Bioresource Technology 122 (2012) 171–180

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Bioresource Technology

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Review

Immersed membrane bioreactors: An overview with special emphasis on anaerobicbioprocesses

Reeta Rani Singhania a, Gwendoline Christophe a, Geoffrey Perchet b, Julien Troquet b, Christian Larroche a,⇑a Clermont Université, Université Blaise Pascal, Laboratoire de Génie Chimique et Biochimique, Polytech Clermont-Ferrand, 24 Av. des Landais, BP 20206, 63174 Aubière Cedex, Franceb Biobasic Environnement, Biopôle Clermont Limagne, 63360 Saint Beauzire, France

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 November 2011Received in revised form 20 January 2012Accepted 23 January 2012Available online 3 February 2012

Keywords:IMBRMembrane bioreactorWastewater treatmentAnaerobiosisMembrane fouling

0960-8524/$ - see front matter � 2012 Elsevier Ltd. Adoi:10.1016/j.biortech.2012.01.132

⇑ Corresponding author.E-mail address: christian.larroche@univ-bpclermo

Immersed membrane bioreactor (IMBR) has emerged as a novel potential technology which is consideredglobally as potent technology, primarily for wastewater treatment. It offers quality improvement in efflu-ents treatment compared to other technological systems. It also offers potential benefits for the biopro-cesses where product formation and separation is desired simultaneously in a compact container. Thisreview gives insight for the wide range applications of IMBR focussing on anaerobiosis. It discusses thesignificance, advantages and drawbacks of IMBR against the conventional methods, highlighting theexternal membrane bioreactors. While the commercial significance of IMBR is obvious for industrialand municipal wastewater treatment, the current focus is shifting on other applications such as anaerobicbioprocesses. Though the IMBR technology is generally considered hand-in-hand as sustainable technol-ogy, the major bottleneck in its application at commercial scale for wastewater treatment seems its eco-nomic feasibility and compatibility. Among the technical issues, the membrane fouling is considered as amajor problem for which several strategies have been developed to overcome the problem, though thereis no complete or universal solution to this problem.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Membrane technology has become a very dignified separationprocess due to its relatively low energy requirement with no addi-tional chemical added. Membrane separation integrated with bio-logical treatment lead to the development of membrane bioreactor(MBR) technology. It refers to a coupled process with these twoelements rather than the sequential process. This review articleis an attempt to highlight potential applications of IMBR other thanwastewater treatment; however, wastewater treatment has beendiscussed as a major model application of the above technology.There are several documents on different issues of MBR relatedto wastewater treatment, membrane fouling, etc. available in theliterature, but very few deal with other applications. It is consid-ered in this document that anaerobiosis could be the most poten-tial emerging application of this technology in coming years.

This technology finds immense potential for municipal andindustrial wastewater treatment applications as it has a significantadvantage that it generates a high grade effluent. It forms the basisof many industrial and wastewater treatment installations, wherea high grade effluent is demanded. The number of the MBRinstalled worldwide has also increased due to reduction in

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nt.fr (C. Larroche).

membrane cost and increasing environmental stringent regulation.In Europe and North America it has been accepted as a preferredapproach for wastewater treatment and reuse applications, partic-ularly in environmentally sensitive regions or where water supplyis limited. Wastewater treatment has been used as a model processinitially to prove the significance of technology, but finds immenseapplications in various other processes where the products haveinhibitory effects in the process and need to be removed forefficient performance. It can be applied for enzymatic reactionswhere product inhibition occur, since the membranes are able toremove continuously the product which in turn helps to avoidthe inhibition and higher productivity can be achieved. For exam-ple, a thermostated membrane bioreactor has been applied for thehydrolysis of pectin by Aspergillus niger polygalacturonase to avoidproduct inhibition. In this process vacuum was used in the perme-ate side in order to increase the trans-membrane pressure, result-ing in higher product removal rate (flux) and productivity (Kisset al., 2009). High specificity membranes are now available which,for example, can separate butanol isoforms.

Mainly two configurations of MBR exist; internal/submerged/immersed where the membranes are immersed in the biologicalreactor or in separate reactor (IMBR) (Figs. 1a and b); and exter-nal/side stream (EMBR) (Fig. 2) where membrane is a separate unitprocess requiring an intermediate pumping step. The difference inthese configurations is that the direction of flow is reverted and theachievable value of trans-membrane pressure (DP) is different. In

Untreated water

Sludge

Bioreactor

Treated water

Pi < Patm

Fig. 1a. Internal immersed membrane bioreactor, Pi = pressure inside,Patm = atmospheric pressure.

Membrane module

Bioreactor

Untreated water

Sludge

Treated water Pi < Patm

Fig. 1b. External immersed membrane bioreactor, Pi = pressure inside,Patm = atmospheric pressure.

Membrane module

Bioreactor

Untreated water

Sludge

Treated water Pi

Po

Pump Pi > Patm

Po = Patm

Fig. 2. Non-immersed/external membrane bioreactor, Pi = pressure inside,Po = pressure outside, Patm = atmospheric pressure.

172 R.R. Singhania et al. / Bioresource Technology 122 (2012) 171–180

EMBR, the liquid is pushed across the membrane while in IMBR, itis pumped. The trans-membrane pressure for EMBR can thus behigher than for IMBR thus decreasing the exchange area neededfor a given permeate flow and increasing the energy requirementfor the former. The MBR was commercialised in 1970s as a side-stream process, where membrane was located outside the mainreactor (biological treatment) separately in a separation device,but the breakthrough in MBR arose in 1989 with the idea ofYamamoto and his co-workers to submerge or immerse the mem-brane in biological reactor, which gave rise to immersed mem-brane bioreactor (IMBR) (Benedek and Cote, 2006). IMBRtechnology offers several advantages over conventional

approaches of wastewater treatment such as small footprint (com-pact process), high grade effluent, low energy input and the mostimportantly environmental sustainability (Pabby et al., 2008; Oronet al., 2008; Le-Clech et al., 2006; Yang et al., 2006). But, its wide-spread implementation and sustainability for viable wastewatertreatment is limited due to its relatively higher cost (Judd and Judd,2010) due to high energy requirement for aeration in particular(Verrecht et al., 2008), which is not required for conventional treat-ment and membrane cleaning management and replacement (Ken-nedy and Churchouse, 2005). In IMBR, aeration is considered as themajor parameter for both the hydraulic and biological process per-formance as it maintains solids in suspended form, scours themembrane surface and also provides oxygen to the biomass forbetter biodegradability and cell synthesis, so energy requirementfor aeration cannot be compromised.

Since IMBR technology is only 20 years old, so long term oper-ational experience and membrane performance data is limitedand if it is; rarely detailed report is available on installation formore years. Hence, it can be said as an emerging technology andextensive research is needed in all the areas including design ofbioreactor, membrane material, type of module, strategies to pre-vent membrane fouling, potential applications, etc.

In this technology the cost of membrane and its maintenancealso shares a major portion of the cost. Recent technical innova-tions and significant reduction in membrane cost have enabled im-mersed membrane bioreactor technology to be accepted aspreferred process for any biological process including wastewatertreatment, though further reduction in cost is required to be aneconomic efficient process.

2. Commercial status of membrane bioreactor

MBR global market is experiencing an accelerated growthwhich is expected to be sustained till next decade. It was in1970s that it was commercialised as side stream process but theintroduction of immersed system after two decades resulted inexponential growth of the technology (Judd, 2008). Europeanmembrane bioreactor market was estimated to be nearly 57$ mil-lion in 2004 and is expected to have sustainable growth in futuretoo. UK/Ireland, France, Germany, Iberia, Benelux and Italy arethe major players in European market for membrane bioreactors(Frost and Sullivan, 2003, 2005) (Fig. 3). United States and Cana-dian MBR market is also expected to have sustained growth inthe next decade mainly due to revenue from membrane basedwater purification, desalination and wastewater treatment (Frostand Sullivan, 2004). It has been estimated that the market willdouble every 7 years (Judd 2008).

In North America, MBR market has been dominated by Zenonand according to review published in 2006 (Yang et al., 2006),71% of the total installations of MBR in USA, Canada and Mexicois provided by Zenon. Memcor is a long established HF membranefilter supplier and represents a potential significant player in MBRmarket which is at par with Zenon on potable water treatment(Judd and Judd, 2010).

3. Evolution of Immersed membrane bioreactor

IMBR is a result of evolution of MBR where the membrane is im-mersed in the bioreactor instead of being outside as a separateunit. The idea of IMBR was conceived in late 1980s or early1990s by independent team at Japan and Canada. Prof Yamamotoand Aya conducted laboratory experiments with fine hollow fibresimmersed in an activated sludge reactor (Benedek and Côté, 2006).The concept was picked up by Japanese companies Kubota andMitsubishi Rayon who continued the research and development

Italy16%

UK and Ireland19%

France12%

Germany18%

Benelux16%

Iberia19%

Fig. 3. European membrane bioreactor survey Frost and Sullivan (2005).

R.R. Singhania et al. / Bioresource Technology 122 (2012) 171–180 173

and then commercialised the technology. Kubota Corporationdeveloped flat sheet panels where Mitsubishi Rayon focussed onfine hollow fibres. Zenon is a Canada based company and is activesince 1980s and is the one which supplies membrane as well asturnkey plants for both water and wastewater treatment process(Judd and Judd, 2010). The ZeeWeed� immersed membrane wasconceived in the early 1990s in an effort to reduce the cost of mem-branes, simplify the MBRs system and reduce energy consumption.Since 1993 ZeeWeed� launched the four generation of membranebioreactor from ZW-150 to ZW-500d with improved membranesand cassettes.

4. General features of the IMBR

Most of the features of the bioreactor in IMBR plays an eminentrole in the overall process efficiency and all the parameters in-volved in the design and operation of MBR have an effect on mem-brane fouling, so the reactor should be designed so as to minimisethe fouling problem. The flux of a membrane module is one of themost important control parameters for the MBR technique, and isrelated to the hydraulic retention time (HRT), volumetric loading,and trans-membrane pressure of the membrane module. Flux isthe quantity of material passing through per unit membrane areaper unit time and occasionally it is referred as permeate/filtratevelocity. It relies on high trans-membrane pressure to maintain fil-tration. The flux of a membrane module is also an important factorin the installation and operating costs of an MBR system. In addi-tion to solid sludge characteristics and effluent quality, solid sludgeconcentration has also effect on the flux of a membrane module.Although a higher concentration of solid sludge indicates highertreatment efficiency, a higher concentration of sludge will alsocause a reduction in the flux of an MBR, and an increase of theoperating pressure thereof. Energy demand of the MBR should beminimised for effective process. Aeration energy models have beensuggested to minimise the energy demand required for aeration.There is a linear relationship between membrane permeabilityand membrane aeration up to a threshold value, beyond whichpermeability is unchanged with membrane aeration (Le-Cletch etal., 2006). It was demonstrated that significant reductions in aera-tion energy could be obtained through operation at lower flux andreducing the membrane aeration requirement accordingly, atlower flows (Verrecht et al., 2008). Gas sparging enables highmembrane permeability in IMBR. In all the membrane processthe high cross flow provides high flux rate. For ceramic membranewhere the fouling is minimal, high cross flow was found detrimen-tal as because thinning cake layer provides less protection againstinternal fouling (Choo and Lee, 2000). At low transmembrane pres-sure crossflow does not influence flux where as at high transmem-brane pressure crossflow becomes important. Combination of flat

submerged membrane with bioreactor is very common for waste-water treatment though hollow fibres are more efficient. Details ofthe membrane material its properties, volume of the bioreactor,operating conditions employed in IMBR and the supplier’s infor-mation have been enlisted in Table 1.

For MBRs several process configurations exists, among them theextractive and diffusive are the major ones. As the name itself sug-gests, in extractive configuration membrane is used to extract thespecific component for their discrete bio-treatment or the biologi-cal treatment of the remaining effluent (Livingston et al., 1998).Extractive membrane bioreactors enhance the performance capa-bilities of biological treatment of wastewater by exploiting themembrane’s ability to achieve a high degree of separation whileallowing transport of components from one phase to another. Thisseparation aids in maintaining optimal conditions within the bio-reactor for the biological degradation of wastewater pollutants.Thus extractive configuration finds applications in various pro-cesses, where extraction of any specific component is necessarydue to having its toxic effect to the microbial culture present inthe medium. For example, acetic acid produced as a by-productin several fermentation process during hydrogen production, couldbe removed by extractive configuration, so as to avoid toxic effectof volatile fatty acids and also drop in pH of the medium from hav-ing deleterious effect on the microorganism. It can be operated inboth modes; external as well as immersed. In diffusive membranebioreactors the membrane is used to pass the gas into the bioreac-tor in the biofilm in its molecular form to enhance its effect on bio-treatment. Thus 100% utilisation of gas takes place than airsparging which helps in making the process economically efficient.

The capital and operating costs associated with a small packageplant MBR for small-scale domestic duty has been appraised basedon a medium-strength municipal wastewater (Fletcher et al.,2007). The three main membrane configurations were considered,these being multi-tube, hollow fibre and flat sheet, with the mostappropriate plant design chosen for each configuration. Resultsindicate that it is possible to produce single household MBR at asimilar current market cost for packaged treatment plants. Deslud-ging and maintenance of these plants is similar but power require-ment for MBR are four times more than the conventional packageplants (Fletcher et al., 2007). Significant work has been performedon modelling MBR biokinetics (Lee et al., 2002; Yildiz et al., 2005)providing a range of values for key parameters for the MBR system.These MBRs have a set application for wastewater treatment andother applications are emerging.

4.1. Membranes – material, properties, preparation and maintenance

There are mainly two different types of membrane material;polymeric and ceramic. Beside these, metallic membrane also

Table 1Immersed membrane bioreactors and detail of the membranes.

Volume of theIMBR

Membrane properties Operating conditions Supplier References

75 L A 0.93 m2 out-to-in immersed Polyvinylidenefluoride (PVDF) hollow-fibre membrane with0.04 lm nominal pore size was used

Piston pump was used to withdraw permeateunder suction and was operated in both forwardand reverse motion. To avoid the entrainment ofair, nitrogen enriched air (>99%) was used to scourthe membrane, produced from compressed air(8 bar g) using a nitrogen selective hollow-fibremembrane

NA McAdam andJudd (2008)

All sixfull-scalemunicipalFS iMBR

Flatsheet (FS) cartridges (Kubota 510), these being0.8 m2 in area and having dimensions of 1 mlength � 0.5 m �width

The mean operating flux across al six plants rangedfrom 16 to 26 LMH, and the specific aerationdemand according to the manufacturer’s designspecifications of 0.53 and 0.75 N m3 (m2 h)�1 fordouble or single deck modules

Kubota, Japan Ayala et al.(2011)

The MBR pilotsystem wasa ZeeWeed-10 (ZW-10),

An outside/in ultrafiltration hollow fibermembrane supplied by Zenon, Nominal membranesurface – 0.93 m2, Nominal pore size – 0.04 lm.Max flowrate – 2.3 L/min, Operating TMP – 6.9–48.25 kPa (1–7 psi), Backpulse TMP, 6.9–20.7 kPa(1–3 psi), Aeration flowrate, 30–100 L/min

When the transmembrane pressure (TMP)exceeded the maximum permissible value(48.25 kPa or 7 psi) due to the high biomassconcentration, sludge wastage was initiated.Sludge wastage was set at the appropriate flowrateso that the operating TMP was stable at 42.3 kPa(6.1 psi) and the MLSS at approximately 5 g/L

ZenonEnvironmental,Canada

Dialynas andDiamadopoulos(2009)

Reactorvolume of9200m3

The total membrane area of 84,480 m2 is dividedinto eight membrane lines (each nitrification tankis equipped with two membrane lines with 24modules each). The applied membrane stageconsists of submerged hollow fibre modules(PVDF, 0.04 lm nominal poresize)

Net flux was 17.1 Lm�2h�1, Permeability 150LMH bar�1, backflush was done 30 s at each30 min interval

Zenon, Environmental,Canada

McAdam andJudd (2008)

3.5 L The membrane module used was SteraporeLHEM03334 hollowfiber membrane fromMitsubishi Rayon Co. The membrane fiber hadnominal pore size of 0.4 mm and total membranesurface area of 0.03 m2 per module.

Five membrane modules were used in parallel atall operating conditions in order to achieve thetargeted lowest HRT. The membranes wereoperated at flux lower than 0.3 m/d and thetransmembrane pressure smaller than 5 kPa. Inorder to minimise cake formation on themembrane surface, the membranes were operatedintermittently (9.5 min on, 0.5 min off)

Mitsubishi Rayon Co. Ng andHermanowicz(2005)

10 L Organic hollow fibres, polysulphone,microfiltration membranes

Peristaltic pump were used for the influent and forthe dead end filtration

Polysulfone,microfiltration,Polymem, France

Clouzot et al.(2011)

1 L The membrane fibers were made of PVDF, with aninner and outer diameter of 0.85 mm and1.45 mm, respectively, the mean pore size of themembrane was 0.01 lm. Each module contained 4membrane fibers, with an effective length of11.0 cm, corresponding to a membrane area of0.002 m2

The initial TMP of the membrane modules used inthe study was determined as 18 ± 1 kPa

Suzhou LitreeUltrafiltrationMembraneTechnology Co. Ltd.,China

Tian et al.(2010)

Workingvolume1514 L

The membrane had a nominal pore size of0.035 mm and an absolute cut off of 0.1 mm

Three full-scale ultrafiltration modules (ZW 500c)were immersed in the membrane tank

Zenon, EnvironmentalCanada

Trussell et al.(2006)

Effectivevolume 2 L

The UF membrane modules were made of PVC,with a nominal pore size of 0.01 lm and a totalmembrane area of 0.4 m2

UF membrane flux was set at 10 L/m2 h,corresponding to a HRT of 0.5 h. The effluentsuction pump was controlled by a timer based on atime sequence of 8 min on and 2 min off in eachcycle. The air was continuously supplied to thereactor at the flow rate of 80 L/h

Litree, China Tian et al.(2008)

174 R.R. Singhania et al. / Bioresource Technology 122 (2012) 171–180

exists. Table 1 gives some of the details available in the literature,about membrane material and its properties. The membrane mate-rial should be configured in such a way so as to allow water to passthrough, so as to have applications in membrane bioreactor tech-nology. A number of different polymeric materials are used to formthe membranes; usually they form a thin surface layer to providethe required permselectivity on the top of the surface, over a thickporous support that provides a strong mechanical strength. Mem-brane should be fabricated in a way to have high surface porosity,must have structural integrity as well as should be resistance tothermal and chemical attack, such as temperature, pH and oxidantconcentration that arise when membrane is cleaned (Judd, 2006).

Synthetic membranes are employed for a variety of applicationsincluding desalination, gas separation, filtration, and dialysis. Thespecificity of the membrane depends largely on its properties suchas symmetry, pore shape, pore size and the polymeric material

used to form the membrane. Hollow porous fibres are made withan extremely large surface area per unit volume to provide a largesurface area and a large filtrate flow can be obtained. Microporoussynthetic membranes are particularly suitable for use in hollow fi-bres and are produced by phase inversion. Microporous phaseinversion membranes are particularly well suited to the applica-tion of removal of viruses and bacteria.

Flat sheet membranes are prepared by bringing a polymer solu-tion consisting of at least one polymer and solvent into contactwith a coagulation bath. Many of the porous membranes are madeof hydrophobic polymers. Hydrophobic microporous membranesare characterised by their excellent chemical resistance, biocom-patibility, low swelling and good separation performance. Thus,when used in water filtration applications, hydrophobic mem-branes need to be hydrophilised to allow water permeation. Somehydrophilic materials are not suitable for microfiltration and ultra-

R.R. Singhania et al. / Bioresource Technology 122 (2012) 171–180 175

filtration membranes that require mechanical strength and ther-mal stability since water molecules can play the role of plasticizers.

Currently, poly (tetrafluoroethylene) (PTFE), polyethylene (PE),polypropylene (PP) and poly (vinylidene fluoride) (PVDF) and zeo-lite are the available hydrophobic membrane materials. Poly vinyl-idene fluoride (PVDF) is a semi-crystalline polymer containing acrystalline phase and an amorphous phase. The crystalline phaseprovides good thermal stability whilst the amorphous phase addssome flexibility to the membrane. PVDF exhibits a number of desir-able characteristics for membrane applications, including thermalresistance, reasonable chemical resistance (to a range of corrosivechemicals, including sodium hypochlorite), and weather (UV)resistance. Novel PVDF/(sulfonated polyethersulfone) SPES mem-branes were prepared by self-assembly of TiO2 along with UV irra-diation. The hydrophilicity of membranes decreased considerablywith deposition of TiO2 and showed high antifouling propertiesdue to photocatalytic and superhydrophilicity effects and also be-cause it possess high antibacterial activity (Rahimpour et al., 2012).

While PVDF has to date proven to be the most desirable mate-rial from a range of materials suitable for microporous membranes,the search continues for membrane materials which will providebetter chemical stability and performance while retaining the de-sired physical properties required allowing the membranes to beformed and worked in an appropriate manner. Zeolite membranesare also getting popularity these days owing to their specificity.

In particular, a membrane is required which has a superiorresistance (compared to PVDF) to more aggressive chemical spe-cies, in particular, oxidising agents such as sodium hypochloriteand to conditions of high pH, i.e. resistance to caustic solutionsduring cleaning.

4.2. Membrane configurations (modules)

There are mainly three types of membrane configurations: mul-ti tube, hollow fibre module, flat membrane module. Usually hol-low fibres and flat sheet modules are used for IMBR technologywhereas; multitube modules are used in external MBR process.Variation exists between flat sheet and hollow fibre configuration,flat sheet operates at higher specific aeration demand and achievecommensurately higher sustainable permeabilities and thusdemanding less cleaning. Hollow fibre modules are attractive asthey offer more filtration surface area per unit volume, but themaintenance of permeability and fouling dynamics remains linkedto good practices in module configuration in association with opti-mised local hydrodynamics conditions (Lebegue et al., 2008). Ahollow fibre (HF) system presents a real advantage over flat sheetsystems because of their high membrane surface development pervolume unit. When using HF in IMBR systems, filtration is gener-ally practiced in outside–inside mode because of the characteris-tics of the concentrated suspensions to be treated (Lebegue et al.,2008). This configuration also allows an easier cleaning of the sys-tem by a back-flush operation because particles released from themembrane are outside the hollow fibres. Hollow fibre low pressuremembrane technologies in terms of ultrafiltration and microfiltra-tion have been recognised as a key technology of 21st century (Xiaet al., 2009; Lesjean and Huisjes, 2008; Tabatabai et al., 2009).Among various types of membrane, the first generation MBR oper-ated with tubular membranes placed in external recirculationloops. The use of recirculation loops leads to increased energy costs2–10 kW h m3 of water produced depending on the internal diam-eter of the tube. In addition the higher shear stresses in the tubesand recirculation pumps can cause the destruction of bioflocs andthis lead to the decrease of biological activity (Brockmann and Sey-fried, 1997). Membrane module costs have decreased dramaticallyover the last years to <US$ 50/m2 (Judd, 2006a,b) leading to a de-crease in capital costs but the membrane fouling abatement leads

to elevated energy demands and has become the main contributionto overall MBR operating costs (Drews, 2010).

Regarding membrane replacement, flat sheet membranes areclaimed to be promising as evidenced from the most establishedIMBR plant globally; Porlock in the UK had only 6.4% of its originalcartridges were replaced even after 10 years of operation (Churc-house et al., 2008). Moreover the damage were due to avoidableexternal errors, mainly handling error and also the decrease inaverage permeability was insignificant during the 10 years whereinitial permeability were recovered by chemical cleaning. Cruciallythere is no indication as to which operating factors contribute sig-nificantly to the life of the membrane (Ayala et al., 2011).

4.3. Problems: membrane fouling

Membrane fouling (colloidal, organic, bio, or a combinationthereof) and scaling, can adversely impact the membrane perfor-mance. It can decrease the permeate flow at a given driving force,lower the permeate quality (purity), and increase energy con-sumed to maintain a given permeate flow. This can necessitatethe cleaning of the membrane separation system in order to re-move the deposits.

There are several reviews available till date on membrane foul-ing, focussing on the fouling reasons and available strategies tocombat the problem (Judd, 2006a,b; Drews, 2010; Le-Clech et al.,2006; Stefanski et al., 2011). The fouling is inevitable and couldbe controlled if the mechanism and responsible substances areknown. Traditionally, there are three factors affecting fouling:membrane, sludge characteristics and operation (Drews, 2010).

Membranes in immersed membrane bioreactor are prone tofouling by microbes, i.e. biofouling, particularly in aqueousstreams, which provide optimum conditions for microbial growth.Mainly the group of compounds responsible for fouling are catego-rised into extra-cellular polymeric substances (EPS) and solublemicrobial products (SMP). EPS and SMP consist of polysaccharides,proteins, lipids, nucleic acid, etc., which are involved in membranefouling. The exopolysaccharides released by the microbial popula-tion and the soluble metabolites produced by them are consideredas the primary foulants by some researchers (Nataraj et al., 2008;Rosenberger and Kraume, 2002) whereas; others do not considerthem as indicator of foulants (Guglielmi et al., 2007; Pollice et al.,2005) rather, proteins are more likely responsible for blockingthe pores than the carbohydrates and also the cake layer was majorcontributor to fouling (Wu et al., 2010). Majority of the view sup-ports that carbohydrate component are the most commonly iden-tified component of the soluble microbial product which areconsidered as the major foulant only at low SRTs (Judd 2008). Thisconfliction makes the understanding of controlling membranefouling difficult (Le-Clech et al., 2006) but all the authors agree thatthere is a strong correlation between the membrane fouling andthe activated sludge structure. Membrane fouling is the majorobstacle for the wider application of the immersed membrane fil-tration technology which leads to the deterioration of the waterpermeability and increase of operational cost (Yamamura et al.,2007).

Although chemical cleaning is effective to combat membranefouling but the frequent chemical cleaning would cause damageto membrane material and shorten the life time of membranemodule (Tian et al., 2010) and may also alter foulant susceptibilityto the membrane (Drews, 2010). Membrane fouling cannot beavoided completely but the design and operation of MBR shouldbe based on strategies to reduce the fouling as well as adoptingproper physical or chemical cleaning methods and in case shouldbe prepared to accept lower flux rates. The frequency of chemicalcleaning method should be minimised as it needs the membraneto be removed offline and also reduces the lifetime of the

176 R.R. Singhania et al. / Bioresource Technology 122 (2012) 171–180

membrane. The physical cleaning (back washing, aeration in bub-bling form) can be done very frequently as it can be done online.The physical cleaning could be done once in few minutes (Jianget al., 2003), whereas chemical cleaning frequency varies and couldbe in several days, months or even in years (Murakami et al., 2000).It has been suggested that the elevation of the membrane in thebioreactor could reduce the fouling due to low concentration ofmixed liquor suspended solids in the upper zone (Kim et al.,2008) which could be good strategy to control fouling to an extent.

Ultrafiltration of river water was investigated by using im-mersed hollow-fibre membrane and the effect of air bubblingmode, air flow rate, size of the air bubble and also the quality ofthe feed water on the membrane fouling was determined. Contin-uous air bubbling was more effective for mitigating membranefouling then intermittent air bubbling also the smaller the air bub-ble size more effective the air bubbling for mitigating the mem-brane fouling of immersed hollow fibre membrane (Tian et al.,2010). Membrane fouling also causes the decrease in the rate offlux. The membrane bioreactors are designed to maintain a con-stant flux (flux is a measure of the rate at which the product or per-meate passes through the membrane per unit of surface area forthe outside membrane surface, net flux also accounts for the actualvolume of permeate lost during backpulsing), so when the mem-brane becomes fouled; vacuum or transmembrane pressure in-creases to pull the clean water through the membrane. When thetransmembrane pressure exceeds 8 psi (vacuum), then the clean-ing is required (Saikkonen et al., 2004). Adjusting the permeabilityfor temperature allows the influence of fouling to be determined.The formula used to calculate permeability at 20 �C is based onthe variation of the viscosity of water with temperature.

Permeability @20 �C ¼ Permeability @T � 1:025ð20� TÞ

Permeability is a calculated parameter of flux normalised bytransmembrane pressure. It is reported in units of GFD/psi. Perme-ability is typically corrected to account for temperature variations.

Hence, the choice or selection of MBR configuration is veryimportant and depends on the application. For anaerobic bioproc-ess, external IMBR could be advantageous due to better accessibil-ity of the membrane module. Hence, this technology makespossible maintenance operations on the filtration unit without dis-turbing the anaerobiosis in the bioreactor itself.

4.4. Strategies to overcome membrane fouling

Relaxation and backwashing have been incorporated in many ofthe membrane bioreactor design as standard operating strategiesto limit membrane fouling (Wu et al., 2010). Excess aeration tothe membrane surface helps to control the membrane foulingbut significant energy is consumed for excess air production. Theeffect of suction time, aeration intensity, and backflush time wereinvestigated for decrease in fouling and suction was having highesteffect to resist fouling followed by aeration density and backflushtime (Schoeberl et al., 2005). The IMBR fouling can be suppressedby coarse bubble aeration (Judd, 2008). The fouling which cannotbe treated by these physical means are treated chemically bystrong oxidative agent (e.g. hypochlorite) to remove organic mat-ter, followed by organic acids along with mineral acids to removemetal hydroxides. But these oxidative agents deteriorate the mem-brane and heavily affect its lifetime (Judd, 2008). Other than airscouring alternate strategies which can minimise energy require-ment would be useful. A new configuration in IMBR was investi-gated to reduce the biosolid concentration near the membranesurface; the position of the membrane was elevated from bottomto the top of the bioreactor (Kim et al., 2008). This configurationhas the advantage of low solid concentration near the membranedue to settling of the solids and also aeration was supplied to the

upper zone where the membrane filtration is carrying over andnot to the bottom zone, thereby saving energy. Recently, there isan increasing attempt to design and manufacture coated mem-branes having antibacterial properties so as to prevent membranefouling. A recent article shows that UV irradiated TiO2 coating overa blend membrane PVDF/SPES attributes antibacterial properties tothe membrane (Rahimpour et al., 2012) which may resist fouling.Anti-biofouling effect of Piper betle extract was evaluated on bio-film formation and exhibited reduced growth rate and EPS produc-tion by Pseudomonas aeruginosa (Siddiqui et al., 2012).

5. Potential applications

5.1. Wastewater management

Scarcity of water resources in arid and semi-arid areas of theworld as well as the need of proper disposal of wastewater haschanged the public view towards the wastewater management. In-creased public concern about health and environment, the need forexpansion of wastewater treatment process due to increased pop-ulation as well as the stringent waste disposal regulation motivatesto search an economically-efficient innovative technology forwastewater treatment. Adequate management of wastewaterwhich was an option earlier now became a necessity. Traditionallysedimentation or decantation was used primarily to remove theparticulate matter or sludge from the wastewater, giving large sizetreatment units. Therefore the impact of membrane systems,which allow to reduce the size of the plant, is rapidly growing.Membrane bioreactors (MBRs) combining biological and physicalprocesses in one stage thus promise to be more compact, efficientand economic. Membrane bioreactors are typically sized to accom-modate community and large-scale sewage treatment, i.e.160,000 gpd, and 20–40 mgd and more (Jordan and Liu, 2009).These large-scale wastewater treatment systems are commonlydesigned to operate while attended, have numerous controls, andtypically require chemical addition. MBRs have achieved consider-able market penetration in the municipal water treatment sectorover the past 15 years (Hanft, 2006). Their advantages over con-ventional processes are well documented (Stephenson et al.,2000; Judd, 2006b), which includes requirement of very less landarea, also the constraints imposed by membrane fouling (Le-Clechet al., 2006). There is very high risk of clogging in external mem-brane bioreactor where the direction of flow is from inside to out-side and is comparatively very difficult to clean. It is difficult tomaintain very stable operating conditions when working with amembrane directly in wastewater due to its variable compositionand high potential risk of fouling of wastewater. In most of the bio-reactors the foulants are converted from their soluble form toinsoluble form so as to get separated easily.

With the advent of IMBR processes where membrane modulesare immersed in a large feed tank and filtrate is collected typicallythrough suction applied to the filtrate side of the membrane thusrequiring less energy, the process became more attractive and pop-ular for the above application (Meng et al., 2007). For wastewatertreatment, which is an aerobic process, internal IMBR is moreadvantageous than external IMBR because it is the sole configura-tion allowing a significant reduction in the plant size. The porediameters of the membranes are generally below 0.1 lm, so theeffluent coming through is almost clear and disinfected (Santoset al., 2011).

IMBR needs very small land area and the management ofdisamenities is also easy so can be installed very near to homesand the smaller sites thereby generating significant saving on thecost of pipes. Application of IMBR has been practised for biologicalnutrient for removal of phosphorous and nitrogen for complete

R.R. Singhania et al. / Bioresource Technology 122 (2012) 171–180 177

separation of liquid and solid (Kim et al., 2008), for bathing waste-water treatment where high rates of removal of CODcr, total nitro-gen and NH4

+–N were documented (Xia et al., 2008) and also fordrinking water purification (Yamamura et al., 2007). Li et al.(2006) supplied submerged membrane bioreactor with inorganicammonium bearing wastewater (NH4

+–N, 500 mg/L) withoutsludge purge and decreased hydraulic retention time (HRT) wherealmost complete nitrification was obtained and Yu et al. (2010)employed three submerged membrane bioreactor at different solidretention times feeded with synthetic inorganic nitrogen NH4

+–N,100 mg/L. All these three reactors could oxidise NH4

+–N to NO3–N where shortest SRT showed significantly higher specific ammo-nium oxidising rate. The market for package MBRs is significantlyinfluenced by the recycling potential of the effluent produced(Fletcher et al., 2007). There is a need for simple, robust small scalewastewater treatment systems designed for relatively unattendeduse, requiring only periodic maintenance.

5.2. Anaerobiosis – hydrogen production

Immersed membrane bioreactor is readily accepted technologyfor anaerobiosis particularly for hydrogen production. Moreprecisely external IMBR serves better for this application compar-atively easier membrane cleaning. The recovery of energy frommunicipal solid waste and domestic wastewater from treatmentplants is a response to the challenges of developing renewablesources of energy while providing sustainable methods of wastemanagement. While common methods for energy production frommunicipal solid waste are anaerobic digestion and incineration(Rhyner et al., 1995), hydrogen production is increasingly per-ceived as a potential pathway, due to hydrogen’s high energy con-tent (142 kJ/g) and the absence of harmful emissions duringutilisation in a hydrogen fuel cell (Das, 2009). The biological pro-duction of hydrogen can be considered as a clean process com-pared to conventional processes and requires a lower amount ofenergy than physico-chemical processes such as pyrolysis and gas-ification (Lin and Lay 2005). Moreover, this process becomes moreattractive if cheap raw materials are used as substrates, such as so-lid organic wastes from agricultural crops, industrial processes anddomestic wastewaters (Das et al., 2008). The microbial productionof hydrogen by fermentation can be broadly classified into twomain categories (dark fermentation and photo-fermentation). Darkfermentation is preferred over the photo fermentation also due tobeing energy efficient process. Here immersed membrane bioreac-tor plays a very important role and provides an immense potentialfor anaerobiosis as in one chamber the process can be done wheremembrane (ion exchange) can be utilised to separate or to recoverhydrogen without need of another chamber which anyway re-solves the problem of toxicity from air. During dark fermentationprocesses, such as classical anaerobic digestion producing primar-ily methane (CH4), the hydrogen production phase is very brief andoccurs before the sulphate-reduction (producing H2S) and metha-nogenic phases take over. The strategy adopted in this studyconsisted in promoting the initial acidogenic and acetogenic stepsby inhibiting the subsequent methanogenic step, which consumeshydrogen (Angenent et al., 2004). In theory, from a stoichiometricviewpoint, only 4 mol of H2 per mole of glucose are produced ifacetate is the sole end product and 2 mol of H2 per mole of glucoseif butyrate is the end product (Liu et al., 2005). There are severalways of inhibiting methanogenesis: for example chemical inhibi-tion of methanogenic bacteria (Chidthaisong and Conrad, 2000).promotion of ferri-reducing conditions (Van et al., 2004), pH con-trol (Kim et al., 2004), control of hydrogen partial pressure (Mizunoet al., 2000), methanogenesis can be inhibited during anaerobicelectro-stimulated degradation of organic waste, thus leading tohydrogen production by application of weak current to hydrogen

producing bacteria (Dictor et al., 2010), inorganic nutrient concen-trations (Lin and Lay, 2005) or temperature control. More recently,studies were performed using single-chamber biocatalyzed elec-trolysis, without a proton exchange membrane (Tuna et al.,2009). In the latter study, hydrogen was produced by electrolysisof volatile fatty acids generated during anaerobic treatment ofwheat powder waste and the method was found to be effectivewith a high energetic efficiency. Most studies have used acetateas a model compound for the biocatalyzed electrolysis experimen-tation, while both mixed and pure bacterial cultures have beenexamined for their potential as hydrogen producers. For example(Prasertsan et al., 2009) found that Thermoanaerobacterium spp.such as Thermoanaerobacterium thermosaccharolyticum, Treponemabryantii were dominant during biohydrogen production from palmoil mill effluent, using a thermophilic fermentative process.

Ethanol fermentation in immersed membrane bioreactor alsopossesses higher advantages as it offers high productivity, easiercontrols, less ethanol inhibition and less pollution, however; theyeast should possess multi stress resistance against the adversecondition offered by the closed circulating system (Ding et al.,2012).

6. Comparison between EMBR and IMBR

Mainly two configurations of MBR exist; internal/submerged/immersed where the membranes are immersed in and are integralto the biological reactor (Figs. 1a and b); and external/side stream(Fig. 2) where membrane are a separate unit process requiring anintermediate pumping step. IMBR/submerged configuration is usu-ally preferred configuration then external/side stream configura-tion especially for domestic water treatment as it needs smallarea and less energy. IMBR also could be either internal or externali.e. inside the biological reactor or inside the separate reactorwhere samples are transferred after biological treatment (Figs. 1aand b). The energy demand of IMBR is at the two orders of magni-tude lower than that of the external MBR configuration but oper-ates at a lower flux demanding more membrane area. Hollowfibre tube modules of the membrane are the most appropriate asit provides a huge membrane area. IMBR relies on coarse bubbleaeration to limit fouling as well as to achieve mixing. Comparisonhas been made between external membrane bioreactors and im-mersed membrane bioreactors that are characterised by differentoperating conditions such as membrane material, filtration mode,shear stress, etc. Effect of these two different processes on sludgestructure and microbial activity was determined (Clouzot et al.,2011). The different hydraulic retention time (HRT) when appliedto both the bioreactors with regard to the easily assimilated glu-cose and carbon source, no impact was exerted on microbial activ-ity. Clouzot et al. (2011) compared the effect of both the membranebioreactor configuration on activated sludge structure and biolog-ical activity. Soluble microbial products (SMP) release was higherin the external MBR (5 mgCOD gMLVSS�1) than in the immersedconfiguration (2 mgCOD gMLVSS�1) (Clouzot et al., 2011). The highshear stress induced by the recirculation pump in the external MBRwas shown to result in decreasing viscosity due to activated sludge(AS) deflocculation. An easier autotrophic microorganism develop-ment was observed in immersed MBR whereas; the external con-figuration gave better conditions for heterotrophicmicroorganism’s development. So the choice of the membrane bio-reactor configuration has to be made by considering the influentcharacteristics and the microorganism responsible for pollutantdegradation.

When compared to crossflow membrane bioreactors or conven-tional bioreactors, several investigators reported that immersedmembrane bioreactors offer significant advantages such as smaller

178 R.R. Singhania et al. / Bioresource Technology 122 (2012) 171–180

footprint, less sludge handling, less operator assistance require-ment, and less energy consumption (Cote et al., 1997) due to lowcross membrane pressure and low permeation flow rate comparedto external configuration. Although the lifetime cost of the sidestream system is high compared to that of the submerged systemthe nature of package plant market, being driven by CAPEX, maymake the low plant capital cost and simple operation the mostattractive option (Fletcher et al., 2007).

When a conventional activated sludge system fails, the qualityof the effluent deteriorates but when the immersed membranebioreactor fails the effluent quality remains unaffected but theeffluent flow rate decreases due to severe membrane fouling.Regardless of the mean cell or hydraulic residence time, IMBReffluent contain undetectable amount of suspended solids andhave low COD demand due to filtration by the membrane (Rosen-berger et al., 2002). Early MBRs were operated at solid retentiontimes (SRT) as high as 100 days with mixed liquor suspended solidsup to 30 g/L, but the recent trend is to apply lower solid retentiontimes (around 10–20 days), resulting in more manageable mixedliquor suspended solids (MLSS) levels (10–15 g/L). It has beenmade possible by advanced immersed membrane bioreactor whichcan operate at modest flux (25% or less of those of side streammembrane bioreactor). Due to these new operating conditionsthe oxygen transfer and pumping cost have tended to decreaseand overall maintenance has been simplified. There are now arange of MBR available commercially, most of which are immersedMBR and the typical hydraulic retention time ranges between 3and 10 h and uses two phase air flow for controlling fouling (Le-Clech et al., 2006).

Despite of several advantages there is still market for the sidestream configuration especially for industrial applications as itcan be installed at low level in a plant building. For a given flowrate the need for membrane exchange area is higher for IMBR com-pared to EMBR. In IMBR, membrane can be replaced only with thespecialist equipment. Both membrane bioreactor configurationsenable biological parameters such as biomass retention time andsludge loading to be adjusted to reduce the production of activatedsludge, but there are two significant differences:

(1) Mechanical breaking up of the floc in the case of non-immersed membrane configurations results in a ten-times-better surface/volume ratio, which helps to reduce produc-tion of excess activated sludge.

(2) The smaller size of the biological basins in non-immersedmembrane configurations, combined with a sludge recircu-lation system, produces a higher temperature in the basin,which also helps to reduce production of excess activatedsludge.

But the excess activated sludge from immersed membrane bio-reactors can be treated by draining or dewatering in a way that isvery similar to the method used to treat conventional activatedsludge in extended aeration. However, excess activated sludgefrom non-immersed membrane bioreactors requires special treat-ment because of the specific nature of the floc.

Appropriate pre-treatment methods are required to preventdamage to the membrane from solids such as sand and chips, foul-ing of the space between the membranes by floating matter orfibres, and, also from the chemical attacks on the membranes bysolvents or other substances for both the configurations, whereverapplied.

In immersed membrane configurations, the membrane is aconsumable used extensively (low cross-membrane pressure, lowpermeation flowrate), which results in very low energy consump-tion in the membrane module. But energy is also required for aer-ation and agitation beyond the needs of the biological basin to

improve the removal of activated sludge that severely fouls theimmersed hollow or flat-fibre membranes used in quasi-frontal fil-tration mode. The latest developments in this configuration there-fore aim for what could be described as ‘‘immersed tangentialfiltration,’’ which results in a continuous increase in the energyused along the membranes.

In non-immersed configurations, the membrane module can becleaned and regenerated, because it is used intensively (high cross-membrane pressure, turbulent regime, high permeate flowrate).This is only possible using more energy to maintain a degree ofturbulence at the membrane level. The non-immersed configura-tion requires more frequent chemical cleaning as clogging of themembrane is more serious than IMBR though it contributes to avery small part of the cost.

There are not many studies on the robustness of membrane per-formance available in public domain to be able to really find theperformance trend of other membrane bioreactor installations.Few reports are there for monitoring the full scale municipal mem-brane bioreactors (Lyko et al., 2008). The article presents the com-plete dataset on the occurrence of extracellular polymericsubstances (EPS) in a full-scale MBR and their relation to mem-brane fouling is reported. On the basis of broad monitoring of per-meability reducing factors, the effect of temperature, settleability,filterability of the sludge and permeate flux on membrane foulingwere investigated (Lyko et al., 2008). Economics of the two pro-cesses have been compared for brine treatment i.e. IMBR and Ionexchange for the removal of nitrate. Membrane cost renders theprocess uncompetitive against ion exchange for brine treatment(McAdam and Judd, 2008). Overall MBR is a very useful technologywhere selection of its different configuration has to be madecarefully based on application and other factors as energy, landarea, membrane properties, etc.

7. Conclusion

MBR offers several advantages, among which the most signifi-cant are small foot print, a high quality effluent generation andrequirement of less energy. Where water scarcity prevails it canconvert salty water or wastewater to drinking water and also haspotential for other bioprocesses. For anaerobiosis; IMBR could bea boon as the energy requirement as well as the rate of the reactionis comparatively less. It is commercially viable technology butneeds to be more economically efficient for its mass adoption. Re-search in membrane manufacturing and its maintenance is contin-uing which can be effective to further reduce the cost of the IMBR.

Acknowledgements

This work is a part of a scientific program supported by theFrench agency ‘‘Agence Nationale de la Recherche’’ (ANR) withthe reference ANR-08-BIOE-013. One of the authors, R.R. Singhaniathanks the FUI (Fonds Unique Interministeriel) for fellowshipthrough the program BAMI.

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