10 12 2.49*10 12

R m (m1) 1.12*10 11 1.12*10 11 1.12*10 11

R f ¼ R t eR m (m1) 2.72*10 12 2.71*10 12 2.38*10 12

NA: not applicable.

a The numbers in the brackets is R 2 of the linear segments of the time to volume

proﬁle.

Table 4 Comparison of fouling potential of sludge fractions (%).

Fraction/sludge type SS CL SL Remark Reference MBR ﬁltering

granulation efﬂuent

2 12 86 No backwash,

HF, PE

This study

MBR sludge 24 50 26 Backwash, HF Bouhabila et al., 2001 MBR sludge

(solute separation)

23 25 52 Backwash,

ceramic membrane

Wisniewski and Grasmick, 1996

Note: HF: Hollow ﬁbre, PE: Polyethylene.

Table 5 Bound EPS of fouling layer, MBR sludge and granule.

Bound EPS bPS

(mgPS/gVSS)

bPN (mgPN/gVSS)

bEPS (mgEPS/gVSS)

bPS/bPN bEPS of fouling

layer (n ¼ 2)

10.5 (0.4) 19.9 (1.9) 30.4 0.5 bEPS of MBR

sludge (n ¼ 6)

18.4 (7.7) 39.9 (11.5) 58.3 0.7 bEPS of granule (n ¼ 7) 10.7 (1.4) 17.0 (2.4) 27.7 0.6 Note: n is number of measurements.

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The concentrations of sPS and sPN decreases in permeate

compared to the MBR supernatant proclaims that they were both

trapped on the surface and inside of the pores of the membrane The

amount of sPS and sPN adsorbed in the membrane was about 31%

and 94%, repsectively, compared to that of the MBR supernatant

This indicates that the sPS in the MBR supernatant was partially

deposited while the sPN was retained completely on the membrane

surface The deposition loading on a unit of the membrane surface

was calculated by the loss of concentration after passing through the

membrane which was at 11 mg sPS/L m2and 8 mg sPS/L m2 The

difference of deposition percentage of soluble macromolecules (sPS

and sPN) shows that they possess different characteristics The

partial deposition of sPS on the membrane could be hypothesized

that there were two main fractions of sPS existing in the MBR

su-pernatant (large and small molecules relative to membrane pore

size) The large ones were deposited on the membrane and the

smaller ones were passed through it (Thanh et al., 2010)

Although the DOC in the MBR supernatant increased compared

to that in the settler, the UVA254 and SUVA showed decreasing

trends This indicates that there is a reduction of double bond

substances (such as humic-like materials, sPN) which are prone to

absorb UV light (Jarusutthirak and Amy, 2006) This correlates with

the majority of the sPS present in the MBR which is usually a long

chain of macromolecules with less double bond linkages The DOC

reduction in the permeate, after passing through the membrane,

again conﬁrms that the DOC (i.e., mainly sPS and sPN) sludge was

deposited on the membrane’s surface UVA254of permeate

reduc-tion means that the double bond compounds (mostly protein) were

trapped on the membrane The passage through the membrane

could be mainly the small MW organic matters such as low

mo-lecular weight PS portions and humic-like materials

3.4 Bound EPS of fouling layer and EPS deposition on membrane

The bEPS of fouling layer was extracted to understand its

char-acteristics, fouling behavior and to compare it with that of the MBR

sludge and granular sludge The bEPS of the fouling layer was

similar to that of granular sludge and approximately half of the

MBR sludge (Table 5) This result implies that the biomass in the

fouling layers on the membrane starts to experience lysis due to the

dense biomass concentration and limitation of substrate transfer

from the bulk liquid The sludge particles may not contribute

signiﬁcantly to fouling propensity when it is moving as a bulk liquid

as mentioned but when it attaches to the membrane as a fouling

layer; it could enhance the reversible fouling This is the reason for

the TMP“jump” observed in this study similar to another lab-scale

submerged MBR (Zhang et al., 2006)

Table 6presents the comparison data of polysaccharides

depo-sition on the membrane and inside the pores for various operating

modes of the MBR It appears that the deposition of the sPS on the

MF membrane is high because the sPS can penetrate into and

adsorb on the surface and pores of membrane This result shows the evidence of sPS deposited inside the pores of the membrane

4 Conclusions The research presented here focused on the simultaneous organic/nitrogen removal and fouling behavior of a batch granu-lation membrane bioreactor The following conclusions are drawn:

The simultaneous nitriﬁcation and denitriﬁcation could be achieved by a single granular sludge reactor even at aerobic conditions The total nitrogen removal is 59% or 22 mg TN/L h (1.76 mg TN/g VSS h) at OLR of 0.86 kg TOC/m3d

Submerged MBR coupling with the granular sludge reactor (BG-MBR) extends theﬁltration duration up to 78 days without any physical cleaning techniques (slow fouling rate of 0.027 kPa/d)

Soluble extracellular polymeric substances are also the main fouling causes in the BG-MBR system that is similar to the case

of conventional MBR Polysaccharides and proteins are both deposited on membrane pores and surfaces where poly-saccharides are found to be the major deposition factor The deposition loading on the membrane is 11 mg/L m2and 8 mg/

L m2for soluble polysaccharides and soluble protein, respec-tively The amount of polysaccharides deposited on membrane ﬁbres after 78 days of ﬁltration is 20mg/cm2

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