27membrane Science

REVIEW ARTICLE

Recent development in membrane science andits industrial applications

I Gede Wenten

Ph.D. (Chemical Engineering), Assoc. Prof., Department of Chemical Engineering, Institut Teknologi Bandung Jl.Ganesha 10 Bandung, IndonesiaCorresponding e-mail : [email protected], 20 December 2002 Accepted, 1 May 2003

The word membrane comes from Latinword, “membrana” that means a skin (Jones, 1987).Today’s word “membrane” has been extended todescribe a thin flexible sheet or film, acting as aselective boundary between two phases becauseof its semi permeable properties. Physically amembrane could be solid or liquid. Its functionis as a separation agent that very selective basedon the difference of diffusivity coefficient, electriccurrent or solubility.

Actually membrane has become an integralpart of our daily lives. All cells composing livingthings, including ours are surrounded with mem-brane. Biological membranes (membrane cells)are very selective that transfer only particularspecies.

Synthetic membrane history began in 1748when French Abble Nollet demonstrated semi-permeability for the first time, that animal bladderwas more semi-permeable to water than to wine.One century later, Fick published his phenome-

nological law of diffusion, which we still usetoday as a first-order description of diffusionthrough membranes. He was also the first man toprepare and study artificial semi-permeablemembranes. These membranes were made froman ether-alcohol solution of cellulose called “col-lodion”. After that many researches were doneand many inventions were found such as dialysis,different permeability of gases at rubber, osmoticpressure, and Donan’s ion equilibrium phenom-ena.

Sartorius Werke GmbH, Germany manu-factured industrial scale membranes, microfiltra-tion membranes, for the first time in 1950. Beforethat, membranes were developed in small scalefor laboratory applications (Lonsdate, et al.,1982). However, the most fundamental break-through in membrane technology came in late1950s when Loeb and Sourirajan discovered verythin membranes for reverse osmosis, the asym-metric membranes.

Development in membrane science and its applicationsWenten, IG.

Songklanakarin J. Sci. Technol.Vol. 24 (Suppl.) 2002 : Membrane Sci. & Tech. 1010

Nowadays membrane applications spreadover various industries: metal industries (metalrecovery, pollution control, air enriching for com-bustion), food and biotechnology industries (se-paration, purification, sterilization and byproductrecovery), leather and textile industries (sensibleheat recovery, pollution control and chemicals re-covery). Other industries that also use membranetechnology are pulp and paper industries (replac-ing evaporation process, pollution control, fiberand chemicals recovery), and chemical processindustries (organic material separation, gas sepa-ration, recovery and recycle chemicals). Medicalsector including health-pharmaceutical-and medi-cal industries (artificial organs, control release(pharmaceutical), blood fractionation, steriliza-tion and water purification), and waste treatment(separation of salt or other minerals and deioni-zation).

Generally, there are several processes tosynthesize membrane, some of them are sintering,stretching, track-etching, phase inversion, andcoating. There are several ways to classify mem-branes. Based on their materials, membranes areclassified as polymeric membrane, liquid mem-brane, solid (ceramics) membrane and ion ex-change membrane. Based on their configuration,membranes are classified as flat (sheet) mem-brane, spiral wound, tubular, and emulsion. Basedon what they do and how they perform, membranesare classified as fine filtration (microfiltration/MF, ultrafiltration/UF, nanofiltration/NF, and re-verse osmosis/RO), dialysis, electrodialysis (ED),gas separation (GS), carried-mediated transport,control release, membrane electrode, and perva-poration (PV).

Membrane processesMembrane processes discussed in this

paper are classified based on various drivingforces, some use pressure difference (microfiltra-tion, ultrafiltration, reverse osmosis, and piezo-dialysis), while others use other driving forcessuch as concentration difference (gas separation,pervaporation, liquid membrane and dialysis),thermal (membrane distillation, thermo osmosis)

and electric (electrodialysis).The principal advantages of membrane pro-

cesses compared to other separation processesare low energy consumption, simplicity and envi-ronmental friendliness. Membrane-based separa-tion is a result of different rate of transfer betweeneach substance in membrane and not a result ofphase equilibrium or mechanically based separa-tion. Therefore, there is no need to add additivematerial such as extractor and adsorber to proceedthe separation. Then we can say that membranetechnology is “clean technology”, in which no ad-ditive materials, which may be potential pollutants,are needed.

One of the major advantages of membranetechnology is low energy consumption. As dis-cussed elsewhere in this paper, membrane-basedseparation is not a result of phase equilibrium thattakes a lot of energy to achieve and maintain. Italso means that the process could be done in nor-mal conditions where no phase change occurs.Phase change may affect the quality of materialsand products. Therefore, membrane technology issuitable for the pharmaceutical, biochemical andfood industries.

Designs of membrane module are verysimple, compact and easy-to-use. In addition, notmuch auxiliary equipment is needed. There is aunique phenomenon in membrane where the scaleof process and operating costs are related propor-tionally. This phenomenon may be caused by themodular-nature of membrane. This nature distin-guishes membrane processes from other pro-cesses such as distillation, in which an increasein the process scale is followed by a decreasein cost until economical condition is reached. Notonly in cost spent, but also in operating condition.Adding several modules including its auxiliaryto existing system can do scaling up membraneprocesses.

Besides the advantages described above,membrane processes also posses several disad-vantages, such as flux optimization and selecti-vity, material sensitivity, fouling and dependabil-ity. Until now there have been several studiesconducted to overcome the disadvantages and



drawbacks in membrane processes.Flux and selectivity problems arise as an

increase in flux is usually followed by a decreasein selectivity, while we aim at increasing both.Therefore membrane processes are suitable forvery selective separation in which flux is not con-cerned such as that carried out in pharmaceuticalindustries.

The dependability problems arise as the char-acteristics of membrane differ from each other. Itis due to the different characteristics of each mem-brane that a direct scale up of membrane processesis virtually impossible. Before a process is appliedin an industrial scale, it is suggested to have a labo-ratory assessment of the membrane. In this way wemay have better prediction on process performance.

Other major problems are material sensitiv-ity and fouling. Polymeric membranes have lim-ited stability (chemically, physically, and biologi-cally), which restrict the conditions of membraneprocesses applied. Nowadays, there are efforts toinvent materials, which may overcome these con-straints. Fouling causes a decline in performanceof membrane processes, in which flux (perform-ance) is very high initially but then decline drasti-cally as materials of foulant accumulate on mem-brane surface. Solutions to the problem may lie inthe hydrodynamic of the process and pretreatmentprocesses.

Membrane industry and market potentialMembrane-based market industry covers

the membrane itself and its module, includingadditional equipment and the systems. Commer-cial success is one of indicator showing the im-portant role played by membrane technology invarious applications. The benefits in membraneprocess applications include reduced operatingcosts relative to competitive technology, savingof product, recovery of by-products, savings ofwater, energy, chemical, etc. In effluent reductionapplications, savings in transport and disposalcost become important (Srikanth, 2000). Mem-branes and membrane processes are used in fourmain areas, which are, in the separation of mole-cular and particulate mixtures, in the controlled

release of active agents, in membrane reactorsand artificial organs, and in energy storage andconversion systems. Membrane has become amulti billion-dollar business and is still growingfast. The worldwide membrane market in 1998particularly in sales of membrane and modulesreached more than 4 billion USD. While sales ofmembrane system reached more than 15 billionUSD (Strathmann, 2001). Figure 1 shows annualsales of membrane and modules for various mem-brane processes. It can be seen that the sales wereincreased over the years and reached a value ofapproximately 4500 million USD in 1998.

From the applications view, approximately40% of membrane sales are destined for waterand wastewater applications, while food and bev-erage processing combined with pharmaceuti-cals and medical applications account for another40% of sales and the use of membranes in che-mical and industrial gas production is growing(Wiesner and Chellam, 1999). The total membranemarket is unevenly distributed, 75% of the marketshare belongs to USA, Japan and Western Europe(Srikanth). The development of membrane mar-ket is determined by energy costs, required prod-uct quality, environmental protection needs, newmedical therapies, and the availability of new andbetter membranes and membrane processes(Strathmann, 2001) .

Some applications of membrane processes,such as water desalination or wastewater treat-ment, have high industrial relevance. However, inthese applications the membrane processes com-pete with conventional water desalination orwater treatment techniques, such as multistageflash evaporation or biological sewage treatmentplants. In other applications of high commercialrelevance, such as in hemodialysis or in fuel cells,membranes are key components, and no econo-mic alternative technique that could compete withmembrane is currently available. There are otherapplications, such as the production of ultrapurewater, where membrane processes compete withconventional techniques, but have a clear advan-tage. There are also a large number of membraneapplications of lower industrial relevance, such as



the dehydration of organic solvents by perva-poration or the recovery of organic vapors fromwaste air streams by gas and vapor permeationmembranes. In certain biosensors and diagnosticdevices, membranes are key components, but interms of the total costs of the final device, the costof the membranes in these devices is negligiblylow. Therefore, this application is often of lesserinterest to the membrane producing industry(Strathmann, 2001). As the membrane markethas grown, the scale of membrane facilities hasbecome more ambitious. The first UF facility forpotable water treatment, inaugurated in 1988 atAubergenville, France, had a design capacity of160 m3/day. Today, facilities’ exceeding 100,000m

3/day is being planned (Wiesner and Chellam,

1999).In spite of impressive sales and growth rate

of the industry, the use of membranes in indus-trial-scale separation process is not without tech-nical and economic problems. Technical problemsare related to insufficient membrane selectivities,relatively poor transmembrane fluxes, generalprocess operating problems, and lack applicationknow-how. Economic problems originate from

the multitude of different membrane productsand processes with very different price structurein a wide range of applications, which are distrib-uted by a great number of sales companies, veryoften as individual products (Mallevialle et al,1996).

In future, the largest market for membranewill continue to be water treatment, with salesto manufactures of consumer water purificationequipment becoming more important. The primarygroup of customers (electric utilities, industrialwater users and municipal water works) will con-tinue to dominate demand in this sector, but thissegment is rapidly maturing and sales are increas-ingly dependent upon replacements for existingsystems. In addition, as both the physical andchemical means of cleaning membranes continueto improve, the average life span of the mem-branes is lengthening, thus reducing replacementsales (Wiesner and Chellam, 1999).

Emerging processes (Strathmann, 2001)Development of improved membranes

and membrane materials Significant progresshas been made during recent years in the develop-

Figure 1. Annual sales of membrane and membrane modules forvarious membranes processes worldwide.

Note: All data only cover membrane and modules membrane sales, and do not include sales of support system(Strathmann, 2001; Manem and Sanderson, 1996)



ment of new membranes and their applications.New inorganic and organic materials, super mo-lecular structures with specific binding properties,are used as membrane materials. For the separa-tion of gases, especially oxygen/nitrogen andmethane/carbon dioxide, new glassy polymersand inorganic materials such as zeolites are usedto produce membranes with better selectivity andhigher fluxes. For the separation of enantiomers,carrier-facilitated transport membranes are pro-duced using molecular imprint techniques. In re-verse osmosis, membranes with better chemicalstability and higher fluxes are now available. Sur-face-modified membranes with better compatibil-ity and affinity membranes for the removal ofendotoxins or other toxic components from bloodmay soon be available. The recent developmentin membrane technology have been assisted bynew research tools, such as atomic force micro-scopy, acoustic time-domain reflectometry, mole-cular dynamic simulations, and computer-aidedprocess design.

High-performance reverse-osmosis mem-branes The Nitto Denko Corporation has led theprogress that has been made in improving re-verse-osmosis seawater desalination membranesduring the last 20 years, which shows the salt re-jection in excess of 99.5% and the water flux ofvarious membranes by a factor of 3. The reasonfor this significant progress is based on the pre-paration technique of the barrier layer of the com-posite membrane, which has many folds, with theresult, that the surface of the actual barrier layeris about three times larger than the area of thesupport structure.

Stabilization of supported liquid mem-branes One of the shortcomings of today’s sup-ported liquid membranes is their short usefullife. In thin membranes the solvent or carrier canbe lost within several hours, which makes themembrane useless. The stability of liquid mem-branes can be increased drastically up to 1000 hoursby placing a thin polymer layer on top of the liquidmembrane.

Preparation of composite hollow fiber bythe triple-nozzle spinneret Asymmetric hollow

fiber or capillary membranes with a denser skinon the in- or outside of the fibers are generallymade by a phase-inversion process. Dip-coatingprocess is mostly used to produce composite hol-low fiber membrane although it require additionalproduction step. A triple-nozzle spinneret wasdeveloped for the preparation of composite hol-low-fiber membranes. The main advantage ofcomposite hollow fibers made in one step withthe triple-nozzle spinneret compared to thosemade by dip-coating is a simplified productionprocess because it can be done in a single pro-duction step. Generally, higher fluxes are alsoobtained in the single-step production, since porepenetration, which is often a problem with dip-coating, is avoided.

Inorganic membranes for gas and vaporseparation with high selectivity Historically,inorganic membranes are produced by a slip-coat-ing and sintering procedure based on metal ox-ides such as α-Al

2O

3. These membranes can be

considered as state-of-the-art structures and areused today in micro-and ultrafiltration. An inter-esting recent development is the preparation ofzeolite membranes. Because of the unique proper-ties of zeolite crystals such as molecular sieving,ion exchange, selective adsorption, and catalysis,these membranes have a large number of potentialapplications in gas and vapor separation and inmembrane reactors and chemical sensors. Denseinorganic membranes based on palladium andpalladium alloys have been used for many yearsfor the selective transport of hydrogen. However,their large-scale industrial applications are limiteddue to high price of the metal. Dense ceramic mem-branes based on perovskites exhibit high mixedelectronic and oxygen ion conductivity, and arewidely studied for applications in solid oxide fuelcells, oxygen sensors, and membrane reactors. Anincreasingly important research area is related tonanoporous ceramic membranes with well-definedpore structures prepared by template-assisted, self-assembling methods. Furthermore, an increasingamount of research effort is concentrated on thedevelopment of proton-conducting membranesfor high-temperature applications in fuel cells and



membrane reactors.Development of improved membrane mo-

dules The overall performance of the state-of-the art membrane modules, such as the plate-and-frame, the spiral-wound, and the hollow-fiber andcapillary membrane modules has been improvedgradually over recent years, and production costshave been reduced significantly. However, onlyvery few completely new module concepts havebeen developed. Two exceptions are the so-calledtransversal flow capillary membrane module andthe spiral-type tubular module. The transversalflow module is used mainly in dialysis. The char-acteristics of these modules are straight mem-brane capillaries and axial flow through the fiberlumen and the shell. In spite of the poor flow dis-tribution, and thus mass transfer, at the shell-sidemembrane surface, this type of module is pre-ferred because of its high packing density and lowproduction costs. Spiral-type tubular membranemodule involves flow around a curved tube at asufficiently high velocity so as to produce centri-fugal instabilities and secondary flow from themembrane surface to the center of the tube, andresults in a substantial increase in flux. However,higher production costs and poor performance ofthe spiral-type membrane modules have so farlimited any large-scale industrial applications.

Development of novel membrane processesand applications New membrane processes thatgive a large breakthrough in its applications aredemineralization by electrodeionization technique,new application in biomedical science, and appli-cation in fuel cells. Actually, a significant deve-lopment has been made in another process, forexample, in controlling waste gas emission bymembrane contactor and membrane reactor, forboth chemical as well as biological conversion.Development in membrane reactor for dehydrogenating reaction, esterification, and enzymaticreaction, seem very prospective that has been de-veloped for a long time, however, until today itsindustrial application has not been seen yet. It isthe same as membrane contactor for emissioncontrol. Typical study and empirical data show alarge potential of this process; however, its ap-

plication has also not been seen yet.Electrodeionization and the use of bipo-

lar membranes Electrodeionization are used forthe production of deionized water of high qualityby combining conventional ion-exchange tech-niques with electrodialysis. The process can beoperated continuously without chemical regen-eration of the ion-exchange resin. The only disad-vantage of the process is the relatively poor cur-rent utilization. Bipolar membranes are usedtoday in combination with regular ion-exchangemembranes for the production of acid and basesfrom the corresponding salts in a process referredto as electrodialytic water dissociation. A bipolarmembrane consisting of a cation- and an anion-exchange layer arranged in parallel between twoelectrodes. As in electrodialysis, up to 100 cellunits can be stacked between two electrodes.Electrodialytic water dissociation is a very energyefficient way. However, there are still severeproblems, such as salt leakage into the productsand low current utilization at high concentrationsof the acid and bases.

Membrane contactors In membrane con-tactors the membrane functions as a barrier bet-ween two phases that avoids mixing but does notcontrol the transport rate of different componentsbetween the phases. The membrane pores are suf-ficiently small that capillary forces prevent directmixing of the two phases. A key advantage ofmembrane contactors is a large mass-transferarea in a relatively small device. A typical large-scale application of a liquid/gas contactor is theremoval or delivery of dissolved gases from or toa liquid, for example, the blood oxygenation dur-ing open-heat surgery, the removal of oxygen dur-ing the production of ultrapure water, and theseparation of olefin/paraffin gas mixtures.

Membrane reactors A membrane reactoris a device that utilizes the properties of a mem-brane to improve the efficiency of chemical orbiochemical reactions. Various forms of membranereactors are applied mainly in catalytic and en-zymatic reactions. In the simplest form of a mem-brane reactor the membrane is used as a contac-tor that separates the catalyst from the reaction



medium. The membrane merely provides a largeexchange area between the catalyst and the re-action medium, but performs no separation func-tion. It is often used in cell culture and fermenta-tion processes such as the enzymatic degradationof pectin in fruit juice. In the second type of mem-brane reactor the membrane shows the selectivemass-transport properties, and is used to shift theequilibrium of a chemical reaction by selectivityremoving the reaction products, for example, indehydrogenation or oxygenation reactions suchas the dehydrogenation of n-butane. The thirdtype of membrane reactor combines the membranecontactor and separation function, such as inenzyme catalyzed deesterification reactions.

Membranes for fuel cells / electrolysis(Srikanth) One breakthrough in the applicationof ionic conducting polymer membranes is theproton exchange membrane fuel cell, a devicethat converts chemical energy directly into elec-trical energy without burning. As the electroche-mical combination of hydrogen (the fuel) andoxygen produce water, the fuel cell is environ-mentally clean and is expected to replace thegasoline engine or rechargeable battery in theautomobiles.

Synthetic membranes in medical appli-cations Biomedical applications are by far themost relevant use of synthetic membranes. Mem-branes are used in medical devices such as he-modialysers, blood oxygenators, and controlleddrug-delivery systems. There is, however, a sub-stantial effort focused on the development of themembrane for the next generation of artificialorgans, such as the artificial liver or artificialpancreas. In this device, as in other novel vehiclesfor the delivery of cell and gene therapy, syntheticmembranes are combined with living cells toform so-called biohybrid organs.

Breakthrough in idustrial applicationMembrane processes cover a wide-range

of application from (waste) water treatment tomedical application. For some application, mem-branes play an important role in which othertechnologies are not capable such as in medical

sector (hemodialysis) and energy sector (fuel cell).Table 1 shows a list of selected industrial appli-cations of membrane processes.

From Table 1, it can be seen that pressure-driven membrane processes dominate the appli-cation rather than other driving-force membraneprocesses. Yet, it doesnot mean that other driving-force membrane processes are not having part in

Table 1. Selected Industrial Applications of Mem-brane Processes (Mulder, 1996; Rautenbachand Albrecht, 1989; Wenten)

Industrial Sector Membrane Processes

Drinking water NF, UF, RODemineralized water RO, ED, EDIWastewater Treatment

Direct (physical) MF, NF, RO, EDMBR MF, UF

Food IndustryDairy UF, RO, EDMeat UF, ROFruit and vegetables ROGrain milling UFSugar UF, RO, ED, MF, NFBeverages

Fruit juice MF, UF, ROWine and brewery MF, UF, RO, PVTea factories MF, UF, NF

BiotechnologyEnzyme purification UFConcentration of fermentation broth MFSCP harvesting MF, UFMembrane reactor UFMarine biotechnology MF, UF

MedicalControl release UFHemodialysis RO, UF

Chemical industryGas separation

Hydrogen recovery GSCO

2 separation GS

Vapor-liquid separationEthanol dehydration PVOrganic recovery PV

Chlor-alkali process Membrane electrolysisEnergy

Fuel-cell Proton exchange membrane



industrial sectors. GS (gas separation) and PV(pervaporation), which are concentration-drivenmembrane process, are used in chemical industrysector for gas separation and vapor-liquid separa-tion. In USA and EU, the most important sectorthat using membrane technology is food industryespecially dairy sector. Membranes are used fordesalting and concentration of whey, and also forconversion of milk into cheese and preparation ofegg white and egg yolk. Fruit and vegetable pro-cessing also utilized membrane technology re-sulted in high energy saving. Beverage industryalso an important sectors in which to apply mem-brane process particularly for separation of alco-hol from beer. In sugar sector, membranes areused in almost 20% of the potential applicationin Netherlands (Rachwal et al., 1994). In chemi-cal industry, beside listed in table above, electro-coat paint recovery still the biggest industry sectorthat utilizes membrane-based processes particu-larly UF. Other sector that predominant for UFapplication is pure water supply for semiconduc-tor industry. High-demand for ultra-high puritychemicals in same industry also fulfilled by theavailability of chemical-resistant membrane. Inbiotechnology, purification of enzyme can be ac-complished using UF. In medical sector, mem-branes play a key role particularly in hemodialy-sis and control release process. Water-oil separa-tion now also become a potential application forUF especially in metal cleaning and wool scour-ing processes (Nuner and Peinemann, 2001).

Membrane technology also makes a break-through in water treatment. A revolutionary watertreatment process was accomplished by corpo-rate membrane into the treatment process result-ing in high effluent quality. Water treatment hasbecome an important issue regarding to the factof the scarcity of clean water sources. A hugesupply of water that covers the earth in form ofocean can not fulfilled the needs of water readilyconsidering conventional desalination processthat must be done before it can be used as potablewater. Desalination with distillation process popu-lar in 1970's is not attractive anymore because ofits high capital cost and wide space demand. In

area with high minerals content, distillation proc-ess are also susceptible to corrosion.

Conventional water treatments are not ca-pable in producing potable water that fulfilledthe requirement of water quality standard that be-coming more stringent nowadays. An advancedwater treatment like ozonation and activated car-bon can improve water quality but on other handadd in a difficulty in operation and cost. Mem-brane technology offers one or two simple steps toovercome it. Membrane as highly selective layeris capable to separate microorganisms’ pathogencompletely. Membranes are also capable of re-ducing hardness, controlling color, and removinginorganic and organic compounds. Four membraneprocesses have direct application for potable watertreatments are RO, NF, UF, and MF. These pres-sure-driven processes differ in the size of themembrane pores, types of constituents removed,and the way removal is achieved (Manem andSanderson, 1996). Water with low quality is ac-ceptable to be processed by membrane. In spacedemand, membrane processes require a smallerspace compared with conventional technology. Anumber of examples from well-known and emerg-ing application of membrane processes in thedrinking water industry are desalination usingRO, softening using NF, and production of drink-ing water from surface water, backwash waterand nitrate removal using ED (Saxena and Bhard-waj, 2001B.) .

The development of membrane scienceand technology offers various membrane mate-rials. Membrane for potable water productioncommonly made from organic polymer (Manemand Sanderson, 1996B.) but ceramic materialsalso can be used (Bottino, et al., 2001). PVDF(polyvinylidenefluoride) as one of fluorocarbonmaterial is suitable to use as membrane-producepotable water because of its resistance againstvarious oxidants (Saxena and Bhardwaj, 2001A.,2001B; Rachwal, et al., 1994). PVDF membraneis a hydrophobic membrane and can easily inte-grate with other technology i.e. pulsed UV light,chlorine dioxide, etc. These integrated systemseconomically fulfilled the SWTR (Surface Water



Treatment Rule) and DBP (Disinfection By Pro-duct) regulations (Saxena and Bhardwaj, 2001B).

Modules configuration mostly used arespiral wound and hollow fiber. HFMF (hollowfiber microfiltration) can be used to produce po-table water for small community (300,000-gphequi-valent to 3000 peoples). Nowadays, HFMFbecome Best Available Technology (BAT) to sup-ply small community by treated surface water/ground water resulted in accomplishment of re-quirement stated by SWDA (Safe Water DrinkingAct). HFMF can operate with dead-end mode orsome degree of circulation. For surface watertreatment, it can achieve a flux range 35-50 gallonper feet quadrate per day (Saxena and Bhardwaj,2001B). In various treated water, MF PVDF 0,1micron with outside-in configuration can achievedflux of 75 gph (Saxena and Bhardwaj, 2001B)with 0.3 bar pressure operation for clean mem-brane and 2 bar for fouled-membrane. High qual-ity, less than 0.05 NTU drinking water can beassured by intact MF membranes, which essentiallyremove 100% of Cryptosporidium and Giardiaand exceed SWTR log removal requirements(Saxena and Bhardwaj, 2001A).

MF and UF started to be used for watertreatment since 1980s. Applications of MF andUF as low-pressure membrane process begin toincrease rapidly in 1994. This is caused by thelowering cost of processes and also a decrease inenergy consumption, which can be attained to lessthan 1 kwh/m3 for 1-10 bar operation pressure(Rachwal et al., 1994). Before the year 1994, thecapacities of all MF and UF factories are lessthan 3 million gallon per day. Nowadays with thegrowing of understanding and the availability ofthe technology, a new facility of membrane withcapacity 2 – 10 million gallon per day is being builtin US (U.S.Water News Online, 1999). RO and EDnow become the chosen methods for desaltingseawater to produce potable water. RO particu-larly have emerged as an effective solution totransform saline, brackish, and contaminatedwater into usable and/or potable product (Manemand Sanderson, 1996C). In 1988, 49,4% of totaldesalination factories worldwide is using RO

(Saxena and Bhardwaj, 2001). Seawater desalina-tion processes for drinking water purposes iswidely practiced in Middle East and claimed tobe 2/3 of desalting capacity in the world. How-ever, the largest plant of RO is located in Yuma,Arizona with the capacity of 660 million gpd fol-lowed by a plant in Saudi Arabian (Saxena andBhardwaj, 2001). RO plants installed in Bahraindesalinate highly brackish water, although thecost is relatively high compared with the cost oftreating fresh water by conventional means, it iscertainly an economically feasible alternative totransporting water over long distance (Mallevialleet al., 1996) .

Technology for wastewater treatment thatmight provide several advantages compared withconventional biological process alone is mem-brane bioreactor. Membrane bioreactor (MBR)can be defined as the combination of two basicprocesses, biological degradation and membraneseparation. Biological process commonly used forwastewater treatment combined with membraneprocess is activated sludge process. Currently, themajority of installed MBR system are being usedfor the treatment of wastewater from the automo-tive, cosmetic, metal fabrication, food and bever-age processing, landfill leachate, and other indus-tries (Adham and Gagiiardo, 1998). MBR can becategorized into three types that are MBR forbiomass separation, MBR for aeration, and MBRfor pollutants extraction. Submerged membranebioreactor for biomass separation is a break-through in membrane bioreactor field for indus-trial wastewater treatment that for the first timewas introduced by Yamamoto (1989). The sub-merged MBR system should be distinguishedfrom other MBR system, in this system membraneis installed inside biological reactor. The drivingforce for submerged MBR is the pressure gra-dient that limits the pressure up to 1 atm. Thispressure gradient can be applied only by a suctionpump. Energy consumption is low because thereis no re-circulation pump. Study which was doneby Yamamoto have succeed to treat waste waterwith an aerobic system and stable flux of about0.1 kg COD/kg MLSS.day F/M ratio, critical



organic loading from 3 to 4 kg COD/m3.day anda very low power consumption of about 0.007kWh/m3.day. In the same manner as the othermembrane bioreactor processes is compared toconventional biological process, this technologyhave a main advantage in high-quality effluents(BOD/TSS< 5 ppm), disinfected, a very low ex-cess sludge production, high biodegradation ef-ficiency and a relatively small place necessity.Therefore, in the future, this technology has alarge potential to be applied in several water andwaste water treatment. The key of this technologyis to overcome washout biomass (that to be aproblem in conventional process), because inMBR, hydraulic retention time (HRT) and sludgeretention time (SRT) are independent.

In the way of using MBR for water reuse,the application can be seen in several developedcountries such as in Japan and USA. The factorthat makes MBR become an attractive alternativeis the superiority of this technology in producinga very low excess sludge or even zero sludgeproduction and it has been properly proven. Themembrane bioreactor process is probably the besttechnology for water recycling inside buildingsrequiring compact system and excellent waterquality. Two different systems are experiencinglarge commercial success: UBIS in Japan (Lam-bert, 1983; Roullet, 1989) and Cycle-let in theUnited States (Irwin, 1989,1990). The Ultra Bio-logical System (UBIS) is a membrane bioreactorsystem where aerobic-activated sludge reactorcombined with membrane unit. This UBIS systemis installed in more than 40 buildings and pro-duces more than 5000 m3/day (Manem and Sand-erson, 1996). The Cycle-let is also a membranebioreactor system, which is based on the combi-nation of a two-phase biological treatment system(anoxic and oxic) with tubular organic mem-branes. Activated carbon for color removal andozone for disinfection are used to finish and im-prove the water quality. Today, the Cycle-let isinstalled in more than 30 installations in USA.Wastewater treatment and recycling in apartmentbuildings is also performed in Europe, where theLyonnaise des Eaux group developed an MBR

process using a ceramic tubular-type membrane.In Japan, the population relies on three domesticwastewater treatment categories: public sewage,on-site treatment tanks, and collected humanexcreta treatment system, which is also called the“night-soil” treatment system (Magara and Itoh,1991). Magara et al. (1994) reported that thereare about 1200 night-soil treatment system acrossJapan that treat more than 42 million populationequivalents of night soil and 30 million popula-tion equivalent of on-site treatment tank sludge.The technology of membrane bioreactor can treatthis high concentration effluent. Several pilotstudies have been reported and at least six full-scale plants are under operation with the ActivatedSludge and Membrane CompleX System (AMEX)technology (Roullet, 1989) (Manem and Sander-son, 1996) .

EDI (electrodeionization) is a continuouschemical-free deionization process that relies onthe same fundamental principle as for mixed-bedion exchange. An EDI stack consists of dilutedcompartments concentrated compartments andelectrode compartments. The diluted compartmentsare filled with mixed-bed ion-exchange resins,which enhance the transport toward the ion-ex-change membranes under the force of a directcurrent. The later configuration, both diluted com-partments and concentrated compartments arefilled with mixed-bed ion-exchange resins. Sincethe concentration of ions is reduced in the dilutedcompartment and is increased in the concentratedcompartment, the process can be used for eitherpurification or concentration.

The concept of electrodeionization (EDI)process has been extensively recognized sincethe mid-1950s. Walters et al (1955) investigateda batch EDI process for concentrating radioactiveaqueous wastes, based on electrolytic regenera-tion of ion-exchange resins, and proposed an ionicconduction mechanism through mixed-bed ion-exchange resins in contact with dilute solution. Inthe late 1950s and early 1960s, Glueckauf (1959)investigated theory, design, and operating param-eters of the EDI process. The proposed theoreticalmodel involved the diffusive transfer from the



flowing solution to the ion-exchange resin beadscombined with the electrolytic transfer of ions alongthe chain of ion-exchange beads. Sammons andWatts (1960) evaluated multi-cell EDI moduleand quantified the relationships among solutionconcentration, flow rate and applied current.

An extended investigation of operatingconditions and performance of the EDI processhas been conducted by Matejka (1971) for high-purity water production from brackish or tapwater. Several researchers were also activelystudying EDI process (Korngold, 1976; Kedemand Maoz, 1976; Govindan and Narayanan, 1981;Ganzi et al., 1992; Neumeister et al., 1996; andWang et al., 2000). During this time, numerouspatents were granted for various types of EDIdevice and its applications (White, 1992; Orenet al, 1992; Giuffrida and Ganzi, 1993; Ganzi,1993; Sugo et al., 1994; Ganzi, 1994).

Millipore introduced the first commercial-ly available module and component systems in1987 under the trade name Ionpure CDI (Korn-gold, 1976). There are presently two sizes of EDImodules available from U.S. Filter. So-called"industrial" module can have 30-240 dilutingcompartments and are capable of flow rates of2.0-64.0 gpm (0.45-14.5 m3/h). The smaller ver-sions, "compact" EDI modules, typically have10-40 diluted compartments and are capable offlow rates of 0.5-4.0 gpm (0.1-0.9 m3/h). U.S. Fil-ter is dominant manufacturer of industrial sizeEDI. Millipore Corporation fabricates low-flowEDI device (under trade name Elix) for labora-tory water purification applications. Recently,other companies (Christ Ltd., Electropure Inc.,Elga Ltd., Glegg Water Conditioning, Ionics Inc.,and Osmonic Inc.) have begun to offer EDI units.Because of inherent process limitations, the EDIprocess may be not competitive with conven-tional technologies. These limitations include:(1) A mixed-bed ion-exchange resins in stackcompartments is a good filter media; (2) Anion-exchange resin tends to adsorb negative charged-colloids; (3) Precipitation of inorganic compo-nents occur at high pH regions; (4) Quaternaryamine is converted into tertiary amine or union-

ized group; (5) Channeling due to the stack designis not proper. In addition, the technology is laborintensive to assemble and requires a multitudeof thin compartments. The inability to produceexcellent ion-exchange membranes results in theloss of electrical efficiency.

Limited success in overcoming the chal-lenges mentioned above has been reported. Newmembranes have been developed but are not costeffective. Hence, carefully pressure control ismuch more effective to minimize electrical effi-ciency loss due to convective transport acrossion-exchange membranes. In order to reducescale formation, Tessier et al (Gallagher, 1996)proposed the addition of scaling agent into con-centrate and electrode rinse in the concentrationrange of 1 to 40 ppm. Special techniques for intro-ducing and removing ion-exchange resins andother particulates from an assembled EDI stackto reduce labor cost also have been patented byParsi et al (Parsi et al., 1993)

Since the initial commercialization in 1987,EDI systems have been applied worldwide tomeet the need of high purity water, such as inpharmaceuticals, semiconductors, power, and highquality optics industries. Limited ability of exist-ing technology to reduce especially the need ofhigh water production cost effectively and environ-ment friendly offers a significant market oppor-tunity for the commercialization of EDI systems.

Although electrodeionization is mainly ap-plied to ultrapure water production, it is increas-ingly used in a wide variety of applications. Somesystems have been installed for wastewater treat-ment (Ganzi et al, 1992; Donovan). There mayalso be EDI applications for starch processing(Widiasa, 2002), recovery of citric acid from fer-mentation broth (Sutrisna, 2002). Millipore Cor-poration has continued to apply EDI to specialtyseparations in the pharmaceutical industry.

The backshock processThe performance of membrane in cross-

flow membrane filtration is strongly influencedby the build up of a fouling layer that finally maycompletely plug the porous membrane surface.



Jonsson et al. showed that for MF membranes, thepore blocking type of membrane fouling is byfar the most predominant. Two approaches havemainly been used to increase the fluxes in MF:applying high crossflow velocities (2-6 m/s) and,the use of backflush technique. In bacfkflush tech-nique, the direction of the permeate flow throughthe membrane is periodically reversed. However,high velocities are energy demanding, and giveproblems with too high-pressure losses in themembrane modules; backflushing also reducesthe effective operation time, and gives a loss ofpermeate to the feed solution.

Very short backflush intervals were re-ported by APV passilac where the backflush du-ration is 1-5 seconds, and they are performed 1-10times per minute at a pressure difference of 1-10bar. This generally means that backflushing ac-counts for about 10-20% of the total operationtime, by which the product flux may improve upto 100%; however, the negative flux during back-flushing has to be taken into account. Since theimpact of backflushing in industrial application isvery limited, because of its fundamental limita-tion, i.e. loss of permeate and operation time, thebackflush process needs adequate optimisation.

The backflush process is optimized bothfor the duration of the backflush and for the back-flush interval. The improvement of the productrate upon backflushing is mainly a function of thebackflush pressure and the interval between twobackflushes. A novel backflush technique with ahigh frequency and extremely short duration timeshas been introduced. It was found that extremelygood results could be obtained using very shortbackflush time (typically 0.06 second) with aninterval time of maximum 5 seconds, preferably1 to 3 seconds. Since the effective backflush timeis very short and the backflush pressure is rela-tively high (typically 1 bar over the feed pressure)the name “backshock” is introduced. The loss ofpermeate during backshocking is very low andhardly affects the net permeate flow.

The novel backflush technique known asbackshock technique in combination with theuse of reversed asymmetric membrane structures

allows filtration at extremely low crossflow velo-cities with very stable permeate fluxes. A hollowfiber membrane made of a very pressure-stablepolymer, with a rather thick skin layer on the out-side of the fiber (pore sizes around 0.6 µm) anda very open structure (pore sizes up to 20 µm) atthe inner surface, is used for clarification of fer-mentation broth (brewing process). Since the larg-est pores are in contact with the feed (beer), theyeast cell can partially penetrate into the porousstructure. Without any backshock at all, thiswould lead to very low fluxes but the very fre-quent backshock prevents the membrane from de-finitive clogging, and enables a filtration processwith an extremely stable flux level (Figure 2).

The fouling layer as such is very likelypresent inside the porous structure of the mem-brane and controlled by the backshock techno-logy. Since the fouling layer is deposited duringthe first few seconds, and the shear stresses both inthe flow channel and at the pore entrance have asignificant effect on how proteins are denaturatedand adsorbed at the membrane surface, a hydrauliccleaning of the membrane surface is performedevery few (1-5) seconds using a pressure of lessthan 0.1 seconds. As a result of the extremely shortdurations, the loss of permeate during backshockingis negligible. In addition, it is found that very goodtransmissions of high MW components could beobtained in combination with a very low turbidityof the feed.Figure 2. Comparison between normal and reverseasymmetric membranes in combination withbackshock (backshock duration is 0.1 s, interval is5 s, crossflow velocity is o.5 m/s; TMP = 0.2 bar(normal) and 0.05 bar (reverse asymmetric); thebackshock was stopped after 72 hours)

Future industrial prospectIntensive researches and development in

membrane area are related closely to the rapidgrowth of membrane technology. It is certainlynot easy to predict the future of this technology.However, below are presented some facts that canroughly describe the future trend of this technol-ogy.



Firstly, current rapid development in alter-natives of membrane materials, membrane pro-duction processes and the increase of membraneproduction, and followed by the membrane qualityenhancement. Consequently, membrane price tendsto decrease and the process is more economical.This is causing wider membrane technology im-plementation especially in application that re-quires high productivity and low cost as in waterand wastewater treatment.

Secondly, for high-pressure and large ca-pacity membrane processes like high-pressure re-verse osmosis, energy recovery units have alreadybeen developed. This allows 70% energy recovery,so that the process becomes less expensive.

Thirdly, ultra low-pressure membrane withhigh productivity has also been developed; hencereverse osmosis process that used to operate onhigh pressure (60-80 bars) could operate on lowerpressure (≈ 20 bars). This condition obviouslylowers the energy consumption. Moreover, proc-ess system and equipment specification becomesimpler.

Next, the fourth factor deals with the envi-ronmental conservation fee. With refers to envi-ronmental regulation, industry that produce wastehas to pay an environmental fee, amount to theproduction capacity or waste volume produced.Since membrane technology is a clean techno-logy, it produces minimum waste or even none. Inaddition, membrane technology is one of thewaste treatment technologies that improve wastequality or even better than that conditioned inthe Industrial WasteWater Standard. It is a tech-nology that can reuse water. Application in starchindustry at PT. Raya Sugarindo, Indonesia, not onlyrecovers tapioca but also produces (reuses) highquality water used for process water.

In Indonesia particularly, membrane technol-ogy still looks exclusive and expensive. Besides,there are some inhibiting factors such as theunstandardized membrane and laboratory assess-ment requirement that resists the widening ofmembrane application. As mentioned in the be-ginning of this discussion, membrane based proc-esses also possess several disadvantage, such as

Figure 2. Comparison between normal and reverse asymmetric membranes in combination with back-shock (backshock duration is 0.1 s, interval is 5 s, crossflow velocity is 0.5 m/s; TMP = 0.2 bar(normal) and 0.05 bar (reverse asymmetric); the backshock was stopped after 72 hours)



flux optimization and selectivity, material sensi-tivity, fouling, and dependability. Until now therehave been several studies conducted to overcomethe disadvantages and drawbacks in membraneprocesses. For example, researches dealing withhydrodynamic techniques as one alternative wayto overcome fouling.

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