Colloids and Surfaces A: Physicochem. Eng. Aspects 264 (2005) 120–125

Influence of the filtration modes on colloid adsorption on themembrane in submerged membrane bioreactor

Li Ma a, Xiufen Li a, Guocheng Dua, Jian Chena,∗, Zhisong Shenb

a Key Lab of Industrial Biotechnology, Ministry of Education and Lab of Environmental Biotechnology, School of Biotechnology,Southern Yangtze University, Huihe Road 170, Wuxi City, Jiangsu Province, 214036, PR China

b Jiangsu Institute of Microbiology, Wuxi City, Jiangsu Province, 214036, PR China

Received 25 October 2004; received in revised form 21 February 2005; accepted 23 March 2005Available online 15 August 2005

Abstract

The influence of three different filtration modes on the colloid adsorption in a submerged membrane bioreactor (SMBR) was evaluatedand the feasibility for the characterization of the colloid adsorption on the membrane using a modified Freundlich isothermal adsorptionequationq = X/A = aC1/n was also explored in this study. Under the same operating conditions, the adsorption equations determined int uctionp draggingw tion. Ther thec e used fort nsequentlya©

K

1

p[bthetMflom

,ing

tion,thathin-are

letelylug-

wnoul-90%rtsure-, the

ions.are

0d

e e

his study were 0.519C1.1623e , 7.416C0.7305

e and 2.590C1.0449e for modes of vacuum pump dragging with air back washing, vacuum and s

umps dragging without air back washing, respectively. Compared with the filtration mode of suction pump or vacuum pumpithout air back washing, the mode of 10 min vacuum pump dragging with 5 min air back washing could reduce the colloid adsorp

esults also demonstrated the modified Freundlich isothermal adsorption equation,qe = X/A = aC1/ne , was applicable to characterize

olloid adsorption on the membrane in the SMBR. Except the characterization of colloid adsorption, this approach also could bhe optimization of operational conditions, the selection of membrane materials and the evaluation of module configuration, colleviating the membrane fouling in a SMBR.2005 Elsevier B.V. All rights reserved.

eywords:Submerged membrane bioreactor; Membrane fouling; Colloid adsorption; Back washing

. Introduction

The submerged membrane bioreactor (SMBR) is aromising process used in the field of wastewater treatment

1–3] with several advantages over the conventional aerobiciodegradation process, including smaller space and reac-

or requirement, better effluent quality, good disinfections,igher volumetric loading and less sludge production. How-ver, the application of microfiltration to aerobic wastewaterreatment suffers from the severe membrane fouling[4–6].embrane fouling leads to the decrease of the filtration

ux. The cost of a membrane system is highly dependentn the surface area required, which is determined by theembrane’s flux. We can contribute to one or more of the

∗ Corresponding author. Tel.: +6 510 5888301; fax: +86 510 5888301.E-mail address:jchen@sytu.edu.cn (J. Chen).

five mechanisms causing flux decline[7]: (a) adsorption(b) steric hindrance, (c) viscosity effects, (d) pore blockand plugging and (e) concentration polarization. Adsorppore blocking and pore plugging are fouling mechanismscan have reversible or irreversible character while stericdrance, viscosity effects and concentration polarizationinherent to membrane processes and cannot be compavoided. Among adsorption, pore blocking and pore pging, adsorption may even occur at zero pressure.

The adsorptive fouling component of flux decline is knoas a significant factor and it is reported adsorptive fing alone can account for permeability losses of up to[8–12]. This finding is a driving force behind many effoto study adsorption. Since the microfiltration is a pressdriven membrane separation process, for a fixed SMBRadsorption fouling deeply depends on operating conditThus, most efforts on the inhibition of membrane fouling

927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2005.03.012

L. Ma et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 264 (2005) 120–125 121

focused on the optimization of operating conditions, suchas feed flow velocity, feed operation mode (dead-end orcross-flow) and pressure difference between two sides of themembrane[13,14]. The similar consideration in the SMBRperformance is to use the strong air bubble aeration[15,16].The strong bubble air aeration agitates the solution in thebioreactor generating a shear force over the membrane sur-face and effectively reducing the deposition and adsorption ofpollutants on the membrane. Another important step for theinhibition of membrane fouling is the periodic back washingthe membrane with air or water[17].

Different from the conventional microfiltration, the driv-ing force of filtration applied to the membrane in a SMBRcomes from the effluent side rather than the influent side.Namely, the driving force is a drag force rather than a press-ing. This drag force not only drives the treated wastewaterpenetrating into lumen of hollow fibers and discharging outof the bioreactor, but also attracts pollutants to deposit andabsorb on the membrane surface and into membrane pores.Obviously, the drag force should be a predominant factor inthe influence of operating conditions on the membrane foul-ing. However, the investigation for the influence of drag forceand back washing on membrane fouling has not been so suf-ficient so far.

In the SMBR system colloidal fouling of membranes isone of the main foulants[4,18–20], thus pointing out to thep erat-i nes.T val-u backw thep ua-t lloida

2

2

d int dt ulew di ere4 ersw ctlyo dert ouldb ted bt pedf nsorc andv upont uracya dule

Fig. 1. Schematic diagram of the submerged membrane bioreactor.

with the same specification was used in the determination ofthe adsorption mass.

2.2. Chemicals and analysis

A turbidity meter was used to determine the turbidityand the standard turbidity solution with the pH 7.0 wasprepared following the APHA standard method[21]. In thestandard turbidity solution, a colloid solution of hydrazinesulphate and hexamethy-lenetetramine, which possessedthe advantages of stable, easy preparation and the uniformdistribution of particles size within the size range of colloidcomparing with P-lactoglobulin[22] and polystyrenelatex [23], was selected as the test absorbate in thiswork.

2.3. Characterization of colloid adsorption

In chemical and environmental engineering, Freundlichisothermal adsorption equation[24] had the form ofqe =X/M = aC

1/ne , whereqe was the adsorption capacity,X the

weight of solute being absorbed (adsorbate),M the weight ofabsorbent,Ce the equilibrium concentration of adsorbate,aand 1/nwere constants dependent on several environment fac-tors. Thus, the adsorption curve could be plotted by ln(X/M)v entw sure.I y them Fre-uw ea

2(

orp-t

aramount importance of understanding the relative opng parameters that govern colloidal fouling of membrahe aim of this study is specified to: (1) determine and eate the influence of filtration modes (drag forces andashing) on the colloid adsorption in a SMBR; (2) exploreossibility of using Freundlich isothermal adsorption eq

ion and turbidity standard solution to characterize the codsorption on the membrane.

. Materials and methods

.1. Experimental apparatus

A schematic diagram of experimental apparatus usehis study was illustrated inFig. 1. The aerobic reactor hahe working volume of 5 L, in which a membrane modas submerged with the effective area of 0.5 m2. Outside an

nside diameters of the polypropylene (pp) hollow fibers w30 and 360�m, respectively. The pore size in the pp fibas 0.1�m. The membrane module was installed direver the diffuser through which air was supplied in orhat the accumulation of foulants on membrane surface ce prevented or weakened by a shearing stress genera

he up-lifting stream of bubbling air. The feed was pumrom the storage tank to the bioreactor. A water level seontrolled the solution level in the bioreactor. A suctionacuum pump were alternatively used, which dependedhe experimental requirement. In order to ensure the accnd repeatability of experiments, a fresh membrane mo

y

ersus lnCe. However, in the case of MBR, the absorbas the membrane, which weight was very hard to mea

t was necessary to replace the membrane weight M bembrane area A in Freundlich equation. Therefore,ndlich equation could be modified to:qe = X/A = aC

1/ne ,

hereX/A was the weight of absorbate (X) per membranrea (A).

.4. Determination procedure of adsorption capacityX/A)

The operation procedure for the determination of adsion capacity (X/A) was as follows:

122 L. Ma et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 264 (2005) 120–125

(1) Prepare 4 or 5 standard turbidity solutions with turbiditybetween 50 and 400 NTU, then take 5 L of the smallestturbidity solution into the SMBR reactor, strongly mixsolution for 5 min with bubbling air, sample and analysisthe initial turbidity (Z0).

(2) Turn on the control system of influent and effluent,adjust the effluent flow rate to 50–60 mL/min, set upthe operation procedure of effluent discharging in thetime sequence of 10 min dragging and 5 min rest[14]for the modes of suction pump and vacuum pump drag-ging, 10 min dragging and 5 min air back washing for themodes of vacuum pump dragging with air back washing,sample and analysis the turbidity of test solution at reg-ular time intervals until the turbidity didn’t change andreached the equilibrium (Ze).

(3) Calculate the adsorption capacityX/A, whereA wasthe membrane area,X the weight of absorbate,X= (C0 −Ce)V,V the solution volume,C0 andCe the ini-tial concentration and equilibrium concentration, respec-tively. One unit of turbidity was equivalent to the con-centration of 13.75 mg/L.

(4) Follow the procedures (2)–(4), repeat to determine andcalculate the adsorption capacity (X/A) of other test solu-tions in the order of small to large initial turbidity (Z0).

(5) Plot adsorption curve with 4 or 5 pairs of ln(X/A) andlnC which determined at various initial turbidity, then

on

2

thiss (2)1 inv s ar test)w

3

3e

, thea ne

Fig. 2. Adsorption curve for vacuum pump dragging with air back washing.

during each filtration mode (suction pump dragging, vacuumpump dragging, vacuum pump dragging with air back wash-ing, and without pump dragging) was measured with fivedifferent initial turbidity solutions of colloids.

Table 1listed the typical experimental data for the deter-mination of the adsorption capacity for the filtration modeof vacuum pump dragging with air back washing in thisstudy.

Fig. 2was the adsorption curve for the filtration mode ofvacuum pump dragging with air back washing plotted withthe data inTable 1. As Fig. 2 shows, there was a good lin-ear relationship between lnCe and ln(X/A), and it was in agood agreement with the characteristic of the modified Fre-undilich isothermal adsorption equationqe = X/A = aC

1/ne .

According to the slope of 1.1623 and the intercept of−0.6552in the adsorption curve, an adsorption capacity equation,X/A = 0.519C1.1623

e , was derived.Following the same procedure as described above, the

adsorption curves for the filtration modes of without pumpdragging, suction pump dragging and vacuum pump draggingwere drawn inFigs. 3–5, respectively.

Similarly, Figs. 3–5indicated that the adsorption curvesfor all of four discharge modes had the good linear relation-ship between lnCe and ln(X/A), and were in good agreementwith the characteristic of the modified Freundilich isother-m 1/n ev , thec acityb modew

TD s and m ashing

Z

12 884 82

E timet=

ecalculate a and 1/n in the Freundlich adsorption equatiaccording to the slope and intercept of the curve.

.5. Design of filtration modes

Three different filtration modes were arranged intudy: (1) 10 min suction pump dragging with 5 min rest;0 min vacuum pump dragging with 5 min rest; (3) 10 macuum pump dragging with 5 min air back washing. Aeference, a mode without pump dragging force (blankas also performed in this experiment.

. Results and discussion

.1. Determination of adsorption capacity, curve andquation between colloids and the membrane

Following the operation procedure described abovedsorption capacity (X/A) between colloids and membra

able 1ata for the determination of the adsorption capacity between colloid

0 (NTU) C0 (g/L) Ze (NTU) Ce (g/L)

55.9 0.77 54.2 0.7480 1.10 78.7 1.0806 1.46 103.5 1.4228.5 3.14 222.6 3.0695 6.81 476.5 6.55

xperimental conditions: temperatureT= 12◦C, volumeV= 5 L, operating

al adsorption equationqe = X/A = aCe . Based on thalues of slope and intercept in four adsorption curvesalculated adsorption equations and the adsorption capetween colloids and membrane for each dischargeere summarized inTable 2.

embrane under the mode of vacuum pump dragging with air back w

X (g) X/A (g/m−2) lnCe ln(X/A)

0.15 0.3 −0.301 −1.2040.30 0.60 −0.077 −0.5110.50 1.00 −0.351 00.90 1.80 1.118 0.52.20 4.40 1.879 1.4

4 h.

L. Ma et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 264 (2005) 120–125 123

Fig. 3. Adsorption curve for without pump dragging.

Additionally, the relationship of the adsorption capacity(X/A) and membrane flux (J) was also determined. As illus-trated inFig. 6, X/A varied in direct proportion toJ. WhenJ increased from 70 to 200 mL/min, the correspondingX/Araised from 1.16 to 3.88.

3.2. Influence of pump dragging force on the colloidadsorption

If the equilibrium concentrationCe in all adsorption equa-tions was 1.375 g/L (100NTU), as the data shown inTable 2,the adsorption capacity between colloids and membranefor four filtration modes was 0.749 g/m2 for without pumpdragging, 0.751 g/m2 for vacuum pump dragging with air

Fig. 5. Adsorption curve for vacuum pump dragging.

back washing, 9.358 g/m2 for vacuum pump dragging and3.613 g/m2 for suction pump dragging, respectively. In otherwords, the adsorption capacities in the modes of vacuumpump dragging with air back washing, vacuum pump drag-ging and suction pump dragging were 1.003, 12.49 and 4.82times higher than that of without pump dragging (blanktest). Obviously, the pump dragging force applied to themembrane greatly increased the adsorption capacity of col-loids on the membrane, or extremely speeded up the col-loid adsorption and membrane fouling, although the strengthof air aeration was the same for each tested mode in thisstudy.

Meanwhile, the result inFig. 6 also showed that theadsorption capacityX/Abetween colloids and membrane var-

TA in a S

D Vacair b

A 0.51C 0.7

Fig. 4. Adsorption curve for suction pump dragging.

able 2dsorption equations and capacities between colloids and membrane

ragging modes Without pumpdragging

dsorption equationX/A = aC1/ne 0.517C1.1645

eapacityX/A atCe = 1.375 g/L (g/m2) 0.749

Fig. 6. Relationship ofX/A andJ.

MBR atCe = 1.375 g/L for four different filtration modes

uum pump withack washing

Suction pumpdragging

Vacuum pumpdragging

9C1.1623e 2.590C1.0449

e 7.416C0.7305e

51 3.613 9.358

124 L. Ma et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 264 (2005) 120–125

ied in direct proportion to filtration fluxJ. It was very clearthat, the larger the fluxJwas, the bigger the pump draggingforce applied to the membrane in a SMBR would be.

3.3. Influence of frequent back washing on the colloidadsorption

Same as the microfiltration process, a periodic back wash-ing with air or water was often used to reduce the colloidadsorption and restore the flux of fouled membrane in theSMBR performance[4,17,25]. Usually, the time interval ofback washing was 30 min to several hours. Visvanathan[25]reported a filtration technique of suction pump draggingwith frequent air back washing in a MBR. This techniqueimproved the effluent flux by up to 371% or reduced the mem-brane resistance by 3.5-fold compared with the continuousoperation (without rest and back washing). A similar resultwas also achieved in this study. As data shown inTable 2,the adsorption capacity of colloids in the filtration mode of10 min vacuum pump dragging with 5 min air back wash-ing was 12.46 times less than that of 10 min vacuum pump.Similarly, it was 4.81 times than that in the suction pumpdragging. It confirmed that frequent back washing had a sig-nificant effect on the colloid adsorption, consequently on themembrane fouling.

In order to increase the efficiency of back washing, twoo asedo uump in thev ctionp ld ber hingw backw ranem peri-m ing.I elyo ge ot

3a

uump tionm and4 ctionp , thec ongt

tionc umpsd asa udy,t suc-t was

why the influence of the filtration modes of suction and vac-uum pumps dragging on the colloid adsorption was prettyclose.

3.5. Modified Freundlich isothermal adsorptionequation

Figs. 2–5illustrated that all of four adsorption curvesfollowed the modified Freundlich isothermal adsorptionequationqe = X/A = aC

1/ne , and the corresponding adsorp-

tion capacities listed inTable 2apparently and reasonablyreflected the degree of colloid adsorption. Consequently, itwas applicable to characterize the colloid adsorption on themembrane using the modified Freundlich isothermal adsorp-tion equation and turbidity standard solution. This approachpossessed advantages of fast, simple and good repeatable inthe characterization of colloid adsorption. Except the char-acterization of colloid adsorption, this approach also couldbe used for the optimization of operational conditions, theselection of membrane materials and the evaluation of mod-ule configuration.

4. Conclusions

low-i

( flu-odeair

rag-col-pac-

the

( Fre-

oid

A

ci-e

R

ro-actor,

actorstew-iron.

perational conditions had been improved in this study bn Visvanathanand’s experimental designs. Firstly, a vacump replaced a suction pump. Since the residual wateracuum pump pipelines was much less than that in the suump, the resistance of back washing in pipelines coueduced. Another benefit from vacuum pump back wasas that the effluent loss, the shortage of suction pumpashing, decreased significantly. Secondly, two membodules were alternatively used in Visvanathanand’s exent, that is, one for filtration and another for back wash

n this study, filtration and back washing were alternativperated in the same membrane module. The advanta

his improvement appeared clear.

.4. Influence of filtration modes on the colloiddsorption

Based on the analysis above, the filtration mode of vacump dragging with air back washing was the best filtraode. The adsorption capacity in this mode was 12.46.81 times less than those in modes of vacuum and suumps dragging without back washing. In other wordsolloid mass adsorbed in this mode was the smallest amhree tested filtration modes.

However, there was no big difference in the adsorpapacity between the modes of suction and vacuum pragging without back washing. Since the filtration flux wrranged to keep constant for all filtration modes in this st

he dragging force applied to membrane no matter byion pump or vacuum pump should be also similar. This

f

Based on the results obtained from this study, the folng conclusions could be drawn:

1) In a SMBR, the filtration mode had a significant inence on the colloid adsorption. Compared with the mof suction pump or vacuum pump dragging withoutback washing, the mode of 10 min vacuum pump dging with 5 min air back washing could reduce theloid adsorption (characterized with the adsorption caity between colloids and membrane), consequentlymembrane fouling.

2) Experiment results demonstrated the modifiedundlich isothermal adsorption equation,qe = X/A =aC

1/ne , was applicable to characterize of the coll

adsorption on the membrane in the SMBR.

cknowledgements

The authors would like to acknowledge Ministry of Snce and Technology, PR China for financial support.

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