Biofiltration of Air 1A

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Critical Reviews in Biotechnology, 25:5372, 2005

Copyrightc Taylor & Francis Inc.

ISSN: 0738-8551 print / 1549-7801 online

DOI: 10.1080/07388550590935814

Biofiltration of Air: A RevieMarie-Caroline Delhomenie

and Michele Heitz

Chemical Engineering

Department, Faculty of

Engineering, Universite de

Sherbrooke, Canada

ABSTRACT In this paper we present a review of the existing air pollu

control technologies (APCT), when used essentially for the eliminatio

volatile organic compounds (VOC). The biotechnologies referred to, biosc

bers, biotrickling filters and biofilters, are also described. A more detailed re

of biofiltration is proposed, presenting the most recent and latest developm

achieved in the field of bioprocessing. In particular, the influence of the fi

bed, the polluted air flowrates, the pollutants, the pressure drop, bed m

ture content, temperature, nutrients, pH and the microorganisms are revie

Models of biofiltration are also presented.

KEYWORDS biofiltration, air treatment, air pollution control, biotechnologies, voorganic compounds.

CONTEXT

The recent pollutant release inventories, published by various governm

and environmental organizations, demonstrate that, since the beginning o

20th century, emissions of atmospheric pollutants in North America have b

in continual increase. Thus, in 2000 in Canada, some 41 Mt of pollutants ticulate matter, SOX, NOX, COXand volatile organic compoundsVOC)

released to atmosphere, representing about 1.3 t/person living in Canada.

industrial sector alone was responsible for 13% of these releases, which conta

about 7% of VOC (Environnement Canada, 2000). On another scale, in 2

the USA emitted some 150 Mt of pollutants (0.5 t/person), containing 10

of VOC, mostly produced and released by the industries (US Environme

Protection Agency, 2003).

Most VOC emitted to the atmosphere are likely to be harmful to hu

health (generating nausea, headaches, irritation and affecting the nervous

breathing systems, initiating cancers, etc.), and they also contribute to substa

damage on fauna and flora (Barnes, 1998). In addition to their toxic effVOC families, such as the chlorofluorocarbons (CFCs), present characteristi

greenhouse effect gases. Some VOC (freons, halons,etc.) are directly involve

the degradation of the stratospheric ozone layer, the natural filter for ultrav

radiation, whereas other VOC, combined with NOX, are precursors for

tropospheric ozone layer formation, which causes the urban smog visible ab

large agglomerations (Barnes, 1998):

Sunlight+ VOC+NOX + air/O2 photosmog

Address correspondence to MicheleHeitz, Chemical EngineeringDepartment, Faculty of Engineering,Universite de Sherbrooke, Sherbrooke,QC, J1K 2R1, Canada. E-mail:[email protected]

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All of this data illustrate the fact that technological

short-term depollution solutions must be developed

and applied to reduce the amplitude of these emissions.

Concerning the VOC emissions problem, existing de-

pollution measures are of three orders:

1. to ban the utilization of certain VOC in industrial

processes (case for the benzene and freons);

2. to review and modify existing processes to reducetheir associated VOC emissions and to make them

less polluting (by installation of recycling systems,

for example);

3. to install depollution systems downstream of the

emitting sites.

This latter category of depollution solutions will form

the subject of the remainder of this work. A brief review

of the air pollution control technologies (APCT), as

applied to VOC reduction, will be presented. Then,

among the various proposed control technologies, theapplicable biotechnologies will be principally discussed,

with the emphasis placed largely on the application of

biofiltration.

DEPOLLUTION TECHNOLOGIES

Due both to the diversity and to the quantities of

VOC released into the atmosphere, a variety of APCT

have been developed. The operating process for all of

these APCT involves either one or several physico-

chemical air/contaminant separation steps. The main

differences between the APCT are related to the treat-

ment of pollutants, following the separation procedure

for their recovery (recycling) or destruction.

The main APCT giving rise to recovery and recycling

of VOC are the following:

phase transfer technologies: adsorption and absorp-

tion. VOC concentration technologies: condensation, cry-

ocondensation and membrane processes. These tech-

nologies permit the extraction of VOC from the ini-

tial gas phase, and to then concentrate them.

Processes leading to the partial or total elimination of

VOC involve the use of more or less drastic oxidative

ways:

combustion processes: incineration and catalytic

oxidation.

chemical or photochemical oxidation technologies. biotechnologies: biotrickling filter, bioscrubber, and

biofilter.

Table 1 presents some characteristics of the main

APCT, as applied to the VOC elimination (Hermia

and Vigneron, 1993; Ruddy and Carroll, 1993; van

der Vaart et al., 1994; Huang et al., 1997; Crocker and

Schnelle, 1998; Goralski et al., 1998; Sylvester et al.,

1998; Hurashima and Chang, 2000; Degreve et al., 2001;

Wang et al., 2001).

The choice of a well-adapted process depends on the

operating conditions (flow rate, VOC concentrations,

temperature, humidity, etc.) and on the pollutants

physico-chemical characteristics (solubility, phase tran-

sition points, biodegradability level, inflammability,

etc.) (Crocker and Schnelle, 1998). Figure 1 presents

the application limits (flow rateVOC concentration)

of the different APCT (Juteau, 1997; Crocker and

Schnelle, 1998; Devinny et al., 1999).

As indicated in Table 1 and Figure 1, biological pro-

cesses constitute pertinent ways for the elimination of

biodegradable VOC (alcohols, ketones, aldehydes, aro-

matic compounds,etc.) emitted at low to moderate con-

centrations (1 ppm to 1000 ppm) (Devinny and Hodge,

1995; Kim, 2004). The industries mainly concerned

with depollution biotechnologies are chemistry, petro-

chemistry, pulp and paper, metallurgy, mining and en-

ergy production (Bailey and Ollis, 1986; Robert and

Pilon, 2000).

In the following sections, bioprocesses (bioscrub-

ber, biotrickling filter and biofilter) are generally de-

scribed, with a more detailed section dedicated to

biofiltration.

THE BIOTECHNOLOGIES

The particular interest of these biotechnologies is that

they do not utilize energy other than the capacity of

microorganisms to metabolize a wide range of VOC:

biocatalytic oxidation of pollutants. The catalysts areheterotrophic microbial strains (bacteria, fungi) that are

able to utilize VOC in two ways:

contaminants, oxidized in the course of the catabolic

pathway (respiratory chain) are a source of energy; contaminants are also a source of available carbon for

the anabolic processes, such as cell growth.

M.-C. Delhomenie and M. Heitz 54


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TABLE 1 Characteristics, advantages and drawbacks of the main APCT applied to VOC elimination.

Technology Principle and characteristics Performances and limitations Costs/(m3 h1) air

Adsorption Transfer of VOC to a porous

solid phase, fixed or fluidized

Materials: activated carbons,

zeolites and polymers

Ex.: Activated carbon adsorbs

1030% VOC on a weight basis

Doubled installations:adsorptiondesorption cycles

Operating temperature


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TABLE 1 Characteristics, advantages and drawbacks of the main APCT applied to VOC elimination.(Continued)

Technology Principle and characteristics Performances and limitations Costs/(m3 h1) air

Membranes Separation of gas mixtures

through semi-permeables

membranes

Materials: polymers (hollow

fibers, silicones), porous

ceramics

Gas flow compressed beforemembrane separation

Conversion: 5098%

VOC are concentrated 5100

times, and valorization (recycle)

possible

Selective membranes, resistant

to halogenated VOC

But . . . Pressure drop

High operating pressures

Membrane cleaning required

Not available

UV/photochemical

oxidation

Complete oxidation by

oxygenated oxidizers (O3,

H2O2), and initiated by UV

radiation

Possible utilization of

photocatalysts (TiO2, FexOy,etc.)

Operating temperature:

ambient

Conversion: 9098%

Moderate energetic costs

But . . .

Inadequate for halogenated

VOC

Deposit of oxidation

by-products on the catalysts

surface (cleaning)

Complex systems

Not available

Biotechnologies(detailed later)

Biocatalytic oxidation of VOC 3 configurations: biofilters

(most frequent), biotrickling

filters and bioscrubbers

Biocatalysts: microorganisms

(bacteria, fungi)

30 s < Residence time < several

min

Operating temperature:

2040C

Filter-bed life time: 35 years

Conversion: 8095% Moderate installation and

operating costs

Low maintenance

But . . .

Strict control of biological

parameters (pH temperature,

moisture level, nutrients,etc.)

Large spaces required for

biofilters

Pressure drop problems

Investment: US $1070 Operation: US $310

The products of the biological reactions engaged inthese bioreactors are essentially: carbon dioxide, water,

inorganic byproducts (e.g. HCl, SOX) related to the

presence of heteroatoms in the VOC squeleton (Cl, S,

N, etc.), new cellular matter, and organic byproducts

(metabolites such as exopolymers). Biooxidations are

exothermic reactions (some kcal/mol VOC oxidized),

thus the associated heat release is also a biodegradation

reaction product (Scriban, 1993).

The effective useof biocatalysts requires strict control

of their environment, normally the biological growth

medium. There are some critical parameters that arecommon to the 3 types of bioreactors: (a) temperature

(optimum between 20 and 35C for a mesophilic mi-

croflora) (Leson and Winer, 1991; Swanson and Loehr,

1997), (b) pH (optimum at about 7) (Leson and Winer,

1991; Leson, 1998), (c) moisture content in the growth

medium, and (d) availability of essential, non-carbon

nutrients (N, P, K, S and micronutrients).

The main differences between the three bioreactorscome from their design and mode of operation: mi-

croorganisms conditioning, respective disposal of fluid

phases (gas and liquid), presence or absence of station-

ary solid phases. Table 2 presents the technical charac-

teristics of the three bioprocesses.

THE BIOSCRUBBER

The bioscrubber, as represented in Figure 2, con-

sists of two subunits: 1) an absorption tower and

2) a bioreactor. In the absorption unit, input gaseouscontaminants are transferred to a dispersed liquid phase

(aerosol). Gas and liquid phases flow counter-currently

within the column, which may contain a packing.

Nevertheless, the addition of inert packing (structured

ceramic for example) provides for increased transfer

surface between the VOC and the aqueous phase

(Kellner and Flauger, 1998). The washed gaseous phase



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FIGURE 1 Application limits (flow rateVOC concentration) of different APCT, based on references of Crocker and Schnelle, Juteau, 1997, and Devinnyet al., 1998.

is released at the top of the column whereas the sepa-

rated contaminated liquid phase is pumped towards an

agitated, aerated bioreactor. This reactor contains the

degrading microbial strains suspended in the aqueousphase, and the nutrients essential for their growth and

maintenance. Most of the presently operating bioscrub-

bers are inoculated with activated sludge, derived from

wastewater treatment plants, for example (Ottengraf,

TABLE 2 The technical characteristics of bioscrubbers, biotrickling filters and biofilters.

Bioprocess Microorganisms Liquid phase Depollution step

Bioscrubber Suspended in the bioreactor,

in the aqueous growth

medium

Mobile

Continuously

dispersed

Recycled

VOC/air separation wit

the absorption column

VOC oxidation in the

aerated bioreactorBiotrickling filter Immobilized on the filtering

material

Mobile

Continuous trickling

over the filter bed

Possible recycling

In the filter bed

In the biofilm

Biofilter Immobilized on the filtering

material

Occasional bed

irrigation with

nutrient solutions

In the filter bed

In the biofilm

1987). In some cases, bioreactors are directly i

ulated with specific degrading strains. The resid

times for aqueous solutions in bioreactors range

tween 20 and 40 days. After a treatment step (filtratsedimentation of biomass), part of the waste solu

may be recycled in the absorption unit while pa

the sedimented biomass may be reintroduced into

bioreactor.

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FIGURE 2 Bioscrubber description.

The advantages of bioscrubbers are as follows:

operational stability and good control of the biolog-ical parameters (pH, nutrients);

bioscrubbers do not generate high pressure drops

(Rho, 2000); their installation does not require large spaces.

However, the major limitations of bioscrubbers are:

bioscrubbers are adapted to treat readily soluble

VOC (alcohols, ketones), with low Henry coefficients

(


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the absorption step (Cox and Deshusses, 1998), the

solubility specifications are less drastic than for bio-

scrubbers (Henry coefficient


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Filter Bed

The filter bed constitutes the heart of the biofiltra-

tion process because it provides the support for micro-

bial growth. Bohn (Bohn, 1996) has established a list

of characteristics that suitable filtering materials should

present. The following criteria are among the more im-

portant of the required specifications:

1. a high specific surface area, which is favorable to

microflora establishment and maintenance, and to

gas/biofilm exchange;

2. a high porosity to facilitate gas convection and

to promote the homogeneous distribution of gases

throughout the bed;

3. a good water retention capacity to avoid bed desic-

cation;

4. the presence and availability of intrinsic nutrients;

5. the presence of a dense and diverse indigenous

microflora.

Peats, soils, composts, and at a smaller scale wood chips,

are the most frequently employed basic materials in

biofiltration, because they satisfy most of the required

criteria, and they are widely available and at low cost.

The main advantage of soils is that they offer a rich

and varied microflora. However, they contain only a

few intrinsic nutrients, they present low specific surface

areas and they generate high pressure drops (Swanson

and Loehr, 1997).

Peat is also an interesting material because it containshigh amounts of organic matter, it presents high specific

surface area, a good water holding capacity and a good

permeability.However, peat contains neither high levels

of mineral nutrients nor a dense indigenous ecosystem.

Composts are the materials that are most frequently

employed for a variety of reasons. They offer a dense

and varied microbial system, a good water holding ca-

pacity, a good air permeability, and they contain large

amounts of intrinsic nutrients. Moreover, their utiliza-

tion in biofilters constitutes a recycling and effective

way for utilization of residual organic matters, such asthe residues and activated sludge of wastewater treat-

ment plants, forest products (branches, leaves, barks),

domestic residues, etc. (Alexander, 1999) However, com-

posts are often less stable than soils or peats because they

tend to break down and to become compacted, leading

to pressure drop increases, due, among other reasons,

to their high water holding capacity.

Some authors have studied biofiltration on wood

chips or barks (Smet et al., 1996a; Smet et al., 1999;

Hong and Park, 2004), but they have shown that per-

formances obtained with such filtering materials are less

satisfactory than those obtained with compost or peat.

This has been explained by the low pH-buffering capac-

ity, the low specific surface areas and the low nutrient

content of such materials. Despite these deficiencies,

wood barks are still widely used in biofiltration as struc-tural materials, in association with peat or compost.

Indeed, to prevent bed crushing and compaction,

most authors employ materials that provide the bed

with good structure maintenance and rigidity, which

delays the clogging phenomena and increases the bed

lifetimes: wood chips or barks (Oh and Choi,2000; Luo,

2001; Mareket al., 2001), perlite (Weigneret al., 2001;

Woertzet al., 2002), vermiculite (Krishnayyaet al., 1999;

Pineda et al., 2000), ceramic (Cardenas-Gonzalez et al.,

1999), glass beads (Zilli et al., 2000), polyurethane foam

(Moe and Irvine, 2000; Woertzet al., 2002), polystyrene(Ottengraf, 1986; Arulneyam and Swaminathan, 2000),

lava rock (Chitwood and Devinny, 2001), etc. More re-

cently, Ibrahim et al., 2001, prepared a bed composed of

activated sludge, immobilized in gel beads, and Marek

et al., 1999, have utilized beds of activated carbon or mi-

croorganisms immobilized in calcium-alginate beads.

Christen et al., 2002, and Sene et al., 2002, developed

a sugarcane-bagasse-based bed, for the treatment of

ethanol and benzene.

Some bed structuring agents also present interest-

ing chemical characteristics such as pH buffering ca-pacity (limestone, for example) (Smet et al., 1996b),

or general adsorbing capacity (activated carbon). Or-

ganic materials such as composts, soils or peats present

only small adsorption capacities: Acuna et al., 1999,

determined a value for the adsorption coefficient of

toluene on peat as 0.137 mg g1 of peat, Tang and

Hwang, 1997, reported partition coefficients for toluene

of 1.43 mg g1 of compost, 2.00 mg g1 of diatoma-

ceous earth, and 0.89 mg g1 of chaff. Beds contain-

ing activated carbon (granulated or powdered) pro-

vide adsorption coefficients for toluene some 10 to20 times greater: 50.6 mg g1 of granulated activated

carbon (Tang and Hwang, 1997), and 0.287 g g1 of

powdered activated carbon (Marek et al., 1999). Thus,

the addition of activated carbon leads to (1) improve-

ments of the degrading capacity of biofilters (Webster

et al., 1995; Abumaizaret al., 1998), (2) elimination of

hydrophobic compounds, which naturally tend to be



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more recalcitrant to biofiltration (Cardenas-Gonzalez,

1999), and (3) to better control loading variations (flow

rate or VOC concentration fluctuations) (Mason et al.,

2000).

Flow Rate

The air flow rate, Q, and the empty bed residence

time (EBRT), , defined as: = Vbed

Q (Vbed, is the bedvolume), are parameters that have significant impact

on the biodegradation performance (Elmrini et al.,

2004). Indeed, two physico-chemical mechanisms may

limit the overall elimination efficiency of a biofilter

(Ottengraf, 1986): the pollutants diffusion transfer from

gas phase to biofilm, and the biodegradation reaction.

Depending on both the flow rate and the VOC concen-

tration, the VOC degradation process is limited by ei-

ther one of these mechanisms, or both simultaneously.

Figure 5 presents the characteristic-time scales of the

various physical, chemical and biological processes oc-curring in a biofilter (Kisselet al., 1984; Picioreanuet al.,

1999). From this time-scale chart, it appears that dif-

fusion mechanisms are slower than the biological re-

actions. Thus, to improve biofiltration performance,

the EBRT should be greater than the time required

for diffusion processes, which is the case for low op-

erating flow rates. Most of the authors studying vari-

able flow rates have effectively verified that long EBRT

give rise to better VOC removal efficiencies (Jorioet al.,

1999; Christen et al., 2002; Delhomenie et al., 2002a;

Martin et al., 2002; Yoon and Park, 2002). However,

FIGURE 5 The characteristic times of the main mechanisms taking place in a biofilter, based on references of Picioreanuet al., and Kisselet al., 1989.

the application of long EBRT requires larger filter

volumes.

On the contrary, when flow rates are too high,

tact times between microorganisms and gases are

short so that the biodegradation reaction canno

completed. Moreover, when the input air velocity i

high, the water contained in the filter bed is strippe

the gas flow, which tends to desiccate the biofilte

most of the operated biofilters, the EBRT ranges f15 seconds to several minutes. The EBRT value also

pends on other operating conditions such as the V

concentration, the VOC biodegradability level and

available bed volumes.

Pollutants

As previously mentioned, the overall biofiltratio

ficiency and the pollutants biodegradability level

on the following microscopic processes: the VOC t

fer rate from gas phase to biofilm, and the Vbiodegradation rate. The magnitude of both param

depends on the operating conditions (flow rate, tem

ature, pH, moisture content, etc.), but also on the

and concentration of pollutants presented. The

taminants absorption in the biofilm is governed by

Henrys law, which correlates the VOC vapor pres

with its solubility in water. The reversible adsorptio

pollutant on the particle surface is described by an e

librium isotherm (linear, Langmuir, Freundlich,

Mass transport in the biofilm is described by mo

ular diffusion in an aqueous phase (Ficks law).

61 Biofiltration o


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biodegradation rate is related to the microbial strains

established in the filter bed, and to the concentration

and chemical configuration of the VOC.

In this way, the removal of slightly soluble com-

pounds, introduced at low concentrations, is limited

by their transfer rate into the biofilm. In contrast, sol-

uble compounds, introduced at higher concentrations,

are easily absorbed into the biofilm, but the biodegrada-

tion rate then tends to limit their removal rate. Takinginto account these parameters, the oxygenated hydro-

carbons (alcohols > aldehydes, ketones > ethers) are

more readily biodegraded than the linear alkanes, fol-

lowed by the aromatic hydrocarbons. The very short

chain alkanes, such as methane, and the branched com-

pounds, are more recalcitrant, and the halogenated hy-

drocarbons (chlorinated) are the most recalcitrant of

VOC.

Further, VOC concentrations introduced beyond a

toxicity threshold value tend to inhibit the microbial

growth and activity. The existence of such thresholdconcentrations has been proved, at the laboratory-scale,

by Tang et al., 1996, with triethylamine, by Jorio et al.,

2000, with styrene and by Krailas et al., 2000, with

methanol. Generally, VOC concentrations do not ex-

ceed 5 g m3 in biofilters inlet gas flow (1000 ppmv

methane equivalent).

Finally, even though most laboratory studies con-

sider single substrates or restricted mixtures of VOC,

large scale gas effluents contain, in fact, more or less

defined, and sometimes variable mixtures of pollutants.

However, authors working with VOC mixtures havedemonstrated that the simultaneous presence of several

compounds can:

have either no effect or they improve the removal

efficiency of single pollutants. This is the case for

the elimination of some recalcitrant compounds that

requires cometabolic oxidation pathways.a For exam-

ple, the trichloroethylene degradation is induced by

the addition of toluene (Ergas et al., 1993; Cox et al.,

1998), propane or methane (Watwood and Sukesan,

1995) in the gas mixture. create interference within the metabolic processes of

each pollutant. Deshusses et al., 1999, reported that

the presence of ethylacetate in the gas flow has an in-

hibitory effect on toluene degradation. Oh et al.,1994,

showed mutual inhibition processes occurring within

aThe VOC-degrading enzymes are activated only in the presence of aprimary substrate.

BTEX mixtures: competitive inhibition between ben-

zene and toluene, benzene and toluene degradation

inhibited by the p-xylene cometabolism. Deshusses,

1997, also illustrated strong mutual inhibitions oc-

curring in a mixture containing hexane, acetone,

propanol, methylisobutylketone, and methylethylke-

tone. Other studies proved that, in some cases, the

observed inhibitions resulted from an accumulation

of inhibitory intermediate metabolites in the micro-bial growth medium (Leon et al., 1999; Yu et al., 2001).

Diauxyb has also been observed as a consequence of

the interactions between compounds (Webster and

Devinny, 1998). This phenomenon was illustrated by

Mohseni and Allen, 2000, who concluded that, in a

mixture of methanol and -pinene, methanol, a hy-

drophilic and easily biodegradable compound, was

consumed prior to the hydrophobic and recalcitrant

-pinene.

Pressure Drop

The bed pressure drop is an important biofiltration

parameter because it is taken into account in the op-

erating costs. Thus, Leson et al., 1995, reported that a

pressure drop increase from 4 to 25 cm of water led

to an increase in energetic needs from 7 to 27 kW,

within only 6 months. The pressure drops recorded in

most of biofiltration systems do not exceed some cm of

water/m. Several factors influence the pressure drop

level through the porous filter bed: filter bed charac-

teristics, flow rate, moisture content, biomass density,

etc.

Among the organic materials that constitute the

biofilter beds, soils are the least air permeable, and

thus give rise to high pressure drops, followed by com-

posts, then peats, and wood chips (Kennes and Thalasso,

1998). Moreover, filter beds that contain small parti-

cles offer high specific surface areas, thus favoring the

microbial activity (Oude Luttighuis, 1998; Kent et al.,

2000), but also constitute a greater resistance to gas flow,

which is even further increased when biomass grows in

bed porosities (Bailey and Ollis, 1986; Allen and Yang,

1991). Even though larger particles favor the gas flow

through the bed (lower pressure drop), they offer less

surface sites for the oxidation reaction, and thus can

lead to lower elimination performances (Delhomenie

bSome compounds, that are easier to degrade, or for example provide

more reaction energy, are sometimes preferred by microorganisms.



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et al., 2002b). As a compromise, Ottengraf, 1986, have

suggested the addition of 35 mm polystyrene beads

to the bed, and Williams and Miller, 1992, concluded

that beds should contain at least 60% particles with a

size greater than 4 mm. As suggested by Eitner and

Gethke, 1987, and Leson and Winer, 1991, most re-

searchers have utilized particles of diameters greater

than the threshold value of 4 mm (Corsi and Seed,

1995; Oude Luttighuis, 1998; Ortiz, 1998; Cardenas-Gonzalez et al., 1999; Delhomenie et al., 2001a).

Overall biofilters dimensions also significantly in-

fluence the bed pressure drop: under the weight

of the overlying layers, the whole bed tends to be

crushed and compacted, which affects its air perme-

ability. Usually, filter bed volumes range from 10 to

3000 m3, with heights ranging from 0.5 to 2 m. In

some cases, bed configurations are also multi-staged or

modular.

The impact of the gas flow rate on the pressure drop

is also important. Several authors have proved that thehigher the superficial gas velocity, the more significant

is the pressure drop. Phillips et al., 1995, proposed a

correlation between the pressure drop (P) and the air

superficial velocity (u), inspired by the Kozeny-Carman

relationship: P = a ub, where 1 < a < 1.9, and b

depends on the filtering material properties and the gas

flow regime (2


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Thus, Delhomenie et al., 2004, have determined that,

in a biofilter treating high concentrations of pollutant,

evaporation and stripping can cause water losses up

to 70 g of water per day per kg filter bed. Van Lith

et al., 1997 developed a calculation procedure to eval-

uate the bed drying rate and proposed moisture con-

trol methods. In biofilters, the systematic humidifica-

tion of the inlet gas stream permits the maintenance

of a minimum moisture content, but Abumaizaret al.,1998, and Devinny et al., 1999, suggest operating an

occasional, manual irrigation of the bed when water

losses are


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element after carbon and oxygen), have been devel-

oped and experimentally validated (Kinney et al., 1998;

Alonsoet al., 2001; Delhomenie et al., 2001a).

Nutrients supply is performed either in the solid

form, directly inserted into the filter bed (Gribbins and

Loehr, 1998), or as mineral salts dissolved in aqueous

solutions, the most frequently used method. Given the

wide range of elements and compounds influencing the

microbial behavior, the optimization of nutrient solu-tions delivered to filter beds is a large subject of study.

Wu et al., 1999, reviewed the most common nutrient

solutions adapted for biofilter use. The most utilized

salts are: KH2PO4, NaxH(3x)PO4,KNO3,(NH4)2SO4,

NH4Cl, NH4HCO3, CaCl2, MgSO4, MnSO4, FeSO4,

Na2MoO4, and vitamins (B1, etc.).

The pH has a similar influence on the biofiltration

efficiency than temperature: there exists an optimum

range of pH, and beyond this interval microbial ac-

tivity is severely affected. Most of the microorganisms

in biofilters are neutrophilic: their optimum pH is 7.Thus, for BTEX, Lu et al., 2002, have observed maxi-

mum degradation rates for pH between 7.5 and 8.0, and

Lee et al., 2002, at around 7.0. Veiga et al., 1999, have also

proved that for pH values ranging between 3.5 and 7.0,

the alkylbenzenes degradation performance is increased

with pH, while Arnold et al., 1997, stated that styrene

elimination was improved within a neutral medium.

Moreover, pollutants that contain heteroatoms (S, Cl,

N) are converted into acid products, which tends to

reduce the pH (Devinny and Hodge, 1995; Christen

et al., 2002), affect the microorganisms, and cause cor-rosion problems in downstream conducts (Webster and

Devinny, 1998).

Kennes and Thalasso, 1998, report that, among the

organic materials employed in biofilters, soils present

the best intrinsic pH buffering capacity, followed by

composts and wood-chips (Smet et al., 1996a). Peats are

naturally acidic (pH 3.04.0), with low buffering ca-

pacity. To maintain the pH constant at around 7.0, some

authors insert buffer materials into their filter beds: cal-

cium carbonate (Ottengraf, 1986; Smet et al., 1996b;

Krishnayya et al., 1999), dolomite (Smet et al., 1999),oyster shells (Ergaset al., 1995; Morgenrothet al., 1996;

Cardenas-Gonzalez et al., 1999). The pH regulation is

also performed via bed irrigation with nutrient solu-

tions that contain pH-buffers: Ca(OH)2 (Acuna et al.,

1996), NaOH (Zilli et al., 1996), NaHPO4, NaHCO3(Tang et al., 1996), urea (Wu et al., 1999; Delhomenie

et al., 2001b).

Microorganisms

Microorganisms are the catalysts of pollu

biodegradation. They are essential for the opera

of the bioprocess. During the elimination of V

heterotrophic microorganismsd are predominant, m

often being bacteria or fungi. The bed inocula

depends on both the nature of the filtering mate

and the VOC biodegradability level. Many authorsfer taking advantage of the ecosystems indigenou

the beds (Mohseni and Allen, 2000; Delhomenie e

2001b; Delhomenieet al., 2002a;). After an acclima

tion period, the most resistant populations are natu

selected and a microbial hierarchy is established in

bed. In many other cases (materials with low biom

density, recalcitrant VOC, reduction of acclimatiza

period), researchers inoculate the beds with conso

extracted from sewage sludge, for example, or strain

rived from either commercial sources or isolated f

previously operated biofilters. Table 3 presents somthe strains employed in biofilters, either as inocul

or as materials isolated from operating systems.

In terms of biomass density, a biofilter contain

tween 106 and 1010 cfue of bacteria and actinomyc

per gram of bed (Ottengraf, 1986; Websteret al., 1

Krailas et al., 2000), and some 103 to 106 cfu of f

per g (Ottengraf, 1986; Tahraoui et al., 1994). La

Pedersenet al., 1997, and Delhomenieet al., 2001a,

reported that, in biofilters, the degrading species re

sent between 1 and 15% of the total populations.

Models

The design and application of large-scale biofi

requires the development of tools to evaluate th

fluence of the main operational parameters (flow

concentrations, nutrient effects, etc.). Studies perfor

at the laboratory and pilot scales provide experime

results that contribute to the understanding of c

plex biofiltration mechanisms and that also suppor

development of process models, which are themse

very useful tools for both performance extrapola

and prediction.Since the beginnings of air biofiltration, progress

been realized in the field of biofiltration mode

and numerous models now exist, describing both

steady-state and the transient behaviors of bioreac

dTheir sources of carbon and energy are the organic compounds.ecfu: colony forming units.

65 Biofiltration o


14/21

TABLE 3 Microorganisms identified during the degradation of VOC by biofiltration.

Pollutants Microorganisms References

Benzene Pseudomonassp. Seneet al., 2002

Benzene Alcaligenes xylosoxidans Yeom and Daugulis, 2001

Benzene Cladosporium sphaeraspermum, Qiet al., 2002

Exophiala lecanii-corni,

Phanerochaete chrysosporium

BTX

Phanerochaete chrysosporium Ohet al., 1998Butylacetate Cladosporium resinae,

C. sphaeraspermum,

Exophiala lecanii-corni,

Mucor rouxi,


Qiet al., 2002

Chlorobenzene Pseudomonassp. Seignezet al., 2001

Dichloromethane Pseudomonas putida Ergaset al., 1996

Dichloromethane Hyphomicrobiumsp. Dikset al., 1994

Dimethyl sulfide Hyphomicrobium Smetet al., 1999

Ethanol Candida utilis Christenet al., 2002

Ethylacetate Rhodococcus fascians Hwanget al., 2002

Ethylbenzene Cladosporium resinae,

C. sphaeraspermum,Exophiala lecanii-corni,


Qiet al., 2002

Ethylene Mycobacteriumsp. Deheyderet al., 1997

Methylethylketone Alcaligenes denitrificans,

Geotrichum candidum,

Fusiarum oxysporum

Agathoset al., 1997

Methylethylketone Cladosporium resinae,

C. sphaeraspermum,

Exophiala lecanii-corni

Qiet al., 2002

Methylethylketone Rhodococcussp. Amanullahet al., 2000

Methylisobutylketone Cladosporium resinae,

C. sphaeraspermum,

Exophiala lecanii-corni,Phanerochaete chrysosporium

Qiet al., 2002

Methyl-tertbutyl-ether Pseudomonas aeruginosa Dupasquieret al., 2002

Pentane Pseudomonas aeruginosa Dupasquieret al., 2002

Phenol Pseudomonas putida Zilliet al., 1996

-pinene Aspergillussp. Diehlet al., 2000

-pinene Pseudomonas fluorescens,

Alcaligenes xylosoxidans

Kleinheinzet al., 1999

Styrene C. sphaeraspermum,

Exophiala lecanii-corni

Qiet al., 2002

Styrene Tsukamurella, Pseudomonas,

Sphingomonas, Xanthomonas

Arnoldet al., 1997

Styrene Exophiala jeanselmei Coxet al., 1997

TEX+ Bacillus sp.,Pseudomonassp.,

Trichosporon beigelei

Veigaet al., 1999

Toluene Acetinobactersp. Mareket al., 1999

Toluene Pseudomonas putida Parket al., 2002; Ergaset al., 1996;

Villaverde and Fernandez, 1997

Toluene Pseudomonas pseudoalcaligenes Oh and Choi, 2000

Toluene Exophiala lecanii-corni Woertzet al., 2001

Toluene Scedosporium apiospermum Garcia-Penaet al., 2001



15/21

TABLE 3 Microorganisms identified during the degradation of VOC by biofiltration.(Continued)

Pollutants Microorganisms References

Toluene Corynebacterium jeikeium,

C. nitrilophilus, Turicella oritidis,

Pseudomonas mendocina,

Sphingobacterium thalphophilum,

Micrococcus lutens

Strausset al., 2000

Toluene Cladophalophoriasp. Woertzet al., 2002

Trichloroethane Pseudomonas putida Ergaset al., 1993Trichloroethylene Pseudomonas putida Coxet al., 1998; Ergas et al.,

Xylene Pseudomonas pseudoalcaligenes Oh and Choi, 2000

Benzene, Toluene, Xylene.+Toluene, Ethylbenzene, Xylene.

Biofilters are three-phase biological reactors involving

complex, physical, chemical and biological phenom-

ena: interfacial transfers, diffusion, convection, disper-

sion, sorption equilibriums, degradation and microbial

kinetics, etc. The differences between the various models

proposed in the literature depend on the assumptionsstated by the authors to establish the mass balances and

equations for the different phases considered.

Ottengraf and van den Oever, 1983, proposed the

first model for air biofiltration, by describing the

steady-state performance. In this model, the cases of

zero and first order degradation kinetics were presented,

and associated with a substrate mass equilibrium at a

unique gas/biofilm interface. This model, analytically

solvable, is still widely used to describe biofilter perfor-

mance (Zilliet al., 1996; Metriset al., 1999; Pinedaet al.,

2000; Delhomenie et al., 2002a; Park et al., 2004). Topredict steady-state biofilter effectiveness, Shareefdeen

et al., 1993, proposed another model by introducing

oxygen limitation and substrate inhibition in the ki-

netic expressions. Deshusses and Hamer, 1993, as well

as Baltzis et al., 1997, developed models for steady-state

biofilters treating mixtures of pollutants, through con-

sidering either Monod or Haldane degradation kinetics.

Even though they provide satisfying descriptions of the

steady-state performance, most of these models reduce

the real three-phase problem to a simplified two-phase

problem involving a unique gas/biofilm interface, andwithout other flow models than the ideal plug flow one.

Dealing with the transient behavior of biofilters led

most of the authors to consider more complex sys-

tems. Hodge and Devinny, 1995, for example, con-

sidered axial dispersion in the plug flow reactor, and

introduced a mass transfer term between the gas and

biofilm/solid phases. Shareefdeen and Baltzis, 1994,

proposed a three-phase development, taking into

count adsorption of the pollutant at the gas/solid

terface. This model also considered oxygen limita

and substrate inhibition. Amanullah et al., 1999,

posed a three-phase model derived from Shareefd

and Baltzis, 1994, combining adsorption and axialpersion phenomena. In their transient model, Tang

Hwang, 1997, established mass balances that took

account the heterogeneity of the packing material

sorption phenomena on the pellets, and they prop

a complex generalized form of the Monod expres

for substrate biodegradation. Deshusseset al., 1995,

Zarook et al., 1997, suggested transient models for

biofiltration of mixtures of pollutants, with kineti

teractions between the pollutants. All of these trans

regime models describe complex mechanisms betw

gas, solid and biofilm, and their solution by numcal methods requires the assumption that both bio

thickness and biomass density remain constant

ing biofilter operation. To this end, it is assumed

biomass growth is counterbalanced by death and m

tenance (Metriset al., 1999). However, as mentione

Morgan-Sagastume et al., 2001, and Delhomenie e

2003, biomass tends to accumulate on the bed p

surface, so that the biofilm thickness in biofilters

reach several hundreds m after several weeks of

eration (Pineda et al., 2000; Cohen, 2001). Cha

of biofilm physionomy alter the transfer mechan(Alonsoet al., 1997; Wubkeret al., 1997) and tend t

fect the overall performance (Delhomenie et al., 2

Song and Kinney, 2002). Despite the recognized im

tance of the clogging problems in biofilters, up to d

only one transient model has been presented in th

erature to describe the consequence of biomass gro

on biofiltration performance (Song and Kinney, 20

67 Biofiltration o


16/21

CONCLUSION

In this paper, the existing air pollution control tech-

nologies (APCT) applied for the removal of volatile

organic compounds have been reviewed. The main

APCT (adsorption, absorption, incineration, etc.) have

been briefly presented. Among them, the emerging

biotechnologies: bioscrubbers, biotrickling filters and

biofilters, have been described. The emphasis of this re-

view focused on biofiltration and presented the recent

and latest developments that concern biofilters. Details

of the main physical, chemical and biological parame-

ters governing this bioprocess have been presented: fil-

ter bed, flowrate, pollutants (nature and concentration),

pressure drop, moisture content, temperature, nutrients

and pH, microorganisms, and models.

ACKNOWLEDGMENTS

The authors are indebted to the Natural Science and

Engineering Research Council of Canada (NSERC) for

their financial support. They also wish to acknowledge

Dr. P. G. Lanigan for text revision.

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