industrial wastewater treatment in a membrane bioreactor: a review

Industrial Wastewater Treatment ina Membrane Bioreactor: A ReviewB. Marrot, A. Barrios-Martinez, P. Moulin, and N. RocheLaboratoire en Procedes Propres et Environnement (LPPE - UMR6181), Universite d’Aix-Marseille, IFR 112, Pole Mediterraneen desSciences de l’Environnement, CNRS Europole de l’Arbois, Batiment Laennec, BP 80, F-13545 Aix-en-Provence cedex, France;[email protected] (for correspondence)

Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ep.10001

This paper provides a detailed literature review ofwastewater treatment in a membrane bioreactor pro-cess (MBR) with special focus on industrial wastewatertreatment. MBR systems are compared with conven-tional wastewater treatment systems. The characteris-tics of the bioreactor treatment process (biomass con-centration and floc size, organic and mass loadingrates, etc.) are examined. The membrane separation ofmicroorganisms from the treated wastewater is dis-cussed in detail. Problems of membrane fouling andmembrane washing and regeneration, linked to acti-vated sludge characteristics, are examined. © 2004American Institute of Chemical Engineers Environ Prog, 23:59–68, 2004

Keywords: Membrane bioreactor, wastewater treat-ment, industrial effluents, membrane fouling, review

INTRODUCTIONA process that uses both a biological stage and a

membrane modules has recently been developed forwastewater treatment: it is called the membrane biore-actor (MBR) process. The bioreactor and membranemodule each have a specific function:

(i) biological degradation of organic pollution iscarried out in the bioreactor by adapted microorgan-isms;

(ii) separation of microorganisms from the treatedwastewater is performed by the membrane module.The membranes constitute a physical barrier for allsuspended solids and therefore enable not only recy-

cling of the activated sludge to the bioreactor but alsoproduction of a permeate free of suspended matter,bacteria, and viruses.

The use of membranes to separate solids and treatedwastewater is the main difference between MBRs andtraditional treatment plants for which the efficiency ofthe final clarification step depends mainly on the acti-vated sludge settling properties.

In this paper, we present a literature review ofwastewater treatment in membrane bioreactors with aspecial focus on industrial wastewater treatment. Wecompare the membrane bioreactor to conventionalwastewater treatment processes before discussing thecharacteristics of the bioreactor (biomass concentra-tion, floc size, etc.) and membrane (fouling, criticalflux, washing, etc.).

MBR CHARACTERISTICS AND COMPARISON WITH A CONVENTIONAL SYSTEMThere are two types of configurations for the mem-

brane array: the membranes can be placed either out-side or inside the bioreactor (Figure 1). For the externalconfiguration, the mixed liquor is filtered under pres-sure in a specific membrane module, whereas for thesubmerged configuration, the filtration is carried out inthe aeration basin by suction removal of the effluent. Inthe external system, the permeate flux generally variesbetween 50 and 120 L � h-1 � m-2 and the transmem-brane pressure (TMP) is in the range of 1 to 4 bar. In thesubmerged configuration, the permeate flux variesfrom 15 to 50 L � h-1 � m-2 and the TMP is about 0.5 bar.The submerged configuration appears to be more eco-nomical based on energy consumption [1] for two mainreasons: no recycle pump is needed since aerationgenerates a tangential liquid flow in the vicinity of themembranes, and the operating conditions are muchmilder than in an external MBR system because of thelower values of TMP and tangential velocities. Gener-

Abbreviations used: AS, activated sludge; BAP, biomass-associated product;COD, chemical oxygen demand; EPS, extracellular polymeric substances; HRT,hydraulic retention time; MBR, membrane bioreactor; PMS, produced microbialsoluble; RO, reverse osmosis; SMP, soluble microbial products; SRT, sludgeretention time; TMP, transmembrane pressure; UAP, utilization-associatedproducts; UF, ultrafiltration.

© 2004 American Institute of Chemical Engineers

Environmental Progress (Vol.23, No.1) April 2004 59

ally, hollow fiber membranes are used in submergedMBR (Table 2).

To avoid membrane fouling, it is often necessary tocarry out tangential filtration in an external MBR sys-tem, especially when concentrated effluents and/orconcentrated biomass are encountered [2]. A higherenergy cost can be justified. In such cases, it is neces-sary to control the shear stress on the biological flocs.The shear stress is greater in the external membranemodules because of the high recycle flow rate. Gener-ally, tubular membranes are used in external MBR sys-tems (Table 1).

When compared to traditional activated sludge sys-tems, the MBR offers many attractive advantages:

(i) The traditional secondary clarifier is replaced bya membrane module. This module is more compact

and the quality of rejected water is independent of thevariations of sludge settling velocity.

(ii) The MBR allows the biomass concentrations tobe higher than for traditional treatment plants. Jeffersonet al. [3] utilized a 20 g � L-1 in biomass concentrationwhile Yamamoto et al. [4] utilized 30 g � L-1, whereasconventional processes utilize biomass concentrationsless than 5 g � L-1 in order to avoid problems inherent tosettling of concentrated flocs.

With poor settling flocs avoided, biological degra-dation is more complete and treatment efficiency ishigher. Nevertheless, as Lubbecke et al. [5] report, in-creasing the biomass concentration involves a reduc-tion in the oxygen mass transfer rate depending on thetype of wastewater and reactor used. Other advantagesof this system are as follows:

Figure 1. Configurations of the membrane bioreactor: (a) external and (b) submerged.

Table 1. Examples of membrane types and membrane characteristics used in external MBR systems.

Membranegeometry Membrane characteristics Wastewater Reference

Tubular Ceramic (0.2 �m) Synthetic [37]Plate UF (20 kDa) Alcohol distillery [31]Tubular Ceramic (0.2 �m)

Zircon (0.05 �m)Food (ice cream) [54]

Tubular Alumina (0.2 �m)Zircon (0.05 �m)

Municipal [14]

Tubular Ceramic (0.1 �m) Kerasep Municipal [13]Tubular Ceramic (Al2O3-TiO2) (300 kDa) Kerasep Synthetic

Municipal[8]

Tubular Ceramic (ZrO2) (0.02 �m–300 kDa) Municipal [30]Tubular UF Zenon (75 kDa) Sanitary and industrial [2]Tubular UF (15 kDa) Synthetic (fuel oil) [55]Tubular Ceramic Kerasep Municipal [56]Tubular MF (0.1 �m) Municipal [57]Tubular UF ceramic (0.02 �m) Municipal [58]Plate UF polyacrylonitrile

Rhone-PoulencSynthetic [59]

Tubular UF-cellulose acetate-Sulfonated polyethersulfone-Hydrophobic polyethersulfone

Synthetic [24]

Tubular MF ceramic (0.2 �m) Municipal [60]

60 April 2004 Environmental Progress (Vol.23, No.1)

(i) The volume of the aeration tank can be alsoreduced since a higher concentration of biomass can bestored in the bioreactor.

(ii) The production of sludge, the disposal of whichis often difficult, is decreased by a factor of 2 to 3 [6],resulting in a reduction of the overall operating costs.

(iii) The membrane bioreactor is perfectly integratedin the industrial process because the wastewater candirectly be treated in situ, allowing water reuse andconcomitant reduction of the manufacturing costslinked to water consumption.

Unlike the conventional activated sludge system, themembrane bioreactor is characterized by a completeretention of the biomass inside the bioreactor becauseof the use of membrane separation, which controls andincreases the sludge retention time (SRT) indepen-dently from the hydraulic retention time (HRT). HighSRTs enable one to increase the sludge concentrationand the applied organic load, thereby increasing thepollutant degradation.

The specific sludge activity during organic matterdecomposition and nitrification depends on the SRT.The SRT is a significant operational factor for the bio-logical process [1]. The nitrifying activity of sludge ismaximal at a SRT of 10 days, but the organic decom-position rate decreases while the SRT increases. Huanget al. [1] have compared variations in the SRT on theperformances of a conventional bioreactor and a mem-brane bioreactor according to the SRT. Chemical oxy-gen demand (COD) removal (70–80%) occurs in theconventional bioreactor. A small reduction in COD

consumption was observed in the bioreactor with shortSRTs (5 to 10 days). In the MBR process, COD removal(90%) remains constant whatever the SRT.

Dufresne et al. [7] were the first to undertake a studyof the traditional activated sludge process (AS) and themembrane bioreactor. Their results on treatment ofpaper wastewater are shown in Figure 2.

This study confirms that the performance of MBR isbetter than that of conventional activated sludge pro-cesses, especially for COD removal and solid suspen-sion separation. The 15-day sludge retention time re-quired for the sludge in a conventional system is notoptimal for the MBR system since its optimum SRTwould be 25 days. This time span would cause opera-tional problems for a conventional activated sludgesystem. However, these results were obtained withnonconstant airflow rate, which is not without influ-ence on the system operation.

Cicek et al. [8] showed that the sludge in a MBRsystem is made up of small flocs of regular size. Theseflocs were composed of zoogleal bacteria and of asmall number of filamentous bacteria. The sludge in aconventional AS system is made up of large flocs andmany filamentous bacteria generally located inside theflocs (floc backbone). Because of the presence of thefilamentous bacteria, settling problems appear on theclarifier at the outlet of the AS process. An excessiveamount of filamentous bacteria indicates a lack of ox-ygen and/or substrate and/or nutrients (N, P). In Ciceket al.’s study [8], in which a synthetic effluent similar toa domestic effluent was used, the operating conditions

Table 2. Examples of membrane types and membrane characteristics used in submerged MBR systems.

Membranegeometry Membrane characteristics Wastewater Reference

Hollow fiber MF-polypropylene Municipal and synthetic [61]Hollow fiber MF (0.1 �m) Mitsubishi Municipal [62]Hollow fiber Zenon (0.1 �m) Refinery [7]Hollow fiber -MF polysulfone (0.2–0.4 �m)

-Zenon (0.1 �m)Municipal [15]

Flat MF polyethylene (0.4 �m)Kubota

Domestic [12]

Hollow fiberPlate

MF polyethylene (0.1 �m)MF polyolefin (0.4 �m)

Municipal [63]

Hollow fiber Hydrophilic polyethylene (0.1�m) Mitsubishi

Municipal [64]

Hollow fiber Polypropylene (0.1 �m) Synthetic Municipal [65]Hollow fiber Polyethylene (0.1 �m) Mitsubishi Municipal [26]Plate and frame Polysulphone (0.4 �m)

Polypropylene nonwoven (5 �m)and glass-filled (0.5 �m)

Domestic [6]

Plate and frameHollow fiber

Polysulphone (0.4 �m) (0.04 �m) Grey water [3]

Flat hollow fiber MF polyolefin (0.4 �m)(0.1 and 0.4 �m)

Municipal [66]

Hollow fiber Polyethylene (0.1 �m) Mitsubishi Domestic [1]Hollow fiber MF (0.1 �m) polyethylene

MitsubishiSynthetic [17]

Hollow fiber Zenon (0.1 �m) Synthetic—raw milk [41]


of both systems MBR and AS were similar except for thesludge retention time: 20 days for conventional acti-vated sludge system and 30 days for the membranebioreactor system. This study also pointed out that theelimination of the COD and dissolved organic carbon isgreater with the MBR system: 99 versus 96.6% and 94.5versus 92.7%, respectively.

Jefferson et al. [3] carried out a comparative study ofthe performances of a membrane bioreactor systemand aerated biological filters for gray water recycling(the wastewater in this study contained shampoo, oil,and soap). Once again, the membrane bioreactor sys-tem was the more efficient process in removal of totalbiochemical oxygen demand for 5 days, turbidity, andcoliforms.

Mignani et al. [9] studied the economics of using themembrane processes to treat textile effluents. Theyused the example of an Italian textile factory, whichafter having treated its wastewater with a conventionalbiological process had to decontaminate the wastewa-ter by an additional process in order to comply with thedischarge standards. In order to avoid the costs of thisadded process, which represented 59% of the totalcosts, they installed an ultrafiltration module(standardFamec-24 assembled modules) following the biologicalsystem. This unit was followed, in turn, by a reverseosmosis stage. This new process enabled them to re-duce the total treatment costs of 122,000 €/year partlyby the reuse of 350,000 m3/year of water. The treatedwastewater is recycled and used as cooling water fordyeing machines and/or as water for washing andrinsing. The return on investment of the membrane

process (270 000 €) is 2 1⁄2 years. This study shows thatthe equipment cost of a membrane process is not abarrier to its use.

Ciardelli et al. [10] studied the treatment of effluentof factories that use dyes. The treatment processesstudied were activated sludge, sand filtration, and ul-trafiltration (UF) and reverse osmosis (RO). The studycontains a technical and economic analysis of the uti-lization of both membrane separation techniques. Thewater quality at the outlet of the membrane processes ismuch better than that obtained using conventional pro-cesses; this treated effluent can be reused at all stagesof production, including the most demanding ones.Finally, in this case, the analysis of investment andoperating costs indicates the potential savings linked tothis process (total cost of 0.97 €/m3).

MEMBRANE FOULINGAs discussed previously, using a membrane biore-

actor for industrial liquid waste processing is a veryattractive technology that offers several advantagescompared to conventional treatment processes. In thepast, the major barrier to the development of this pro-cess was the cost of these membranes [11]. Since 1992these costs have been reduced by developments thatinclude the following:

(i) longer life time;(ii) cheaper replacement cost (by a factor of 15 since

1992 and a factor of 4 since 1995);(iii) reduction of the energy consumption compared

to the permeate flux obtained, by the use of gravity inthe case of the submerged MBR [12];

Figure 2. Comparative results of paper wastewater treatment in a conventional activated sludge system (AS)and in a MBR system (MBR) [7].


(iv) reduction of the product manufacturing costs(water reuse);

(v) the possibility of using more specific mem-branes.

One must now take into account the fact that thelimiting factor for further process development is mem-brane fouling resulting from the following:

(i) formation of a layer or cake on the membraneand/or the intrusion of molecules, colloids, and parti-cles in the porous structure;

(ii) preferential adsorption on the membrane sur-face. Fouling induces transmembrane flux reduction:when the flux reaches a threshold value, membranewashing becomes necessary.

Many studies have focused on the above problems[6, 13–17] and some procedures have been developedto reduce cake formation on the membrane surface.Conventional techniques for limiting membrane foul-ing are as follows:

(i) reduction of the membrane fouling by aeration inthe vicinity of membranes by filtration below the criti-cal flux, by the addition of coagulants, by high-fre-quency backpulsing, or by utilizing a high recycle ve-locity;

(ii) removal of the fouling material after formationby chemical washing (backwashing or backpulsing).

Unfortunately, the complexity of fouling is increasedby a biological activity, and the progression in this fieldof research is relatively slow [18].

Membrane fouling is influenced by the membrane’schemical nature, but also, as Ramesh Babu and Gaikar[19] emphasize, by the membrane operational param-eters. For example, the use of hollow fiber microfiltra-tion membranes introduces transmembrane pressuregradients, which have an impact on flux rates. Themagnitude of the flux depends on the design of thehollow fiber (length, internal diameter, permeability)and on the properties of the cake [20]. In the MBR, theresistance of the cake, generally composed of microor-ganisms and inorganic and organic substances includ-ing extracellular polymers, is the main contributor toresistance [17].

Hwang and Lin [21] also show that the structure ofthe membrane pores has a significant effect on fouling.Vrijenhoek et al. [22] describe reverse osmosis andnanofiltration membrane fouling, which depends onthe surface morphology, i.e., the rougher the surfacethe faster the fouling by attachment of colloids on themembrane surface. Atomic force microscopic imagesreveal that the particles accumulate mostly in the smallhollows of rough membranes, leading to their foulingand a severe flux decline.

Hirose et al. [23] show the existence of a linearrelation between the surface roughness of the reverseosmosis membrane and flux, but unlike the previousresearchers, they found that the permeate flux in-creases with membrane roughness. They conclude thata greater membrane roughness increases the local tur-bulence and wall shear stress and then the permeateflux.

Despite many investigations, the role of membranesurface properties in colloidal fouling of RO and nano-

filtration membranes is not yet entirely understood [22].Studies have shown that it would be better to usehydrophilic membranes rather than hydrophobic onesbecause the flux decreases much more slowly [24].

Membrane: Concept of a Critical FluxField et al. [25] were the first to introduce the con-

cept of critical flux. As long as one operates below thiscritical flux, the membrane fouling can be neglectedand thus membrane cleaning is not required. It is im-portant therefore to choose an adequate initial perme-ate flux or TMP.

Liu et al. [26], studying a bioreactor with submergedmembranes, determined that the hydrodynamic char-acteristics have a strong influence on liquid mass trans-fer. The airflow rate affects the liquid tangential circu-lation, which in turn affects the transmembranepressure; furthermore, the dimensions of the bioreactoralso act on liquid circulation. Sondhi et al. and Sondhiand Bhave [27, 28] show that at steady state the per-meate flux increases with the transmembrane pressureand tends to stabilize around a value called the limitingflux. A decrease of the permeate flux to less than thetransmembrane pressure value (3 bar) was also ob-served by Srijaroonrat et al. [29] during filtration utiliz-ing ceramic membranes treating an emulsion of waterand grease. Under this pressure, the layer in the vicinityof the membrane can be compacted, thus increasingthe resistance. The same results can be expected for amembrane bioreactor where the biological particles arenot inert and interact at first between them as well aswith the membrane surface.

Various biological conditions within the reactor aretherefore supposed to act differently on membranefouling. Based on this phenomenon, Tardieu et al. [30]concluded that a purely hydrodynamic analysis is notsufficient to explain the fouling problem.

Critical flux depends on hydrodynamics, particlesize (it is reached very quickly for small particles),interactions between colloids and membrane [16], andsuspension properties (pH, salinity, conductivity).Choo and Lee [31] stated that operational changes ofTMP and fluid velocity have very little influence on thepermeate flux. Tardieu et al. [30] noted that a ceramicmembrane installed externally to the membrane biore-actor is quickly subject to fouling (due to the formationof a thick cake) when the critical flux is exceeded. Theyalso studied the critical flux variation with the biologi-cal load and reported that when the COD increasesfrom 50 to 150 g � L-1 in the bioreactor, which operateslargely under the critical flux, the TMP increases grad-ually. However, the TMP decreases to its former valuewhen the initial COD is restored, meaning that thecritical flux has not been reached during this CODoverload.

Defrance et al. [13] investigated whether it would bebetter to have a constant TMP or permeate flux in orderto avoid the fouling of the ceramic membranes usedwith a membrane bioreactor. They established that itwould be better to filter utilizing a constant permeateflux rather than with constant TMP. Field et al. [25] havehighlighted this preference and other authors agreed


[17]. Moreover, Field et al. [25] and Tardieu et al. [30]observed two distinct zones: an undercritical zone,where the flux can be maintained constant over longperiods (from 1 week to 1 month according to [30]) andan overcritical zone, where the operation cannot bemaintained over a long period of time. When Jeffersonet al. [3] operated their membrane bioreactor below theconditions of critical flux, the low transmembrane pres-sure prevented an irreversible membrane fouling, al-lowing a stable flux to develop, a reduction of themembrane cleaning frequency, and, consequently, acost reduction for operation.

Concentration and Sizes of the Particles and the FlocsBarker et al. [32] showed that the main part of the

soluble organic matter present in the effluents of bio-logical treatment processes was produced microbialsoluble (PMS). PMS is composed of the organic com-pounds released by the metabolism of substrates (usu-ally associated with biomass growth) and by biomassdecay. PMS must be regarded as one of the significantfactors in the processes of biological treatment [33].Namkung and Rittmann [34] have classified PMS as [1]utilization-associated product (UAP) and [2] biomass-associated product (BAP). The UAP represent the PMSassociated to the substrate metabolism and the biomassgrowth; the rate of production of biomass is propor-tional to the substrate consumption rate. The BAP arePMS associated with biomass death and biomass pro-duction is proportional to the biomass concentration;the BAP is a by-product of the endogenous respirationof the cell mass. Because of the high concentration ofactivated sludge in the reactor and the long sludgeretention times, it is impossible to ignore the formationof microbial products in the membrane bioreactor sys-tem.

The disposal of undesirable organic and mineralmatter occurs with the help of flocculating bacteriawhich form flocs that are the base of sludge. However,these bacteria are not distributed uniformly in the flocand, as Peignen-Seraline and Manem [35] noted, thebacteria located in the central part of the floc are not incontact with the pollutant to be eliminated.

In the case of a biological suspension, the heteroge-neity of the aqueous solution and floc size complicatesthe fouling phenomenon. Wisniewski and Grasmick[14] established the influence of the size distribution ofa biological suspension on membrane fouling. Theynoted that the biological flocs are disintegrated by the(re)circulation inside the membrane bioreactor.Changes in particle size distribution modify the foulingproperties of the suspension; the presence of smallerparticles, resulting from the breakdown of the flocs,increases the membrane fouling. Choi et al. [24]showed as well that for an external type of membranebioreactor, the membrane fouling depends on the in-tensity of shear stress imposed on the bacterial flocs bythe recycling pump. The pump breaks down the flocs,generating more fine colloidal particles and releasingextracellular polymeric substances (EPS, principal com-pounds of the soluble organic matter) from the interiorof the floc to the bioreactor. Nagaoka et al. [36] deter-

mined that the EPS seems to be a key factor responsiblefor membrane fouling in the MBR. Despite a very largemolecular weight distribution, it is generally admittedthat EPS molecular weight is above 35,000 Da [24].These EPS imply an increase of the cake resistance, i.e.,a flux reduction. The efficiency of COD elimination bya MBR is directly linked to the concentration of thesolids in suspension in the reactor [37].

Vyas et al. [38] and Cabassud et al. [39] explainedthat the increase in the permeate flux results from anincrease of turbulence due to an air–liquid–large par-ticle tangential flow. Wisniewski and Grasmick [14]divided the bacterial suspension into three fractions:(1) the soluble fraction (or dissolved; d � 10-3 �m), (2)the colloidal fraction (polymers, fragments of cells; 10-3

� d � 1 �m), and (3) the particulate fraction (solids insuspensions, mainly bacterial flocs with the solids con-centration depending on the sludge age; d � 1 �m).The soluble fraction is responsible for approximately52% of the total resistance whereas the colloidal andparticulate fractions only represent 24% each. Theseresults are completely different from those obtained byDefrance et al. [40], who attributed only 5% of themembrane fouling to the soluble fraction, 30% to thecolloidal fraction, and 65% to the particulate fraction.This difference is probably due to the membrane chem-ical washing conducted after each filtration byWisniewski and Grasmick [14]. This chemical washinginduced an elimination of the soluble fraction adsorbedby the wall [40]. Bouhabila et al. [41] believed that thecolloidal fraction was 50% responsible for the mem-brane fouling, the dissolved fraction for 26%, and thesuspended solids for 24%. These results show a differ-ence that originates simultaneously from the substrateused (milk containing synthetic substrate), the biomass,the methods used to separate sludge, the membranes,and the application or not of a chemical washing.However, the lack of any pH measurements is prejudi-cial; actually, Wakeman and Tarleton [42] noted astrong influence of pH on the colloidal membranefouling, which could indicate some divergences in theprevious studies. However, according to Tardieu et al.[30], it seems that both the characteristics of the bacte-rial suspension and the quantity of soluble and colloi-dal compounds are responsible for membrane fouling.

Regarding the permeate flux variation with the solidconcentrations, Vyas et al. [38] found two distinctzones. Increasing the biomass concentration from 0.65to 2.5 g � L-1 yielded a flux reduction. On the otherhand, beyond 2.5 g � L-1 the biomass concentration hadno significant effect on the membrane fouling and thepermeate flux remained stable. Defrance et al. [40]reported this same phenomenon with a first zone offast decrease of the permeate flux for biomass concen-trations ranging between 0.8 and 1.5 g � L-1; this firstzone is then followed by a second zone, between 1.5and 5 g � L-1, corresponding to the stabilization of thefouling layer. Yet, when the concentration is increasedbetween 5 and 10 g � L-1 they find a third zone corre-sponding to a flux decrease (less marked than the firstzone). This result could be due to the suspension


rheological properties (increase of non-Newtonian vis-cosity).

Membrane: Washing and RegenerationEffective washing requires an understanding of the

interactions between the fouling products and themembranes as well as the effect of the washing proce-dures on elimination of the deposit. Marcucci et al. [43]cleaned UF membranes by backwashing for 90 secevery 20 min under a TMP of 0.4 bar. When the hy-draulic performance markedly decreased, the mem-brane must undergo chemical washing using acid andalkaline compounds.

High-frequency backpulsing is different from back-washing. Backwashing consists of reversing the filtra-tion direction for 5 to 30 sec every 30 to 60 min or everyhour, possibly accompanied by air sparging [44]. High-frequency backpulsing can be performed every coupleof seconds after a few minutes of filtration. High-fre-quency impulses (typically 0.1–2 Hz) can also be ap-plied over a very short period (usually around a secondwith microfiltration and UF). By means of sudden vari-ations of pressure (the transmembrane pressure is re-versed under high-frequency force) the permeate goesback through the membrane and declogs it. Moreover,as stated by Ramirez and Davis [45], the backpulsingdoes not deteriorate the quality of the permeate in anycase. The problem with this technique is that it requireshigh-pressure-resistant membranes. At the presenttime, only ceramic membranes for microfiltration andultrafiltration seem to meet this requirement. Many au-thors [28, 46–50] show that the technique is particularlyefficient because it totally eliminates the membranefouling but also maintains a flux 2 to 5 times higherthan the average flux without backpulsing. During fil-tration tests with a bentonite suspension using tubularceramic membranes, Ramirez and Davis [45] found amaximum flux with high-frequency backpulsing whichwas 10 times larger than the flux obtained withoutbackpulsing. For an optimal output it is therefore nec-essary to optimize the application time and the pres-sure.

Kennedy et al. [47] tested UF hollow-fiber mem-branes. They reported that the effectiveness of thehigh-frequency backpulsing depends on the period ofapplication rather than on the pressure. Ma et al. [50]and Redkar et al. [46] also confirmed that long back-pulses induce an useless loss of permeate while veryshort duration pulses are not advantageous becausethey do not remove the fouling particles effectively.

For a given application, there is an optimal combi-nation of frequencies and backpulsing times maximiz-ing the permeate flux. Moreover, time intervals be-tween two backpulsing applications must be of thesame order of importance as cake development char-acteristic time. Ramirez and Davis [45] presented aneconomic analysis of the process.

CONCLUSIONIn this bibliographical review we have discussed the

use of membrane bioreactors for wastewater treatment

in general and specifically for industrial effluents. Incomparison with the conventional activated sludge sys-tem, the MBR systems have better removal efficiencyand a potential for water reuse in manufacturing.

Nevertheless, this treatment process strongly de-pends on the biomass concentration, which controlsthe mass transfer in the bioreactor and the level ofmembrane separation. Consequently, it is very impor-tant to be able to quantify and understand the factorslimiting mass transfer.

Many authors insist that the problem of membranefouling in the presence of microorganisms is linked tomicrobial products, concentration, and sizes of parti-cles. Different strategies of membrane washing orbackwashing are proposed in order to maintain a stablepermeate flux in the MBR systems.

To date, the oxygen contribution in the membranebioreactor has not been sufficiently studied so that thecosts of oxygen supply cannot be neglected. Someauthors have shown the strong influence played by thebiomass presence on the reduction of the aerationcapacity of the biological systems [51, 52]. Sutapa [52]notably showed the strong dependence of the gas/liquid transfer in a bioreactor with the biomass concen-tration. The aeration capacities of a bioreactor can bedivided by a factor of 4 for a biomass concentration of20 g � L-1 because of the viscous nature of the mixedliquor. This condition could limit the aerobic biologicalactivity in a MBR system due to the lack of oxygen.Thus, it is necessary to increase the oxygenation of themedium to provide the oxygen necessary to the micro-organisms. This can be done by increasing the airflowrate. However, this results only in a limited increase ofoxygen transfer [52]. In order to optimize the oxygentransfer, it may be necessary to enrich air with pureoxygen and/or limit viscosity connect medium due tothe high biomass concentration. Indeed, the activatedsludge suspensions are characterized by a non-New-tonian behavior, with a reduction of viscosity accordingto the gradient of imposed shear stress [53].

It therefore seems necessary to determine whetheroxygen becomes a limiting factor in MBR operation,considering the biological degradation activity, andthus for high biomass concentrations in an aerationtank. Rheological studies should enable us to defineshear conditions in the MBR inducing a decrease ofapparent viscosity of the medium in order to optimizethe oxygen transfer.

ACKNOWLEDGMENTSFinancial support of this study was provided by

FEDER, IFR CNRS 112 “PMSE,” and CONACYT of Mex-ico.

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industrial wastewater treatment in a membrane bioreactor: a review

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