Biotechnology Advances 25 (2007) 410–422www.elsevier.com/locate/biotechadv

Research review paper

Two-phase partitioning bioreactors for treatmentof volatile organic compounds

Raul Muñoz a, Santiago Villaverde a, Benoit Guieysse b,⁎, Sergio Revah c

a Valladolid University, Department of Chemical Engineering and Environmental Technology,Paseo del Prado de la Magdalena, s/n, Valladolid, Spain

b School of Civil and Environmental Engineering, Nanyang Technological University,Block N1, Nanyang Avenue, Singapore 639798, Singapore

c Universidad Autónoma Metropolitana–Cuajimalpa c/o UAM-I, Departamento de Ingeniería de Procesos e Hidraúlica,Av. San Rafael Atlíxco No. 186, Col. Vicentina, C.P. 09340, México, Distrito Federal, Mexico

Received 30 August 2006; received in revised form 27 March 2007; accepted 27 March 2007Available online 1 April 2007

Abstract

Two-phase partitioning bioreactors (TPPBs) allow the biological removal of volatile organic compounds (VOCs) fromcontaminated gas streams at unprecedented rates and concentrations. TPPBs are constructed by adding a non-aqueous phase (e.g.hexadecane, silicone oil) to an aqueous phase that contains the microorganisms responsible for degrading the VOCs. Presence of awater-immiscible phase improves the transfer of hydrophobic substrates (e.g. hexane, oxygen) or reduces the toxicity of inhibitorysubstances (e.g. benzene, toluene) to the microorganisms present in the aqueous phase. The non-aqueous phase is selected based oncost, safety, good partitioning properties towards the target pollutants, biocompatibility, and non-biodegradability. TPPBs havehitherto been designed as laboratory-scale well-mixed stirred-tank reactors or as biofilters that contain a non-aqueous phase. Scale-up and industrial use of TPPBs require elucidation and modeling of the mechanisms of substrate transfer and uptake; understandingof the mechanisms of microbial selection; identification or synthesis of new inexpensive and robust non-aqueous phases; andgeneration of suitable guidelines for process design and control.© 2007 Elsevier Inc. All rights reserved.

Keywords: Biphasic bioreactor; Two-phase partitioning bioreactors; biological gas treatment

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4112. Improving substrate mass transfer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4143. Improving stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4174. Process design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417

4.1. Non-aqueous phase selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4174.2. Parameters affecting interfacial areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418

⁎ Corresponding author. Tel.: +65 6790 5282; fax: +65 6791 0676.E-mail address: bjguieysse@ntu.edu.sg (B. Guieysse).

0734-9750/$ - see front matter © 2007 Elsevier Inc. All rights reserved.doi:10.1016/j.biotechadv.2007.03.005

411R. Muñoz et al. / Biotechnology Advances 25 (2007) 410–422

5. Limitations of TPPBs and future prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4196. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420

1. Introduction

Atmospheric organic pollutants represent a majorenvironmental and human health problem (Table 1) thatwas recently illustrated by evidence linking childhoodcancers with prenatal or early postnatal exposure toatmospheric carcinogens (Knox, 2005a,b). Volatile organ-ic compounds (VOCs) are therefore strictly regulated (e.g.EuropeanUnion directive 1999/13/EC) and their release tothe atmosphere must be prevented. Among the technolo-gies available for air pollution control from stationarysources (Table 2), biological systems (e.g. biofilters,biotrickling filters) represent cost-effective solutions fortreating low concentrations of pollutants (Delhoménie andHeitz, 2005; Moo-Young and Chisti, 1994). Thesetechnologies are based on the natural ability of micro-organisms to convert organic pollutants into carbondioxide, water, inorganic compounds, and biomass (i.e.the biocatalyst) under mild conditions of temperature and

Table 1Example and properties of various hazardous atmospheric organic pollutants

VOCa CAS Formula MWb Powb Solubilty b H b

Benzene 71-43-2 C6H6 78.12 2.13 1790 5.55×Toluene 108-88-3 C7H8 92.14 2.73 526 6.64×o-Xylene 95-47-6 C8H10 106.2 3.12 178 5.18×

1,3-butadiene 106-99-0 C4H6 54.09 1.99 735 7.36×

Chloroform 67-66-3 CHCl3 119.4 1.97 7950 3.67×

Trichloroethylene 79-01-6 C2HCl3 131.4 2.42 1280 9.85×

Tetrachloroethane 79-34-5 C2H2Cl4 167.9 2.39 2830 3.67×

Formaldehyde 50-00-0 CH2O 30.03 0.35 4×105 3.37×

Acetaldehyde 75-07-0 C2H4O 44.05 −0.34 1×106 6.67×a Representative VOCs found at various US urban locations (Mohamed et a

The US EPA defines VOCs as substances with vapor pressure greater than 0.based on carbon chains or rings (and also containing hydrogen) with a vapor pvapor pressure greater 0.074 mm Hg at 20 °C. Many substances are also exb Molecular weight (MW), logarithm of the partitioning octanol–water ra

coefficient (H, atm m3 mol−1), and Vapor Pressure (mm Hg) according toCorporation (http://www.syrres.com/esc/physdemo.htm).c According to theMaterial SafetyData Sheets ofThe Physical andTheoretica

T = toxic, VT = very toxic, H = harmful, C = carcinogen, M = mutagen, and Td According to the Australian National Pollutant Inventory (http://www.np

pressure (Devinny et al., 1999; Shareefdeen and Singh,2005). Unfortunately, biological processes are not suitablewhen microbial activity is inhibited (i.e. high VOCconcentration or toxicity) or limited by the slow transferof hydrophobic pollutants and oxygen from the gaseousphase to the microorganisms present in the aqueous phase.

Two-phase partitioning reactors (TPPBs, also knownas biphasic bioreactors) have been proposed for reducingthese limitations and extending the range of applicabilityof biological treatment of contaminated air (Table 3).TPPBs contain a non-aqueous phase (e.g. hexadecane,silicone oil, polymer beads) in addition to an aqueousphase. The non-aqueous phase improves the transfer ofhydrophobic VOCs and oxygen to the microorganismsand can reduce the exposure of microorganisms to inhib-itory substances by lowering their concentration in theaqueous phase. For example, Daugulis and Boudreau(2003a) reported a toluene elimination capacity (EC) of727 g/m3 of reactor volume per hour in a TPPB containing

VPb Risks c Common uses d

10−3 94.8 T, C Manufacture of chemicals, solvent10−3 28.4 T Solvent10−3 6.61 H by

inhalationSolvent, manufacture of chemicals,sterilizing agent

10−2 2110 Suspected C,probable Te

Production of synthetic rubber,plastics and acrylics

10−3 197 Toxic,probable C

Manufacture of chemicals, solvent.

10−3 69 Toxic, C, M,possible Te

Solvent, manufacture of chemicals.

10−4 4.62 VT, C, M Solvent, production of wood stainsand varnishes.

10−7 3890 VT, probableC, M

Manufacture of chemicals

10−5 902 Suspected C Manufacture of chemicals.

l. 2002). There is however no clear and unanimous definition of a VOC.1 mm Hg, the Australian National Pollutant Inventory as any chemicalressure greater than 2 mm Hg at 25 °C, and the EU as chemicals with acluded such as CO, CO2, CH4, and sometimes formaldehyde.tio (log Pow, dimensionless), aqueous solubility (mg l−1), Henry lawthe Interactive PhysProp Database Demo of the Syracuse Research

l Chemistry Laboratory ofOxfordUniversity (http://physchem.ox.ac.uk/):e = teratogen.i.go.au/ ).

Table 3Characteristics and properties of TPPBs used for the treatment of VOCs from gaseous emissions

VOC/TPPB system Microorganism Load a [VOC] b RE c ECd Reference

BenzeneSTRe with 33% f hexadecane Alcaligenes xylosoxidans 140 6 95 133 Davison and Daugulis (2003a)STR with 33% hexadecane Achromobacter xylosoxidans 1240 60 99 1200 Nielsen et al. (2005b)

TolueneSTR with 33% hexadecane Alcaligenes xylosoxidans 235 9 99 233 Davison and Daugulis (2003b)STR with 33% hexadecane Alcaligenes xylosoxidans 748 15 97 727 Daugulis and Boudreau (2003a)

HexaneBiotrickling filter with 5% silicone oil Activated sludge 100 10 90 90 Van Groenestijn and Lake (1999)TPPBs with 10% silicone oil Pseudomonas aeruginosa 180 3 77 140 Muñoz et al. (2006)Fungal TPPBs with 10% silicone oil Fusarium solani 180 3 67 120 Arriaga et al. (2006)Fungal biofilter with 1% silicone oil Fusarium solani 180 3 90 160 Arriaga et al. (2006)

StyreneBiotrickling filter with 20% silicone oil Mixed bacterial culture 555 1 96.8 537 Djeribi et al. (2005)a 1 VOC volumetric loading rate (g m−3 reactor h−1).b VOC inlet concentration (g m−3).c Removal efficiency (%).d Elimination capacity (g m−3 reactor h−1).e STR = stirred-tank reactor.f Amount of non-aqueous phase volume added per volume of total working reactor volume (%).

Table 2Current technologies for air pollution control

Technology Principle Range ofapplication

Advantages and limitations Cost a

Gas flowm3 h−1

VOCg m−3

Adsorption Adsorption to activated carbon,zeolites, or polymers

5–50000 b10 Proven and efficient. High maintenanceand pressure drop problems.

25–155

Incineration Thermal oxidation at 760 – 1200 °C N10000 2–90 Efficient. High costs, can releasehazardous incomplete degradation products.

30–600

Catalyticoxidation

Thermal catalysis (Pt, Pd, alumina,ceramics…) at 300–650 °C

N10000 2–90 Efficient and less energy demanding thanincineration. Catalyst deactivation and disposal,by-product formation.

30–330

Absorption Contaminated gas is washed with water 100–60000 8–50 Possible VOC recovery. Not suitable for lowconcentrations. Generates wastewater.

40–190

Condensation Liquefaction via cooling and/or compression 100–10000 N60 Possible VOC recovery. Suitable for highconcentrations only. Condensates must befurther treated.

30–200

Membrane Separation through semi-permeablemembranes

5–100 N50 Possible VOC recovery. Membrane fouling andneedfor high pressure.

–

Photochemicaloxidation

The contaminants are oxidized by UV–photocatalysis (TiO2).

– – High efficiency and moderate energy demand.Catalyst poisoning.

–

Biotechnologies The contaminated gas is passed through asuspended microbial culture in bioscrubbers or,in the case of biofilters or biotrickling filters,through a packed bed of a solid support on thesurface of which a biofilm is formed.

80–500000 b5 Moderate costs, high efficiency, environmentallyfriendly. Need for control of biologicalparameters, large space requirement, pressuredrop problems.

13–90

According to Devinny et al. (1999), Revah and Morgan-Sagastume (2005),Van Groenestijn and Hesselink (1993), and WERF (1997).a Cumulated investment and operation costs US$ m−3

air treated h (Delhoménie and Heitz, 2005).

412 R. Muñoz et al. / Biotechnology Advances 25 (2007) 410–422

Table 4Characteristics and properties of various conventional biofilters or bioscrubbers used for VOC removal

VOC/bioreactor Microorganism Load a [VOC] b RE c ECd Reference

BenzenePowered compost based biofilter Mixed bacterial culture 25 0.4 81 20 Zilli et al. (2005)Peat biofilter Pseudomonas sp. 30 0.5 85 26 Zilli et al. (2004)

TolueneExternal loop airlift bioreactor Pseudomonas putida 35 15 100 35 Harding et al. (2003)Bubble column Mixed bacterial culture 30 10 97 29 Neal and Loehr (2000)Biofilter with compost and perlite Mixed bacterial culture 29 0.8 97 28 Neal and Loehr (2000)Biofilter with ceramic pellets Mixed bacterial culture 46 0.8 100 46 Song and Kinney (2005)Flat composite membrane bioreactor Pseudomonas sp. 238 3.2 84 198 Jacobs et al. (2004)Fungal biofilter Scedosporium apiospermum 255 6 98 250 García-Peña et al. (2001)

HexanFungal biofilter Aspergillus niger 100 4 100 50 Spigno et al. (2003)Fungal biofilter with perlite Fusarium solani 160 3 66 100 Arriaga and Revah (2005)Bacterial biofilter with peat and perlite Activated sludge 290 2 20 60 Kibazohi et al. (2004)Internal loop airlift bioreactor Pseudomonas aeruginosa 0.2 1.2 100 0.2 Oliveira and de Franca (2005)

StyreneBiotrickling filter with lava rock Mixed bacterial culture 473 1 98.3 464 Djeribi et al. (2005)a Volumetric loading rate (g m−3 reactor h−1).b VOC inlet VOC concentration (g m−3).c Removal efficiency (%).d Elimination capacity (g m−3 reactor h−1).

413R. Muñoz et al. / Biotechnology Advances 25 (2007) 410–422

hexadecane, that even exceeded the performance ofmembrane or fungal biofilters (Table 4). Likewise,Arriagaet al. (2006) achieved hexane EC of 160 g mreact

−3 h−1 in afungal biofilter supplied with silicone oil. This eliminationcapacity was significantly higher than the highest EC of

Fig. 1. Estimated flow rates of hydrophobic gaseous substrates (hydrophobic V(B) TPPBs. The substrate flow rate is determined as the product of the mass tand the bulk substrate concentration. In conventional systems (A), microbialthe aqueous phase. In TPPBs (B), the aqueous substrate concentration is maisubstrate affinity for the non-aqueous phase. This increases the overall subadhere to the non-aqueous phase and directly take up the contaminants with

100 g mreact−3 h−1 and 60 g mreact

−3 h−1 reported in classicalfungal and bacterial biofilters, respectively (Kibazohi etal., 2004; Arriaga and Revah, 2005). Similarly, Cesário etal. (1997b) increased oxygen transfer by 120% using 10%of a perfluorocarbon FC40 in an oxygen limited toluene

OCs and O2) in (A) conventional systems (without organic phase) andransfer coefficient and the concentration gradient between the interfaceactivity (and VOC removal) is limited by the slow substrate transfer tontained at a very low level due to microbial consumption and the highstrate transfer to the aqueous phase. Certain microorganisms can alsoout needing their prior transfer to the aqueous phase.

Table 5Partition coefficient of selected solutes in various non-aqueous phases

Solute Non-aqueous phase KG /NA Reference

Hexane Silicone oil 0.003 Arriaga et al. (2006)Hexane Hexadecane 0.004 Arriaga et al. (2006)Hexane Tetradecane 0.003 Arriaga et al. (2006)Hexane Undecane 0.004 Arriaga et al. (2006)Hexane Undecane 0.0012 Abraham and Acree (2004)Toluene Undecane 0.0004 Abraham and Acree (2004)Benzene Undecane 0.0015 Abraham and Acree (2004)Methane Silicone oil 1.9 Jakob et al. (2006)Methane Mineral oil 3.0 Jakob et al. (2006)Oxygen Silicone oil 5.7 Jakob et al. (2006)Oxygen Mineral oil 7.2 Jakob et al. (2006)Oxygen C10F8 2.5 Dias et al. (2003)Oxygen Undecane 3.8 Abraham and Acree (2004)Oxygen Dodecane 4.5 Rols et al. (2004)Oxygen Perfluorocarbon,

Forane F66E2.1 Rols et al. (2004)

414 R. Muñoz et al. / Biotechnology Advances 25 (2007) 410–422

biodegradation process. Perfluorocarbons have been usedto enhance oxygen transfer in various other kinds ofbioreactors (Chisti, 1999).

Here we review the merits and limitations of two-phase partitioning bioreactors (TPPBs) for overcomingsome of the operational limitations of biological systems.Recommendations are made for design and operation ofTPPBs. Areas needing further research are identified.Although TPPBs contain several non-aqueous phases(biosolids, packing material, gaseous phase, etc.), in thisreview the “non-aqueous phase” refers only to the solidor liquid phases that have been deliberately selected forimproving the mass transfer of pollutants or reducingtheir effective toxicity. This terminology is consistentwith most other publications in this area.

2. Improving substrate mass transfer

The treatment of VOCs in a bioreactor is based on thecapacity of aerobic heterotrophic microorganisms to usethese substances as carbon and energy sources. In mostcases, this implies that the pollutants and oxygen mustfirst be transferred from the gas phase to the aqueousphase where they can be metabolized by the microorgan-isms (Fig. 1). Therefore, pollutant degradation in suchsystems occurs only in the aqueous phase. The volumetricmass transfer rate (mol m−3 s−1) of gaseous substrates(pollutants or oxygen) to the aqueous phase is given as:

FG=A ¼ KlaG=ASG

KG=A� SA

� �ð1Þ

where KlaG/A represents the global volumetric masstransfer coefficient (h−1), SG and SA are the substrate (i.e.VOC) concentrations (mol m−3) in the bulk gas andaqueous phases, respectively, and KG/A is the substratepartition coefficient (dimensionless) between the gaseousand aqueous phases. KG/A is calculated as follows:

KG=A ¼ SGS⁎A

ð2Þ

where S⁎A is the substrate concentration at the gas/aqueousinterface (mol m−3, Fig. 2).

Poorly soluble compounds exhibit high KG/A values(e.g. 74 for hexane and 8.7 for ethane; Sander, 1999).Therefore, only low values of concentration gradientsare established between the gaseous and aqueousphases. This severely reduces transport of pollutants tothe aqueous phase and, therefore, the microbial deg-radation of pollutants is poor. Similarly, because of thelow solubility of oxygen in water (KG/A(O2)=31) therate of microbial oxidation of pollutants is low espe-

cially during the degradation of moderately solublepollutants present at high concentrations (e.g. toluene,KG/A=0.25; benzene, KG/A=0.22), or during the degra-dation of soluble vapors present at low concentrations(e.g. acetone, KG/A=0.001; ethyl acetate, KG/A=0.0051; ethanol, KG/A= 2×10−4; Sander, 1999). Oxygenlimitation is related to the consumption rates and theavailability of both oxygen and the target VOC. These inturn depend on their concentrations in the gas phase,their diffusion coefficient in the liquid phase, and thestoichiometric requirement for complete oxidation ofthe pollutant present in the liquid. These variables canbe expressed as a single parameter A (Diks andOttengraf, 1991), as follows:

A ¼ DO2

DVOC

VVOC

VO2

O2½ �GVOC½ �G

KG=A VOCð ÞKG=A O2ð Þ : ð3Þ

For any order of the degradation reaction, pollutantlimitation can be expected if AN1. An A value of lessthan unity indicates oxygen limiting conditions. In Eq.(3), DO2

and DVOC are the effective diffusion coeffi-cients for oxygen and the VOC, respectively, VVOC/VO2

(g pollutant/g O2) is the reaction stoichiometry forcomplete oxidation, [O2]G and [VOC]G are the concen-tration of oxygen and VOC in the gas phase, respectively;and KG/A (VOC) and KG/A (O2) are the gaseous/aqueouspartition coefficients for the target VOC and oxygen,respectively.

To overcome the limitation caused by the poor transferof hydrophobic gaseous substrates to the aqueous phases,various authors added to the mixture a non-aqueous phaseof high affinity towards the substrate (Table 5). For

415R. Muñoz et al. / Biotechnology Advances 25 (2007) 410–422

instance, Bruining et al. (1986) and Junker et al. (1990)showed that the addition of perfluorocarbon, hexadecane,or decane improved oxygen transfer in fermentation.Similarly, Cesário et al. (1992) proved the ability ofsilicone oil to trap hexane in a spray tower prior to itsdestruction in a liquid impelled loop bioreactor. Thesepioneering studies demonstrated the potential of TPPBsfor air pollution control, as the presence of a non-aqueousphase opens a new path for the transport of hydrophobicsubstrates from the gaseous phase to the aqueous phase(e.g. to the microbial cells) via the non-aqueous phase(Fig. 1). Under steady state, no substrate accumulates inthe non-aqueous phase; thus, all the substrate entering thenon-aqueous phase is transferred to the aqueous phase.The overall substrate transfer from the gaseous to aqueousphase (FG→A, mol m− 3 s−1) can be expressed as follows:

FGYA ¼ FG=A þ FNA=A ð4Þ

where FNA/A represents the substrate mass transfer fromthe non-aqueous to the aqueous phase (mol m−3 s−1)expressed as:

FNA=A ¼ KlaNA=AS⁎⁎NAKNA=A

� SA

� �: ð5Þ

In Eq. (5), KlaNA /A represents the volumetricnon-aqueous/aqueous mass transfer coefficient (h− 1)

Fig. 2. Typical concentration profiles of hydrophobic substrates (VOCs andrepresents the concentrations of VOC and O2, respectively, in the treated gas[ ]⁎⁎ represent the equilibrium concentrations at the gaseous/non-aqueouswere estimated using the data in Table 5 for air contaminated with 5 g VO

and KNA/A represents the substrate non-aqueous/aqueous partition coefficient. The latter is calculatedas follows:

KNA=A ¼ S⁎⁎NAS⁎⁎A

dimensionlessð Þ ð6Þ

where S⁎⁎NA and S⁎⁎A are non-aqueous and aqueoussubstrate concentrations (mol m− 3) at non-aqueous/aqueous interface, respectively (Fig. 2).

The substrate transfer from the gas to the non-aqueousphase (FG/NA) can be mathematically expressed asfollows:

FG=NA ¼ KlaG=NASG

KG=NA� SNA

� �ð7Þ

where KlaG/NA is the global volumetric gaseous/non-aqueous mass transfer coefficient (h−1) and KG/NA is thesubstrate gaseous/non-aqueous partition coefficient. Thelatter is calculated as follows:

KG=NA ¼ SGS⁎NA

dimensionlessð Þ: ð8Þ

In Eq. (8), SNA and S⁎NA are the substrate concentra-tions (mol m−3) in the bulk non-aqueous phase and atthe gaseous/non-aqueous interface, respectively (Fig. 2).

As the non-aqueous phase of a TPPB is selected forbeing immiscible in water (see Section 4.1), it normally

O2) in a conventional system (A) and in a TPPB (B). [VOC] and [O2]phase ([ ]g), aqueous phase ([ ]A), non-aqueous phase ([ ]NA), [ ]⁎ andand non-aqueous/aqueous interfaces, respectively. All concentrationsC m−3.

416 R. Muñoz et al. / Biotechnology Advances 25 (2007) 410–422

exhibits a high affinity to hydrophobic substrates (i.e. alow KG/NA value; Table 5). This, together with the lownon-aqueous substrate concentration (SNA) resultingfrom biological consumption, generates a high substrategaseous/non-aqueous transfer (Eq. (7)). This high flowof substrate entering the non-aqueous phase is thenreadily transferred to the aqueous phase because theglobal volumetric gaseous/non-aqueous transfer coeffi-cient (KlaG/NA) is normally lower than the volumetricnon-aqueous/aqueous transfer coefficient (KlaG/NA)(Cesário et al., 1997a; Zhao et al., 1999). Cesarioet al. (1997a,b) defined the total substrate transport fromthe gas phase (mol m−3 h−1) as follows:

FGYA ¼ KlaGYASGPK

� PS

� �ð9Þ

where K―

is the average partition coefficient (dimen-sionless) between gas and the liquid phase when theaqueous and non-aqueous phases are considered as asingle homogeneous phase. KlaG→A is the overallvolumetric gaseous/liquid mass transfer coefficient(h−1) and S

―

is the average substrate concentration(mold m−3) in the liquid phase (aqueous and non-aqueous) considered as a single homogeneous phase. K

―

and S―

are given by:

1PK

¼ xNAKG=NA

þ xAKG=A

ð10Þ

and

PS ¼ xNASNA þ xASA ð10Þ

where xNA and xA are the non-aqueous and aqueousphase fractions, respectively.

The presence of the non-aqueous phase thereforeslightly decreases the overall volumetric transfer coef-ficient (KG→Aa) as a result of the introduction of a newresistance to transport (transfer from the non-aqueous tothe aqueous phase, equivalent to a 25% reduction inKG→Aa using 1% of silicone oil; Dumont et al., 2005;Nielsen et al., 2003b). However, there is a net increaseof substrate transfer rate from the gas to the aqueousphase (Davidson and Daugulis, 2003a,b; Nielsen et al.,2003b, 2005a) due to the higher affinity of hydrophobicsubstrates for the non-aqueous phase (K

―is lower than

KG/A due to the low KG/NA value) and the “relativelylow” average substrate concentration (as compared withthe saturation concentration) due to biological con-sumption. This increases the concentration gradientscompared with conventional systems and therefore thevolumetric mass transfer is increased (Fig. 2). Conse-

quently, performance improves with lower KG/NA

(Cesário et al., 1997a). This theoretical analysis basedon the two films theory is supported by experimentalobservations. For example, Van Groenestijn and Lake(1999) reported a higher hexane removal in a bacterialTPPB with silicone than in any conventional bacterialbiotrickling filters (Tables 3 and 4). Similarly, Nielsenet al. (2003b) reported a 58% increase in oxygen transferby using 27% of a silicone oil phase. Cesário et al.(1997b) increased oxygen transfer by 120% by using10% of a perfluorocarbon FC40 in an oxygen-limitedtoluene biodegradation process.

The transfer mechanisms described above are basedon the hypothesis that substrate uptake only takes placein the aqueous phase. Various authors however haveobserved bacterial adhesion at the non-aqueous/aqueousinterface (Muñoz et al., 2006; Deziel et al., 1999; Fig. 1),and suggested that direct pollutant uptake from the non-aqueous phase improved the overall removal efficiencyof the process. Direct interfacial uptake can be difficult toprove experimentally when the non-aqueous phase is aliquid. MacLeod and Daugulis (2005) however clearlyshowed that bacteria existed exclusively on the organicside of the interface during the degradation of phenan-threne using bis(ethyl hexyl)sebacate as the non-aqueousphase. The mechanism of the substrate uptake in TPPBsdepends on the type of pollutant, the selected non-aque-ous phase, the culture conditions, and the physiologicalstate of the cells, and is therefore difficult to predict(Deziel et al., 1999). Furthermore, the two uptakemechanisms described above can occur simultaneouslyduring the microbial treatment of multiple substrates bymicrobial consortia. This was for instance shown byGauthier et al. (2003) who reported that among variousmicroorganisms isolated from a TPPB with silicone oil,those that biodegraded highly hydrophobic high molec-ular weight polycyclic aromatic hydrocarbons (PAHs)preferentially grew at the non-aqueous/aqueous inter-face. In contrast, the microorganisms that degraded theless hydrophobic low molecular weight PAHs grew bothin aqueous phase and at the interface. Many microorgan-isms are also capable of actively accumulating hydro-carbons inside their cells, produce biosurfactants that cansolubilize the pollutants, or generate hydrocarbon micro-droplets that are then taken up (Watkinson and Morgan,1990; Bouchez et al., 1999; van Hamme et al., 2003).Surfactants might also improve cell adhesion to the non-aqueous interface. In fact, the presence of a non-aqueousphase has also been shown to favor the selection ofmicroorganisms that are capable of interfacial uptake(see Section 4.1). It also induced the production ofbiosurfactants during phenanthrene removal by an

417R. Muñoz et al. / Biotechnology Advances 25 (2007) 410–422

algal–bacterial consortium in a TPPB with silicone oil(Muñoz et al., 2003). More research is needed for betterunderstanding these mechanisms that might open newpossibilities for improving the biological treatment ofhydrophobic VOCs.

3. Improving stability

In biological processes, irreversible or temporarylosses of microbial activity (and pollutant removal ca-pacity) can be caused by either long-term exposure tochronic toxicants or short-term exposure to highconcentrations of acute toxicants (Jones et al., 1997;Mirpuri et al., 1997; Tresse et al., 2003). For instance,Leddy et al. (1995) and Villaverde et al. (1997a,b,c)showed that prolonged exposure of Pseudomonasputida 54G to toluene vapor (toluene liquid concentra-tions of approximately 7 mg l−1) caused importantfractions of the culture to die or loose capacity tobiodegrade toluene. Likewise, Song and Kinney (2005)reported decline in the toluene elimination capacities ofseveral biofilters subjected to high toluene loadings.Oliveira and Livingston (2003) also demonstrated thathigh concentrations of toxic monochlorobenzene in-duced washout and instability in a bioscrubber.

TPPBs have also been used to extract inhibitorycompounds (e.g. ethanol, citric acid, or carboxylic acid)in-situ in order to support higher production yields andfacilitate downstream processing (Daugulis, 1997;Malinowski, 2001). This concept was later applied toimprove the biodegradation of toxic contaminants inaqueous wastes (Daugulis, 2001; Déziel et al., 1999;Rosenberg, 1989) by reducing their aqueous concen-tration and, thereby, their toxicity towards the microbialcommunity (Fig. 2). This is of particular interest in thetreatment of moderately soluble VOCs which aqueousconcentration can reach levels high enough to inducesevere phenotypic and genotypic modifications onmicrobial cultures (Jones et al., 1997; Leddy et al.,1995). As a typical example, a gaseous effluent con-taining up to 60 g benzene per cubic meter was suc-cessfully treated at 99% removal efficiency by usinghexadecane as the non-aqueous phase (Nielsen et al.,2005b). Without the solvent, such a high gaseousconcentration would have resulted in benzene aqueousconcentration of up to 270 mg l−1 (based on a Henrylaw constant of 0.22). This high concentration exceedsthe benzene EC50 value of 202 mg l−1 towards a bio-luminescent bacterium that is representative of activat-ed sludge microflora (Ren and Frymier, 2002). Collinsand Daugulis (1997) also achieved phenol biologicaltreatment by P. putida at a total load of 6.7 g l−1, which is

far above the maximum concentration (approximately1 g l−1; Geng et al., 2006) that even highly resistant strainscan tolerate. Similarly, Munro and Daugulis (1997)achieved PCP biodegradation with Arthrobacter sp. in aTPPB with diethyl sebacate at a total (non-aqueous+aqueous) initial concentration of 6.7 g l−1, a value muchhigher than its IC50 toward activated sludge bacteria (i.e.9 mg l−1 according to Liu et al., 1982).

Fast-transfer and high absorption capacity of theVOCs in the non-aqueous phase can protect micro-organisms from surges in the gaseous pollutant con-centration by buffering the aqueous concentration. Thus,Oliveira and Livingston (2003) prevented microbialwashout by using a silicone oil-based absorber prior tothe biodegradation step during monochlorobenzeneremoval by Pseudomonas sp. under fluctuating inletconcentration of the pollutant. Similarly, Nielsen et al.(2005b) showed that the performance of bioscrubberssupplied with n-hexadecane could be maintainedconstant under changing loadings of benzene.

4. Process design

There is still a need for more knowledge on how todesign and scale up TPPBs for air pollution control. Toour knowledge, no pilot or large scale TPPB system hasever been tested. The affinity of the non-aqueous phasefor the target VOC and oxygen and its stability over longperiods of operation are key parameters that need to beconsidered. Operational parameters must then beoptimized to enlarge the interfacial areas involved inmass transfer and ensure an efficient and robust bio-degradation over long periods of operation.

4.1. Non-aqueous phase selection

According to Bruce and Daugulis (1991), a non-aqueous phase suitable for use in TPPBs should beinexpensive, readily available, immiscible in water, non-biodegradable, biocompatible (i.e. non-toxic to themicrobial community) and exhibit a high affinity (i.e.high KNA/G) to the limiting substrates in order toincrease their mass transfer or reduce their aqueousconcentration. However, only few studies have system-atically selected the non-aqueous phase for the treatmentof gaseous streams. In recent studies, both Muñoz et al.(2006) and Arriaga et al. (2006) selected silicone oilagainst hexadecane, tetradecane, 1-decanol, diethylsebacate, and 2-undecanone as the best non-aqueousphase for the treatment of hexane in a stirred-tankbioreactor inoculated with Pseudomonas aeruginosaand in a biofilter inoculated with the fungus Fusarium

418 R. Muñoz et al. / Biotechnology Advances 25 (2007) 410–422

solani. None of non-aqueous phases tested inhibited thebacterium and only silicone oil, was not biodegraded.Diethyl sebacate, 1-decanol, and 2-undecanone inhib-ited the fungus and silicone oil was the only on-aqueousphase tested which was biocompatible but not biode-graded. These studies illustrate well the dilemma facedwhen selecting the non-aqueous phase: solvent toxicityand biodegradability are species-dependant and itmight be hard to combine biocompatibility, non-biodegradability, and good partitioning properties.

As seen above for the case of hexane biodegradationby P. aeruginosa, a non-biodegradable non-aqueousphase can be difficult to find during the treatment ofhydrophobic pollutants, probably because microorgan-isms capable of biodegrading hydrophobic substratesare also more likely to biodegrade the hydrophobic non-aqueous phase. In general, the biodegradability oforganic compounds decreases in the presence of longalkyl chains or hydroxyl, ester, and acid groups in themolecule (Loonen et al., 1999). It is therefore not sur-prising that polydimethylsiloxane polymers (e.g. sili-cone oil) or large branched alkanes (e.g. 2,2,4,4,6,8,8-heptamethylnonane) have been preferred to linearalkanes, ketones and hydroxylated solvents for thetreatment of hydrophobic pollutants in TPPBs (Dézielet al., 1999; Guieysse et al., 2001a, b; Muñoz et al.,2006).

Although biodegradability screening can be carriedout, it will always remain difficult to predict the stabilityof the non-aqueous phase during long-term operationwith microbial communities where co-metabolism,recombination, acclimation, and selection could ulti-

Fig. 3. Gaseous/non-aqueous hexane partition coefficient (KG/NA) invarious organic solvents according to the data of Arriaga et al. (2006). Theleft cluster represents solvents toxic to Fusarim solani whereas the rightcluster shows biocompatible solvents that were biodegraded by the fungus.Silicone oil (KG/NA=0.0034; unknown log Pow) was the only non-aqueous phase tested that was both biocompatible and non-biodegradable.

mately lead to the emergence of microorganismscapable of degrading the abundant non-aqueous phase.In fact, Djeribi et al. (2005) concluded that the mainadvantage of TPPBs during gas treatment is to favor theselection of microorganisms with high membranehydrophobicity that are capable of interfacial adhesionand direct substrate uptake at the non-aqueous phase, forimproved removal of hydrophobic pollutants. Thiscorroborates the observations of Ascon-Cabrera andLebeault (1993) and Gauthier et al., (2003) during theselection of microorganisms capable of degradingxenobiotics in TPPBs with silicone oil.

The problem of finding a suitable non-aqueous phasethat is stable under long-term operation can be overcomeby using biocompatible and highly stable solid polymersas the non-aqueous phase (Daugulis, 2003b; Amsdenet al., 2003). Although these systems are slower torespond to sudden changes in pollutant concentrationcompared with the liquid–liquid systems (Boudreau andDaugulis, 2006), they can be more stable.

In regard to the toxicity of the non-aqueous phase,Arriaga et al. (2006) reported that non-aqueous phaseswith log Pow values lower than 4.5 were toxic toF. solani whereas non-aqueous phases with log Pow

values higher than 6.5 were biocompatible (Fig. 3).Similarly, Collins and Daugulis (1997, 1999) reportedthat two different Pseudomonas strains were inhibitedby solvents with log Pow values lower than 3.1 and 3.2(i.e. the critical log Pow) and selected 2-undecanone andAdol 85 NF (an industrial oleyl alcohol) for the bio-degradation of aromatics by these bacteria in TPPBs.The log Pow is indeed often correlated with the toxicityof organic solvents (de Bont, 1998). Solvents with a logPow greater than 4 are generally considered as non-toxicat saturation (Ramos et al., 2002). However, experimen-tal assessment of toxicity is recommended as solventtolerance greatly varies among microbial species. Asseen above, non-aqueous phases with high log Pow arerecommended for hydrophobic pollutants and, therefore,solvent biocompatibility is generally not a problem inthese cases. Nonetheless, it can be difficult to find a non-aqueous phase that is biocompatible and at the same timeexhibits a high affinity for moderately-soluble pollu-tants. Unfortunately, there is insufficient data for pre-dicting the KG/NA values of a given pollutant/non-aqueous phase system and this parameter should besystematically evaluated.

4.2. Parameters affecting interfacial areas

Large interfacial areas are crucial for optimal masstransfer of substrate and for microbial adhesion in cases

419R. Muñoz et al. / Biotechnology Advances 25 (2007) 410–422

of direct pollutant uptake. The interfacial areas involvedin mass transport (i.e. non-aqueous/aqueous, gas/non-aqueous, and gas/aqueous) are determined by the non-aqueous/aqueous phase ratio, the turbulence intensity,the solvent properties, and the gas flow rate (Chatziet al., 1989; Nielsen et al., 2003b). For example, Ascón-Cabrera and Lebeault (1995) showed that the interfacialarea in a TPPB with silicone oil increased when theorganic/aqueous phase ratio was increased up to 40%.Further increase in phase ratio did not affect the inter-facial area. The amount of non-aqueous phase used inTPPB for gas treatment varies greatly (Table 3). Forexample, Van Groenestijn and Lake (1999) used 2 L of a1:1 (vol/vol) mixture of silicone oil and mineral mediumin a 20 L lab scale biofilter (total non-aqueous volu-metric loading of 5%) for hexane removal. In contrast,Arriaga et al. (2006) used 0.4 L of a 5:95 (vol/vol)mixture of silicone oil in mineral medium to inoculate a2.5 L biofilter (total loading of 0.8%). Higher loadings(up to 33%) have been used in stirred-tank (ST) bio-reactors (Table 3) because they do not have any packingmaterials.

Aeration significantly contributes to turbulence in aTPPB. For example, degradation of phenanthrene in-creased from 22 mg L−1 h−1 to 32 mg L−1 h−1 when theaeration rate was increased from 1 to 3 vvm in a TPPBwith silicone oil as the non-aqueous phase (Muñoz et al.,2005). These results were achieved in a standardindustrial fermentor where turbulence was generatedby mechanical agitation. A high level of turbulencehelps in breaking up the non-aqueous liquid phase andthe VOC-laden gas into microscopic droplets andbubbles, to significantly increase the interfacial areasavailable for mass transfer. However, this processinvolves increased energy consumption and consequent-ly increased cost of operation. Gas–liquid mass transferphenomena in conventional aqueous system are welldescribed but little information is available on transferphenomena in TPPBs (Cesário et al. 1996a,b, 1997a,b;Nielsen et al., 2003a, 2005a; Van Ede et al., 1995).Some of the correlations that have been developed forestimating interfacial mass transfer coefficients inprocesses such as liquid–liquid extraction are potential-ly useful for the design of TPPBs (Chisti, 1999).

5. Limitations of TPPBs and future prospects

The biological treatment of VOCs from gaseousstreams is traditionally conducted in bioscrubbers,trickling biofilters or biofilters (see the recent reviewsof Iranpour et al., 2005; Shareefdeen and Singh, 2005;Delhoménie and Heitz, 2005). TPPBs for air treatment

are often constructed by adding a non-aqueous liquidphase to the liquid phase of bioscrubbers or tricklingbiofilters (Daugulis and Boudreau, 2003a; Van Groe-nestijn and Lake, 1999; Arriaga et al., 2006).

Many authors have also constructed TPPBs in me-chanically stirred aerated tank bioreactors by adding aliquid or solid non-aqueous phase to the aqueous cellsuspension. This configuration makes use of the entirebioreactor volume in contrast to conventional biofilterswhere some form of a packing material is needed(Daugulis and Boudreau, 2003a; Deshusses, 1997;McNevin and Barford, 2000). Stirred tanks also allowefficient control of environmental conditions, such astemperature and pH, and prevent biomass overgrowth,channeling, shrinkage, drying, and deterioration of thefilter bed structure, or process inhibition that occurs athigh pollutant concentrations that may be found in plug-flow reactors (Garcia-Peña et al., 2001). On the otherhand, stirred tanks require higher energy inputs formechanical agitation and aeration (to overcome thehydrostatic pressure). Extensive data and experienceexist on design of multiphase stirred reactors as they arewidely used in the chemical industry. High interfacialareas can be achieved in stirred-tank reactors at relativelylow power inputs by adding surfactants (Srivastava et al.,2000), a practice that is sometimes used in conventionalliquid–liquid extraction processes. Presence of surfac-tants significantly increases the interfacial gaseous/non-aqueous and non-aqueous/aqueous areas, allowingsubstrate volumetric mass transfer coefficients that are5 to 15 times higher than in the absence of surfactants(Bredwell and Worden, 1998; Lye and Stuckey, 1996).

Most of the studies hitherto reported of VOCsremoval in TPPBs were short-term experiments. Long-term stability is crucial as the non-aqueous phase mightdegrade or be lost during continuous operation. Thelongest operation time (6 months) was reported by VanGroenestijn and Lake (1999) in a biotrickling filterpercolated with a mixture of mineral medium andsilicone oil. Although stability was not systematicallyinvestigated, the authors did not report any biodegrada-tion of the non-aqueous phase. Approximately 30% ofthe silicone oil initially introduced was lost during thefirst 100 days of operation. This was attributed tosampling losses. No further loss apparently occurredduring the next 100 days of operation.

Review of literature reveals that fungal and mem-brane-based bioreactors display a pollutant removalperformance that is similar to TPPBs for similar rangesof loading. In fungal degradation processes, aerialmycelia of fungi that are in direct contact with the gasphase supposedly directly uptake VOC. This uptake is

420 R. Muñoz et al. / Biotechnology Advances 25 (2007) 410–422

faster than if a flat biofilm of bacteria directly contactsthe gas phase because of a high gas–mycelium inter-facial area of the fungal mat and the highly hydrophobicnature of the fungal cell wall (Arriaga et al., 2005;Kennes and Veiga, 2004; Groenestijn and Kraakman,2005). Although fungal biofilters often exhibit a betterperformance than bacterial biofilters for the removal ofhydrophobic VOCs (Table 4), they still suffer from thesame operational problems (i.e. biomass clogging, beddrying, high recovery times after process inhibitioncaused by VOCs surges) as do bacterial systems(Garcia-Peña et al., 2001; Kennes and Veiga, 2004;Nielsen et al., 2005b). Potentially, fungal bioreactorsmay pose a hazard to human health because of thepathogenic nature of some of the fungi present in thesereactors and the possible atmospheric release of fungalspores (Chisti, 1998; Prenafeta-Boldú et al., 2006). Inmembrane bioreactor the VOCs diffuses from the gasphase into the water phase through a membrane ofhydrophobic nature such as silicone tubes. Despite at-taining high ECs as a result of the large surface for masstransport and the high affinity of the selected membranefor VOCs and oxygen, membranes based bioreactorsare still limited by their high cost of construction andapparently poor long-term stability (Reij et al., 1998).

6. Conclusions

By protecting microorganisms against acute andchronic toxicants and increasing the transfer of hydro-phobic gaseous substrates to the cells, TPPBs allowscost-efficient and stable biological treatment of toxic andhydrophobic volatile organics at high loadings andelimination capacity values (Tables 3 and 4). Neverthe-less, various technical problems must be solved beforeTPPBs can be used at a large scale. In particular, meth-odologies are needed for solvent selection, especially forsolvents having long-term stability, and new hydropho-bic solvents need to be identified or made synthetically.Mechanisms of substrate transfer need to be elucidatedand modeled. Understanding is needed of the mechan-isms of microbial adhesion to the aqueous/non-aqueousinterface and of how microbes might directly access thesubstrate from the non-aqueous phase. How micro-organisms are selected in TPPBs needs to be understood.Guidelines are needed for design and control of TPPBs.

Acknowledgements

The financial support from the Spanish Ministry forScience and Education (contracts PPQ2006-08230 andJCI-2005-1881-5), SIDA (Swedish International Devel-

opment Cooperation Agency; project SWE-2002-205)and Conacyt (project Semarnat 120-2002) are gratefullyacknowledged.

References

Abraham MH, Acree WE. Correlation and prediction of partitioncoefficients between the gas phase and water, and the solventsdodecane and undecane. New J Chem 2004;28:1538–43.

Amsden BG, Bochanysz J, Daugulis AJ. Degradation of xenobiotics ina partitioning bioreactor in which the partitioning phase is apolymer. Biotechnol Bioeng 2003;84:399–405.

Arriaga S, Revah S. Improving hexane removal by enhancing fungaldevelopment in a microbial consortium biofilter. Biotechnol Bioeng2005;90:107–15.

Arriaga S, Muñoz R, Hernandez S, Guieysse B, Revah S. Gaseoushexane biodegradation by Fusarium solani in two liquid phasepacked-bed and stirred tank bioreactors. Environ Sci Technol2006;40:2390–5.

Ascon-Cabrera M, Lebeault JM. Selection of xenobiotic-degradingmicroorganisms in a biphasic aqueous–organic system.Appl EnvironMicrobiol 1993;59:1717–24.

Ascón-Cabrera MA, Lebeault JM. Interfacial area effects of a biphasicaqueous/organic system of growth kinetic of xenobiotic-degradingmicroorganisms. Appl Microbiol Biotechnol 1995;43:1136–41.

Bouchez-Naïtali M, Rakatozafy H, Marchal R, Leveau JY, Vandecas-teele JP. Diversity of bacterial strains degrading hexadecane inrelation to the mode of substrate uptake. J ApplMicrobiol 1999;86:421–8.

Boudreau NG, Daugulis AJ. Transient performance if two-phasepartitioning bioreactors treating a toluene contaminated gas stream.Biotechnol Bioeng 2006;94:448–57.

Bredwell MD, Worden RM. Mass-transfer properties of microbubbles.1. Experimental studies. Biotechnol Prog 1998;14:31–8.

Bruce LJ, Daugulis AJ. Solvent selection strategies for extractivebiocatalysis. Biotechnol Prog 1991;7:116–24.

Bruining WJ, Joosten GEH, Beenackers AACM, Hofman H.Enhancement of gas–liquid mass transfer by a dispersed secondliquid phase. Chem Eng Sci 1986;41:1873–7.

Cesário MT, Beeftink HH, Tramper J. Biological treatment of wastegases containing poorly-watersoluble pollutants. In: Dragt AJ, HamJ, editors. Biotechniques for Air Pollution Abatement and OdourControl Policies, Proceedings of an International Symposium,Maastricht, The Netherlands. Amsterdam: Elsevier1992. p. 135–40.

Cesário MT, Beeftink HH, Tramper J. Feasibility of using water-immiscible organic solvents in biological waste–gas treatment.Bioprocess Eng 1996a;12:55–63.

Cesário MT, de Wit HL, Tramper J, Beeftink HH. New technique forkla measurement between gas and water in aerated solvent-in-water dispersions. Biotechnol Tech 1996b;10:195–8.

Cesário MT, Beverloo WA, Tramper J, Beeftink HH. Enhancement ofgas–liquid mass transfer of apolar pollutants in the biological wastegas treatment by a dispersed organic solvent. Enzyme MicrobTechnol 1997a;21:578–88.

Cesário MT, de Wit HL, Tramper J, Beeftink HH. Dispersed organicsolvent to enhance the overall gas/water mass transfer coefficientof apolar compounds in the biological waste–gas treatment.Modeling and Evaluation. Biotechnol Prog 1997b;13:399–407.

Chatzi EG, Gavrielides AD, Kiparissides C. Generalized model forprediction of the steady-state drop size distribution in batch stirredvessels. Ind Eng Chem Res 1989;28:1704–11.

421R. Muñoz et al. / Biotechnology Advances 25 (2007) 410–422

Chisti Y. Biosafety. In: Subramanian G, editor. Bioseparation andBioprocessing: A Handbook, vol. 2. New York: Wiley-VCH1998.p. 379–415.

Chisti Y. Mass transfer. In: Flickinger MC, Drew SW, editors.Encyclopedia of Bioprocess Technology: Fermentation, Biocatalysis,and Bioseparation, vol. 3. New York: Wiley1999. p. 1607–40.

Collins LD, Daugulis AJ. Biodegradation of phenol at high initialconcentrations in two-phase partitioning batch and fed-bachbioreactors. Biotechnol Bioeng 1997;55:155–62.

Collins LD, Daugulis AJ. Benzene/toluene/p-xylene degradation. Part 1.Solvent selection and toluene degradation in a two-phase partitioningbioreactor. Appl Microbiol Biotechnol 1999;52: 354–9.

Daugulis AJ. Partitioning bioreactors. Curr Opin Biotechnol 1997;8:169–74.

Daugulis AJ. Two-phase partitioning bioreactors: a new technologyplatform for destroying xenobiotics. Trends Biotechnol 2001;19:457–62.

Daugulis AJ, Boudreau NG. Removal and destruction of high concentra-tions of gaseous toluene in a two-phase partitioning bioreactor byAlcaligenes xylosoxidans. Biotechnol Lett 2003a;25: 1421–4.

Daugulis AJ, Amsden B, Bochanysz J, Kayssi A. Delivery of benzeneto Alcaligenes xylosoxidans by solid polymers in a two-phasepartitioning bioreactor. Biotechnol Lett 2003b;25:1203–7.

Davison CT, Daugulis AJ. Addressing biofilter limitations: a two-phase partitioning bioreactor process for the treatment of benzeneand toluene contaminated gas streams. Biodegradation 2003a;14:415–21.

Davison CT, Daugulis AJ. The treatment of gaseous benzene by two-phase partitioning bioreactors: a high performance alternative to theuse of biofilters. Appl Microbiol Biotechnol 2003b;62: 297–301.

de Bont JAM. Solvent-tolerant bacteria in biocatalysis. TrendsBiotechnol 1998;16:493–8.

Delhoménie MC, Heitz M. Biofiltration of air: a review. Crit RevBiotechnol 2005;25:53–72.

Dias AMA, Freire M, Coutinho JAP, Marrucho IM. Solubility ofoxygen in liquid perfluorocarbons. In: Ely JF, editor. Proceedingsof the Fifteenth Symposium on Thermophysical Properties,Colorado, USA2003. p. 325–30.

Deshusses MA. Biological waste air treatment in biofilters. Curr OpinBiotechnol 1997;8:335–9.

Devinny JS, DeshussesMA,Webster TS. Biofiltration for Air PollutionControl. Boca Raton, FL: Lewis Publishers/CRC Press 1999.

Déziel E, Comeau Y, Villemur R. Two liquid-phase bioreactors forenhanced degradation of hydrophobic/toxic compounds. Biodeg-radation 1999;10:219–33.

Diks RMM, Ottengraf SPP. Verification studies of a simplified modelfor the removal of dichloromethane from waste gases using abiological trickling filter (Part I). Bioprocess Eng 1991;6:93–9.

Djeribi R, Dezenclos T, Pauss A, Lebeault JM. Removal of styrene fromwaste gas using a biological trickling filter. Eng Life Sci 2005;5:450–7.

Dumont E, Andrès Y, Le Cloirec P. Enhancement of oxygen transfer inbioprocesses by the use of an organic phase: Effect of silicone oil onvolumetric mass transfer coefficient of oxygen (kla). In: Kennes C,Veiga MC, editors. Proceedings of the International CongressBiotechniques for Air Pollution Control; 2005. p. 163–73.A Coruña, Spain.

García-Peña EI, Hernandez S, Favela-Torres E, Auria R, Revah S.Toluene biofiltration by the fungus Scedosporium apiospermumTB1. Biotechnol Bioeng 2001;76:61–9.

Gauthier E, Déziel E, Villemur R, Juteau P, Lépine F, Beaudet R. Initialcharacterization of new bacteria degrading high-molecular weight

polycyclic aromatic hydrocarbons isolated from a 2-year enrichmentin a two-liquid-phase culture system. J Appl Microbiol 2003;94:301–11.

Geng A, Soh AEW, Lim CJ, Loke LCT. Isolation and characterizationof a phenol-degrading bacterium from an activated sludge. ApplMicrobiol Biotechnol 2006;71:728–35.

Guieysse B, Cirne MDTG, Mattiasson B. Microbial degradation ofphenanthrene and pyrene in a two-liquid phase partitioningbioreactor. Appl Microbiol Biotechnol 2001a;56:796–802.

Guieysse B, Lundstedt S, van Bavel B, Mattiasson B. Biologicaltreatment of PAH contaminated soil extracts. In: Magar VS, JohnsonG, Ong SK, Leeson A, editors. Bioremediation of Energetics,Phenolics, and Polynuclear Aromatic Hydrocarbons — The 6thInternational In-situ and On-site Bioremediation Symposium, SanDiego, California, vol. 6(3). 2001b. p. 181–8.

Harding RC, Hill GA, Lin YH. Bioremediation of toluene-contami-nated air using an external loop airlift bioreactor. J Chem TechnolBiotechnol 2003;78:406–11.

Iranpour R, Cox HHJ, Deshusses MA, Schroeder ED. Literaturereview of air pollution control biofilters and biotrickling filters forodor and volatile compound removal. Environ Progress 2005;24(3):254–67.

Jacobs P, de Bo I, Demeestere K, Verstraete W, van Langenhove H.Toluene removal from waste air using a flat composite membranebioreactor. Biotechnol Bioeng 2004;85:68–77.

Jakob F, Jakob K, Perjanik N. Silicone Dielectric Fluids. AnalyticalChemTech Iternational, Inc. 2006. (http://www.weidmann-acti.com/knowledge_center/).

Jones W, Mirpuri R, Villaverde S, Lewandowski Z, Cunningham AB.The effect of bacterial injury on toluene degradation andrespiration rates in vapor phase bioreactors. Water Sci Technol1997;36:85–92.

Junker BH, Wang DIC, Hatton TA. Oxygen transfer enhancement inaqueous/perfluorocarbon fermentation systems: II. Theoreticalanalysis. Biotechnol Bioeng 1990;35:586–97.

Kibazohi O, Yun S, Anderson WA. Removal of hexane in biofilterspacked with perlite and a peat-perlite mixture. World J MicrobBiotechnol 2004;20:337–43.

Kennes C, Veiga MC. Fungal biocatalysts in the biofiltration of VOC-polluted air. J Biotechnol 2004;113:305–19.

Knox EG. Childhood cancers and atmospheric carcinogens.J Epidemiol Community Health 2005a;59:101–5.

Knox EG. Oil combustion and childhood cancers. J EpidemiolCommunity Health 2005b;59:755–60.

Leddy MB, Philips DW, Ridgway HF. Catabolite-mediated mutationsin alternate toluene degradation pathways in Pseudomonas putida.J Bacteriol 1995;177:4713–20.

Lye GJ, Stuckey DC. Predispersed solvent extraction of microlideantiobiotics using colloidal liquid aphrons. In: Shallcross DC, PaiminR, Prvic LM, editors. Proceedings of the International SolventExtraction Conference, Melbourne, Australia1996. p. 1399–405.

Liu D, Thomson K, Kaiser KLE. Quantitative structure–toxicityrelationship of halogenated phenols on bacteria. Bull EnvironContam Toxicol 1982;29(2):130–6.

Loonen H, Lindgren F, Hansen B, Karcher W, Niemela J, HiromatsuK, et al. Prediction of biodegradability from chemical structure:modeling of ready biodegradation test data. Environ Toxicol Chem1999;18:1763–8.

MacLeod CT, Daugulis AJ. Interfacial effects in a two-phasepartitioning bioreactor: degradation of polycyclic aromatic hydro-carbons PAHs by a hydrophobicMycobacterium. Process Biochem2005;40:1799–805.

422 R. Muñoz et al. / Biotechnology Advances 25 (2007) 410–422

Malinowski JJ. Two-phase partitioning bioreactors in fermentationtechnology. Biotechnol Adv 2001;19:525–38.

McNevin D, Barford J. Biofiltration as an odour abatement strategy.Biochem Eng J 2000;5:231–42.

Mirpuri R, Jones W, Bryers JD. Toluene degrading kinetics forplanktonic and biofilm-grown cells of Pseudomonas putida 54 g.Biotechnol Bioeng 1997;53:535–46.

MohamedMF, KangD, Aneja VP. Volatile organic compounds in someurban locations in United States. Chemosphere 2002;47: 863–82.

Moo-Young M, Chisti Y. Bioreactor applications in waste treatment.Res Cons Recycl 1994;11:13–24.

Munro DR, Daugulis AJ. The use of an organic solvent and integratedfermentation for improved xenobiotic degradation. Resour EnvironBiotechnol 1997;3:207–25.

Muñoz R, Guieysse B, Mattiasson B. Phenanthrene degradation by analgal–bacterial consortium in two-phase partitioning bioreactors.Appl Microbiol Biotechnol 2003;61:261–7.

Muñoz R, Rolvering C, Guieysse B, Mattiasson B. Aerobic phenan-threne biodegradation in a two phase partitioning bioreactor. WaterSci Technol 2005;52:265–71.

Muñoz R, Arriaga S, Hernandez S, Guieysse B, Revah S. Enhancedhexane biodegradation in a two phase partitioning bioreactor: over-coming pollutant transport limitations. Process Biochem 2006;41:1614–9.

Neal AB, Loehr RC. Use of biofilters and suspended growth reactors totreat VOCs — the role of humidity and microbial heat generation.Waste Manag 2000;20:59–68.

Nielsen J, Villadsen J, Lidén G. Scale-up of bioprocesses. In: Nielsen J,Villadsen J, LidénG, editors. Bioreaction Engineering Principles. 2ndEdition. Kluwer Academic/Plenum Publishers2003a. p. 477–517.

Nielsen DR, Daugulis AJ, Mclellan PJ. A novel method of simulatingoxygen mass transfer in two phase partitioning bioreactors.Biotechnol Bioeng 2003b;83:735–42.

Nielsen DR, Daugulis AJ, Mclellan PJ. A restructured framework formodeling oxygen transfer in two-phase partitioning bioreactors.Biotechnol Bioeng 2005a;91:773–7.

Nielsen DR, Daugulis AJ, Mclellan PJ. Transient performance of atwo-phase partitioning bioscrubber treating a benzene-contami-nated gas stream. Environ Sci Technol 2005b;39:8871–977.

Oliveira TAC, Livingston AG. Bioscrubbing of waste gas–substrateabsorber to avoid instability induced by inhibition kinetics.Biotechnol Bioeng 2003;84:552–63.

Oliveira FJS, de Franca FP. Performance of an internal-loop airliftbioreactor for treatment of hexane-contaminated air. Appl BiochemBiotechnol 2005;121:581–91.

Prenafeta-Boldú FX, Summerbell R, Sybren de Hoog G. Fungi growingon aromatic hydrocarbons: Biotecnology's unexpected encounterwith biohazard? FEMS Microbiol Reviews 2006;30: 109–21.

Ramos JL, Duque E, Gallegos MT, Godoy P, Ramos-González MI,Rojas A, et al. Mechanisms of solvent tolerance in gram-negativebacteria. Ann Rev Microbiol 2002;56:743–68.

Reij MW, Keurentjes JTF, Hartmans S. Membrane bioreactors forwaste gas treatment. J Biotechnol 1998;59:155–67.

Ren S, Frymier PD. Estimating the toxicities of organic chemicals tobioluminescent bacteria and activated sludge. Water Res 2002;36:4406–14.

RevahS,Morgan-Sagastume JM.Methods for odor andVOCcontrol. In:Shareefdeen Z, Singh A, editors. Biotechnology for Odour and AirPollution. Heidelberg, Germany: Springer-Verlag 2005. p. 29–64.

Rols JL, Condoret JS, Fonade C, Goma G. Mechanism of enhancedoxygen transfer in fermentation using emulsified oxygen-vectors.Biotechnol Bioeng 2004;35:427–35.

Rosenberg E. Biofilms on water-insoluble substrates. In: CharacklisWG, Wilderer PA, editors. The Structure and Function of Biofilms(Dahlem Workshop Reports). Wiley1989. p. 59–71.

Sander R. Compilation of Henry's Law Constants for Inorganic andOrganic Species of Potential Importance in EnvironmentalChemistry. 1999. (http://www.mpch-mainz.mpg.de/~sander/res/henry.html).

Shareefdeen Z, Singh A. Biotechnology for Odour and Air Pollution.Heidelberg, Germany: Springer-Verlag 2005.

Song J, Kinney KA. Microbial response and elimination capacity inbiofilters subjected to high toluene loadings. Appl MicrobiolBiotechnol 2005;68:554–9.

Spigno G, Pagella C, Fumi MD, Molteni R, de Faveri DM. VOCsremoval from waste gases: gas-phase bioreactor for the abatementof hexane by Aspergillus niger. Chem Eng Sci 2003;58:739–46.

Srivastava P, Hahr O, Buchholz R, Worden RM. Enhancement of masstransfer using colloidal liquid aphrons: Measurement of masstransfer coefficients in liquid–liquid extraction. Biotechnol Bioeng2000;70:525–32.

Tresse O, Lescob S, Rho D. Dynamics of living and dead bacterialcells within a mixed-species biofilm during toluene degradation ina biotrickling filter. J Appl Microbiol 2003;94:849–55.

Van Ede CJ, Van Houten R, Beenackers ACM. Enhancement of gas towater mass transfer rates by a dispersed organic phase. Chem EngSci 1995;50:2911–22.

Van Groenestijn JW, Hesselink PGM. Biotechniques for air pollutioncontrol. Biodegradation 1993;4:283–301.

Van Groenestijn JW, Lake ME. Elimination of alkanes from off-gasesusing biotrickling filters containing two liquid phases. EnvironProg 1999;18:151–5.

Van Hamme JD, Singh A, Ward OP. Recent advances in petroleummicrobiology. Microbiol Mol Biol Rev 2003;67:503–49.

Villaverde S, Mirpuri RG, Lewandowski Z, Jones WL. Physiologicaland chemical gradients in a Pseudomonas putida 54G biofilmdegrading toluene in a flat plate vapor phase bioreactor. BiotechnolBioeng 1997a;56:361–71.

Villaverde S, Mirpuri RG, Lewandowski Z, Jones WL. Study oftoluene degradation kinetics in a flat plate vapor phase bioreactorusing oxygen microsensors. Water Sci Technol 1997b;36:77–84.

Villaverde S, Fernández MT. Non-toluene associated respiration in aPseudomonas putida 54G biofilm grown on toluene in a flatplate vapor phase bioreactor. Appl Microbiol Biotechnol1997c;48: 357–62.

Watkinson RJ, Morgan P. Physiology of aliphatic hydrocarbon-degrading microorganisms. Biodegradation 1990;1:79–92.

Zhao S, Kuttuva SG, Ju LK. Oxygen transfer characteristics ofmultiple-phase dispersions simulating water-in-oil xanthan fer-mentations. Bioprocess Eng 1999;20:313–23.

Zilli M, Daffonchio D, di Felice R, Giordani M, Converti A. Treatmentof benzene-contaminated airstreams in laboratory-scale biofilterspacked with raw and sieved sugarcane bagasse and with peat.Biodegradation 2004;15:87–96.

Zilli M, Guarino C, Daffonchio D, Borin D, Converti A. Laboratory-scale experiments with a powdered compost biofilter treatingbenzene polluted air. Process Biochem 2005;40:2035–43.