APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Aug. 1994, p. 2717-2722 Vol. 60, No. 80099-2240/94/$04.00+0Copyright © 1994, American Society for Microbiology

Design and Performance of a Trickling Air Biofilter forChlorobenzene and o-Dichlorobenzene Vaporst

YOUNG-SOOK OHt AND RICHARD BARTHA*Department of Biochemistry and Microbiology, Cook College, Rutgers University,

New Bnrnswick New Jersey 08903-0231

Received 3 March 1994/Accepted 17 May 1994

From contaminated industrial sludge, two stable multistrain microbial enrichments (consortia) that werecapable of rapidly utilizing chlorobenzene and o-dichlorobenzene, respectively, were obtained. These consortiawere characterized as to their species composition, tolerance range, and activity maxima in order to establishand maintain the required operational parameters during their use in biofilters for the removal ofchlorobenzene contaminants from air. The consortia were immobilized on a porous perlite support packed intofilter columns. Metered airstreams containing the contaminant vapors were partially humidified and passedthrough these columns. The vapor concentrations prior to and after biofiltration were measured by gaschromatography. Liquid was circulated concurrently with the air, and the device was operated in the tricklingair biofilter mode. The experimental arrangement allowed the independent variation of liquid flow, airflow, andsolvent vapor concentrations. Bench-scale trickling air biofilters removed monochlorobenzene, o-dichloroben-zene, and their mixtures at rates of up to 300 g of solvent vapor h-1 m-3 filter volume. High liquid recirculationrates and automated pH control were critical for stable filtration performance. When the accumulating NaClwas periodically diluted, the trickling air biofilters continued to remove chlorobenzenes for several months withno loss of activity. The demonstrated high performance and stability of the described trickling air biofiltersfavor their use in industrial-scale air pollution control.

Over the past decades, chlorobenzenes (CBs) have beenused extensively as solvents, heat transfer agents, insect repel-lents, deodorants, degreasers, and intermediates in dye andpesticide synthesis (11). They have been also used as capacitorfluids to replace polychlorinated biphenyls (22). The extensiveuse of CBs has led to their widespread release into theenvironment, and they have been detected in many surfacewater and groundwater samples, in sewage, and in somebiological tissues. Both CB and o-dichlorobenzene (o-DCB)are chemically stable, and their photochemical degradation insoil and aquatic environments is limited. CB and o-DCB areidentified as priority pollutants by the U.S. EnvironmentalProtection Agency (13).CBs are known to be mineralized under appropriate condi-

tions in the laboratory by bacteria isolated from soil and water.Growth on CB (19), m-DCB (5),p-DCB (21), and o-DCB (10)involves conversion of CBs to chlorocatechols in consecutivedioxygenase and diol dehydrogenase reactions. The chlorocat-echols are further oxidized to chloromuconic acids via modi-fied ortho-ring fission catalyzed by relatively nonspecific cate-chol 1,2-dioxygenase enzymes. These reports suggest thatbiodegradation might be an effective method for the removalof CBs accidentally released into the environment. Namkungand Rittmann (15) reported that biodegradation was theprevalent mechanism for the removal of CB from wastewatercompared with evaporation and adsorption. Therefore, biolog-ical treatment was considered feasible for the removal of CB

* Corresponding author. Mailing address: Department of Biochem-istry and Microbiology, Rutgers University, Cook College, LipmanHall, New Brunswick, NJ 08903-0231. Phone: (908) 932-9739. Fax:(908) 932-8965.

t New Jersey Agricultural Experiment Station publication D-01511-01-94.

t Present address: AgBiotech Center, Cook College, Rutgers Uni-versity, New Brunswick, NJ 08903-0231.

vapors emitted from chemical process industries or during airstripping of contaminated aquifers and soils. To date, airbiofiltration has been used chiefly for the abatement of odorsfrom sewage treatment and composting, but pioneering workon dichloromethane (DCM) removal by Diks and Ottengraf(7-9) was inspirational for our studies. Work presented heredemonstrates that with appropriate process control, biofiltra-tion is capable of the efficient and sustained removal of CBsfrom air and appears suitable for industrial-scale air pollutioncontrol.

MATERIALS AND METHODS

Enrichment and isolation of CB degraders. The bacterialconsortia used in this study were obtained from a contami-nated industrial sludge. Portions (1 g) of the sludge weresuspended in a mineral salts medium containing 4 g ofNa2HPO4, 1.5 g of KH2P04, 1.0 g of NH4Cl, 0.2 g ofMgSO4 * 7H20, and ferric ammonium citrate (5 mg/liter, pH7.0) (1). As the only source of carbon and energy, CB ando-DCB were added at the initial concentrations of 55.3 and 6.5mg liter-', respectively. Erlenmeyer flasks with a 9:1 airspace/liquid ratio were closed with Teflon-wrapped rubber stoppersand were incubated with rotary shaking (200 rpm) at 30°C. Theflasks were aerated daily, and CB and o-DCB were added asneeded. Microbial growth was quantified by protein determi-nations (4). After several serial transfers, stable microbialconsortia developed. The individual members of the consortiawere isolated by streaking on mineral agar and incubationunder solvent vapors. The isolated strains were characterizedwith the Rapid NFT system (Sherwood Medical, Plainview,N.Y.).Measurement of CB and o-DCB utilization in flasks. The

utilization of CB and o-DCB was quantitatively measured bygas chromatographic headspace analysis. Cells grown on min-eral medium with CB or o-DCB were harvested by centrifuga-

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tion (16,000 x g, 20 min), and the cell pellet was resuspendedin a fresh mineral medium. Aliquots (10 ml) of this suspensionwere placed into serum bottles (160 ml) closed with Teflon-faced silicon rubber septa and aluminum crimp caps. Theserum bottles were incubated on a rotary shaker (200 rpm) at30°C. Liquid volume and shaking were designed to avoid ratelimitation by oxygen transfer. Utilization of CB or o-DCB wasmonitored by taking 100-,ul samples of the headspace of theserum bottle with a gastight syringe (Hamilton, Reno, Nev.)and determining the CB or o-DCB content by gas chromatog-raphy, with a Hewlett-Packard Model 5890 instrumentequipped with a flame ionization detector and a 30-m capillarycolumn (DB-608; J & W Scientific, Folsom, Calif.). Operatingconditions were as follows: injector, 150°C; oven, 150°C;detector, 250°C; nitrogen carrier gas flow rate, 6 ml min-1;retention times, 1.99 min (CB) and 4.94 min (o-DCB). Stan-dard curves were generated by evaporating completely mea-sured amounts of CB or o-DCB in a known volume of air. Allexperiments were performed in duplicate, and any abioticlosses of CB or o-DCB were subtracted by sampling anidentical but uninoculated bottle. The utilization of CB oro-DCB was also confirmed by biomass production and chloriderelease. Liquid samples for these purposes were also removedby syringe through the septum. Biomass was calculated fromprotein (4) by use of factor 2. Chloride concentration wasmeasured as described by Bergmann and Sanik (3).

Effect of pH and salinity on CB and o-DCB utilization. Thebiodegradation of haloaromatics produces acidity and chlorideions, which may accumulate and poison the degrading micro-organisms. To assure continued microbial activity in biofiltersfor removal of CBs, the tolerance range of the degradingconsortia had to be determined and, subsequently, processcontrol measures had to be taken to keep conditions within thedetermined tolerance ranges. The measurements of tolerancewere performed in sealed serum vials as described in theprevious section. For measurement of the pH effect on CB ando-DCB removal, washed consortium bacteria were suspendedin phosphate buffers in the pH range of 4.0 to 10.0. Initialbiomass for CB and DCB consortia was 137 and 548 mgliter- 1, respectively; CB and o-DCB were added at 0.1 and 0.05,ul ml-', respectively. Removal of CB and o-DCB vapor fromthe headspace of the serum vials was measured during incu-bation, with shaking (200 rpm) at 30°C. The amounts (milli-grams) of CB or o-DCB removed per gram of biomass perhour were measured as specific activity and were plottedagainst the pH value. The pH shift during these experimentsfrom the set initial value was less than 0.5 U.

For experiments on salt tolerance, washed consortium cellswere suspended in potassium phosphate buffer (pH 6.5, opti-mal for both the CB and the DCB consortia) and NaCl wasadded in the range of 0 to 1,000 mM. Initial biomass in theseexperiments was 109 and 548 mg liter-' for the CB and theDCB consortia, respectively. As before, CB and o-DCB wereadded at 0.1 and 0.05 ,ul ml-1, respectively. Specific CB ando-DCB removal activities were calculated compared with thespecific activities measured at 0 mM NaCl.Measurement of CB and o-DCB removal in trickling air

biofilters. The removal of CB and o-DCB vapors from air-streams was measured with the arrangement shown in Fig. 1.This type of device is a trickling air biofilter designed accordingto the work of Ottengraf (17). For preparing a filter, a sufficientamount of dry perlite (Grace & Co., Cambridge, Mass.) wasweighed out to pack 5-cm-diameter glass columns. The perliteoccupied 59% of the total column volume (1,570 ml). Micro-bial consortia pregrown on CB or o-DCB were harvested bycentrifugation and suspended in fresh mineral medium. The

AIRPUMP

FIG. 1. Schematic diagram of a trickling air biofilter with pHcontrol and water recirculation. For details, see the text.

perlite packing received sufficient microbial suspension to fill50% of the available pore space, and the glass columns werepacked with this moistened packing material. CB or o-DCBvapor was passed through the packed columns with airstreamsgenerated by aquarium pumps (Wisper 700; Willinger Bross,Inc., Oakland, N.J.). Split airstreams passed through eitherwater or solvent traps and were combined to pass through thefilter column. The split airstreams were metered by low-capacity rotameters (Gow-Mac, Bridgewater, N.J.) to allow theindependent variation of the total airflow and CB or o-DCBconcentration in the air stream. Concurrently with the airflow,mineral medium (500 ml) diluted with distilled water (1:9[vol/vol]) was circulated with a peristaltic pump (Manostat,New York, N.Y.). At any one time, about a quarter of thisvolume was on the column and three quarters were in thereservoir. The pH of the liquid was adjusted by an automaticpH control module salvaged from a defunct fermentor (NewBrunswick Scientific, Edison, N.J.) with NaOH to remain inthe neutral range. The trickling air biofilters were operated atambient room temperatures of 20 to 25°C. CB or o-DCBconcentrations in the airstreams were measured prior to andafter exit from the trickling air biofilter column and also atseveral sampling ports along the column, by gas chromatogra-phy. By knowing the concentration drop and the volume of airpassing through the column, CB or o-DCB removal rates couldbe calculated. Removal rates were expressed as grams ofsolvent removed per hour per cubic meter of filter volume.

RESULTS

CB and o-DCB utilizers and their substrate ranges. Twobacterial consortia were obtained by serial transfers on CB ando-DCB and were designated CB and DCB consortium, respec-tively. The CB consortium consisted of nine distinct microbialstrains, but only three of these were capable of growingindividually on CB as sole carbon and energy source. All threewere oxidase-positive gram-negative rods and were identifiedas Pseudomonas spp. Two of these were able to grow also onbenzene and on toluene, but none of the three strains grew onxylene isomers or on o-DCB. The CB consortium was notinhibited by CB concentrations as high as 330 mg liter-' andshowed a growth yield (Y) on CB of 0.29.Three distinct microbial strains were isolated from the DCB

consortium, and one of the three strains was an oxidase-positive gram-negative rod that was capable of growing on

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160- 60~ 60- 60~r A

~40 40~E 940- -40

20 120 -2 20.20-

0 0 0 00 1 0 1 2

Time (h) Time (h)

FIG. 2. Removal of CB (a) and o-DCB (b) vapor (0) from theheadspace of a serum bottle and the simultaneous production ofchloride (0). CB and o-DCB were added at 0.2 and 0.5 p.l to 10 ml ofmineral medium, respectively.

o-DCB as only carbon and energy source with a growth yield(Y) of 0.12. This isolate was identified as a Pseudomonas sp.and was capable of growing also on CB and on benzene but noton toluene or xylenes. o-DCB was more toxic than CB, ando-DCB concentrations in excess of 130 mg liter-' inhibited thegrowth of the DCB consortium. Figures 2a and b show theutilization of CB and o-DCB by the CB and DCB consortia,respectively. Disappearance of CB was balanced by the stoi-chiometric release of chloride, strongly suggesting the com-plete mineralization of CB. Only 70% of the expected chloridewas released during the disappearance of o-DCB. Apparently,the mineralization of degradation products was not completeduring this 2.5-h experiment. The CB and DCB consortiatolerated higher solvent concentrations and showed higherremoval efficiencies than the pure cultures isolated from them(data not shown). Therefore, the following flask and biofiltra-tion experiments were conducted with the consortia ratherthan the isolates. Although the DCB consortium was capableof utilizing CB, it did so with only 50% specific activitycompared with the CB consortium, and o-DCB was preferredto CB when both substrates were present as a mixture (datanot shown). The CB consortium was unable to utilize o-DCB.Therefore, a mixture of the CB and DCB consortia was usedwhenever a mixture of CB and o-DCB had to be removed.

Effect of pH and salinity on CB and o-DCB utilization. Theremoval of both CB and o-DCB had broad pH optima with80% or more of the maximal activity observed between pH 5.0and 8.5. However, at pH extremes, o-DCB removal activitydeclined more sharply than CB removal activity (data notshown). o-DCB removal was also more sensitive to elevatedchloride ion levels than CB removal. At 200 mM NaCl, 80% ofthe CB removal activity was preserved, but only 40% of theo-DCB removal activity was (data not shown). From theseexperiments, it was concluded that in a biofilter columndesigned for removal of CB, o-DCB, or both, provisions had tobe made to keep pH within the 5 to 8.5 range and NaCl at leastbelow 200 mM, but preferably below 100 mM. The mainte-nance of these parameters on a biofilter column was attainedby adding a liquid circulation system with an automatic pHcontrol unit and by a periodic dilution of the recirculationliquid.Removal of either CB or o-DCB vapors in a trickling air

biofilter. Figure 3 shows the removal of CB vapor in a tricklingair biofilter with pH control compared with an identical butuninoculated filter and a filter without pH control. For thethree sets of experiments, filter volume (5 by 20 cm), superficialvelocity (11 m h-1), CB vapor concentration (1.2 g m-3) and,

§-

c

80^

40

20-

0 _ _12 14 16 18

Time (day)

FIG. 3. Removal of CB vapor in an inactive trickling air biofilter(-), in an active trickling air biofilter without pH control (0), and inan active trickling air biofilter with pH control (-).

in the active filters, the initial biomass were adjusted to thesame values. The removal rate of CB vapor obtained in thetrickling filter with pH control was 5.1 g m-3 h-1. Similarexperiments were performed also with o-DCB vapor removal,with the DCB consortium (data not shown). Superficial veloc-ity of the air was 11 m hV1, and the concentration of o-DCB inthe air was 0.3 g m-3. The removal efficiency of o-DCB vaporwas 85.4%, and the removal rate was 14.1 g m-3 h-1. Theserelatively low removal rates were a consequence of low inoc-ulum size and low liquid circulation rate.Removal of CB-o-DCB mixtures in trickling air biofilters.

Removal of CB-o-DCB vapor mixtures was performed in acolumn packed with perlite. Microbial suspensions of the CBconsortium and the DCB consortium were prepared and mixedtogether to be used as the inoculum of the trickling air biofilter(5 by 80 cm). The biomass (dry weight) of the two suspensionswas adjusted to the same value (0.99 g liter-1, each). Ascalculated from CB consumption and yield coefficient, thisinitial biomass may have increased during the first week ofoperation to 1.57 g in the total column (0.67 volume %). Afterthis initial increase, there was little further change in biomassbecause of mineral nutrient limitation (20).

Superficial velocity of the air was 9.2 m h-1, and the flowrate of the recycling water was 0.04 liter h-1. The concentra-tion of CB-o-DCB vapor mixtures was in the range of 1.12 to4.8 g m-3, and the concentration ratio of CB and o-DCB wasmaintained at around 1:1. Figure 4 shows the removal of CBvapor during the removal of CB-o-DCB vapors in a tricklingair biofilter. The concentrations of CB-o-DCB vapors weremeasured at the inlet, at the middle point of the filter, and atthe exit. The change in the CB concentration of the incomingair was plotted, and the bars represent the percent removal ofCB vapors by each section of the trickling air biofilter. At lowCB concentration, the half of the filter close to the inlet(section 1) completely removed CB vapor from the incoming

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80- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~3

*|~~~~~~~~160-~~~~~~~~~~

o o0.5 1 1.5 2 2.5 3 3.5 4 5 6 7 8 9 10

Time (day)

FIG. 4. Removal of CB vapor from a CB-o-DCB vapor mixture ina separated trickling air biofilter. The curve (U) shows concentrationof CB in the inlet during operation of the trickling air biofilter.Removal of CB vapor by section 1 (M) and section 2 (El) wasmonitored. Water flow rate was initially 0.04 liter h-l and wasincreased to 0.72 liter h-1 on day 3 (see arrow).

air and the other half of the filter (section 2) close to the exitwas not exposed to CB vapor. When the concentration of CBwas increased to 2.5 g m-3 on day 2, only 50% of the incomingCB was removed in section 1 and yet section 2 failed to showany removal activity. As during column packing microbialbiomass was evenly distributed, the absence of removal activityin section 2 was most likely due to low pH. The recycling waterwas collected and neutralized at the bottom of the filter andreintroduced to the top of the filter. This configuration couldbe expected to establish a decreasing pH gradient in the flowdirection of the recycling water along the filter. Therefore, at alow liquid recirculation rate (0.04 liter h-1), section 2 wascontinuously exposed to low pH, restricting its biofiltrationactivity. In fact, the pH of the recirculated liquid prior toneutralization during days 1 to 3 was 4.0 to 5.0, i.e., below thefavorable range for CB and o-DCB removal. When after day 3the recycling rate was raised to 0.72 liter h-1, CB removal insection 2 became detectable. At the same time, CB removalincreased also in section 1 of the column. Removal of o-DCBvapor showed a pattern similar to CB removal (data notshown). This experiment indicated that water recirculation rateis closely tied to effective pH control and maintenance ofbiological activity in the trickling air biofilter.

Subsequently, the effect of liquid recycling rate on removalof CB-o-DCB vapor mixtures was systematically explored andis shown in Fig. 5. In this experiment, the superficial velocity ofair was fixed at 102 m h-1, and the rate of liquid recycling wasvaried. At each liquid recycling rate, the concentration ofCB-o-DCB (1:1 ratio) in the air was raised until removalactivity ceased to increase or started to decline. At this time,liquid recirculation rate was increased, and the previouslydescribed process was repeated. At the highest liquid recircu-lation rate tested (3.2 liter h-1), the trickling air biofilterremoved in excess of 200 g of CB-o-DCB mixture h-1 m-3filter volume on a sustained basis. When the 3.2 liter h-l liquidrecirculation rate was used from the start of the experiment(data not shown), CB-o-DCB removal rates of over 300 g h-1m-3 were achieved. At slow liquid circulation rates, the HClproduced in the filter could not be removed and neutralizedfast enough, and the microbial population was inhibited byacidic conditions. Therefore, the highest performance of atrickling air biofilter for CB-o-DCB removal can be achievedat high liquid flow rates which assure the removal and neutral-ization of the HCl as soon as it is produced.As the neutralization process produces NaCl and previous

200-

I-

0

50-

,, II I

0 0.5 1 1.5 2Concentration (g m-3)

FIG. 5. Removal of CB-o-DCB vapors in a trickling air biofilter.Concentration of CB-o-DCB vapors was varied in the range of 0.1 to3.02 g m-3, and removal rate of CB-o-DCB vapors was measured atthe water flow rates of 0.04 (U), 0.72 (A), 1.8 (@), and 3.2 (0) literh-V.

experiments showed that high NaCl concentrations were in-hibitory to biofiltration activity, it was necessary to dilute orexchange the circulation water periodically. During the re-moval of CB-o-DCB vapors in the trickling air biofilter, theconcentration of NaCl in the recirculation water was moni-tored and the removal rate of CB-o-DCB vapors was calcu-lated (Fig. 6). In this experiment, the superficial velocity of theair was 97 m h-' and CB-o-DCB concentration was 0.95 gm-3. Volume of the recirculating liquid was 0.5 liter, and waterflow rate was 3.2 liter h-1. The trickling air biofilter showed asharp decrease in its activity above 200 mM NaCl. During thefirst 2 days, 2.26 g of chloride was released from degradation ofCB-o-DCB vapors, and this amount corresponded to 72% ofthe calculated chloride in the CB-o-DCB removed. The miss-ing chloride was probably retained by the filter bed and othercomponents of the apparatus. In our experiments, the recyclingwater was exchanged with diluted mineral medium wheneverthe NaCl concentration reached 200 mM. In a full-scaletrickling air biofilter, it would be more advantageous to usecontinuous and automated dilution of the recirculated liquid toa preset electrical conductivity and to drain the excess liquid byvolume sensors (8, 9).

DISCUSSION

Characteristics and substrate range of the CB-utilizingconsortia and cultures. The utilization of both CB and o-DCBby isolated microbial cultures has been reported previously,and their biodegradation pathways have been described (10,19). We have no evidence that our consortia used differentpathways of degradation. Our consortia were dominated byPseudomonas strains, and CB utilizers isolated previously werealso identified as Pseudomonas or Alcaligenes species (14, 16,19). From a practical point of view, it is important for

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3

Time (day)

FIG. 6. Removal of CB-o-DCB vapors in a trickling air biofilter.Removal rate (0) of CB-o-DCB vapors and production of chloride(0) were monitored. The removal rate decreased sharply above 200mM NaCl.

biofiltration that the consortia we obtained are relativelyresistant to elevated CB concentrations. While in previousreports CB at 100 mg liter-' prevented growth (19), our CBconsortium tolerated CB at 330 mg liter-' and the DCBconsortium tolerated o-DCB at 130 mg liter-'. Both consortiaand the pure cultures isolated from them were free of require-ments for yeast extract or other growth factors. Two of thePseudomonas strains isolated from our CB consortium grew onboth CB and toluene. The simultaneous utilization of CBs andmethylbenzenes, such as toluene and xylenes, by CB-degradingbacteria is rare. Pettigrew et al. (18) described a Pseudomonassp. (JS6) which degraded toluene and CB simultaneously.The CB consortium showed a greater tolerance to extremes

of pH and to high salinity than the DCB consortium. Thetolerance of the DCB consortium more closely resembledresults reported previously for DCM degraders. Hartmans andTramper (12) reported that 200 mM NaCl decreased DCMutilization by three bacterial strains to 20 to 40% of themaximal rate, and Diks (7) reported a 50% decrease in DCMutilization by Hyphomicrobium spp. at 200 mM NaCl. Noutilization was observed above 500 mM NaCl. Both selectivityfor and cross-utilization of CB and o-DCB were reported inearlier studies. The CB degrader described by Reineke andKnackmuss (19) was unable to utilize o-DCB, but the Pseudo-monas strains described in the studies of Haigler et al. (10)utilized CB and o-DCB at similar rates. Nishino et al. (16)reported that Pseudomonas putida strains isolated on CB alsogrew on o-DCB, but at much slower rates than on CB.

Biofilters for control of odors from sewage treatment or

composting do not require specific microbial inocula (17). Inbiofiltration of potentially biogenic solvents such as methanol,a metabolic inhibitor was required to create a biologicallyinactive control filter (20). However, in the case of clearlyxenobiotic solvents such as CB, o-DCB, and DCM, withoutspecifically selected microbial inocula no measurable biofiltra-tion activity develops within a reasonable time period (Fig. 3).Inoculation of the biofilter may be accomplished by adding a

microbial suspension as outlined previously. Immobilization ofthe suspended bacteria, in our case, was a spontaneous pro-cess. The microbial biomass actively adhered to the solidsupport, leaving the recirculating liquid essentially clear. Thus,dilution of the recirculating liquid for NaCl removal did notwash out biomass. Inoculation of new filter columns could beaccomplished quite effectively also by mixing approximately5% old packing material with biomass into the solid support ofa new filter column.

Liquid circulation rates and CB-o-DCB removal in tricklingair biofilters. The strong effect of liquid circulation rate onbiofiltration performance evident in Fig. 5 was observed also instudies with other chlorocarbon volatiles, particularly DCM (8,9, 12). Hartmans and Tramper (12) observed a linear relation-ship between liquid circulation velocity and DCM removal in atrickling air biofilter. However, at low liquid flow rates, theauthors did not observe significant differences between the pHof the entering liquid and that of the outflowing liquid.Therefore, they concluded that the low filter performance atslow water circulation rates was caused not by a decrease in pHbut by a mass transfer resistance (transfer of DCM from thegas phase to the aqueous biofilm). This theory does not explainour result shown in Fig. 4. Between days 2 and 3, section 1 ofthe column removed only 50 to 60% of the incoming CB vapor.At this time, section 2 of the column was exposed to similar CBconcentrations as section 1 was between days 1 and 2 and yetfailed to remove any measurable amounts of CB. This is notexplainable by mass transfer resistance. When on day 3 theliquid flow rate was increased to 0.72 liter h-1 at essentiallylevel CB concentrations, section 2 showed a gradual ratherthan an instantaneous increase of CB removal. This result wasconsistent with a gradual improvement in pH control, while adecrease in mass transfer resistance would have produced aninstantaneous effect.

Diks and Ottengraf (8, 9) also reported that water flow ratesstrongly influenced the removal of DCM in a trickling airbiofilter and proposed that an axial (longitudinal) pH gradientin the filter column was partially responsible for this phenom-enon. Our analysis of filter performance under low liquidrecirculation rates (Fig. 4) showed that only the proximalportion of the column was active, lending support to the axialpH gradient explanation.Our results demonstrate that selected microbial consortia

immobilized on a suitable porous support can remove from airCB and o-DCB vapors with high efficiency. With appropriateprocess controls such as pH adjustment and salt removal,biofiltration activity could be maintained for long periods oftime. Biofiltration shows great promise for the control ofvolatile organic emissions from chemical manufacturing pro-cesses and also from the air stripping type of subsoil remedia-tion activities (2, 6). Advanced biofiltration technology isexpected to play an important part in meeting the goals of the1990 Clean Air Act Amendment.

ACKNOWLEDGMENTS

This work was supported by a grant (BICM 31) from the HazardousSubstances Management Research Center and by New Jersey statefunds.

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