Biofiltration: An Emerging Technology - NISCAIR ONLINE PERIODICALS

Indian Journal of BiotechnologyVol 2, July 2003, pp 396-410

Biofiltration: An Emerging Technology

Carlos R Soccol'>, Adenise L Woiciechowski', Luciana P S Vandenberghe', Marlene Soares', Georges Kaskantis Netol

and Vanete Thomaz-SoccofiBiotechnology Processes Laboratory, Department of Chemical Engineering, UFPR,Federal University of Parana, POBox 19011, CEP 81531-970 Curitiba-PR, Brazil

2Federal Center of Technological Education of Parana, CEFET -PR, Brazil3Pathology Department, Molecular Parasitology Laboratory, Federal University of Parana,

CEP 81531-970 Curitiba-PR, Brazil

Received 20 December 2002; accepted 21 February 2003

Gas biofiltration is a relatively new technology used to purify contaminated air from volatile organic and inor-ganic compounds (VOCs and VIes). This biotechnological process is now gaining popularity among industries due toits low cost, operational simplicity, removal efficiency, comprehension, modeling and mainly because it is intrinsicallyclean, as it reduces or eliminates the need for additional treatment of the end-products. Since 1980s different chemi-cal processes have been utilizing biofiltration with different flow rates, up to 200 thousand m3 hr', Technologies con-sidered being forms of biological gas purification include bioscrubbers, biotrickling filters and biofilters, operatingwith the same fundamental mechanisms of biodegradation. Biofilters, subject of this review, can be regarded as solidfermenters, The polluted gas is forced to flow through a bed packed support on which microorganisms are immobi-lized or attached as a biofilm. As a biological purification process, it is based on the ability of microorganisms to de-grade organic and inorganic compounds, and their complete oxidation to generate energy. This review describes andevaluates some techniques, apparatus and support media used for biofiltration of gases, focusing industrial applica-tions. The encapsulation of microbial cells has also been considered.

Keywords: biofiltration, solid state fermentation, cell immobilization, biofllter, volatile organic and inorganiccompounds

IntroductionBiofiltration is a relatively emerging new technol-

ogy, applied to waste gas purification or to the controlof volatile organic and inorganic compounds (VOCs)and (VICs), aromatic compounds (BTEX) and othertoxic and. odorous compounds, generated in waste-water treatment facilities, rending plants, compostingfacilities, and other odour-producing operations, usedfor over 40 years in the USA and Europe, and a tech-nology that is being used at industrial level for treat-ing high-volume, low-concentration air steams. Thegas passes through a reactor in which microorganismsare fixed in a water phase within a filter supportmaterial.

Gaseous emissions are often involved in problemslike odour nuisance, health impacts, smog formation,acid rain and the greenhouse effect. Growing publicawareness in health aspects and environmental im-

*Author for correspondence:

Tel: 55-41-361-3183; Fax: 55-41-266-0222

E-mail: [email protected]

pacts, combined with the implementation of manytitles of the US 1990, Clean Air Act Amendments(CAAA) and similar regulations in Europe, have beenforcing many industrial and agricultural processes,transport functions, energy production and effluenttreatment systems to meet the emission standards laiddown in guidelines.

Most of the techniques that are being applied in thetreatment of these off-gases are physico-chemicalmethods, including separation, adsorption, scrubbing,incineration and catalytic oxidation (Adler, 2001). Inspite of that, biological methods of gas filtration, suchas biofiltration, bioscrubbing and biotrickling, havebeen attracting an increasing popularity in Europe,because of their low cost, operational simplicity andmainly because they are intrinsically "clean technolo-gies", as they reduce or eliminate the need for addi-tional treatment of end-products.

Technologies considered to be forms ofbiofiltration include biofilters, bioscrubbers,biotrickling filters and engineered biofilters, alloperating with the same fundamental mechanisms of

SOCCOL et al.:BIOFILTRATION

contaminant sorption and biodegradation (Adler,2001). Bioscrubber consists of a scrubber and anactivated sludge system. The contaminated gas is putin contact with water, by a current or contra flow, in aspraying tower, resulting on its absorption in thismobile phase, which passes through the unit beingsubsequently treated. In biotrickling filters, the off-gas is forced through an inert support covered with abiofilm. A liquid phase (water and cells) is sprayed onthe packed bed, being continuously circulated.Different from bioscrubber system there is no need ofan activated sludge system for the regeneration of thisliquid phase, because the absorption andbiodegradation of the target compounds are combinedin the same reactor. The liquid phase flows steadilyover the porous bed and it is continuously collectedand recirculated. Biofilters, subject of this review, canbe regarded as solid fermenters. The gas is forced toflow through a bed packed support on whichmicroorganisms are attached as a biofilm orimmobilized. As a microbial purification process,biofiltration is based on the ability of microorganismsto degrade organic and inorganic compounds presentin waste gases and their complete oxidation togenerate energy for metabolism.

This review describes and evaluates some tech-niques, apparatus, support media and the encapsula-tion of microbial cells used at gas biological treat-ment, with a focus on industrial applications.

HistoryBiofiltration is the oldest biotechnological method

for the removal of undesired off-gas componentsfrom air. Since 1920' s biofilters have been applied toremove odorous compounds from wastewater treat-ment plants or intensive animal farming (van Groe-nestijn & Hesselink, 1993). Initially, they were madeby digging trenches, laying an air distribution systemand refilling the trenches with permeable soil, woodchips or compost. From the late 1970's most of thedevelopment work on biological off-gas treatment hasbeen carried out in Europe, especially in Germany andThe Netherlands, in response to increasingly nationalregulatory requirements. Only up to 1980's intensiveprogress has started in Western Europe and UnitedStates (Ottengraf et aI, 1986), and since then re-searches on biofiltration are being focused also on thedegradation of toxic volatile chemicals and on indus-trial applications, using .different supports, types offilters and microorganisms

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The first biofilters were built in Unites States in the1960s (Gerrard et al, 2000). During the late 1980s tothe late 1990s, approximately thirty large full-scalesystems about more than 100m3 of filter material havebeen built for the control of volatile organic com-pounds (VOCs), hazardous air pollutants (HAP) andodour (van Lith et al, 1997). Biofiltration has hadmore industrial success in Europe and in Japan whereover 500 biofilters are in operation (Bohn, 1992).

Industrial ApplicationsMany chemical process industries have already

been utilizing biofiltration to treat waste gases. Forexample, S C Johnson & Son, Inc, Racine, USA, treatair for propane and butane removal from room air,about 5,000 m3 hr', with a removal efficiency of90%; Monsanto Chemical Co, Springfield, USA, treatethanol and' butyraldehyde removal from dried air,about 50,000 m3 h·l, with a removal efficiency of 99%(Bohn, 1992). CYDSA S A Group (Mexico) has in-stalled several biofilters in Mexico and United Statesto treat odorous compounds, hydrogen sulphide andcarbon disulphide, having flow rates from 4,000 m3

h(l to 45,000 m3 hr" and efficiency for each contami-nant of 90%,95-98% and 40-70%, respectively. Thisgroup has also treated carbon disulphide and hydro-gen sulphide from a cellulose sponge manufacturingfacility in biotrickling filters previously inoculatedwith an adapted microbial consortium (Hugler et al,1999). During the pilot test stable removal efficiencyand elimination capacity of +90% and 220g CS2 nr'hr-l were respectively attained, with an empty bedresidence time of 33 seconds. For H2S, efficienciesgreater than 99% were always obtained.

According to Hubber (van Groenestijn & Hes-selink, 1993) flow rates up to 200 thousand rrr' h(lcan be successfully treated by biofiltration, filter vol-umes exceeding 3,000 rrr' are reported and volumetricloads between 10-200 m3 m-3 hr' are usual. As a bio-logical process an important aspect concerning itsindustrial application is the interruption on workingand feeding during holidays or weekends. Paca (1995)has investigated how the interruption on working timein a painting shop affected the biofilter performance.Up to 48hrs of interruption the degradation efficiencyafter a new start-up remained as high as before theinterruption. After long interruptions the degradationefficiency at first strongly dropped 25%, but in a fewhours it achieved the original level of degradationagain. His research also showed that the time interval

398 INDIAN J BIOTECHNOL, JULY 2003

of decreased degradation efficiency could be signifi-cantly shortened using lower volumetric gas flow rateafter the start up of the biofilter performance.

Biological treatment of contaminated air is not al-ways appropriate. First, because biotechniques for aircleaning are only efficient and cost effective in treat-ing large volumetric airstreams with low level ofpollutants, up to 1-5 g m-3 (van Groenestijn & Hes-selink, 1993). The process becomes more expensivewhen concentrations are greater than 50 g m', be-cause of moisture and temperature control require-ments (Gerrard et al, 2000). Second, as a biologicalsystem the micro flora present must be capable andvery efficient on the conversion of the contaminantsinto harmless compounds. Third, air contaminatedwith dust, oil and grease accumulates and clogs thefilter bed. Finally, the contaminant degradation is de-pendent also in its water solubility and on its biode-gradability. For example, biofiltration may be unsuit-able for highly halogenated compounds, such as tri-chloroethylene (TCE), trichloroethane (TCA) andcarbon tetrachloride because of its low aerobicdegradation, which means longer residence times andlarger bed volumes (Bohn, 1992). Also the size of abiofilter is inversely proportional to the degradationrate (Chitwood et al., 2000). Data from some studiesdone with variations of the component to be treatedand the media used with reference are given inTables 1 and 2.

MicroorganismsRegarding the microbiological potential of biofil-

ters, different aspects have been recently studied:isolation and characterization (Bendinger et al, 1990;Cho et al, 1991; Shareefdeen et al, 1993; Mallakin &Ward, 1996; Andreoni et al, 1997; Lipski et al, 1997;Reichert et al, 1997); the use of pure cultures of bac-teria (Diks et al, 1994; Megharaj et al, 1998; Krishnaet al, 2000); fungi (Cox et al, 1997; Woertz & Kin-ney, 2000); mixed microbial populations (Cox & De-shusses, 1999); effect of enrichment culture includingapplication of special strains, types of microorganismsand their metabolic activities (Cox et al, 1997; Cox &Deshusses, 1999; Kennes et al, 1995; Zilli et al,1996); effects of external conditions on microbial ac-tivity (Kennes et al, 1995; Weigner et al, 2001); andrelease of microorganisms from biofilters (Ottengraf& Konongs, 1991; Becker & Rabe, 1997).

Microorganisms presented in biofiltration systemsare predominantly aerobic. Oxygen supply is abun-

dant in the incoming air but it has to be dissolved intothe water phase to be available to the micro flora inthe biofilm. Depending on the composition of the off-gas and physico-chemical conditions in the filter, dif-ferent mixed populations may develop, but most ofthe microorganisms found in biofilters are bacteria(van Groenestijn & Hesselink, 1993). Rieneck (1992)found that most of the bacteria are coryneforms andendospore formers and only occasionally pseudomo-nads. Actinomycetes are mainly represented byStreptomyces spp. Yeasts and fungi are less abundantin biofilters. Most of the filamentous fungi belong tothe Mucorales (Mortierella, Rhizopus) and Deutero-mycetes (Penicillium, Aspergillus, Cladosporium,Fusarium, Trichoderma, Alternaria and Botrytis) (vanGroenestijn & Hesselink, 1993).

Due to fungi, enzymatic oxidative non-specificsystems, like lacases and peroxidades, tend to degrademore complex molecules. In biofilters, their aerialhyphae form a very large specific surface area, whichis in direct contact with air flowing through the filter.Different of what happen for bacteria, the pollutant istransferred directly to the cell surface, bulk transitionproblems. This may improve absorption of hydropho-bic volatile compounds. Majcherczy et al, 1990, didexperiments with biofilters based on white rot fungigrowing on straw or other agricultural residues. Whengrowing, the fungus secreted oxidative enzymes,which catalyze the degradation of lignin. These en-zymes are non-specific and able to degrade manyaromatic compounds as well and the elimination ofstyrene, a-pinene and chlorophenols from air (vanGroenestijn & Hesselink, 1993).

In order to achieve high elimination capacity anddegradation efficiency of the biofilter an adaptationperiod of the micro flora is necessary. During this pe-riod a gradual increase of organic load has to be per-formed (Paca, 1995). As in all biosystems, a newlyinstalled biofilter will have to adapt itself to its micro-bial ecology and a faster start up can be achieved byinoculating microorganisms, which can be specializedin the degradation of the contaminant or not, like: ac-tivated sludge, pure culture or consortia of specializedmicroorganisms isolated from contaminated areas oralso achieved. It can be expected that in biofilters mi-crobial population is in a constant growing state afterthe adaptation period, which means constant biomassweight but not necessarily of constant composition. Inorder to give to the industrial necessities more reli-able information concerning steady state or transient

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Table l-Biofiltration applications on pure air component

Air waste/Component Microorganism Support Reference

Ammonia Celite pellets Sorial et at, 200 1Bacteria Compost + activated carbon Liang et al, 2000

Perlite Joshi et al, 2000Sponge cubes coated with activated Kim et al, 2002carbon and zeolite

a-pinene Perlite + forest soil as inoculant van Groenestijn & Liu, 2002Expanded clay granules<p I cm. + forest soil as inoculantPolyurethane foam cubes + forest soilas inoculant

Benzene Pseudomonas sp. Sugar cane bagasse Sene et al, 2002Alcaligenes xylosoxidans Yeom & Daugulis, 2001

Ethanol Wood bark Ramirez- Lopez et al, 2000Hydrogen sulphide Pig manure and sawdust Elias et al, 2002

Lava rock Chitwood & Devinny, 2001Pellets of agricultural residues Elias et al, 2000

Pagella & De Faveri, 2000

Methyl acetate Lu et al, 2001 a

Methyl ethyl ketone Rhodococcus sp. Compost + activated carbon Amanullah et al, 2000Methyl tert-butyl ether Bacteria Fortin & Deshusses, 1999N,N dimethylacetarnide Lu et ai, 200lbNitrogen oxide Lee et al, 200 1

Styrene Perlite Paca et al, 200 1Exophiala yeanselmei Wood bark Juneson et al, 2001

Yard waste Lu et al, 2001c

Toluene Pseudomonas putida Yu et al, 2002P. putida Cox & Deshusses, 2002

Park et al, 2002Polyurethane foam Moe & Irvine. 2001

Moe & Irvine, 2000a,bPeat Acuna et al, 2002

Corynebactrium jeikeium Strauss et al, 2000Corynebacterium nitrilophilusMicrococus luteusPseudomonas mendocinaSphingobacteriumthalphophilumTuricella oritidis

Xylene Bacteria + Yeast Cellulose Bibeau et al, 2000

responses of biofilters, models which provide greaterscope for understanding the process in different stud-ies are discussed (Deshusses et al, 1995; Gerrard et al,2000; Schwarz et al, 2001).

Biofiltration-Concept, Development and BiofiltersBiofiltration can be described as the removal and

oxidation of harmful gases, mainly volatile organiccompounds, from contaminated air. The contaminatedair passes through a support of compost or soil whichis disposed into a horizontal or vertical reactor. Under

the bed lies a distribution system, consisting of perfo-rated pipes designed specially to provide an equalflow of the gas through the porous bed (Bohn, 1992).

Gas treatment by biological processes can also bedescribed as a diffusion of gaseous phase into anaqueous phase, where microorganisms convert biode-gradable pollutant components into harmless prod-ucts. Biofilters are systems in which the gas is blownthrough a bed of compost or soil, where the naturalmicroorganisms presented into the support consumethe gaseous organic pollutants or an inert support


Air waste mixture

Table 2-Examples of biofiltration applications on air waste mixtures of components

Reference

Acetaldehyde + propionaldehyde

Air from piggery facilities

Aromatic hydrocarbons (benzene.ethyl benzene. toluene. and styrene),ketones (methyl ethyl ketone,methyl isobutyl ketone and methylpropyl ketone), and organic acids(n-butyl acetate. ethyl 3-ethoxypropionate

Benzene, toluene, ethyl benzene, m-xylene, p-xylene, and o-xylene

Ethanol and toluene

H2S e CS2

H2S, MeSH, Me2S

H2S, Methylmercapatan

H2S and/or NH3

Isoprene, dimethyl sulphide,chloroform, benzene,trichloroethylene, toluene, in-xylene, o-xylene and styrene

Methanol, PineneMono-chlorobenzene (m-CB) andortho-dichlorobenzene (o-DCB)

2-propanol

Toluene and benzene

Toluene, benzene and xylene

Toluene and styrene

Toluene and xylene

Microorganism

Cladosporium resinaeC. sphaerospermumExophiala lecanii-corniMucor rowxiiPhanerochaete chrysosporium

Bacteria

Pseudomonas putida

Thiobacillus sp.

Thiobacilllus thioparusThiobacillus sp.

Arthrobacter oxydansArthribacter oxydans ePseudomonas putidaThiobacillus thioporusNitrosomonas europaea

Bacillus and Fungi

Consortium

Acinetobacter sp.

Pseudomonas pseudoaalcaligenes

Support

Sludge gel beads +hollow plastic balls

Wood chips> 20 mmWood chips 10 a 10 mrn

Ceramic

Peat

Peat

Peat

Ca-alginateAttached cells

Ca-alginateCompost and wood chips

Peat

Sol-gel

Acti vated carbonPorou's peat

Peat and glassPerlitePeat, Bark chips, Vermicu-lite, Hydroballs

Ibrahim et al, 2001

Sheridan et al, 2002

Qi et al, 2002

Quinlam et al, 1999

Villaverde et al, 2000Hartikainen et al, 2001

Ruokojarvi et al, 2001

Kim et al, 1999Chung et al, 200laChung et al, 200lbChung et al, 200lcChung et al, 2000Sun et al, 200 1

Yoon & Park, 2002

Kong et al, 200 IBaltzis et al, 200 I

Leethochawalit et al,2001

Li et al, 2001Choi & Oh, 2002

Metris et al, 2001Oh & Choi, 2000

where a special microorganism or pool of microor-ganisms are cultivated. In these cases, the degradationof environmentally harmful compounds is due to anactive biofilm deposited onto a package consisting ofactive and selected microorganisms. The aqueousphase is presented in the form of humidity, at the solidsupport (Soto et al, 1995; Reij et al, 1998).

Biofilters=-composed of a solid support, coveredby an active biofilm, a bed through which the gas withharmful compounds pass to the environment-a tech-nology available and is being investigated now-a-days. The mechanism of the process consists of re-

moving the unwanted odorous organic compoundfrom the gaseous phase, concurring its absorption oradsorption onto the porous solid phase of the biofilteror dissolution into the liquid phase, and then oxidationof this compound by microorganisms (Adler, 2001;Burgess et al, 2001).

A range of technologies is available to treat odor-ous air from wastewater treatment plants, sulphur ef-fluents, industrial processes and industrial wastewatersuch as organic solvents, volatile organic compoundsand sulphur compounds. Many kinds of apparatus canbe designed and many kinds of media are available

SOCCOL et al.:BIOFILTRATION

for this purpose. Biofiltration is advantageous becauseit does not require large amounts of energy duringoperation and besides produces a relatively low-toxicity waste steam (Burgess et al, 2001; Adler,2001).

Advantages and Disadvantages of BiofiltrationCompared to other conventional physico-chemical

treatments biofiltration has some advantages and dis-advantages that are given in Table 3.

TheoryIndustrially, the waste gases are being treated tra-

ditionally by physico-chemical processes, such asscrubbing, adsorption, condensation and chemicaloxidation, but during the last decade, biologicaltreatment of waste gases are being accepted as an ef-fective and economic alternative. When the removalof harmful VOCs from the gaseous phase is due tosolubilization, the general principle consists of thediffusion of the pollutants into the liquid phase wherethey are degraded by microorganisms into productslike CO2, H20 and other mineral compounds (Kennes& Thalasso, 1998). The process can be expressed as:

Pollutant gas + oxygen + microorganisms ~microorganism cells +·C02 + H20

In aerobic process, bacteria consume ionic sulphidegroups and oxidize them to non-odorous sulphurgroups. It suggests that the odorous compounds areremoved from the gaseous phase by adsorption anddissolution in the liquid phase with simultaneouschemical and biological oxidation (Ostojic et al,1992). Adsorption and absorption of the gas onto the

401

solid phase of the biofilter bed is a phenomenon thatconcurs to retain the pollutant to be biologicallytreated (Adler, 2001).

Biofilter MediaThe sorption or dissolved capacity of the bed media

is relatively low, but the oxidation process regeneratesthe sorption and the solution capacity, setting a dy-namic process involving the removal of odorousvolatile organic compounds from the gaseous phaseand its microbial oxidation, so the beds are effectivedue to the continuous odorous VOC oxidation (Bohn,1992; Schroeder, 2002).

Since all of the filter media allow polluted air tointeract closely with degradative microorganisms,oxygen and water, the choice of the biofilter media isvery important for the performance of the process.The material used as filter media must have somecharacteristics that will be important for theperformance of the biofilter. Physical mediaconstitution must provide fine porous, large surfacearea and a uniform pore size distribution. Poreuniformity strongly defines the flow, and so theefficiency of the biofilter. Fine or narrow pore sizeuniformly distributed allover the media, increases theuniformity of air and water flow through the bed(Bohn, 1992). The degree of porosity of the beddingmaterial is important to improve the adsorption ofmicroorganisms to it. Inorganic bed material,consisting of a variety of metal oxides, glass orceramics beads, is said to have good flow properties,due to its uniform shape. In inorganic material, thedegree of porosity can, in certain limits, be controlled.(Cohen, 2001). PVC is frequently used as a packingmaterial, but due to its smooth surface, it took much

Advantages

Table 3-Advantages and disadvantages of biofiltration (Anit & Artuz, 2002)

Disadvantages

2

Lower capital costsLower operation costsLower chemical usageNo combustion source

Biofiltration units can be designed to fit in shape and size to theindustrial unit setting, optimizing spaces

System versatile to treat odors, toxic compounds and VOCs

Efficiency > 90% for low contaminant cone.« 1000ppm)

Possibility of different media, microorganisms, and operationalconditions, for many emission points

3

4

System is not fitted for compounds, which have low ad-sorption and degradations rates, mainly chlorinated VOCs.

Large biofilters units or large areas are required to' treatcontaminated sources with high chemical emissions.

Source of emission that vary severely or produces spikes,can be detrimental to the biofilter performance and to themicrobial population.

Biofilters require long periods of acclimation for microbialpopulation, weeks or even months, mainly for VOC treat-ment.


more time to reach a maximum loading rate thanwhen porous red clay and grey potter's clay is used(Van den Berg & Kennedy, 1981).

Besides, as the active microorganism film will ad-here onto the biofilter media, the amount of microor-ganisms presented will be dependent on the availablesurface, increasing the biofilter efficiency. Usually,the packing material is a mixture of natural fibrousmaterial with a large specific area and a coarse frac-tion. An appropriate biofilter media must have largesurface area for both adsorption of contaminants andfor supporting the microbial growth (van Groenestijn& Hesselink, 1993; Anit & Artuz, 2002).

The material chosen for biofilter media must bevery well structured and physically stable to ensurethat the media does not compact, shrink or aggregateduring the time of operation. This will develop prefer-ential flow in the bed and decrease the available me-dia area, or cause the filter plugging. Biofilter mediamaterial must have physical characteristics, such asphysical stability and ease of handling (Bohn, 1996;Anit & Artuz, 2002).

If the media is inert, it must have large microbialpopulation on it, which will oxidize all organic com-pounds. Synthetic or inert media must be inoculatedwith soil, compost or sewage sludge. These materialshave a big and complex population of microorganismsavailable to develop the proper microbial culture forthe process (Schroeder, 2002; Bohn, 1996). Pure cul-ture can also be tested as inoculum.

Other points to be evaluated are operation andmaintenance costs. Lifetime of the media must be dis-cussed, because to replace dirty, disruptive bed mate-rial is costly task. The old media though not a hazard-ous material must be dumped, which is sometimesvery costly (Bohn, 1996). Overtime compost and de-composed peat need to be replaced every three to fiveyears (Schroeder, 2002). During the operation period,maintenance is needed to keep the biofilter effective.Plastic and ceramic media must be cleaned every fewmonths to break up surfaces of biofilm, to avoid plug-ging. It is necessary to choose materials that are ableto keep hydrophilicity, to avoid dryness, because de-gradative microorganisms need moist condition toflourish (Bohn, 1996). Appropriate biofilter materialmedia must have the ability to retain moisture to sus-tain biofilm layer, and also the capacity to retain nu-trients and supply them to the active biofilm formedby microbes, when required (Anit & Artuz, 2002).Finally, all these technical factors, the media cost

must be analyzed to make the installation feasible.Low price and accessible material must be found, be-cause the material cost and the transportation feesmay make the treatment unviable.

Many materials are available to be used as media inbiofilters. Among them are: compost, peat, soil, acti-vated carbon, wood chips or bark, perlite, vermicu-lete, lava rock and inert plastic material (Morton &Caballero, 1997). Typical biofilter media materialincludes compost-based material, earth, plastic, orwood-products based. The purpose of biofilter mediais to provide a large surface area for the adsorptionand absorption of contaminants. The media alsoserves to provide nutrients for the microbial popula-tion. And for some types of media lack of proper nu-trients will require addition of nutrients, such as nitro-gen or phosphorus compounds, in order to supply thenutritional requirements of microorganisms (Anit &Artuz, 2002). Plastic packing, such as plastic rings ofvarious sizes and porous diatomaceous earth pelletsare tested in laboratory studies (Sorial et al, 1995;Wright et al, 1998). Mixture of media types aresometimes used to provide operational advantage.Using a soil, peat or compost bed, the media can pro-vide some or all essential nutrients required for themicrobial growth. Other agents can be added, de-pending on the requirements (Adler, 2001). Normallythe biofilter bed media is composed of a mixture ofactivated carbon, alumina, silica and lime, alterna-tively, soil, peat, or more refined material, such asinert material, cellulous material or mineral material,combined with a microbial population that enzymati-cally catalyzes the oxidation of the absorbed, ad-sorbed or dissolved gases. The most common materialused as bed media is a mixture of compost and a inertcharge to give support to the bed such as wood chipsor bark, silica, perlite or synthetic media, basically,polystyrene beads (Bohn, 1992).

Organic bedding material is expected to have ahigher adsorbtivity when compared to inorganic mate-rial. For instance, microbial adsorption is 248 mg/gfor wood chips and 2 mg/g for inorganic silica. It isattributed to the larger variety of reactive groups car-boxyl, amino hydroxyl, etc, and the presence of cer-tain quantity of nutrients presented on organic mate-rial that help the attachment of the microorganisms(Cohen, 2001). Nutrients or buffer for the gas treat-ment process can be added to the packing or can beextracted form natural packing such as compost, soilor peat. The amount of nutrients or buffer added must

SOCCOL et al. :BIOFILTRA nON

be based on the expected contaminants and the pro-jected life of the packing. Nutrient limitation can re-sult in rapid loss of performance (Schroeder, 2002;Corsi & Seed, 1995; Maria et al, 1999). Bed materialmust be selected depending upon many factors suchas resistance to the microorganism attach, mechanicalresistance, cost, available surface parameters, etc.Many kinds of materials have been tested in biofiltersto treat volatile organic compounds. Some of theseworks with the media and microrganism used arementioned in Tables 1 and 2.

Mechanisms of BiofiltrationRemoval of contaminants is a multi-step process,

beginning with the transportation of the contaminantto the liquid phase, its transportation to the bacterialcell in the biofilm, and a transport across the cellmembrane where the compound biodegradation oc-curs, concurring it for the cell metabolism. (Schroe-der, 2002). The treatment process relies on two pri-mary fundamental mechanisms: sorption and biodeg-radation. As the contaminated gas steam passesthrough the bed, contaminants are transferred fromthe gaseous phase to the liquid or solid phase onto themedia. Three mechanisms are responsible for thistransfer to the phase where the contaminant will de-grade:

(1) Adsorption on organic media-desorp-tion/dissolution in aqueous phase-biodegradation.

Treatedgastrearn

403

(2) Direct adsorption in biofilm-degradation.(3) Dissolution in aqueous phase-degradation.

Once adsorbed in the biofilm layer, or dissolved in thewater layer around the biofilm, the contaminant, usu-ally an organic molecule is available as food for themicroorganism metabolism, serving as carbon andenergy source for support life a growth. Then thecontaminant is exhausted from the biofilter (Adler,2001).

These physical, chemical and biological phenom-ena that concur to the contaminated gas treatmenthappen in an apparatus, which has some special char-acteristics due to operational variations. Variations ingas-phase bioreactors are shown in Fig. 1 (Schroeder,2002).

Biofilters and biotrickling filters are packed bedunits, where an up flow of contaminated gas is ap-plied, which passes through the bed. On the surface ofthe bed media grow an active biofilm of microorgan-isms, responsible for the biodegradation contami-nants. The difference between biofilter and biotrick-ling filter is that in the latter, water is continuouslysprayed over the packing surface and the drained liq-uid is collected at the bottom and recycled to the topof the column. Typical water spray rates are 1 to 20m3im2.d (Sorial et al, 1995; Schroeder, 2002). If thewater is returned to the top of the reactor, the totalvolume of the water in biotrickling filters is greaterthan in biofilters. So, great amount of contaminants

8. bIofiler

Fig. I-Gas-phase bioreactors (Schroeder, 2002)


can be dissolved in the liquid phase, increasing thespecific treatment rates, and decreasing the size of thebioreactor, when compared to the biofilters. Biotrick-ling filters are appropriates to be installed where highconcentrations of contaminants are found (Chitwoodand Devinny, 2001).

Scrubber is an apparatus where the contaminatedgas is treated in two steps: In first step, the inertpacking results in dissolution of gas components inthe water phase. Then the water with dissolved haz-ardous components is treated in another tank orpacking bed and either reused or discharged. In thecase of bioscrubbers, gas treatment also occurs in twosteps, in which contaminant adsorption occurs in thefirst step and biological treatment occurs in the sec-ond step (Schroeder, 2002; van Groenestijn & Hes-selink, 1993). The advantages and disadvantages ofthe three techniques used to treat polluted air aresummarized in Table 4.

Microorganisms are the most critical component ofthe biofilter, because they are responsible for thetransformation or destruction of contaminants. Natu-ral microorganisms are usually proper for the process,as they are being gradually adapted to the operation orcontaminant applied. They can come from sewagesludge and acclimated to specific contaminants thatare presented. However, in some cases, and for spe-cific chemicals, a specialized microbe or genetically

engineered cultures may be required (Adler, 2001). Inall the systems, microbial community includes, bacte-ria, fungi, protozoa and invertebrates. The microor-ganisms obtain their nutrients from the liquid phase ofthe biofilter, so moisture content of biofilters, bio-trickling and bioscrubbers is a very important con-trolling parameter (Schroeder, 2002).

Immobilization of Microorganisms to the BeddingMaterial in Biofilters

The self attachment of microorganisms can be di-vided into two main immobilization processes: a) theself attachment of cells to the filter bedding material,which is defined as attached growth; b) the artificialimmobilization of microorganisms to the beddingmaterial (Fig. 2).

Attached GrowthSince the concentration of attached microorgan-

isms is usually higher than that of suspended micro-organisms, a higher microbial concentration could bemaintained within systems treating fluids with at-tached microorganisms. In many cases, a highermetabolic activity was measured within attachedgrowth treatment systems (Cohen, 2001). Accordingto Polprasert (1989), the biofilm is more efficient inwaste decomposition than the suspended microorgan-isms. This is probably due to the increased biodegra-

Table 4--Characteristics, advantages and disadvantages of the three basic techniques used to treat polluted air(adapted from Groenestijn & Hesselink, 1993)

Biofiltration

Characteristics:• Immobilized biomass• Immobile water phase• Single reactorAdvantages:• High gas-phase surface area• Easy operation and start-up• Low operation costs

Bioscrubbing

Characteristics:• Suspended biomass• Mobile water phase• Two reactorsAdvantages:• Better control of reactionconditions• Compact equipment• Low pressure dropDisadvantages:• Low surface area for masstransfer• Wash out of low growingmicroorganisms• Disposal of excess sludge• Difficult start-up procedures• Extra air supply needed athigh degradations rates• High investments, mainte-nance and operation costs

Disadvantages:• Poor control of operation conditions• Slow adaptation to variation in gasconcentration• Large area required

Biotrickling filtration

Characteristics:• Immobilized Biomass• Mobile water phase• Single reactorAdvantages:• Better retention togrowing microorganisms• Single reactor

•

slow

Disadvantages:• Low surface area for masstransfer• Disposal of excess sludge• Difficult start-up procedures• High operation costs

SOCCOL et at. :BIOFIL TRA nON 405

Fig. 2-Schematic illustration of immobilization methods

dation activity, which results in the higher amount ofactive biomass concentration within the attachedgrowth systems. The nutrient concentration aroundbiofilms is usually higher than that of the free fluid.This high nutrient concentration is responsible for theincrease of microbial growth rate, which consequentlyenhances the degradation activity. Physiologicalchanges are also observed for attached microorgan-isms (Lazarova & Manem, 1995). A great resistanceto toxicity is usually seen within attached biofilmcompared to suspended microorganisms. This is ex-plained by the diffusion limitation of nutrient in thesupport, which reduces the concentration of toxiccompounds (Pandey, 1992, 1994; Pandey & Soccol,2000; Pandey et al, 2001; Soccol, 1994 a, b). On theother hand, the diffusion restriction of the biofilmmight reduce oxygen penetration into a thick biofilmand limit the amount of aerobic decomposition. Thiscan lead to variable reactor performance and uncon-trolled sloughing.

It is very important to analyze the structure andproperties of biofilms such as biofilm thickness andheterogeneity. The biofilm thickness is influenced byseveral factors including the bedding material con-struction and the different system designs. Biofilm'sthickness usually varies from tens of micrometers tomore than 1 em depending on the characteristics ofsurfaces (smooth or porous). The activity increaseswith thickness. Above this level, the diffusion of nu-trients becomes a limiting factor (Cohen, 2001).

Although biofilm seems to consist of a homogene-ous layer, there is a considerable non-uniformitywithin biofilms. Wanner and Gujer (1984) showedthat the dissolved oxygen diffusivity is not constantthrough the biofilm, but decreases with depth. This isprobably due to the different growth rates of microor-ganisms, which are located at different depths withinthe biofilm.

The attachment of microorganisms to a surface de-pends on the microbial structure. The main microbial

structure involved in this process is the glycocalyx,which consists of extracellular polysaccharides. Sev-eral forces also participate in the microbial attachmentto surface. Not only one force is dominant, but acombination of forces, which change with differentenvironmental conditions, microbial species, surfaceproperties and fluid properties. These forces includeelectrostatic interactions, covalent bond formation,hydrophobic interactions and partial covalent bondbetween microorganisms and hydroxyl groups on sur-faces (Cohen, 2001).

To select a bed material some points must be con-sidered. Organic material has higher adsorbtivitycompared to inorganic material. It also contains acertain amount of nutrients, which help the microor-ganisms to attach and grow. Inorganic bedding mate-rial is usually considered to be resistant to microbialattack, to exhibit high thermo stability and to havegood flow properties (Gemeiner et al, 1994). Thesetypes of bedding usually consist of a variety of metaloxides. Since the attachment of microorganisms to thebedding material occurs in aqueous conditions, metalhydroxides are formed on the surfaces of inorganicmaterials, such as glass or ceramics, which are nega-tively charged. The high biomass accumulation ofnegatively charged inorganic material suggests thatprobably partial covalent bonds are formed betweeninorganic metal hydroxides and the microbial surface(Kolot, 1988).

The degree of porosity of the bedding material isan important factor in the adsorption of microorgan-isms to the bedding material. According to Tampionand Tampion (1987), the maximum accumulation ofbiomass occurred when pore sizes were one to fivetimes the bacterial size.

Fluid Properties and the Microbial AttachmentpH. It is observed that at lower pH there is higher

attachment of microorganisms. This is probably dueto the influence of pH on the microbial surfacecharge. At low pH values, the negative charge of themicrobial surface is reduced, which leads to areduction in the electrostatic repulsion between themicroorganisms and the negatively charged beddingmaterial. This reduction in the electrostatic repulsionmight be the cause of greater microbial attachment atlow pH values (Cohen, 2001). The microbial surfacecharge is influenced by the presence of salts in thefluid.

Salt content. Kolot (1988) reported that the in-crease of in Ca-ion concentration from 0.01 % to 1%


was responsible for the decrease (eight-fold) of chargeon the microbial cell surface. Besides the influence onthe surface charge of microbial cell, the presence ofsalts also affects the attachment of microorganisms tothe bedding material. Meadows (1971) has showedthat it was possible to detach microbial cells fromsand by washing with distilled water, but when saltwas present in the water the microbial leaching de-creased considerably. According to DiCosmo et al(1994), in cells of C. roseus the maximum level ofadhesion occurred in the presence of O.IM AlCh,which is probably the achievement of an optimal sur-face charge for the attachment (Cohen, 2001).

Immobilization of Microorganisms at Filter BeddingMany papers and reviews have been published on

the immobilization of microbial cells (Chibata et al,1983; Chibata & Wingard, 1983; Mattiason, 1983).However, non-ideal general methods applicable toimmobilization of all types of microbial cells havebeen developed. In practice, it is necessary to choosesuitable methods and conditions for immobilization ofeach type of cell. Immobilization techniques havebeen used for several applications such as productionof organic compounds. It is also used in biofiltration.A large number of immobilization methods have beendeveloped. These techniques can be classified intofive categories: carrier binding, cross-linked, entrap-ment, microencapsulation and membrane methods.

Carrier binding method. The carrier bindingmethod is based on direct binding of cells to water-insoluble carriers by physical adsorption, ionic bondor covalent bonds. As carriers, water insoluble poly-saccharides (cellulose, dextran and agarose deriva-tives), proteins (gelatin and albumin), synthetic poly-mers (ion-exchange resins and polyvinyl chloride)and inorganic materials (brick, sand and porous glass)are used (Cohen, 2001).

The surfaces of microbial cells contain large quan-tities of a variety of reactive groups. The covalentbond method includes the creation of covalent bondsbetween reactive groups and different ligands on thebedding material (Tampion & Tampion, 1987). Themain problem associated with this technology is thatmicroorganisms are exposed to potent reactivegroups, which exert toxic effects (Cohen, 2001).

Cross-linking method. Cells can be immobilized bycross-linking with bi or multifunctional reagents suchas glutaraldehyde, toluene diisocyanate and others. Itinvolves the joining of microorganisms (covalent

bond) to each other to form a large three dimensionalcomplex structure. This immobilization method usu-ally suffers from the toxicity problem associated withthe covalent bonding method (Tampion & Tampion,1987).

Entrapment method. The method consists of en-trapping cells into polymer matrices. It has been ex-tensively investigated. The pores in the matrix aresmaller than the microbial cells, keeping them trappedwithin the material. The substrate can stilI penetratethrough the polymer matrix (Cohen, 2001). The fol-lowing matrices have been employed: collagen, gela-tin, agar, alginate, carrageenan, cellulose, triacetate,polyacrimide, epoxy resin, photo-cross linkable resinpolyester, polystyrene, polyurethane, etc.

Some disadvantages of this method are associatedwith high diffusion restriction with some polymermaterials. On the other hand, this method benefitsfrom some advantages including: the achievement ofa high viable biomass concentration, higher resistanceto toxic compounds within the treated fluid, the possi-bility of immobilizing together different species ofmicroorganisms separated physically from each other,greater plasmid stability within genetically engineeredmicroorganisms etc, which are immobilized in thismethod.

Microencapsulation. Microorganisms are immobi-lized in wrapped droplets with a thin membrane. Thecells can freely move within their own capsule, con-suming substrates that penetrate through the mem-brane cover. The microcapsules are mainly con-structed of nylon and cellulose nitrate. Usually, thediameter of these microcapsules varies from 10 to 100urn (Cohen, 2001).

Membrane separation method. In this method mi-croorganisms are isolated from the bulk fluid by theuse of sheets of membrane. The membranes allow thesubstrates to penetrate to the cell's zone. Normally,the membranes used in this method are usually porousultrafiltration membranes (0,0002-0,1 urn). Non-porous membranes can also be employed. After use,membranes are cleaned by chemical and/or physicalmethods, which cannot be applied directly in biofil-tration systems, due to the harmful effect they causeto immobilized microorganisms (Iorio & Calabro,1995).

Comparison between Attached Growth Systemsand Entrapment Technology for Fluid Treatment

Cohen (2001) presented a comparative study bet-ween attached growth systems and entrapment tech-

SOCCOL et al.:BIOFlLTRATION

nology for fluid treatment. The author affirmed thateach treatment system is considered to be most effec-tive at different fluid characteristics. For example,attached growth is indicated for biofilter, which treat afluid containing many different contaminants and/or afluid that changes its content with time. On the otherhand, the entrapment method can be used for biofilterthat treats a fluid containing few contaminants. Thesesystems normally need a complex sequencing degra-dation process. It is clear that neither of the immobili-zation processes can be considered superior to theother.

It is very important to note that a good system forbiofiltration provides the best conditions for the mi-croorganisms and, consequently, will achieve a highefficiency. This means that the microbial needs, thefluid physics, mass transfer calculations, degradationpathways and ways of microbial immobilization willcontribute to utilize all possibilities in the control offluid pollution.

Conclusions

For the environmental protection many technolo-gies have been developed along the years, for thetreatment of solid, liquid and gaseous waste. Re-cently, the interest in gas, volatile organic and inor-ganic waste treatment is increasing and many studiesare being carried out all over the world, because theprotection of soil, water and atmosphere is consideredas fundamental to maintenance of life. One of themost promising knowledge that is being developed totreat gases, volatile organic and inorganic wastes, arethe biofiltration techniques. This is a technique usedto treat, degrade and transform harmful products,toxic gases and volatile pollutants that are generatedin industrial processes, into productive activities, andproducts that have to be disposed of, such as odorousgases and vapours from refinery products, piggeryfacilities and others. This review showed that the bio-filtration techniques can be employed successfully totreat these kinds of pollutants, and many tools areavailable to be used. Besides, when it comes to theuse of biological tools, it is always possible to find amicroorganism associated to an appropriate apparatusto reach a treatment process fitted to the necessities.Depending on the application and the compost or thecompost mixing to be treated, some systems are con-sidered to be more effective than others. Analysis ofall the factors that interfere during the process, is themost important step for success. A good equipment

407

and process will achieve a high efficiency of pollutantremoval. Appropriate design and operational condi-tions are the prerequisites for the microorganisms thatare responsible for the degradation of·pollutants.

AcknowledgementCRS thank Conselho nacional de Desenvolvimento

Cientffico e Tecnol6gico-CNPq (Brasilia, Brazil) fora scholarship.

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