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Journal of Environmental Management 91 (2010) 1039–1054

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Journal of Environmental Management

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Review

Bioreactors for treatment of VOCs and odours – A review

Sandeep Mudliar*, Balendu Giri, Kiran Padoley, Dewanand Satpute, Rashmi Dixit,Praveena Bhatt, Ram Pandey, Asha Juwarkar, Atul VaidyaEnvironmental Biotechnology Division, National Environmental Engineering Research Institute (NEERI), Nehru Marg, Nagpur 440 020, India

a r t i c l e i n f o

Article history:Received 25 August 2008Received in revised form12 December 2009Accepted 3 January 2010Available online 23 February 2010

Keywords:VOCOdourBiological treatmentBioreactorBiofilterMembrane bioreactor

* Corresponding author. Tel.: þ91 712 2240097; faxE-mail address: [email protected] (S. Mudli

0301-4797/$ – see front matter � 2010 Elsevier Ltd.doi:10.1016/j.jenvman.2010.01.006

a b s t r a c t

Volatile organic compounds (VOCs) and odorous compounds discharged into the environment createecological and health hazards. In the recent past, biological waste air treatment processes using biore-actors have gained popularity in control of VOCs and odour, since they offer a cost effective and envi-ronment friendly alternative to conventional air pollution control technologies. This review provides anoverview of the various bioreactors that are used in VOC and odour abatement, along with details ontheir configuration and design, mechanism of operation, insights into the microbial biodegradationprocess and future R&D needs in this area.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Volatile organic compounds (VOCs) and odorous compoundsemitted from various industries pose problem to human andenvironmental health. With increasing population, and new resi-dential and industrial developments, the demand for VOC andodour control systems to provide nuisance-free breathable air isincreasing. Stringent environmental legislations enforced bygovernment agencies, have led polluting industries to adopteffective air pollution treatment processes in order to comply withthese regulations. As a consequence, biological treatment tech-niques for VOC and odor control have gained tremendous popu-larity in view of the several advantages they offer in comparison totraditional physical and chemical removal methods. Biologicalwaste air treatment processes are not only cost effective ascompared to conventional techniques such as incineration oradsorption but are also environment friendly (Devinny et al., 1999;Delhomenie and Heitz, 2005; Shareefdeen and Singh, 2005).

Biological waste air treatment technology makes use of severaltypes of bioreactors depending on the load and kind of pollutant tobe treated. The type of bioreactor used for abatement has a directconsequence on the efficiency of the treatment process. An under-standing of the bioreactors used for VOC and odour treatment, their

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design and configuration, as well as necessary parameters for theiroperation will not only help in increasing the efficiency of thetreatment process but also give insights to develop newer, betterand robust treatment techniques. This review attempts to providean overview of the various bioreactors used for the control of VOCsand odours, their merits and demerits, their important operationalparameters and future R&D needs in this area.

2. Bioreactors in VOC and odour control

Bioreactors play a very important role in the control of VOCs andodorous gases that are emitted by polluting industries. Althougha number of different configurations exist, the main types ofconventional air phase biological reactors include biofilters, bio-trickling filters and bioscrubbers. Among the newly developedreactors are the membrane reactors (Shareefdeen and Singh, 2005;Kumar et al., 2008a,b), which have been used for VOC and odourabatement. Although, the basic pollutant removal mechanisms of allthe reactors are more or less similar, differences exist in the use ofmicroorganisms (may be either in suspended (in liquid) or immo-bilized (biofilm) form), packing media, pollutant concentration etc.

3. Biofilter

Biofilters (BFs) are reactors in which a humid polluted air streamis passed through a porous packed bed on which a mixed culture of

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pollutant-degrading organisms is immobilized (Fig. 1). Generally,the pollutant gases flow through the porous packing media, aretransported from the gaseous phase to the microbial biofilm(through liquid phase or moisture) and the biological oxidation ofVOCs occurs.

BFs are used to treat a wide variety of organic and inorganicpollutants in industrial and municipal exhaust streams. Although,traditionally used for treatment of odorous gases from sewagetreatment plants and composting facilities, BFs now find wideapplication in treatment of several VOCs and odours. Among theseare odourants such as ammonia, hydrogen sulphide, mercaptan,disulphides, etc., and VOCs like propane, butane, styrene, phenols,ethylene chloride, methanol, etc. Bench and pilot scale studieshave shown that 60 out of 189 hazardous air pollutants (HAPs) canbe successfully treated with biofiltration (Devinny et al., 1999;Shareefdeen and Singh, 2005). In Europe, more than 600 chemicalprocessing industries use BFs for deodorization and treatment ofVOCs.

BFs are typically used for the treatment of large volumes of airstreams containing low concentration of VOCs or odorants. Theadvantages and disadvantages of BFs are discussed below.

Advantages:

(a) Cost effective with (low operating and capital costs)(b) Low pressure drop(c) Treat large volumes of low concentration VOCs or odorants(d) Secondary waste streams are not produced

Disadvantages:

(a) Clogging of the medium due to particulate matter(b) Problem of medium deterioration(c) Less treatment efficiency at high concentrations of pollutants(d) Difficulty in moisture and pH control

There are two BF configurations conventionally used for VOCand odour treatment viz.

� The open design BFs, with ascending gas flows, installedoutside the VOC/odour generating units. These reactors requirelarge areas, and are also exposed to climate changes.

Fig. 1. Schematics of a biofilter unit.

� The close design BFs, with either ascending or descending gasflows, installed in closed rooms. These reactors require lessspace than the open configuration.

3.1. Biofilter operation

The operation of BFs involve a series of steps beginning with thetransfer of the pollutant from the air to the water phase, adsorptionto the medium or absorption into the biofilm, and finally biodegra-dation of the VOC/odorant within the biofilm (Devinny et al., 1999).The most important physical, chemical and biological parametersinfluencing the biofiltration process are described below.

3.1.1. Transfer and partitioning of pollutantThe first step in the biofiltration process is the transfer of

contaminants from the air to the water phase. This is generally nota rate-limiting step, and so one frequently assumes that the gas andliquid are at equilibrium. At equilibrium, the partition between airand water is generally described by Henry’s law, which is given bythe equation:

Cgi ¼ HiCli

where Cgi is the concentration of pollutant i in the gas phase, Hi isHenry’s coefficient and Cli is the concentration of i in the liquidphase (Shareefdeen and Singh, 2005).

Henrys’ coefficient (constant of proportionality Hi in aboveequation) has been described in different units in literature. Usinga non-dimensional Henrys’ coefficient, substances with values over0.01 are considered volatile, and the higher the value, the lesssoluble the substrate is in water. For example, the non-dimensionalHenrys’ coefficient (at 25 �C) for ammonia is reported to be 0.0005,while for H2S it is 0.92. Henrys’ coefficient depends on thetemperature and the chemical potential in the liquid phase(Shareefdeen and Singh, 2005).

In general, the elimination capacity of a BF declines withincreasing Henry’s law constant since this indicates a tendency topartition away from the liquid/biofilm phase where degradation istaking place.

3.1.2. BiofilmThe biofilm is a key element of the BF, which brings about the

biodegradation of the pollutants viz. VOC and odorous compounds.‘‘Biofilm’’ is the mass of organisms growing on the surface of thesolid support; it carries out the catabolic activity and transforms thepollutants to harmless products.

The thickness of the biofilm is influenced by several factors.These include the type of pollutant, its rate of flow through the BF,the bedding material used, and the design and configuration of thetreatment system being used. Biofilm thickness usually varies fromtens of micrometers to more than 1 cm, although an average of1 mm or less is usually observed (Shareefdeen and Singh, 2005).The activity increases with the thickness of the biofilm, up to a leveltermed the ‘active thickness’. Above this level, the diffusion ofnutrients becomes a limiting factor (Devinny et al., 1999). Varioussteady state and dynamic mathematical models have been reportedin literature to predict the substrate, oxygen and nutrient pene-tration profile in the biofilm and facilitate evaluation of overallbiofilm effectiveness factor (Mudliar et al., 2008a,b; Shareefdeenand Singh, 2005; Metris et al., 2001).

3.1.3. Biofilter bedThe BF bed constitutes the heart of the biofiltration process

because it provides the support for microbial growth. Bohn (1992)established a list of characteristics that an ideal BF bed should

S. Mudliar et al. / Journal of Environmental Management 91 (2010) 1039–1054 1041

possess. The most important desirable characteristics of the BF bedinclude (a) high specific surface area for development of a microbialbiofilm and gas-biofilm mass transfer, (b) high porosity to facilitatehomogeneous distribution of gases, (c) a good water retentioncapacity to avoid bed drying, (d) presence and availability ofintrinsic nutrients, and (e) presence of a dense and diverse indig-enous microflora.

Peat, soil, compost, and wood chips, are the most frequentlyemployed basic materials in BF beds. These materials satisfy most ofthe required desirable criteria, and are widely available at low cost.Each of these materials has their own merits and demerits. Themain advantage of soil is that, it offers a rich and varied microflora.It however, contains only a few intrinsic nutrients, presents lowspecific surface area and generates high-pressure drops (Swansonand Loehr, 1997). Peat has high amounts of organic matter, highspecific surface area, and good water holding capacity and goodpermeability. However, peat contains neither high levels of mineralnutrients nor a dense indigenous microflora as in the case of soil orcompost. Composts are materials that are most frequentlyemployed in biofiltration for a variety of reasons. Compost offersa dense and varied microbial system, good water holding capacity,good air permeability, and contains large amounts of intrinsicnutrients. Moreover, the utilization of compost in BFs constitutes aneffective way of recycling and utilizing waste residual organicmatter, such as activated sludge from wastewater treatment plants,forest products (branches, leaves, barks), domestic residues, etc.(Alexander, 1999). However, composts are often less stable than soilor peat and have the tendency to break down and become compact,leading to increase in pressure drop in BF beds. This among otherreasons is attributed to their high water holding capacity. Someauthors have studied biofiltration using wood chips or barks aspacking material (Smet et al., 1996a,b, 1999; Hong and Park, 2004).However, in general these authors have concluded that perfor-mances obtained with such filtering materials are less satisfactorythan those obtained with compost or peat. This has been explainedby the low pH-buffering capacity, the low specific surface areas andthe low nutrient content of such materials. Despite these defi-ciencies, wood barks are still widely used in BFs as support mate-rials, in association with peat or compost.

Indeed, to prevent bed crushing and compaction, most authorssuggest materials that provide the bed with good structure, easymaintenance and rigidity, which consequently delay the cloggingphenomena which thereby increases the bed lifespan. Examplesinclude wood chips or barks (Luo, 2001), perlite (Woertz et al.,2002), vermiculite (Pineda et al., 2000), glass beads (Zilli et al.,2000), polyurethane foam (Moe and Irvine, 2000), polystyrene(Arulneyam and Swaminathan, 2000), lava rock (Chitwood andDevinny, 2001), etc. Ibrahim et al. (2001) prepared a filter bedcomposed of activated sludge immobilized on gel beads. Christenet al. (2002) and Sene et al. (2002) developed a sugarcane-baggase-based bed, for the treatment of ethanol and benzene. Some bed-structuring agents also possess interesting chemical characteristicswhich they impart on the bed such as pH-buffering capacity(limestone), or general adsorbing capacity (activated carbon;Abumaizar et al., 1998).

The efficiency of a BF material with respect to the pollutant fortreatment is given by its adsorption coefficient or partition coeffi-cient. Tang and Hwang (1997) reported partition coefficients oftoluene as 1.43 mg g�1 with compost, 2.00 mg g�1 with diatoma-ceous earth, and 0.89 mg g�1 with chaff. Beds containing activatedcarbon (granulated or powdered) provide adsorption coefficientsfor toluene approximately 10–20 times greater (50.6 mg g�1 ofgranulated activated carbon) (Tang and Hwang, 1997; Acuna et al.,1999). Literature reports indicate that addition of activated carbonleads to improvement in biofilter degrading capacity (Abumaizar

et al., 1998), elimination of hydrophobic compounds, and bettercontrol of loading variations (Mason et al., 2000).

3.1.4. Oxygen levelsOxygen levels play a very vital role in the performance of a BF.

However, this is very case specific as can be seen from experimentsdiscussed below, conducted by separate authors, on the effect ofoxygen limitation on biofiltration. Experiments by Shareefdeenet al. (1997) using air enriched with oxygen improved the perfor-mance of the BF and demonstrated that oxygen was indeeda limiting factor. In another experiment, Deshusses et al. (1996)found that there was no significant improvement in the simulta-neous removal of a mixture of methyl ethyl ketone (MEK) andmethyl isobutyl ketone (MIBK), when the oxygen content in air wasincreased. In this case, the fact that significant cross-inhibition ofMEK and MIBK biodegradation occurred suggested that kineticeffects were more important than diffusion effects. This was furtherdemonstrated in transient experiments where spikes of eithercompound were injected into BFs, and both cross- and self-inhi-bitions were observed. Thus, role of oxygen in BF performanceseems to be case specific. Oxygen is most likely to affect high-performance BFs or when thick biofilms exist.

In general principal in most applications, BF operations seek toavoid anaerobic conditions. This is because, existence of evenmicro-anaerobic conditions lead to the formation of compounds,which themselves are odorous and this deviates from the overallgoal of eliminating odorants and VOCs. Some studies, howeversuggest that, fortuitous anaerobic microenvironment conditionsthat exist in BFs, help in the degradation of organic pollutants(Shareefdeen et al., 1997).

3.1.5. NutrientsThe pollutants introduced into the BFs, form the major carbon

and energy source for microbial activity. Hydrogen and oxygen arefound in the air, in the growth medium, and sometimes in the VOCs.The availability of the other macronutrients (N, P, K, and S) andmicronutrients (vitamins, metals) is partially fulfilled by thefiltering materials used in the BF. Materials such as composts arewell known to contain various nutrients.

Studies have demonstrated that irrespective of the filteringmaterial employed, the steady addition of nutrients is necessary tosustain a satisfactory microbial degradation activity. For example,some studies have shown that long-term utilization of compost-based beds lead to progressive exhaustion of the intrinsic nutritiveresources (Morgenroth et al., 1996). This progressive nutrientdeficiency then becomes a limiting factor for the long-term bio-filtration performance (Delhomenie et al., 2001a,b).

Models of biofiltration performance as a function of nutrientsupply, and of nitrogen in particular have been developed andexperimentally validated (Delhomenie et al., 2001a,b; Alonso et al.,2001; Metris et al., 2001; Dorado et al., 2008). Nutrients formicrobial growth are supplied either in the solid form which isdirectly inserted into the filter bed (Gribbins and Loehr, 1998), or asaqueous solutions, which is the most frequently used method. Wuet al. (1999) reviewed the most common nutrient solutions used inBFs. These include KH2PO4, NaxH(3�x)PO4, KNO3, (NH4)2SO4, NH4Cl,NH4HCO3, CaCl2, MgSO4, MnSO4, FeSO4, Na2MoO4, and vitamins(B1, etc.). Given the wide range of elements and compoundsinfluencing microbial behavior, the optimization of nutrient solu-tions for BFs is a challenging area of study.

3.1.6. pHAs is the case with several biological processes, pH has an

important influence on biofiltration efficiency. Above or below anoptimum pH range, microbial activity is severely affected. Most of


the microorganisms in BFs are neutrophilic i.e. their optimum pH is7. Lu et al. (2002), observed maximum degradation of BTEXbetween pH values of 7.5 and 8.0. Lee et al. (2002) also reporteda pH of 7.0 to be optimal for BTEX degradation. Veiga et al. (1999),studied the effect of pH on alkyl benzene degradation (between pH3.5 and 7.0), and found that alkyl benzene degradation increasedwith pH. Arnold et al. (1997) stated that styrene eliminationimproved in a neutral medium.

VOCs that contain hetero-atoms (S, O, and N) are converted intoacidic products, which tend to reduce the pH (Christen et al., 2002),affect microorganisms and cause corrosion problems in down-stream conduits (Webster and Devinny, 1998). Similar observationsduring VOC degradation due to formation of acidic intermediateshave also been reported by various authors (Shareefdeen and Singh,2005; Maestre et al., 2007). Kennes and Thalasso (1998) reportedthat among the organic materials employed in BFs, soil exhibitedthe best intrinsic pH-buffering capacity followed by composts andwood chips. Peats are naturally acidic (pH 3.0–4.0), and have lowbuffering capacity. To maintain the pH (at neutral) some authorshave reported insertion of buffer materials into the filter beds, fore.g. – calcium carbonate (Smet et al., 1996a,b), and dolomite (Smetet al., 1999). The pH can also be controlled by bed irrigation withnutrient solutions that contain pH buffers, for example Ca (OH)2,NaOH (Zilli et al., 2000), NaHCO3 (Tang and Hwang, 1997), and urea(Delhomenie et al., 2002), etc.

3.1.7. Moisture contentThe moisture content of the filter bed is a critical factor for

biofilter performance because microorganisms require water tocarry out their normal metabolic activity. Sub-optimal moisturelevels leads to drying of the bed and development of fissures thatcause channeling and short-circuiting (Shareefdeen and Singh,2005). Deprivation of water to microorganisms causes a significantreduction in the biodegradation rate. Excess water inhibits transferof oxygen and hydrophobic pollutants to the biofilm, therebypromoting the development of anaerobic zones within the bed andlimiting the reaction rate. Too much water can also result in foulsmelling emissions due to the lack of oxygen, increasing back-pressure due to reduced void volume, and channeling of the gaswithin the bed.

Optimal water levels vary with different filtering material,depending on medium, surface area, porosity and other factors.Moisture content for optimal operation of the biological filtershould be within 30–60% by weight, depending on the filteringmedium used. Moisture levels in a biofilter are often maintainedthrough pre- humidification of the inlet gas stream. Also, it is oftennecessary to provide direct application of water to the bed througha sprinkler system at the top of the bed. More advanced controlsinclude the use of load cells that sense the weight of filter bed andare connected to sprinkler controls. Supplemental moisture supplymay be required because bio-oxidation is an exothermic reaction,and so drying can occur within the bed. Drying of the packingmaterial can lead to localize dry spots, and can result in non-uniform gas distribution and reduction in the activity of microor-ganisms (Shareefdeen and Singh, 2005). Recently, it has beenreported that, biofilters tend to experience drying at the air inletport, which causes decreased pollutant removal over time (Sakumaet al., 2009).

Control of moisture requires a better understanding of thedrying of the support due to changes in inlet air temperature andrelative humidity and from production of metabolic heat duringpollutant oxidation. Various models are now available to studydrying and its effect on biofilter performance. These modelsdescribe the variations in pollutant concentration, air relativehumidity, temperature, and water content of the media, to predict

water evaporation from the packing material as a consequence ofmetabolic heat generation and variations of the relative humidityof the inlet air stream, and also the resulting decrease in biofilterperformance (Devinny and Hodge, 1995; Devinny et al., 1999;Metris et al., 2001; Morales et al., 2003).

3.1.8. MicroorganismsMicroorganisms are the catalysts for biodegradation of VOCs

and odours. For the degradation of VOCs, heterotrophic microor-ganisms have been extensively reported (most often bacteria orfungi). The bed inoculation depends on both the nature of thefiltering material and the biodegradability level of the VOC to betreated. Many scientific workers prefer taking advantage of theecosystems already prevailing in the beds (Delhomenie et al.,2001a,b, 2002; Mohseni and Allen, 2000). After an acclimatizationperiod, the most resistant population to the toxic VOC is naturallyselected and a microbial hierarchy is established in the bed. In othercases (such as for recalcitrant VOCs), researchers inoculate the BFbeds with consortia extracted from sewage sludge, or strainsderived from either commercial sources or isolated from previouslyoperated BF. In general, in terms of biomass density, a BF containsbetween 106 and 1010 cfu of bacteria and actinomycetes per gram ofbed (Krailas et al., 2000). Pedersen and Arvin (1995), Pedersen et al.(1997), and Delhomenie et al. (2001a), have reported that in BFs,the degrading species represents between 1 and 15% of the totalpopulation.

3.1.9. Biofilm architectureVOC elimination is the result of many, interdependent processes

that simultaneously take place inside the biofilter. To date, littleinformation exists about biofilm architecture in BFs. Previous workwith scanning confocal laser microscopy has revealed the existenceof cell-free channels extending from the biofilm-liquid interface tothe substratum and their possible role in enhancing pollutant andoxygen mass transfer (Cox and Deshusses, 1998). A new andpromising development is the use of computed axial tomography(CAT) X-ray scanning to characterize the biofilm macro- architec-ture (Shareefdeen et al., 1997). CAT scans of a toluene-degrading BFcontaining a large amount of biomass immobilized on poly-propylene pall rings showed a heterogeneous distribution ofbiomass with large areas completely filled with biomass whereasother sections of the reactor covered by <1 mm thick biomass.Further, image analysis revealed the presence of air/water channelsranging in area from <5 to380 mm2, with smaller channels (0–60 mm2) contributing to more than 80% of the interfacial area. Infuture, further application of high resolution X-ray and possibly CATscanning techniques could contribute to a better understanding ofthe architecture of biofilms. Such progress could lead to a betterunderstanding of pollutant mass transfer in BFs and ultimately toa better design of materials for stable culture support (Shareefdeenet al., 1997).

3.2. Future needs

Biofiltration has clearly been shown to be a cost and energyefficient technology for treatment of a range of waste emissionscontaining VOCs and odours. There is a need to work on innovativestrategies such as pretreatment of VOCs and odours to removeparticulates and/or enhance biodegradability and improve tech-niques to treat more complicated polluted airstreams especiallymultiple pollutant mixtures. Moreover, BF technology found fieldapplication well before its fundamental principles were under-stood. This has resulted in several cases of unsuccessful or subop-timum operation of large-scale BFs. Today, with much betterunderstanding of the fundamental principles underlying the


biofiltration process, scope exists for designing better bioreactorswith optimal operating conditions. A number of fundamentalquestions however remain unanswered These include the quanti-fication of biomass turnover, the understanding of biodegradationkinetic relationships and factors influencing these relationships,complex ecological complexity of BFs, and the interrelationshipsbetween pollutants, oxygen and essential nutrients (Cox andDeshusses, 1998). All these factors have been shown to significantlyinfluence both the performance and the long-term stability of BFs,and thus require further investigation. In particular, quantitativestudies are necessary. This would be made easier with theexpanding use of modern tools of biotechnology. Further, there isneed to develop design correlations on mass/heat transfer, diffu-sion coefficient in biofilm, gas/liquid holds up to facilitate improvedbiofilter design. Furthermore, the conventional phenomenologicalmodels are best with difficulties such as requirement of thedetailed knowledge of the underlying physicochemical phenomenaand extensive time required for their development, testing andvalidation. Hence, there is also a need to develop generic mathe-matical models of the biofiltration process using biological inspiredcomputing techniques for quantitative robust prediction anddesign optimization (Narendra et al., 2006; Omkar et al., 2008).Finally, the largest problem to overcome will be the translation ofrecent and future basic advances into real process improvements.

4. Biotrickling filter

The schematic description of a typical biotrickling filter (BTF) isprovided in Fig. 2 (Delhomenie and Heitz, 2005). In such a filter, thegas is carried through a packed bed, which is continuously irrigatedwith an aqueous solution containing essential nutrients required bythe biological system. Several studies have shown that the choice ofa co- or counter-current configuration for liquid and gaseous pha-ses does not influence the biodegradation performance (Cox andDeshusses, 1999). Microorganisms grow on the packing material ofthe biofilter as biofilm. The pollutant to be treated is initiallyabsorbed by the aqueous film that surrounds the biofilm, and thenthe biodegradation takes place within the biofilm.

Fig. 2. Schematics of a biotrickling filter unit.

The filtering material used in a BTF has to facilitate the gas andliquid flows through the bed, favor the development of themicroflora, and should resist crushing and compaction. BTF packingthat best meet these specifications are made from inert materialssuch as resins, ceramics, celite, polyurethane, foam (Centinkayaet al., 2000) etc. However, most of these materials present limitedspecific surface areas between 100 and 300 m�1 (Roh, 2000) withexceptions in some cases where >1000 m�1 for polyurethane-based beds have been reported. As they are made from inert orsynthetic material, BTFs need to be inoculated with suitablemicrobial culture. The use of activated sludge as initial microbialinoculum has been extensively reported (Lu et al., 2002; Oh andBartha, 1997).

The advantages and limitations of BTFs include the following.Advantages:

(a) Less operating and capital costs(b) Low pressure drop(c) Capability to treat acid degradation products of VOCs

Disadvantages:

(a) Accumulation of excess biomass in the filter bed(b) Complexity in construction and operation(c) Secondary waste streams

Due to the permanent trickling mechanism, biofiltrationprocesses are more adapted for the elimination of water solubleVOCs. Nevertheless, as the contact between microorganisms andthe pollutants occur simultaneously (Cox and Deshusses, 1999), thesolubility specifications are less stringent than for bioscrubbers(Henry coefficient <0.1; Van Groenestijn and Hesselink, 1993). Ina typical BTF, VOC inlet concentrations are generally less than0.5 g m�3. The continuous distribution of the nutrient solutionfacilitates the control of the biological operating parameters (viz.pH etc.).

As the contact between the microorganisms and the pollutantsoccurs after the VOC diffusion in the liquid film, the liquid flow rateand the recycling rate are recognized to be critical parameters forBTF operation. Research has suggested that an increase in the liquidflow rate should result in proportional increase in the activeexchange surface for gas–liquid mass transfer, and then improvethe degradation rate (Alonso et al., 2000). Some researchers haveshown that maintaining minimum water and nutrient supply issufficient to achieve good performance (Lu et al., 2002; Thalassoet al., 1996). In addition, as the distribution and the recycling ofnutrient solutions add to energy costs, other studies suggest thatthe optimum recycling and distribution flow rates have to be foundexperimentally and on a case-by-case basis (Dolfing et al., 1993).

The major drawback of BTFs is the accumulation of excessbiomass in the filter bed. Some researchers have demonstratedthat, in the course of the degradation process, the biofilm thicknesscan be several millimeters (Janni et al., 2001; Cohen, 2001) whichcan cause clogging, an increase in pressure drop, bed channeling,creation of anaerobic zones and can ultimately lead to performanceloss (Alonso et al., 2001). Several studies have been attempted todevelop solutions to the clogging problem in BTF. The controlstrategies suggested are of three types: mechanical, chemical orbiological. Mechanical treatment includes bed stirring (Wubkeret al., 1997; Laurenzis et al., 1998) or bed back washings with water(counter-current washings), which permit the draining of excessaccumulated biomass (Smith et al., 1996). Chemical treatmentsinclude dissociation of the chemical binding between the biomassand the bed particle surface either by damage to the biomass bycreating nutrient or water deficiency, or utilization of disinfecting


agents (Diks et al., 1994; Schonduve et al., 1996; Cox andDeshusses, 1999; Armon et al., 2000; Chen and Stewart, 2000).Biological methods utilize biomass predators, such as protozoa(Cox and Deshusses, 1999). Amongst all of these methods, backwashings with water are the most efficient and certainly cause lessdisruption to the biotrickling filter ecosystem and performance(Cai et al., 2004).

4.1. Operation and applications of biotrickling filters

The important parameters of bioreactors in general (nutrients,pH, microorganisms, oxygen levels, etc.) have been described indetail for BFs in the preceding section. They will therefore not bedealt with individually for all reactors.

BTFs find wide application in VOC and odour treatment. Ascompared to conventional compost or soil bed BFs which aregenerally limited to the elimination of odorous compounds and non-chlorinated volatile organic compounds, a wider range of pollutantscan potentially be treated in BTFs. This is because, environmentalconditions can be better controlled in the BTFs and potentially toxicdead-end metabolites can be purged out of the system. Also, labo-ratory BTFs offer the opportunity to work with monocultures,especially using genetically engineered microorganisms.

Oh and Bartha (1997) first reported the elimination of nitro-benzene vapors in a laboratory scale BTF. They used a stablemicrobial consortium enriched from sewage sludge and immobi-lized it on perlite. During the start up period of four weeks, the inletnitrobenzene concentration was kept relatively low (<80 mg m�3)to avoid poisoning of the culture, after which high and sustainednitrobenzene elimination was observed with 80–90% degradationfor inlet concentrations ranging from 100 to 300 mg m�3 and anempty bed gas contact time of 21 seconds. This corresponds to anelimination capacity of 50 g m�3 h�1, a high value that could lead toan economically viable process. A nitrogen balance showed that98% of the nitrobenzene nitrogen was converted into ammoniawhile a small amount of nitrite was produced.

Two other compounds of interest, namely diethyl ether (Bauerleand Fischer, 1987) and gasoline additive methyl tert-butyl ether(MTBE) (Fortin and Deshusses, 1999) were reported to be bio-degraded in laboratory BTFs. Fortin and Deshusses (1999) achieved75% removal efficiency for an inlet MTBE concentration of 0.8 g m�3

with empty bed residence time less than a minute in BTF. Thiscorresponds to an elimination capacity of 50 g m�3h�1, anextremely high value for a compound for which biodegradation insitu still remains a challenge. The reactor studied by Fortin andDeshusses (1999) was originally inoculated with various samples ofaquifer material and soil contaminated with MTBE. Interestingly,MTBE removal was significant only after addition of traces of a peathumic substance (PHS) extract to the recycle liquid. As biomassaccumulated in the reactors, the benefits of the PHS were no longersignificant. While several reports exist on bio-stimulation usingPHS in wastewater treatment, the exact mechanisms involved inbio-stimulation using PHS are yet to be elucidated.

Also noteworthy, is a study by Sun and Wood (1997), whoimmobilized a pure culture of Burkholderia cepacia PR123 (TOM23C)constitutively expressing toluene ortho-monooxygenase to co-metabolize the biodegradation of trichloroethylene (TCE) vapors ina BTF. Aerobic biodegradation of TCE only occurs through co-metabolism, and addition of a growth substrate (usually toluene,methane, propane, phenol, or ammonia), which is required toinduce the expression of the appropriate TCE-degrading enzyme.Bacterium B. cepacia PR123, however, expresses toluene ortho-monooxygenase constitutively, which circumvents the problem ofcompetitive inhibition of TCE oxidation by the usual inducersduring the growth phase. The authors Sun and Wood (1997) used

glucose as a carbon and energy source and observed TCE elimina-tions up to 200 times higher than previously reported. As observedin other bioreactors for TCE aerobic co-metabolism, rapid inacti-vation of the TCE-degrading enzyme by TCE breakdown products(e.g. TCE epoxide) remained a problem.

Biotrickling filtration is a maturing technology, and the numberof full-scale BTFs is rapidly increasing. In the past few 2–3 years,several successful conversions of full-scale chemical scrubbers tobiotrickling filters have been demonstrated (Kraakman, 2001;Kraakman, 2003; Gabriel and Deshusses, 2003). Until recently, itwas thought that successful bio-treatment in BTFs required a gascontact time ranging from 10 to 30 s (Wu et al., 2001). However,some researchers have shown that in BTFs the residence time canbe reduced to 5 s (Shareefdeen and Singh, 2005). Recently, analkaline biotrickling filter was shown to be very effective fortreatment of H2S odours (Sanchez et al., 2008).

4.2. Future needs

Recent research in the field of biotrickling filtration for airpollution control have focused on various aspects pertaining to themicrobiology of pollutant-degrading microorganisms, kinetics ofpollutant uptake, and means to control biomass accumulation.Nevertheless, additional information on the fundamental princi-ples underlying biotrickling filtration is needed. Key questions to beaddressed are mainly concerned with the complex ecology of bio-films. In particular, studies are needed to understand the overallrole of secondary processes (i.e. those processes not directly asso-ciated with the elimination of the primary pollutant) and howthese can be controlled in practice. In the future, the ability tocontrol the ecology of biofilms in BTFs may enable optimalbalancing of the net growth of biomass, so that reactor stability canbe ensured over a very long period. Additional research is neededfor better understanding of the kinetic relationships for pollutantbiodegradation. Particularly, understanding of the biodegradationof mixtures of pollutants, role and impact of oxygen and ancillarynutrients on the rate of biodegradation and on the biomass yield,and to determine the influences of various stresses, such aschanging conditions and mass transfer limitations, is important.These studies are extremely relevant for future implementation ofBTFs in actual field conditions (Cox and Deshusses, 1999).

In a nutshell, review of the recent research work emphasizes onthe need of fundamental understanding of the degradation processthrough in situ analysis and extended application of modern toolsin biotechnology. This is essential in order to establish baselineinformation (presently not available) for rational reactor design andoptimum process operation. This, together with number of pilotscale application and demonstration of techno-economic viability,would transfer this technology from lab to the field.

5. Bioscrubber

A bioscrubber unit (Fig. 3, Delhomenie and Heitz, 2005) consistsof two subunits namely (1) an absorption unit and (2) a bioreactorunit. In the absorption unit, input gaseous contaminants aretransferred to the liquid phase. Gas and liquid phases flow counter-currently within the column, which may contain the packingmaterial. Nevertheless, the addition of inert packing providesincreased transfer surface between the VOC and the aqueous phase(Van Groenestijn and Hesselink, 1993). The washed gaseous phaseis released at the top of the column whereas the separatedcontaminated liquid phase is pumped to an agitated, aeratedbioreactor. This reactor unit contains the appropriate microbialstrains suspended in the aqueous phase in nutrient solution(media) essential for their growth and maintenance.

Fig. 3. Schematics of a bioscrubber unit.


Most of the bioscrubbers being operated presently use activatedsludge derived from wastewater treatment plants as inoculum(Ottengraf, 1987). In some cases, bioreactors are directly inoculatedwith specific degrading strains. The residence time for suchbioreactors range between 20 and 40 days and these are operatedpractically as activated sludge processes including recycle of sludge.Part of the treated solution is recycled for absorption of VOCs to theabsorption unit.

The advantages of bioscrubbers are as follows.

(a) Operational stability and better control of operating parame-ters (pH, nutrients);

(b) Relatively lower pressure drops(c) Relatively smaller space requirement.

Disadvantages of bioscrubbers include-

(a) Bioscrubbers are adapted to treat readily soluble VOCs (alco-hols, ketones), with low Henry coefficients (<0.01), and atconcentrations less than 5 g m�3 in the gaseous phase

(b) Provides low specific surface area for gas–liquid mass transfer(generally <300 m�1)

(c) Excess sludge generation(d) Generation of liquid waste

Some studies have shown that the addition of emulsifyingagents (silicon oil, phthalate) in the aqueous solution can signifi-cantly improve the elimination of less soluble compounds, becausethey favor the VOC mass transfer from gas to the liquid phase(Mortgat, 2001).

5.1. Variations in bioscrubber design

Substantial modifications in bioscrubber design have been donein the recent past to enhance their performance for VOC and odourtreatment. Some modified bioscrubber units are listed below.

5.1.1. Sorptive-slurry bioscrubberThe sorptive-slurry bioscrubber consists of a suspended growth

bioscrubber with powdered activated carbon (PAC) added to thebiomass slurry (Kok, 1992). Gaseous VOCs partition into the slurryin the absorber unit and adsorb onto the carbon. Adsorption on the

column and any biodegradation in the absorber lowersthe pollutant concentration in the liquid, which increases theconcentration gradient and gas transfer. Biodegradation in thebioreactor unit continues to drive the pollutant concentration tolow levels. The carbon is regenerated as the VOCs desorb anddegrade, and is then recycled to the absorber. PAC also providesa surface for the growth of biofilms, and may assist in treating peakloads and in decreasing inhibition by toxic compounds. Addition of2–5% PAC to biomass slurry is recommended to improve bio-scrubber performance (Hammervold et al., 2000).

5.1.2. Anoxic bioscrubberAerobic bioreactors are used to transform inorganic pollutants

and degrade organic pollutants in conventional bioscrubbers,whereas systems combining scrubbers and up flow anaerobicsludge blanket (UASB) bioreactors have been used for the degra-dation of perchloroethylene and also to treat waste gases contain-ing NOX and SOX (Centinkaya et al., 2000; Janssen et al., 2000;Shareefdeen and Singh, 2005).

5.1.3. Two-liquid phase bioscrubberThe concept of two-liquid phase bioscrubber unit originates

from the fact that application of conventional bioscrubbersbecomes limited for the treatment of pollutants that are readilysoluble in water. Consequently, the addition of an organic solvent tothe water phase can enhance biodegradation of more hydrophobiccompounds (Deziel et al., 1999), and facilitate the elimination ofa range of hydrophilic and hydrophobic compounds. Besidesimproving bioavailability, solvents can reduce the toxicity ofcontaminants, and can act as a buffer system for fluctuating loads ofpollutants. The addition of 10–30% water immiscible high boiling-point solvent to the liquid phase facilitates the absorption ofhydrophobic compounds from the gas phase in the absorber. Two-liquid phase bioreactor systems have been tested for the removal ofalkanes, benzene, styrene, phenol, naphthalene, and pentachloro-phenol. Solvents such as silicon oil, paraffin oil, dibutyl phthalate,di-n-octyl phthalate, di-n-nonylphthalate, and pristine are goodcandidates for this application, and among them silicon oil has beenfound to be the best for two-liquid phase systems (Yeom andDaugulis, 2001).

5.1.4. Airlift bioscrubberEdwards and Nirmalakhandan (1999), have described an airlift

bioscrubber having a combined absorption/biodegradation reactorconfiguration, for the removal of air phase benzene, toluene andxylene (BTEX) compounds. The reactor comprises of two concentrictubes, with the inner tube shorter than the outer tube. As in someconventional airlift bioreactors, the inner tube serves as the downcomer, and the annular space between the two tubes works asa riser (Ward, 1989). Air is introduced into the reactor through thesparger, which is located near the bottom of the riser. The combinedmedium of mixture of air and water in the riser has lower densitythan the water in the down comer, resulting in a fluid circulationwithin the reactor. A mathematical model developed for thisprocess indicated that removal rates of >99% can be achieved forbenzene and toluene in the air stream with concentrations<1000 ppmv, and that the airlift bioscrubber should be operated atbiomass concentrations of 2 g 1�1 or greater for operationalstability. Recently, Jianping et al. (2005) reported the simultaneousremoval of ethyl acetate and ethanol in air streams using a gas–liquid-solid three-phase flow airlift bioreactor.

5.1.5. Spray column bioscrubberBioscrubbers have been suitably modified as per the demand

and nature of the odorants to be treated. A modified bioscrubber for

Fig. 4. Schematics of a membrane bioreactor containing microporous hydrophobicmembrane, a biofilm and suspended cells.


treatment of large volumes of waste gas and industrial emissionswith low concentration of odorants with poor solubility wasinvestigated by Cesario et al. (1992). The modified system consistsof two bioreactor units, which include (a) a spray column bioreactorwith liquid impelled loop reactor (LILR) and (b) a spray columnbioreactor connected to an airlift loop reactor (AIR). In both thesystems, gaseous odorants/liquid contactor is a spray column fortransfer of the odorants from gas to the water phase. This is inter-connected to either liquid impelled loop reactor or airlift loopreactor. Both the attached systems (LILR and ALR) contain culturemedium and the biomass. The spray column and LILR/ALR areinterconnected having a provision for recycling of the contents ofthe reactor (Cesario et al., 1992).

The spray column reactor with LILR contains nutrient medium.It is necessary that the nutrient medium in the reactor must possesscharacteristics such as immiscibility with water, non-biodegrad-ability, should be non-toxic to the biocatalyst, should have lowvapor pressure, relatively low viscosity, and a density different fromthe density of water. The water immiscible liquid is recycledbetween the absorber and the bioreactor. However, the operation(recycling) depends upon (a) the volume of industrial emission tobe treated, (b) physicochemical characteristics and (c) theconcentration of the odorants in the gaseous emission and also (d)the targeted removal efficiency.

The novel reactor of a spray column system with LILR/ALR is stillon laboratory scale and requires extensive investigations to ascer-tain the odorant transfer from gas to the liquid absorber (ALR).

5.2. Future needs

Bioscrubber are generally considered to be useful for the treat-ment of waste gases containing water soluble pollutants (Henry’slaw coefficient, H< 0.01), but depending on the concentration andtype of pollutant, bioscrubbers may also efficiently be used forodorous gases with other characteristics. Bioscrubbers, providesubstantial advantages for waste gas treatment because of theirsmaller space requirements, high loading rates, reliable operation,process control, low risk of clogging, and low operating cost. Whenhigh concentrations of contaminants are to be treated, bioscrubbersoffer more advantages than conventional BFs, BTFs, and chemicalscrubbers. Combination of bioscrubbers and some polishing stepsmay improve the treatment efficiency for gases with mixture ofhydrophobic and hydrophilic compounds.

Bioscrubbers offer higher elimination efficiencies for gasesgenerated from liquid wastes, viz. H2S, NH3, and organic sulfurcompounds. However, due to the acidifying nature of thesesubstances, substantial oxidation and sulfuric acid production inthe scrubber may cause the pH to drop and decrease mass transferefficiency. The capacity of the adsorption section for handlinghigher H2S concentration needs to be further improved. This couldbe achieved by increasing the buffering capacity of the scrubbingmedium and also through pH control.

The bioscrubber application can be made more attractive thanthe use of other conventional BF or BTF at relatively high pollutantconcentration (>0.5 g m�3) with necessary modifications. There isalso a need to develop integrated/hybrid reactor configurations toachieve optimal and efficient bio-treatment of VOCs/odours, indi-vidually and in mixtures.

6. Membrane bioreactors

Membrane bioreactors were designed as an alternative toconventional bioreactors for waste gas treatment. The membranebioreactor allows the selective permeation of the pollutant, whichis not allowed in any of the reactors discussed previously.

Application of membrane bioreactors for waste gas treatment hasbeen reviewed before (Reij et al., 1998; Kumar et al., 2008a,b). Theconcentration difference between the gas phase and the biofilmphase provides the driving force for diffusion across the membrane.A pressure difference is not applied. The driving force dependsstrongly on the air–water partition coefficient of the diffusingvolatile component. For components with a high partition coeffi-cient the driving force for mass transfer is small. The concentrationin the liquid, which depends on the biodegrading activity of themicrobial population, also affects the driving force. The surface ofthe membrane forms the contact area (Reij et al., 1998).

In a BF bioreactor unit, waste gas is blown through a bed ofcompost or soil, where microorganisms consume or degrade thegaseous organic pollutants. No separate water phase is present. Anadvantage of the membrane bioreactor over the BF is the presenceof a discrete water phase allowing optimal humidification of thebiomass and removal of the degradation products, thus avoidinginactivation of the biomass. In a membrane bioreactor, themembrane serves as the interface between the gas phase and theliquid phase (Fig. 4, Reij et al., 1998). The gas–liquid interface thuscreated (e.g. in hollow fibre reactors) is larger than in other types ofgas–liquid contactors (Yang and Cussler, 1986). Moreover, in bio-trickling filter and the bioscrubber, a packed bed of inert material ispresent on which water is continuously sprayed. The pollutants inthese reactors have to diffuse through the water phase before it isconsumed by the microorganisms. For pollutants with poor water-solubility such a layer of water causes a substantial additionalresistance for mass transfer (De Heijder et al., 1994). In themembrane bioreactor, on the contrary, the liquid phase is situatedat the opposite side of the biofilm and hardly forms a barrier formass transfer of the poorly water soluble pollutants (Fig. 4). Asmentioned before, large gas–liquid interfaces of 1000–10,000 m2m�3 can be created in hollow fibre reactors (Rautenbachand Albrecht, 1989), allowing high mass transfer rates. The pressuredrop in the gas phase is much lower than observed in BFs, wherepressure drop may become significant.

Advantages of membrane bioreactors include.

(a) No moving parts(b) Process easy to scale up(c) Flow of gas and liquid can be varied independently, without the

problems of flooding, loading, or foaming

Disadvantages of the bioreactor are-

(a) High construction costs(b) Long-term operational stability (needs investigation)


(c) Possible clogging of the liquid channels due the formation ofexcess biomass

6.1. Membrane materials

Two types of membrane materials have been used to preventmixing of the gas and liquid phases and simultaneous transfer ofvolatile components. These types are hydrophobic micro porousmembrane and dense membrane.

6.1.1. Micro porous membranesA micro porous membrane has a highly porous structure, with

a typical commercial membrane containing 30–85% pore space(Hartmans et al., 1992), including open pores in the surface of themembrane. The surface pores are generally sub micrometer in size,which prevents organisms from passing through the pores. If thepore size distribution is not sufficiently controlled or pore size is toolarge, then intrusion by organisms and organics will occur, signifi-cantly reducing mass transfer and potentially plugging the gasphase (Hartmans et al.,1992; Attaway et al., 2002; Fitch et al., 2003).Hydrophobic micro porous membranes consist of a polymer matrixof polypropylene or Teflon and contain pores with a diameter in therange of 0.01–1.0 mm. Since the membrane material is hydrophobic,the pores are filled with gas. Water does not enter the pores, unlessa certain critical pressure at the liquid side is exceeded.

If the transferred component disappears by chemical reaction,its mass transfer rate increases (Prasad and Sirkar, 1992). Activemicroorganisms present in the liquid will thus enhance it signifi-cantly. If the microorganisms are present as a biofilm on themembrane, liquid flow close to the membrane is absent and asa consequence the mass transfer coefficient does not apply at all. Inthis case (biofilm), simultaneous diffusion and reaction in a stag-nant layer need to be calculated (Reij et al., 1995).

6.1.2. Dense membranesA dense-phase membrane has no pores for removal to occur. The

contaminant must dissolve in to the membrane and diffuse throughthe membrane. In case of transport through a dense membrane, thediffusing volatile component is absorbed in the membrane materialand diffusion takes place in the dense polymer (Reij et al., 1998).

The mass transfer coefficient inside a dense membrane dependson both the solubility and the diffusivity of the volatile componentin the dense matrix (Crank and Park, 1968). Interaction between thevarious gaseous components is assumed to be absent, i.e. inde-pendent diffusion. For each volatile component, the solubility anddiffusivity are different and the mass transfer resistances of densemembranes for various gases may differ considerably due tospecific interactions between the components in the gas phase andthe membrane material. As a consequence, components can beselectively extracted from or retained in the gas phase by a properchoice of the membrane material (Reij et al., 1998).

6.2. Gas–liquid contactors for membrane bioreactors

Both micro porous and dense membranes have been used fora variety of processes that involve gas–liquid contact. Micro porousmaterial is generally applied in hollow fibres, although spiral-wound and plate-and-frame modules have also been used (Sirkar,1992; Wickramasinghe et al., 1992). Micro porous membranes canbe applied as gas–liquid contactors when selective action of themembrane is not required. Volatile components diffuse throughthis material depending on their diffusion coefficient in air andtheir vapor pressure, while the membrane serves as gas–liquidcontact area.

Dense material is available as tubes (usually silicone tubing),with a wall-thickness of at least several hundred micrometers, andas composite membranes. Composite membranes consist of a thin,selective top layer (<1–30 mm) of dense material, supported bya highly porous support layer of, for, e.g. non-woven polyester ora microfiltration membrane. Composite membranes can be appliedin spiral-wound, plate-and-frame and in hollow fibre modules.Examples of the application of both micro porous and densemembranes as gas–liquid contactors are given in the followingsection (Reij et al., 1998).

6.3. Applications of membrane bioreactors in waste gas treatment

In addition to the relatively new process of membrane-basedgas absorption (Sirkar, 1992), membrane contactors have recentlybeen tested for biological treatment of gas streams. In sucha process, the pollutants diffuse through the membrane and aredegraded by the microbial population present in the liquid phase.In this context, various publications were critically reviewed andthe summarized results are presented in Table 1. In general, thebiomass is supplied with carbon and oxygen from the gas phase,while water and mineral nutrients are supplied through the liquidphase. Microorganisms grow as a biofilm on the membrane, butmay also be suspended in the liquid phase.

Most studies on membrane bioreactors concern the removal ofhydrophobic pollutants from air. Hydrophobic pollutants, likexylene, toluene, hexane, and propene, have a high air–waterpartition coefficient. The driving force for the transfer of thesepollutants to the water phase is very small and as a consequence,mass transfer limits the biodegradation and therefore the design ofthe bioreactor becomes critical (Dingemansa et al., 2008a,b; Witteet al., 2009). The large gas–liquid interface and excellent masstransfer properties of membrane reactors (Yang and Cussler, 1986;Karoor and Sirkar, 1993) have inspired several workers to testmembrane bioreactors for the removal of less water solublepollutants from air (Reij et al., 1995; Bauerle et al., 1986, 1987). Adual tube dense-phase silicone membrane bioreactor was investi-gated for control of cyclohexane-contaminated air as part of a jetpropulsion fuel remediation investigation strategy (Roberts, 2006).Mass transfer characteristics for VOC permeation through flat sheetporous and composite membranes showed that the contribution ofthe porous ‘‘backing’’ layer for mechanical support can besubstantial in comparison to the porous layer in contact with thedense layer (Dingemans et al., 2008). Removal in the bioreactorranged from 29.4 to 596.6 mg m�2 m�1 in and measured elimina-tion capacities ranged from 46.7 to 947.9 g m�3 h�1.

The membrane materials used in several studies were chosensuch that they were impermeable to microorganisms (Hartmanset al., 1992; Freitas dos Santos et al., 1995). As a consequence, theseorganisms could not contaminate the gas phase. This precautionwas considered to be important in case the membrane bioreactorwas applied for the treatment of indoor air or manned space cabin(Binot et al., 1994).

Freitas dos Santos et al. (1995) tested a reactor with siliconetubes to remove 1,2-dichloroethane from air. For the destruction oftrichloroethene (TCE), Parvatiyar et al. (1996a,b) designed a newmembrane bioreactor in which both an aerobic and an anaerobicregion were present (Fig. 5). In the anaerobic zone, TCE is partiallydechlorinated and the products are supposedly degraded further inthe aerobic zone of the biofilm.

The silicone membranes due to their selectivity for hydrophobiccomponents, retains acid vapors (SO2) that hamper biodegradationof 1,2-dichloroethane (Freitas dos Santos et al., 1995).

Dense membranes may also serve as a buffer, in case the supplyof pollutants is variable. It should be noted, however, that due to

Table 1Application of membrane bioreactor for VOC and odour treatment.

Contaminant(g m�3)

Membrane Load(g m�3 h�1)

EC(g m�3 h�1)

Operation(days)

Gas HRT(s)

Flux(mg m�2 h�1)

References

Xylol (0.1–0.6) HF silicone rubber 1.8-8.1 1.8–7.9 n.r.a 2.5-14 67–302 Bauerle et al. (1986)Weckhuysen et al. (1993)Butanol (0.1–0.5) 1.8–7.3 1.3–5.4 48–207

DCM (0.2–0.4) 2.4-11 1–4 37–151Toluene (0.38) HF polyporous pp 2-31 33–1500 w90 0.9–6.3 13–180 Ergas and McGrath

(1997)Toluene (0.7–3.4) HF polyporous PP 23-121 16-42 > 20 0.9-1.8 15–35 Ergas et al. (1999)Nitrogen oxide (0.1) HF polyporous PP 1.3 0.1–0.9 105 1.9 3–8 Min et al. (2002)DCM (0.65) Spiral-wound silicone rubber 14–35 12–28 11 n.r.a 25–55 Freitas dos Santos et al.

(1995)Methane, TCE in

liquid (264)Silicone rubber tubing n.a.b 12 w120 n.a.b 130 Clapp et al. (1999)

Dimethyl sulfide(0.036–1.81)

Flat composite siliconerubber on polysulfone

5–270 5–200 79 8–24 De Bo et al. (2002)(De Bo, 2003)

Toluene (0.75–1.5) HF polyporous polysulfone,two in series

n.r.a n.r.a >140 16 or 32 80 Parvatiyar et al.(1996a,b)

Methanol (0.01–2.6) Composite with siliconerubber

36 25 n.r.a n.r.a 420 Resier et al. (1994)Toluene (0.03–4.2) 58 40 – – 670Hexane (0.03–2.4) 33 24 – – 400Xylol (0.1–0.6) Composite with silicone

rubber600 360 52 4.3–15 170 Attaway et al. (2001)

BBTEX (7.7–15.4)BTEX (2.2–9.8) HF polyporous PP 118 105 20 8–16 312 Attaway et al. (2002)DCM (0.16) Flat polyporous PP 180 80 <1 1.6–9.6 320 Hartmans et al. (1992)Toluene (0.075) Flat composite, 84 29 117 De Bo et al. (2002)Toluene (0.004–3.18) Silicone rubber on PVDF 32–470 30–395 339 2–24 790Toluene (0–0.97) Flat composite, silicone

rubber on PVDF0–170 0–170 20 24 n.r.a De Bo et al. (2002)

TCE (0.04) Flat composite, silicone rubber 6 0–6 n.r.a De Bo et al. (2002)Dimethy1 sulfide

(0.011–1.63)On PVDF 5–170 5–130 20–144 4–24

Toluene (0.5–0.75) HF polyporous PP 16–96 32–72 >8 1.8–7.2 6–60 Dolasa and Ergas (2000)TCE (0.04–0.2) 3–9 0–6 3.6–7.2 0–5 Dolasa and Ergas (2000)Butanol (0.6–2.3) HF polyporous polysulfone 13–26 3.8–13 105 1.6 and 2.9 13–326 Fitch and England (2002)TCE (0.13–0.21) HF polyporous PP 0.03–0.1 0.01–0.06 21 96–300 0.06–0.2 Pressmann et al. (2000)Benzene (0.1) HF polyporous PP 7–60 4.8–58 100 0.17–1.4 4.2–20.4 Fitch et al. (2003)Benzene (0.1) Latex rubber tubing 7-28 2.5–18 40 0.55–1.4 670–2700 Fitch and England (2002)Cyclohexane Dual tube silicone rubber 395–2189 47–947 40 – 29–597 Robert et al. (2006)DMS Thermophilic membrane 64 54 270 24 128 Munkhtsetseg et al.

(2008)Toluene Composite porous PAN 0.72 0.6 165 2–24 – Kumar et al. (2008a,b)Cyclohexane Dual tube silicone rubber 46.7 947 46.7 40 1764–35,760 Robert et al. (2006)

Configurations: HF: hollow fibre (i.d.< 0.5 mm); C: capillary (0.5 mm< i.d.< 10 mm); PP: polypropylene; PDMS: polydimethylsiloxane; PVDF: polyvinylidenefluoride;Zrf: zirfon.Compounds: MeOH: methanol; BuOH: 1-butanol; NH3: ammonia; BENZ: benzene; TCE: trichloroethylene; TOL: toluene; PROP: propylene; NO: nitric oxide; HEX: hexane;DMS: dimethylsulfide; BTEX: mixture of benzene, toluene, ethylbenzene and xylenes; DMS: dimethylsulfide; DCM: dichloromethane; DCE: dichloroethane.

a n.r., Not reported or not sufficient data to calculate.b n.a., Not applicable (e.g., static gas phase).


simple thermodynamics, the equilibrium concentration in thewater phase would never change upon the insertion of any type ofmembrane between the gas phase and the water phase.

Irrespective of the membrane resistance, the driving force formass transfer depends on the concentration to which the pollutantis reduced in the liquid phase. Therefore, the removal rate ina membrane bioreactor depends largely on the activity of themicrobial population. In most of the reported studies, biofilmformation was observed to be an essential part of reactor operation(Ergas et al., 1999). Both mixed cultures and pure cultures wereused for biofilm generation. The hydrophobic nature of both microporous and silicone membranes facilitates microbial adhesion. Themicroorganisms located close to the membrane are exposed tohigher substrate concentrations than suspended cells, making itmore likely that most cell growth occurs close to the membrane.

Biofilm growth may cause serious problems if excess biomass isnot sloughed off. Freitas dos Santos et al. (1995) attributed thedecreasing reactor performance and the increasing pressure dropover the liquid phase to extensive biofilm formation in the spiral-wound membrane module they studied. Clogging of hollow fibreswith a biofilm of propene-degrading Xanthobacter cells could beprevented by applying a very high liquid velocity, but still the

reactor performance decreased over a period of 3–6 months as thebiofilm matured. These results suggest that, even if clogging isprevented, biofilms are prone to aging (Reij and Hartmans, 1996).

During the degradation of dichloromethane (Bauerle et al.,1986)a good biofilm did not develop on the reactor membrane. The bio-film growing on dichloromethane sheared off the membrane after94 days, causing a drop in reactor performance (Reij et al., 1995).Recently investigation on DMS removal in thermophilic membranebioreactor indicated an elimination capacity of 54 g m�3 h�1 withthe removal efficiency 84% at gas retention time (GRT) 24 s(Munkhtsetseg et al., 2008). The reason suggested for this was thathydrochloric acid was produced during the degradation ofdichloromethane, which accumulated in the biofilm to toxic levelsand destabilized the biofilm.

Aziz et al. (1995) purposely repressed biofilm formation ina membrane reactor for wastewater treatment by the addition ofa sequestering agent. This membrane reactor was part of a two-stage bioreactor, in which methanotrophs were circulated. In themembrane reactor, the methanotrophs degraded trichloroethylene(TCE) and in a separate reactor, growth substrate was supplied.Separate reactors were required for the study, since TCE itself didnot support microbial growth and could be degraded only co-

Fig. 5. Aerobic and anaerobic biodegradation of trichloroethene (TCE) in membranebioreactor.


metabolically when a primary substrate was supplied. Such a two-stage process may not only be used for wastewater treatment butalso for the removal of TCE from waste gas.

A completely new strategy for the removal of TCE from air wasdesigned by Parvatiyar et al. (1996a,b). In their newly designedmembrane bioreactor, acetate was added to the liquid phase ascarbon source and as electron donor to lower the oxygen tension inthe biofilm. Under anaerobic conditions created in this way, TCEwas partially dechlorinated. Subsequently, the products of theanaerobic dechlorination were degraded further in the aerobiczone of the biofilm (Fig. 5, Cesario et al., 1992). Their work, however,did not contain experimental evidence that both the aerobic andthe anaerobic zone are present, but it is the first report on thecontinuous removal of TCE from air in the absence of volatilegrowth substrates.

While reviewing literature (Table 1), it was found that theexperiments reported are very varied and the approach in each caseis different. For example-in most studies oxygen was made to diffusethrough the membrane along with the compound to be removedfrom air, but Freitas dos Santos et al. (1995) supplied oxygen in thewater phase. Parvatiyar et al. (1996a,b), on the other hand, main-tained the liquid phase anaerobic, to allow anaerobic degradation.Secondly, the time period in each experiment varied, while somelasted for more than a year, some were completed within a few days.

6.4. Future needs

All studies carried out on membrane reactors are laboratory scaleexperiments. To the best of our knowledge, no reports are availableon pilot-plant investigations or full-scale applications of membranereactors in biological waste gas treatment. Membrane modulesappear relatively easy to scale up given their modular nature (Karoorand Sirkar, 1993); however an extensive long-term performancetesting is necessary before they can be applied on full-scale.

The effect of biomass on the membrane material, in the long runhas not sufficiently been tested. During prolonged operationmicrobial polysaccharides might get absorbed to the membranematerial, decreasing the critical pressure, and allowing liquid topenetrate the pores of hydrophobic micro porous membrane. Thiswetting of the membrane may significantly increase membraneresistance (De Heijder et al., 1994) and will be a bottleneck for long-term operation of reactors with micro porous membranes. Noexperimental evidence on this subject has been reported so far.Wetting of the membrane could be prevented by a thin coating ofdense material applied on the liquid side of a porous membrane.Such composite membranes have been used in blood oxygenationto suppress blood-trauma and prevent the pores from filling withliquid and cell debris (Sirkar, 1992).

In addition to the durability of the membrane material, thestability of the biomass is essential as well. The formation of thickbiofilms (Freitas dos Santos et al., 1995) and clogging of the liquidchannels (Kreulen et al., 1993) were shown to deteriorate reactorperformance. Even when clogging was prevented by a very fastliquid flow, the performance of hollow fibre modules decreasedwith time (Kreulen et al., 1993). Therefore, strategies have to bedeveloped to monitor the biofilm, to stabilize its activity, and toremove excess biomass from the membrane modules.

The removal of poorly water soluble pollutants from air can beconsidered to be the most promising application for membranebioreactors. The mass transfer resistance of membranes for thisgroup of pollutants is negligible. Recently, Kumar et al. (2009), havereported enhance performance of a composite membrane biore-actor for treating toluene vapors. The composite membrane con-sisting of a porous poly acrylonitrile (PAN) support layer coatedwith a very thin (0.3 microm) dense polydimethylsiloxane (PDMS)top layer indicated maximum elimination capacity of609 g m�3 h�1 along with the flux 1.2 g m�3 h�1. Moreover, thelarge gas–liquid interface of membrane modules enables efficientremoval of these pollutants that in general are difficult to removefrom air.

Other niches for the application of membrane bioreactors areindoor applications and the removal of pollutants that requirea specific microbial population (like TCE and nitrogen monoxide).Recently, the membrane bioreactor with both an anaerobic and anaerobic zone was proposed (De Heijder et al., 1994). Such a biore-actor might enable the biodegradation of pollutants, such as highlychlorinated hydrocarbons, that until now were considered to bebeyond the reach of aerobic biological waste gas treatment.

7. Comparison of conventional bioreactors and membranebioreactor for VOC and odour control

The application areas and comparative performance evaluationof bioreactors (biofilter, biotrickling filter, bioscrubber, membranebioreactor) widely reported for VOC and odour control is presentedin Tables 2a and 2b, respectively. The target pollutant concentrationfor bioscrubber is relatively higher than biofilter and biotricklingfilter, while in case of membrane bioreactor the membrane fluxlimit should allow for higher VOC and odour concentration. Bio-filters usually have a media layer of 1–2 m to prevent excessive airvelocities through the media, which easily results in high-pressuredrop or airflow preferences. Biotrickling filters and bioscrubbers donot require an upfront humidifier, like biofilter do, to increase theinlet air humidity up to a preferable 100%. The footprint of bio-trickling and bioscrubber reactors is normally much smaller, sincethey usually contain a more open packing that can be more than2 m high. The media in biofilters needs to be replaced frequently,due to the deterioration of the (usually organic) media, or theincreasingly worse process conditions like pH decrease, nutrientdepletion, or extensive biomass accumulation, salt content orpressure drop. In biotrickling and bioscrubber reactors, no mediumchange-out is required, since inert packing material is used andprocess conditions can be better controlled. Operational costs canbe saved with biotrickling or bioscrubber reactors, since up to 40%of the operational cost of a biofilter is typically related to themedium change-out. Further, the biofilter reactor do not havea continuous and distinct liquid phase as found in the case of bio-trickling and bioscrubber. Therefore, important process conditionslike pH, salt content, nutrients, toxic intermediates or end productsof microbial degradation are much easier to analyze and to controlin biotrickling and bioscrubber. Also, the liquid phase itself (thewater content) can be better controlled to obtain the optimal watercontent through the bioreactor system, and to minimize drying out

Table 2aComparative performance evaluation of bioreactors for VOC and odour control.

BioreactorType

Target VOCsand Odoursconc. g/m3

Treatment efficiency for Pressuredrop

Capitalcost

Operationalcost

Bioprocesscontrola

Low conc.of VOCs/Odours

High conc.of VOCs/Odours

High watersolubleVOCs

Low waterinsolubleVOCs

Fluctuatingfeed conditions

Biofilter <1 High Low High Low Low Low Low Low LowBiotrickling

filter<0.5 High Low High Low Low Low Low Low Low

Bioscrubber <5 High High High Low High Very low Medium Medium HighMembrane

reactorHigh High High High High Need long-

term evaluationNeed long-term evaluation

High High Need long-term evaluation

a Comparative evaluation on bioprocess control parameter is given in Table 2b.

Table 2bCritical bioprocess control parameters for different bioreactor configurations.

Type Moisture Nutrient/pH Clogging Transient response Airflow channeling Startup

Biofilter Highly sensitive Highly sensitive Sensitive Sensitive Highly sensitive SensitiveBiotrickling filter Not sensitive Not sensitive Highly sensitive Highly sensitive Sensitive Highly sensitiveRotating contactors Not sensitive Not sensitive Not sensitive Highly sensitive Not sensitive Highly sensitiveBioscrubber Not sensitive Not sensitive Not sensitive Highly sensitive Not sensitive SensitiveSuspended growth Not sensitive Not sensitive Sensitive Sensitive Not sensitive SensitiveMembrane reactor Not sensitive Not sensitive Highly sensitive Sensitive Not sensitive Highly sensitive


of the packing material or the biofilm on the packing material.Heslinga (1994) mentioned that probably 50–75% of the problemswith conventional biofilters are related to a poor control of thewater content in the biofilter media.

An important disadvantage of biotrickling and bioscrubberreactors is the higher complexity to construct and to operate. Thestartup of biotrickling and bioscrubber reactors is also morecomplicated, since the inert medium does not contain microor-ganisms at the start. As soon as the microorganisms are present,they can be washed out by the required drainage of the processwater; a problem that is encountered with full-scale operations ofbioscrubbers. Pressure drop control is more complex especiallywith biotrickling reactors. Compared to a biofilter, higher airvelocities through the media results more easily in a higher pres-sure drop. When biotrickling or bioscrubber reactors are applied forthe treatment of pollutants like hydrogen sulfide, ammonia orchlorinated compounds, the degradation produces acid end prod-ucts in the drain water, which needs further processing. Biotricklingand bioscrubber reactors make a better process control possible,which require, on the other hand, measurement or controlinstrumentation.

The location of the water film with respect to the biomassdiffers in biotrickling filter, bioscrubber and membrane bioreactor.In the trickling bed reactor and in the bioscrubber pollutants haveto diffuse through the water phase, before they can be consumedby the microorganisms. For pollutants with a poor water-solu-bility, such a layer of water causes a substantial additional resis-tance for mass transfer. In the membrane bioreactor, on thecontrary, the liquid phase is situated at he opposite side of thebiofilm and hardly forms a barrier for mass transfer of the poorlywater soluble pollutants. Therefore, the removal of poorly watersoluble pollutants from air can be considered as the most prom-ising application for membrane bioreactors. The mass transferresistance of membranes for this poorly water soluble pollutants isnegligible. Moreover, the large gas–liquid interface of membranemodules enables efficient removal of these pollutants, which ingeneral are difficult to remove from air. Other niches for theapplication of membrane bioreactors are indoor application andthe removal of pollutants that require a specific microbial pop-ulation, like TCE and nitrogen monoxide. Very recently,

a membrane bioreactor with both an anaerobic and an aerobiczone was proposed. Such a bioreactor might enable the biodeg-radation of pollutants, such as highly chlorinated hydrocarbons,that until now are considered to be beyond the reach of (aerobic)biological waste gas treatment.

Disadvantages of membrane bioreactors are the high invest-ment cost, particularly compared to other bioreactor, and possibleclogging of the liquid channels due the formation of excessbiomass. Compared to other types of bioreactors, the membranemay form an additional barrier for mass transfer. So far, membrane-based biological waste gas treatment has only been tested onlaboratory scale. If the long-term stability of these reactors can bedemonstrated, membrane bioreactor technology can be useful inthe treatment of gas streams containing poorly water solublepollutants and highly chlorinated hydrocarbons, which are difficultto treat with conventional methods for biological waste gastreatment.

8. Other bioreactor configurations

Apart from bioreactors described in the preceding sections forVOC and odour treatment, several other bioreactors have also beenreported. An innovative design was reported by Yang et al. (2002),which consists of a rotating drum BF. Open pore reticulated poly-urethane foam was used as the BF packing medium and this newdesign resulted in better distribution of VOCs, oxygen, nutrients,and biomass, over conventional BFs. Two types of rotating drum BFswere investigated to study the effect of medium configuration onBF performance for VOC treatment. One was a single-layer BF thatconsisted of a thick layer of open pore reticulated polyurethanefoam media. The other was a multi-layer BF that used a set of fourconcentric thinner layers of the media. The effect of the twodifferent media configurations was examined using diethyl ether atvarious organic loading rates. The results showed that the multi-layer BF could maintain more stable and higher ether removalefficiencies at gas empty bed contact time (EBCT) of 30 s, whencompared to the single-layer BF at gas EBCT of 90 s, and at organicloading rates ranging from 32.1 to 128.4 g (ether) m�3 l�1. The multi-layer BF also exhibited a more even biomass distribution on theconcentric surface at medium depth than the single-layer BF, which


suggested a reduced possibility of short-circuiting of gas streamsand, consequently, better performance (Yang et al., 2002).

A novel rotating rope bioreactor was described by Mudliar et al.(2008a,b), especially for the treatment of VOCs characterized byhigh volatility along with high water-solubility. The bioreactorcould be used for treatment of vapor phase VOCs by suitablyscrubbing the compound in water and subjecting it to the newbioreactor referred to as the rotating rope bioreactor (RRB) fortreatment. The novel bioreactor provided higher interfacial area(per unit reactor liquid volume) along with high oxygen masstransfer rate, greater microbial culture stability and consequentlyhigher substrate loadings and removal rates in comparison to otherconventional reactors (e.g. BFs) widely used for the treatment ofVOCs. Pyridine was used as a model compound to demonstrate theenhanced performance of RRB. The experimental results showedthat the novel RRB system was able to degrade synthetic waste-water containing pyridine with removal efficiency of more than85% up to a loading of 66.86 g m�3 h�1. Further, the authors alsodescribed a single stage reactor called the rotating rope biofilter fordirect treatment of VOCs instead of a two-stage process describedabove. This reactor is a modified closed RRB where the waste aircontaining the VOC is directly sparged through the water hold-upof the reactor and the water soluble VOCs are absorbed in theaqueous phase. It is then degraded by the microbial consortiumimmobilized on the RRB rope media.

Kan and Deshusses (2003) reported a new type of bioreactor forair pollution control referred to as a foamed emulsion bioreactor(FEBR). The new reactor was based on an organic-phase emulsionand actively growing pollutant-degrading microorganisms, madeinto foam with the air being treated. As there is no packing in thereactor, the FEBR is not subjected to clogging. Mathematicalmodeling of the process and proof of concept using a laboratoryprototype revealed that the foamed emulsion bioreactor greatlysurpasses the performance of existing gas phase bioreactors.Experimental results showed a toluene elimination capacity as highas 285 g (toluene) m�3

(reactor) h�1 with removal efficiency of 95% ata gas residence time of 15 s and a toluene inlet concentration of1–1.3 g m�3.

Study of toluene degradation in a flat plate vapor phase biore-actor by a Pseudomonas putida 54 G biofilm using oxygen microsensors was reported by Villaverde et al. (1997). Oxygen micro-electrodes were used to measure oxygen concentration profilesthrough the gas, liquid, and biofilm phases. The linear shape of thedissolved oxygen concentration profile in the outer 87% of thebiofilm thickness suggested an absence of reaction in this layer.Oxygen consumption in the remaining 13% biofilm layer (0.3 mm)followed zero order kinetics with a rate constant of 102.2 g m�3 h�1,for toluene gas concentration of 1.5 g m�3. The increase in respi-ratory activity near the substratum was confirmed by microscopicstudy of cryogenic biofilm sections, and the lack of activity in thesurface film was interpreted as a consequence of injury exerted bythe toxic substrate. The accumulation of dead cells on the top of thebiofilm contributed resistance to the transport of substrates todeeper layers of the biofilm suggesting a protective role of the outerlayer against the harmful effect of the toxic substrate. These resultshighlight a new conceptual biofilm model in which both microbialgrowth and inactivation are controlled by substrate transport,leading to a structure that by itself controls substrate availability.

9. Conclusions

This review provides an overview of the various bioreactorsused for waste gas treatment, limitations of existing bioreactors,and new avenues required in bioreactor development and design.Clearly, many of the bioreactor designs discussed herein still

require improvement, and confirmation of significantly betterperformance compared to existing designs. For example, consid-ering the volatile nature of VOCs and odourous compounds, sus-pended growth reactor systems/bioscrubbers may find limitedapplication in the field at higher substrate concentration andloadings, and oxygen transfer can become a limitation, since airpurging cannot be used in such systems. In future, improved‘‘rational bioreactor design’’ techniques need to be graduallydeveloped. The knowledge needed to do this can be developed byfurther research on topics such as the mechanisms of clogging, thekinetics of biofilm growth, and fundamental microbial ecology.Flow characterization through the bioreactor is important, since gasflow, liquid flow and gas velocity have an important impact onprocess parameters like mean gas resident time, gas dispersion inthe reactor, and pressure drop over the system. These parametersare important to scale up and to operate a bioreactor at optimumconditions. The bioreactor development should be also focused onissues like robustness (flexible to process fluctuations/failures),large pollutant loadings, high temperatures, halogenatedcompounds and poorly water soluble compounds. It is a big andchallenging task to design a bioreactor from fundamental theory,but definitely the understanding of biological treatment is growingwith time.

Further, developments of innovative combined bioreactordesigns remain a high priority, since a single bioreactor configu-ration will never provide a universal solution to existing VOC andodour problems. In many instances, progresses in reactor designand development will require similar advances in understandingthe fundamentals of the bioprocess, so that a more logical, creativeand focused approach in bioreactor design can be implemented.Hence, to improve the performance of the biological air treatmentsystem for VOCs and odours, there is a need for continuous inno-vation in bioreactor configurations.

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