Journal of Membrane Science 227 (2003) 81–93

Critical flux determination by the flux-step method ina submerged membrane bioreactor

Pierre Le Clecha, Bruce Jeffersona, In Soung Changb, Simon J. Judda,∗a School of Water Sciences, Cranfield University, SIMS, Building 61, Cranfield MK43 OAL, Bedfordshire, UKb Department of Environmental Engineering, Hoseo University, Asan City, Chungnam 336-795, South Korea

Received 28 October 2002; received in revised form 10 July 2003; accepted 16 July 2003

Abstract

A standard flux-step method has been developed for assessing fouling in a membrane bioreactor operating at constantflux. Three keys parameters based on transmembrane pressure (TMP) were derived to depict fouling behaviour and identifythe onset of fouling at the so-called critical flux. A zero rate of TMP increase (dP/dt) was never attained during the trial,such that no critical flux, in its strictest definition, could be defined in this study for a submerged MBR challenged with realand synthetic sewage. The three TMP-based parameters all indicated roughly the same flux value at which fouling startedto become significant (the weak form of critical flux), this being 10 and 18 l m−2 h−1 when the MBR was fed with syntheticand real sewage, respectively. Long-term filtration trials featured much lower dP/dt values compared to those obtained duringshort-term fouling test, but sub-critical flux operation led to a catastrophic increase in fouling rate after some critical timeperiod apparently dependent on feedwater matrix and flux. It was concluded that flux-step determination of the critical fluxcannot be used to predict long-term TMP behaviour in real MBR systems, but nonetheless provides useful data on comparativefouling propensity.© 2003 Elsevier B.V. All rights reserved.

Keywords: Submerged membrane bioreactor; Critical flux; Flux-step method; Fouling; Sewage

1. Introduction

Membrane bioreactors (MBRs) are becoming anestablished process option for wastewater treatment.The combination of membrane filtration with a sus-pended growth bioreactor is now widely used for mu-nicipal and industrial waste treatment[1–3]. AlthoughMBRs offer many advantages over conventional pro-cesses, such as small footprint and better effluentquality, membrane fouling remains a major drawback.

∗ Corresponding author. Tel.:+44-1234-754842;fax: +44-1234-751671.E-mail address: s.j.judd@cranfield.ac.uk (S.J. Judd).

Fouling leads to a significant increase in hy-draulic resistance, manifested as permeate flux de-cline or transmembrane pressure (TMP) increasewhen the process is operated under constant-TMP orconstant-flux conditions, respectively. Different foul-ing mechanisms, such as macromolecule adsorption,pore plugging, or cake build-up, can take place atthe membrane surface. For the filtration of monodis-perse particles filtration fouling phenomena are rel-atively well understood, and can be predicted fromfirst-principles[4] and semi-empirical models[5,6].However, the particle matrix within the MBR liquor islike that of activated sludge and, as such, is composedof biological flocs formed by a large range of living

0376-7388/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.memsci.2003.07.021

82 P. Le Clech et al. / Journal of Membrane Science 227 (2003) 81–93

micro-organisms along with soluble and colloidalcompounds. Additionally, bulk biomass physiologi-cal characteristics, such as concentrations of mixedliquor suspended solids (MLSS) and extra-polymersubstances (EPS), change according to the operatingconditions. As a result, fouling in MBR processes isvery unpredictable and difficult to control.

Key to the identification of appropriate operatingconditions is the so-called “critical flux” (Jc), a con-cept originally presented by Field et al.[7]. Theseauthors stated that: “The critical flux hypothesis formicrofiltration is that on start-up there exists a fluxbelow which a decline of flux with time does not oc-cur; above it, fouling is observed”. Two distinct formsof the concept have been defined. In the strong form,the flux obtained during sub-critical flux is equated tothe clean water flux obtained under the same condi-tions. However, clean water fluxes are rarely attainedfor most real feedwaters due to irreversible adsorptionof components not removed other than by chemicalcleaning. In the alternative weak form, the sub-criticalflux is the flux rapidly established and maintainedduring the start-up of the filtration, but does not nec-essarily equate to the clean water flux. Alternatively,stable filtration operation, i.e. constant permeability(K) for an extended period of time, has been definedas sub-critical operation even when preceded by aninitial decline in flux due to solute adsorption[8].

A number of slightly different manifestations ofsub-critical flux operation have been proposed, largelydepending on the method of determination employed(Table 1). Kwon and Vigneswaran[9] equatedJc tothe inertial lift velocity (VIL ) as defined by lateral mi-gration theory introduced by Green and Belfort[10].In this model,VIL is defined mathematically and com-

Table 1Critical flux definitions

Definition Method of determination Restriction Reference

Stable operation for long period Observation of TMP andflux behaviour

Initial flux decline not take account [8]

Transition between pressure-dependantand pressure-independent flux

Hydraulic tests (changes inTMP for different fluxes)

Short-term experiment [13]

Inertial lift velocity (VIL ) Determination ofVIL Based on theoretical model [9]No material deposition DOTM Soluble deposition not visible [11]No material deposition Mass balance Soluble deposition not visible [12]Stable operation (constant specific flux)

from the start-upFlux-step method Short-term experiment [14]

Limiting flux Stepwise increase of TMP Less fouling control [16]

pared to the permeate flux. If the latter is smaller thanVIL , particles do not deposit and fouling would not beexpected. This rigorous definition is difficult to applybecause of the relative complexity of the determinationof VIL , particularly for heterogeneous matrices, andis in any case limited to larger colloidal particles. Ex-perimental determination ofJc by direct observationthrough membrane (DOTM) of material depositiononto the membrane has been conducted using a modelof homodispersed suspensions of polystyrene latexparticles[11], and some authors have also used massbalance determination[12]. Neither of these methodscan take account of either deposition or adsorptionof soluble compounds or transient changes in cakeproperties at zero deposition, for example, due to cakecompaction and migration of fines through the cake[13]. As result, none of these determination techniquescould be practically applied to real MBR systems.

Given the limitations of applying particle hydro-dynamics to the identification ofJc in real systemswhere both solute adsorption and transient cake be-haviour can be important, recourse generally has to bemade to experimental determination. By plotting fluxagainst TMP it is possible to observe the transition be-tween constant and non-constant permeability at theonset of fouling. The flux at this transition has beentermed “secondary critical flux”[13], but is also de-fined as the weak form ofJc. Recent studies suggestthis transition to mark the boundary between foulingby EPS and sludge solids[14]. Whilst potentially use-ful in providing a guide value for the appropriate op-erating flux, the absolute value ofJc obtained by thismethod is likely to be dependent on the exact methodemployed and, specifically, the rate at which the fluxis varied with time[9].

P. Le Clech et al. / Journal of Membrane Science 227 (2003) 81–93 83

A common practice is to incrementally increase theflux for a fixed duration for each increment, giving astable TMP at low flux but an ever-increasing rate ofTMP increase at fluxes beyondJc [13,15–17]. Thismethod is preferred over the TMP-step method sinceit provides better control of the flow of material depo-sition on the membrane surface, as the convective flowof solute towards the membrane is constant duringthe run[16]. However, no single and precise agreedprotocol exists for critical flux measurement, mak-ing comparison of reported data difficult. Variablesinclude step duration, step height, initial state of themembrane (new/backwashed), feed characteristics andsystem hydraulics. The aim of the paper is to achievea better understanding of the impact of these differentparameters used in the determination method on theresult of the test, and to compare data derived fromflux-step analysis with that from prolonged operation.

2. Materials and methods

2.1. Membrane and bioreactor

The pilot scale rig (Fig. 1) comprised a 40 l biore-actor (0.275 m diameter base by 1.7 m wetted height)with a vertical-mounted submerged tubular membranemodule (Millenniumpore Limited, Sunderland)[18]operating under conditions of constant cross flow ofair (Table 2). Constant-flux filtration was carried out

PermeateFR

Air

PT

FeedConcentrat

DosingPump

FRPT

FeedConcentrate

Water

Pressure transducer

Liquid flow meter

Fig. 1. Schematic of the submerged MBR fed with synthetic sewage.

Table 2Membrane and operating characteristics of the submerged MBR

Module total length (m) 0.9Membrane working length (m) 0.6Lumen internal diameter (mm) 8.5Pore size (�m) 0.2Number of lumens 12Total membrane area (m2) 0.19Initial membrane permeability (l m−2 h−1 bar−1) 1030Cross section area (mm2) 550Bioreactor air-flow (l min−1) 4Module air-flow (l min−1) 6Module air loading (m s−1) 0.2

using a peristaltic pump on the permeate stream. Thepermeation rate was incrementally increased and thepressure change continuously monitored by means ofpressure transducer (model 68001-04, Cole Parmer)situated in the permeate line, allowing accurate mea-surements to be recorded on a computer (Pico datalogging system, Pico Technology). Under the experi-mental conditions used, the pressure of the retentatewas constant and the TMP assumed to vary only withchanges in permeate pressure due to fouling. TMP wastaken to equate to the difference between the aver-aged hydraulic head at the mid-point of the membranemodule and the permeate pressure.

Biological treatment was performed under aero-bic conditions with compressed air supplied to thesludge through a porous dome diffuser at the base ofthe tank. An additional diffuser provided air to each

84 P. Le Clech et al. / Journal of Membrane Science 227 (2003) 81–93

individual membrane lumen, allowing independentcontrol of aeration of the membrane surface and thebioreactor[19]. The hydraulic retention time (HRT)ranged from 16 to 24 h. No wasting of biomass tookplace except for sampling, thereby giving an infinitesolids retention time (SRT). The MLSS concentrationstabilised at 3 g l−1 for both real and synthetic sewagetests, the level being limited by the low strength ofthe feedstock and the relatively high HRT applied.

2.2. Feed solutions and analysis

The bioreactor was fed with a synthetic sewage,using a recipe adapted from the Organisation for Eco-nomic Cooperation and Development (OECD) guide-lines (Table 3), and employed previously[20,21],which yielded a mean chemical oxygen demand(COD) of 460 mg l−1 and total nitrogen (TN) of44 mg l−1. EPS protein and carbohydrate levels (EPSpand EPSc, respectively) were measured in samplesextracted from the sludge by the heating extractionmethod adapted from Zhang et al.[22]. Followingan acclimatisation period of a few weeks, the exper-iments were repeated for a bioreactor fed with realsewage (Table 4), data from which are reported inSections 3.3–3.4.

Fig. 2. Schematic representation of the critical flux determination by the flux-step method.

Table 3Synthetic sewage recipe

Constituent Concentration (mg l−1)

Peptone 0.20Meat extract 0.14Urea 0.01CaCl2·2H2O 0.004MgSO4·7H2O 0.002K2HPO4 0.011NaCl 0.007

Table 4Selected feed and biomass characteristics

Synthetic Real

COD (mg l−1) 460 139TN (mg l−1) 44 31EPSp (mg g−1 SS) 73 60EPSc (mg g−1 SS) 30 17

2.3. Cleaning protocol

The membrane module was systematically cleanedprior to each critical flux test. For experiments re-ported inSection 3.2, ex situ chemical cleaning of themembranes was carried out by soaking in 0.5 wt.%hypochlorite at 50◦C for 20 h. A negative TMP of

P. Le Clech et al. / Journal of Membrane Science 227 (2003) 81–93 85

50–75 mbar applied for 5 min was adopted as thestandard backwash protocol for tests reported inSection 3.3. Under these conditions, the volume ofpermeate used to backwash the membrane was around300 ml.

2.4. Data processing

For each flux-step, two TMP values are reported:the initial TMP (TMPi), defined as the TMP ob-tained after the initial sudden increase in filtrationresistance following the step increase in flux, andthe final TMP (TMPf ), defined as the TMP at theend of the step (Fig. 2). Practically, the TMPi wastaken arbitrary as the TMP value 30 s after thebeginning of the flux-step. From these two TMPvalues, parameters pertaining to fouling can thenbe defined, these being the initial TMP increase(�P0 = TMPn

i − TMPn−1f ), the rate of TMP increase

(dP/dt = (TMPnf −TMPn

i )/(tnf − tni )) and the average

TMP (Pave = (TMPnf + TMPn

i )/2) (Fig. 2).

3. Results

3.1. Fouling reversibility

To assess the stability and reversibility of foulingduring Jc determination with the MBR fed with syn-

0

100

200

300

0 1 2 3 4 5Time (h)

TM

P (m

bar)

0

5

10

15

20

25

Flux

(L

.m-2

.h-1

)

Fig. 3. Critical flux determination, synthetic sewage.

thetic sewage, flux-step experiments were carried out.A step height of 2 l m−2 h−1 and 15 min duration werearbitrary chosen (Fig. 3). According to Fig. 3, forfluxes up to 10 l m−2 h−1 the TMP appears constant,while its rate of increase becomes significant at higherfluxes due to fouling. However, close observation ofthe initial flux-steps (Fig. 2) indicates the TMP toincrease even at the lowest flux of 2 l m−2 h−1, from2.2 to 2.6 mbar over the 15 min flux-step period.

During this cycle test it was observed that the TMPvalues obtained during the descending phase weregreater than the corresponding values recorded dur-ing the ascending phase. For example, at the initialflux-step of 2 l m−2 h−1, Pave was 2.4 and 8 mbar forthe ascending and descending phases, respectively(Fig. 3). When consecutive cycles were carried out,without intermediate cleaning (i.e. neither backwashnor chemical cleaning), critical hydraulic performanceappeared to change little following the first cycle withTMP values remaining around 8 mbar for the lowestflux value of 2 l m−2 h−1 for both the second and thirdconsecutive cycles. These observations indicate theformation of an initial irreversible fouling layer afterthe first flux-step cycle, on which some reversiblefouling forms.

A clear difference can be observed in the foulingrate surrogate term dP/dt between virgin and non-virgin membranes (Fig. 4). Although dP/dt obtainedfor the three states of membrane followed the same

86 P. Le Clech et al. / Journal of Membrane Science 227 (2003) 81–93

0.001

0.01

0.1

1

10

0 5 10 15 20 25

Flux (L.m-2.h-1)

dP/d

t (m

bar.

min

-1)

Backwashed

Chemically cleanedNew

Fig. 4. Effect of membrane state on fouling rate, synthetic sewage.

exponential trend, the dP/dt value recorded for thevirgin membrane material was around 10 times lowerthan that of the backwashed and chemically-cleanedmaterials. During the tests on synthetic sewage,dP/dt was never zero—even with virgin membranes.Although the chemical clean employed presumablyremoves more material than a hydraulic backwash,similar results were obtained from both cleaningprocedures.

3.2. Step length and height

For an MBR fed with synthetic sewage, dP/dt versusflux curves obtained for a 5, 30 and 120 min step du-

0

10

20

30

40

50

60

0 5 10 15 20 25 30

Flux (L.m-2.h-1)

∆P0

(mba

r)

120min30min15min10min5min

Fig. 5. Effect of step duration on�P0 for synthetic sewage.

ration were similar to those recorded for a 15 min steplength for fluxes below 15 l m−2 h−1. However, in-creasing the step length to 120 min produced a signifi-cant increase in�P0 at flux values above 15 l m−2 h−1

(Fig. 5). For example, at 24 l m−2 h−1, �P0 increasedto 51 mbar at a 120 min step duration, compared toonly 15 mbar a 30 min step.

The effect of step height was also assessed byconductingJc determinations at step heights of 3, 6and 9 l m−2 h−1 in consecutive experiments. Resultsshowed dP/dt to increase with increasing step height.At the final flux of 25 l m−2 h−1, the fouling ratewas 2.0, 3.5 and 4.1 mbar min−1 for step heights of3, 6 and 9 l m−2 h−1, respectively. At flux-steps of

P. Le Clech et al. / Journal of Membrane Science 227 (2003) 81–93 87

0.001

0.01

0.1

1

10

0 10 20 30 40

Flux (L.m-2.h-1)

dP/d

t (m

bar.

min

-1)

Synthetic

Real

Fig. 6. Fouling rate for real and synthetic sewage as a function of flux.

3 l m−2 h−1 or less the impact on fouling rate wasinsignificant.

3.3. Synthetic versus real sewage

In a second set of experiments, the fouling propen-sity of the biomass fed with the synthetic sewage wascompared to that of real sewage with flux-step dura-tions of 15 min and step heights of 2 and 3 l m−2 h−1

for synthetic and real sewage matrices, respectively.The membrane was backwashed with permeate beforeeach test and each series of tests was replicated fivetimes.

0

10

20

30

40

50

60

0 10 20 30 40

Flux (L.m-2.h-1)

∆P0

(mba

r)

Synthetic

Real

Fig. 7. �P0 of real and synthetic sewage as a function of flux.

From TMP raw data, the value of theJc for the syn-thetic sewage can be determined from visual observa-tion to be 10 l m−2 h−1, although closer examinationof the initial flux-steps reveals that the TMP is neverabsolutely constant at any point during the test. Sim-ilar observations could be made for the real sewagewhere the critical flux in its weak form is around10 l m−2 h−1.

Fig. 6reveals the similar relationship between dP/dtand low flux values (up to 12 l m−2 h−1) obtained forthe two types of sewage used,. Because of the large ex-perimental error obtained for low fluxes, it is possibleto consider dP/dt to be constant, though nonetheless

88 P. Le Clech et al. / Journal of Membrane Science 227 (2003) 81–93

0

10

20

30

40

0 100 200 300 400 500Pave (mbar)

Flux

(L

.m-2

.h-1

)

Real

Synthetic

Clean water permeability

Fig. 8. Permeability graph for tests carried out with synthetic and real sewage.

non-zero, up to a certain flux value—this being 10 and19 l m−2 h−1 for synthetic and real sewage, respec-tively. For higher fluxes, an exponential relationshipbetween dP/dt and the flux could be discerned and thecomparable fouling rates, i.e. dP/dt values at the sameflux, for real sewage were observed to be much lowerthan those measured for the synthetic matrix.

Trends in the instantaneous TMP increase (Fig. 7)show �P0 to be stable at the lower fluxes em-ployed, ranging from 1.6 to 3 mbar for the syntheticsewage (with an experimental error of around 20%).However, for the synthetic sewage at fluxes above12 l m−2 h−1, �P0 increases with increasing flux,reaching 21.5 mbar at a flux of 22 l m−2 h−1. For

Fig. 9. TMP transients for longer-term trials.

real sewage,�P0 is unchanged at 1.7 ± 0.4 mbar forfluxes up to 19 l m−2 h−1, and then increases expo-nentially to reach 40 mbar for the highest flux tested(41 l m−2 h−1). Flux versusPave is similarly linear(and the permeabilityK thus constant) for flux val-ues up to 12 l m−2 h−1 for the synthetic sewage, andthen rapidly decreases to reach 132 l m−2 h−1 bar−1

for the highest flux tested (Fig. 8). For real sewage,the linear region extends to a flux of 19 l m−2 h−1,beyond whichK decreases in much the same way aswith the synthetic sewage. Finally, it can be observedthat the clean water permeability of the membranewas always found to be higher than the permeabilityvalues observed for both types of sewage, indicating

P. Le Clech et al. / Journal of Membrane Science 227 (2003) 81–93 89

the non-existence of theJc in its strict form underthese operating conditions.

3.4. Long-term experiments

Long-term experiments have been carried out forboth synthetic and real sewage matrices at 7 and9–18 l m−2 h−1, respectively (Fig. 9). In the case ofthe synthetic sewage, TMP increased only very slowlyfor the first 4 days (from 11 to 17 mbar, correspondingto dP/dt = 0.001 mbar min−1) and then exponentiallyup to 176 mbar at around day 8. The greater perme-ability the membrane displayed for this trial comparedwith previous runs may be indicative either of poorlydefined membrane rejection characteristics, variabil-ity in the biomass fouling propensity or simply dif-ferences in the cleaning efficacy. For the real sewageoperating at the lower flux of 9 l m−2 h−1, the TMPincreased only marginally for the first 10 days, from14 to 22 mbar (dP/dt = 7.10−4 mbar min−1). There-after, the fouling rate accelerated to produce a TMPvalue of 54 mbar by day 12. When the higher flux of18 l m−2 h−1 was imposed, an exponential trend re-sulted with no apparent lag time: within the first 2 daysof the experiment, TMP increased from 40 to 90 mbar,and rose to 540 mbar by day 4. Clearly, the syntheticsewage has a significant fouling propensity even at thelower, supposedly sub-critical, flux value—and cer-tainly greater than that of the real sewage. Finally, it isnoticeable that much lower dP/dt values were obtainedduring the early stage of these long-term experimentscompared to those recorded for the short-term foul-ing experiments (Fig. 6). For example, during theJcdetermination the dP/dt calculated at 9 l m−2 h−1 was0.1 mbar min−1 for the real sewage, while the dP/dtwas only 7× 10−4 mbar min−1 for the first 10 daysof operation during the long-term trial with the MBRfed with the same feed and operated at the same flux.

4. Discussion

4.1. Critical flux determination with the flux-stepmethod

The three parameters calculated from the TMP rawdata presented in the current study appear to be goodindicators of membrane fouling. Results reveal that

Table 5Critical flux values (in l m−2 h−1) under a number of criteria

Syntheticsewage

Realsewage

Visual observation of the raw data 10 10dP/dt = 0a <2 <2dP/dt < 0.1 mbar min−1 10 15dP/dt (Fig. 6) 10 19�P0 (Fig. 7) 12 19Pave (Fig. 8) 12 19�(dP/dt)/�J (Fig. 10) 10 19

a According to the strict form of critical flux.

the three TMP-based parameters indicate the similarflux value at which fouling starts to become signifi-cant (Table 5), and thus the weak form of critical flux.

The constant value of�P0 observed for low fluxesincorporates the equivalent increase that would oc-cur under clean water conditions, and this value (i.e.�P0,mem) is directly related to the membrane resis-tance. Rapid fouling due to rapid (and mostly re-versible) accumulation at the membrane surface atthe start of the flux-step, reflected in the�P0 value,takes place only at higher fluxes leading to an ex-ponential relationship between�P0 and flux abovesomeJc value. Even though, since�P0 > �P0,mem,some fouling occurs when�P0 is constant, the fluxfor which �P0 starts to increase could be consid-ered as the critical flux, according to the weak formdefinition.

According to the strict definition of the critical flux,sub-critical operation is characterised by dP/dt = 0,which was never achieved during these trials. This iscontrary to a number of publications[23,24] in whichstable operation (or constantK) has been reported. Al-though dP/dt values observed at low fluxes could beconsidered to be insignificant, they revealed, like the�P0 parameter, the marginal fouling occurring un-der so-called sub-critical conditions[14]. A Jc valuecould be defined from an arbitrary dP/dt limit (like0.1 mbar min−1, for example, as given inTable 5), orfrom the calculation of�(dP/dt)/�J (second momentof TMP with respect to flux, or rate of change of foul-ing resistance rate), where a clear discontinuity in thetrend can be observed (Fig. 10).

Plots of flux versus TMP are often used to de-termine membrane permeability for a specific ap-plication. However, the precise identification of flux

90 P. Le Clech et al. / Journal of Membrane Science 227 (2003) 81–93

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40Flux (L.m-2.h-1)

∆(dP

/dt)

/ ∆J

Synthetic

Real

Fig. 10. The second moment of TMP with respect with flux (�(dP/dt)/�J) vs. flux.

and TMP, and the exact method used for their de-termination is rarely reported. Specifically, data mayrefer to steady-state or unsteady state. By using theflux-step method as a standard technique to obtainPave, the system permeability is precisely identifiedfor specific unsteady state condition. Finally, anotherJc value can be defined from the flux versusPavecurve as the transition between the TMP-dependentand TMP-independent flux.

The main parameter impacting on dP/dt was foundto be the flux-step height. The phenomenon observedcould be explained by the nature of the materialdeposit, varying with the way the flux is increased.A previous study with colloidal silica[15] has re-vealed mechanistic information pertaining to theflux-step. Gradually increasing the flux was reportedas allowing a small and ordered fouling layer to slowlydevelop, leading to a smaller dP/dt value. On the otherhand, fouling layers formed more quickly by a greaterflux increase were observed to be more disordered instructure and had a greater hydraulic resistance. Theseresults illustrate the importance of filtration start-up,and especially the way in which the flux is imposed onthe system to allow the membrane–biomass interfaceto adjust to the hydraulic conditions. The importanceof the start-up strategy was also revealed by the effectof step duration on�P0. Because of the higher�P0observed at larger step durations, a compromise be-tween step length and height is required to limit theoverall TMP increase produced.

4.2. Long-term experiments

Since each flux-step was maintained only for amaximum period of 120 min, the flux-step methodfor the determination of the critical flux as employedin the current study cannot be used to predict abso-lute values of fouling rate for longer-term operation.It has been clearly shown that an MBR system fedwith real sewage with aJc of around 18 l m−2 h−1

could not sustain stable operation at this flux formore than around 2 days. Even with the lower per-meate flux, fouling rates, as represented by dP/dt,tend to decrease with time over the longer term andmay be 10–100 times lower than those measured forthe short-term flux-step trials. Moreover, during thetime period of the experiment only components thatrapidly and apparently reversibly foul the membraneare likely to be accounted for in the TMP increaseobserved. Irreversible foulants may be assumed to bepresent at low concentrations and to contribute littleto the measured short-term TMP increase[25].

Although it is claimed that operating MBRs at a suf-ficiently low flux allow extended runs without mem-brane cleaning[24], TMP behaviour observed duringlong-term experiments suggest that no such sustainedoperation is possible indefinitely. The same observa-tion has already been reported by Ahn et al.[2] andOgnier et al.[26]. The latter group attempted to modelthe phenomenon, assuming the flux to decrease in pro-portion to the deposition of foulant material. Such a

P. Le Clech et al. / Journal of Membrane Science 227 (2003) 81–93 91

model predicts the observed exponential increase inTMP at extended lengths of time but fails to take intoaccount the period of pseudo steady-state operation.So far, no models are available for the prediction of theduration of this steady-state critical time period. How-ever, if, for the purposes of the current study, this crit-ical time is arbitrarily defined as the point of the TMPcurve where the correlation factor for an exponentialtrend is more than 95%, the critical time is 2.5 daysfor the test with synthetic sewage at 7 l m−2 h−1, and0 and 8.5 days for real sewage at 18 and 9 l m−2 h−1,respectively (Fig. 9). These results could be comparedwith the critical time period of around 23 days ob-tained by Ognier et al.[26], operating at a flux of10 l m−2 h−1 with a synthetic sewage (2 g MLSS l−1)on a side-stream MBR.

4.3. Fouling propensity of biomass fed by differenttypes of sewage

The direct comparison of the MBR fed with twotypes of sewage showed the MBR feed characteris-tics to play an important part in determining foulingpropensity. The higher COD and TN concentrationsmeasured for the synthetic feed may contribute to thelarger fouling propensity observed in that trial. Morespecifically, the EPS level has been identified as be-ing primarily responsible for fouling in MBRs[20],representing up to 90% of the total filtration resis-tance[27], whilst solutes in general appear to providebetween 5 and 52% of the membrane fouling in anMBR [28]. The significantly lower proteinaceous EPSlevel measured for real sewage may account for thelower fouling rate recorded for this matrix (Table 4),though clearly more work is required to verify thisand ascertain the relative significance of the variousphysical and chemical fractions.

5. Conclusions

From results presented here, it is apparent thatmembrane fouling in an MBR takes place even atlow flux rates, but changes dramatically when theso-called critical flux (in its weak form) is reached.Visual determination of critical flux as the maximumflux for which the TMP remains stable from theflux-step method is insufficiently rigorous. A closer

examination of the TMP behaviour is required tomore precisely define the critical flux value. Specifichydraulic parameters calculated from the flux-steptest, these being�P0, dP/dt, K and Pave, yield nearidentical critical flux values (Table 5). Critical fluxanalysis based on specific values for flux-step andduration and interpreted with reference to thesepressure-related parameters would permit comparisonof fouling behaviour between different studies.

The dP/dt values measured for long-term exper-iments are always lower than the equivalent valuesmeasured for the short-term flux-step experiments.The critical flux value determined by the flux-stepmethod thus indicates the point at which foulingstarts to become severe, but does not yield predictiveabsolute permeability data for extended operation.

Finally, the benefits of gradually imposing the de-sign flux during the incipient stages of operation havebeen demonstrated. For start-up of the process, a smallstep height coupled with a moderate step duration(15–30 min) appears to be the best option. However,the exact rate at which the flux should be imposed andfor how long is unclear, and is certainly dependenton the feedwater matrix: the synthetic sewage used inthe current study generates mixed liquors of substan-tially greater fouling propensity than those of the realsewage matrix studied (Table 5).

Acknowledgements

This work was sponsored by the Engineering andPhysical Sciences Research Council, Anglian Water,Millenniumpore Limited, Severn Trent Water, ThamesWater, United Utilities and Yorkshire Water, whosesupport the authors gratefully acknowledge.

Nomenclature

EPSc fraction of carbohydrate containedin extracted solution from thesludge (mg g−1 SS)

EPSp fraction of protein contained inextracted solution from thesludge (mg g−1 SS)

J flux (l m−2 h−1)Jc critical flux (l m−2 h−1)

92 P. Le Clech et al. / Journal of Membrane Science 227 (2003) 81–93

K membrane permeability(l m−2 h−1 bar−1)

Pave average TMP (mbar)�P0 initial TMP increase (mbar)�P0,mem initial TMP increase occurring

with clean water and newmembrane (mbar)

dP/dt rate of TMP increase(mbar min−1)

�(dP/dt)/�J second moment of TMP withrespect with flux((mbar min−1)(l m−2 h−1)−1)

TMPf TMP value at the end of theflux-step (mbar)

TMPi TMP value at the beginning ofthe flux-step, after initial TMPincrease (mbar)

VIL inertial lift velocity (m s−1)

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