Development of a membrane-aerated biofilm reactor to completely mineralise perchloroethylene in wastewaters

Biodegradation of PCE in a Hybrid Membrane AeratedBiofilm Reactor

Dieudonné-Guy Ohandja1 and David C. Stuckey2

Abstract: A continuous flow flat sheet hybrid membrane aerated biofilm reactor �MABR� was used to treat a synthetic wastewatercontaining perchloroethylene �PCE�; 1.25–2.5 g chemical oxygen demand �COD�/L of glucose was also added to the synthetic waste-water as a source of COD representative of a real wastewater. The reactor was able to biodegrade 70 mg L−1 of PCE in 9 h without theaccumulation of any intermediate compounds, resulting in a removal rate of 247 mmol of PCE h−1 m−3 in a reactor with a specificmembrane area of 4.048 m2 m−3. MABRs have never been used before for PCE degradation, and this rate is one of the highest volumetricPCE degradation rates reported in the literature. COD removal was also good and varied from 85 to 92%. Since very few volatile fattyacids accumulated in the system, most of the residual COD was attributed to soluble microbial products as reported by previousresearchers. A mass balance on chloride during this study showed that only 72–81% of it could be accounted for. It is probable that someof the chlorinated ethenes were adsorbed onto the biofilm or that aerobic intermediates of low-chlorinated compounds such as trichloro-ethanol, dichloroacetyl, and chloroacetaldehyde were produced in the system. Nevertheless the chloride mass balance in this workcompares well with the literature. Due to their high PCE and COD removal rates, hybrid MABRs have the potential to be used for anumber of refractory organics which require combined anaerobic/aerobic biological treatment for degradation.

DOI: 10.1061/�ASCE�0733-9372�2007�133:1�20�

CE Database subject headings: Biodegradation; Biofilm; Membranes; Reactors; Hybrid methods.

Introduction

Perchloroethylene �PCE� is commonly used in the dry-cleaningand degreasing industries as a solvent. Due to improper disposal,spills, and incomplete degradation, PCE and its biodegradationintermediates are among the most commonly encounteredgroundwater pollutants. Some chemical methods, such as the useof surfactant modified zeolite/zero-valent iron pellets �Zhanget al. 2002�, have been used to degrade PCE, however, in mostcases their use is precluded due to the costs they incur. In con-trast, biological degradation of PCE is often used due to its lowcost. It has been reported that PCE can only be biodegraded underanaerobic conditions, however, degradation of PCE under anaero-bic environments often results in the accumulation of less chlori-nated biodegradation intermediates such as trichloroethylene�TCE�, dichloroethylene isomers �DCEs�, and vinyl chloride�VC�. These biodegradation intermediates are quite toxic, VCbeing even more toxic than PCE, however, evidence of their bio-degradation under aerobic conditions has been documented �Vliegand Janssen 2001�. Although some cases of complete mineraliza-

1Ph.D. Student, Dept. of Chemical Engineering and ChemicalTechnology, Imperial College London, Prince Consort Rd., London SW72AZ, UK. E-mail: [email protected]

2Professor of Biochemical Engineering, Dept. of ChemicalEngineering and Chemical Technology, Imperial College London, PrinceConsort Rd., London SW7 2AZ, UK �corresponding author�. E-mail:[email protected]

Note. Discussion open until June 1, 2007. Separate discussions mustbe submitted for individual papers. To extend the closing date by onemonth, a written request must be filed with the ASCE Managing Editor.The manuscript for this paper was submitted for review and possiblepublication on January 4, 2005; approved on July 10, 2006. This paper ispart of the Journal of Environmental Engineering, Vol. 133, No. 1,

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tion of PCE have been observed under anaerobic conditions, oftenan aerobic polishing step is required in order to degrade the lowconcentration of chlorinated biodegradation intermediates.

Different approaches have been proposed in order to provideboth aerobic and anaerobic conditions for the complete mineral-ization of PCE. Gerritse et al. �1997� achieved complete mineral-ization of PCE using an anaerobic reactor followed by an aerobicunit, while Tartakovsky et al. �1998� used coimmobilization ofanaerobic and aerobic organisms on granules in the same reactor.Both studies used air bubbling, however, this method can result inproblems such as gas stripping of the volatile organic compounds�VOCs� into the environment.

Membrane-aerated biofilm reactors �MABRs� are commonlyused as an alternative to conventional aeration methods �Coteet al. 1988�. These reactors have the potential to improve aerationand overcome some of the problems associated with conventionalaeration as they provide bubbleless aeration, thus avoiding thestripping of volatile compounds. The membrane used in the reac-tor acts as a means of providing oxygen, and as a support for anactive biofilm. MABRs are capable of maintaining both high con-centrations of active bacteria in the biofilm, as well as a high rateof oxygen transfer �Stephenson et al. 2000�. It has also been re-ported that the development of a biofilm on the gas-permeablemembrane, combined with appropriate control of gas pressureinside the membrane, reduces the stripping of volatile pollutantsinto the gas phase �Semmens 1991�. Xylene, a highly volatilecompound, has been successfully degraded in a MABR �Debusand Wanner 1992�, and evidence of complete TCE degradation ina MABR has been documented by Clapp et al. �1999�. Moreover,the MABR is potentially capable of combining both anaerobicand aerobic layers in the biofilm, enabling both aerobic andanaerobic processes to occur simultaneously in the same reactor.Processes necessitating both anaerobic and aerobic conditions

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occur in this reactor �Semmens et al. 2003�, and these processescan be associated with chemical oxygen demand �COD� removal.To our knowledge there are no reports in the literature of PCEbeing mineralized in a MABR. Since a MABR can enhance PCEdegradation �minimize volatility, and provide both aerobic andanaerobic conditions in one reactor�, it seems logical to apply thisreactor design for this purpose. Therefore, this study examinedthe use of a dead-ended flat sheet MABR to provide both anaero-bic and aerobic environments in the same reactor for PCE degra-dation, and evaluated the effect of system parameters on PCEdegradation efficiency.

Material and Methods

Source of Inoculum

The initial biomass was collected from a municipal wastewaterplant �Mogden, West London� and acclimated in laboratory-scalereactors. The acclimation procedure was carried out as follows:for the anaerobic biomass, digested sludge from the municipalwastewater plant was screened and inoculated into a suspendedgrowth batch reactor under anaerobic conditions. The reactor wasfed each week with PCE �2 mg L−1� and glucose�1.25 g COD L−1�, and PCE biodegradation intermediates weremonitored to see whether the bacteria were acclimatizing. For theaerobic biomass, the screened sludge was inoculated into an aer-ated suspended-growth batch reactor, and TCE �2 mg L−1� wasadded regularly, while 1.25 g COD L−1 of glucose was addeddaily. This phase lasted for more than 6 months before the cul-tures were used for inoculation. The biomass used to seed theMABR reactor was an equal volume mixture of the anaerobicallyand aerobically acclimated sludge.

MABR Setup

A laboratory-scale membrane reactor was made primarily ofstainless steel �Fig. 1�. The reactor had two compartments; a liq-uid one �the upper compartment� with a working flow throughvolume of 190 mL which contained sampling ports, and one ofair �the lower compartment� of 192 mL which was dead end. Thesynthetic wastewater was pumped through the upper compartmentwhile the lower compartment was connected to either an air oroxygen cylinder to maintain a positive gas pressure, and this pres-sure was monitored by a low pressure meter. A microporous

Fig. 1. Schematic of experimental setup of flat sheet membraneMABR

membrane made of polypropylene �3M Corporate Process Tech-

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nology Center, Minneapolis� separated the two compartments.The membrane had the following characteristics; a thickness of1.4 mm, bubble point pore diameter of 0.36 �m, and an air per-meability of 6355 g m−2 h−1 at 8.6125 kPa. The specific area ofthe membrane was 4.048 m2 m−3, and it lay on top of a stainless-steel support screen �Millipore� to keep it flat. The connections tothe reactor were either in stainless steel or Teflon to avoid the lossof volatile compounds, and the reactor was fitted with a dissolvedoxygen probe on the liquid side. The gas produced during me-tabolism was partially vented through a gas sampling outlet port�10 mL dead gas volume� which was an opening on the lid of thereactor sealed with a Teflon coated septum. The air flow into thereactor was measured with a rotameter periodically, and wasfound to be around 5 mL min−1.

Reactor Startup

The MABR was seeded with 100 mL of acclimated sludge with atotal volatile suspended solids �TVSS� of around 2000 mg L−1 ,and made up to volume with feed media, and air was supplied tothe reactor via the lower compartment. A low pressure of0.689 kPa �gauge� was maintained in the gas compartment usinga pressure-regulating valve to prevent air bubbling through themembrane. The reactor was operated in batch mode for 5 days toallow for biofilm development on the membrane, and during thisperiod only glucose was fed to the reactor. After 5 days PCE wasintroduced into the liquid compartment via a pump from the feedtank, initially at a concentration of 5 mg L−1, and the reactor wasswitched to a continuous mode. Glucose was used as the growthsubstrate, and was pumped from the biomedium reservoir at aninitial concentration of 1.25 g COD L−1; the concentration usedwas based on past work to determine the optimal amount of glu-cose required for PCE degradation �Ohandja and Stuckey 2006�.The hydraulic retention time was 48 h �4 mL h−1� and the recyclerate was 70 mL min−1, resulting in completely mixed hydraulicconditions in the reactor. The feed composition was �per liter ofdeionized water�: 0.65 g K2HPO4; 0.20 g NaH2PO4·2H2O;0.44 g NH4HCO3; 0.11 g CaCl2 ·2H2O; 0.10 g MgCl2 ·6H2O;3.73 g NaHCO3; 0.1 g yeast extract; 1 mL of trace elements;2 mL of vitamin solution; and 1.25 g glucose as COD �modifiedfrom the nutrient solution used by Holliger et al. 1993�. The vi-tamin solution contained �per liter of deionized water�: 0.02 gbiotin; 0.05 g folic acid; 0.5 g pyridoxine; 0.1 g riboflavin; 0.2 gthiamine; 0.1 g panthothenic acid; 0.2 g nicotinic acid; and 0.1 gamino benzoic acid. The trace elements solution contained �perliter of deionized water�: 2 g FeCl2 ·4H2O; 0.1 g MnCl2 ·4H20;0.19 g CoCl2 ·6H2O; 0.07 g ZnCl2; 0.002 g CuCl2; 0.01 gAlCl3 ·6H2O; 0.006 g H3BO3; and 0.036 g Na2MoO4.

Experimental Procedure

During the acclimation period �Days 0–140�, the inlet PCE con-centration was gradually increased until it reached 70 mg L−1.When an accumulation of intermediates occurred at Day 33 theinitial glucose concentration of 1.25 g L−1 in the feed was in-creased to 2.5 g L−1 �as COD� to assess whether increasing con-centrations of electron donor would enhance degradation rates.This was followed by a change in the gas providing oxygen; airwas changed to an oxygen-rich gas �30% O2: 70% N2� on Day100, and finally to pure oxygen on Day 126. Again, this was doneto assess whether increased amounts of oxygen enhanced the deg-radation of PCE degradation intermediates �Ohandja and Stuckey

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was reduced gradually from the original 48 h to 36, 24, 12, and9 h over Days 125–234. These changes were made only whenthere were virtually no chlorinated intermediates left in the efflu-ent. At an HRT of 6 h, the reactor did not achieve steady state,and hence the HRT was increased back to 9 h. During the HRTstudies, the influent PCE concentration was maintained at70 mg L−1.

Analytical Procedures

The gas-phase concentrations of chlorinated compounds �PCE,TCE, cis-DCE, VC� and ethylene in the headspace of the uppercompartment and in the gas within the lower compartment of thebioreactor were determined using a Pye Unicam gas chromato-graph equipped with a flame-ionization detector �FID� and a8 ft�1/8 in. stainless-steel column packed with Haysep Q �80–100 mesh porous polymer-Alltech�. The injector temperature was260°C and the detector was 300°C. The column was held at100°C for 2 min and then increased to 200°C at the rate of16°C min−1. 500 �L was taken from either the upper compart-ment headspace or from the gas within the lower compartmentand injected into the gas chromatograph �GC�; the limits of de-tection for PCE, TCE, cDCE, VC, and ethylene were: 0.07, 0.3,0.013, 0.16, and 1.8 �mol L−1, respectively. Aqueous concentra-tions of these compounds were calculated using the dimensionlessHenry’s law constants as reported in Gossett �1987�. The coeffi-cient of variance �COV� for all VOC measurements was ±12%,and peak separation was good. COVs were average values forreplicate measurement of ten identical samples for quality control.Dissolved oxygen �DO� was measured using a 703P meter �Uni-probe� equipped with a Uniprobe oxygen probe. Chloride ionmeasurement was carried out using an ion chromatograph �Di-onex DX-120 with an IonPAC AS114 4�250 mm column�. Themobile phase was 3.5 mM Na2CO3 and 1.0 mM NaHCO3 at1.1 mL min−1, and the COV was ±5%. COD was measured usingthe “closed reflux, colorimetric method” described in APHA�1992� and the COV was ±5%. Glucose and volatile fatty acids�VFAs� were measured using a Shimadzu �model 10A� high-pressure liquid chromatography �HPLC� system with AminexHPX-87H ion exclusion column �Biorad�, methane was measuredusing a GC-thermal conductivity detector �TCD� according to themethods used by Aquino and Stuckey �2004�, and the COVs were±10, 6, and 5%, respectively.

Results and Discussion

PCE Degradation

An increase in PCE concentration in the influent resulted in theappearance of PCE and biodegradation intermediate compoundsin the effluent �Fig. 2�, indicating that the biomass present in thereactor initially could not completely mineralize PCE. In MABRsbiofilms are generally layered, and theoretically comprise a com-plete aerobic layer for aerobic oxidation processes, an intermedi-ate anoxic layer, and a completely anaerobic layer on the top ofthe biofilm �Casey et al. 1999�. Since the DO in the bulk liquid inthis MABR was zero �data not shown�, it is probable that such alayered biofilm had developed on the membrane. It is also knownthat PCE and TCE can be dechlorinated under anaerobic condi-tions, and PCE yields TCE, cDCE, VC, and ethylene as interme-diate biodegradation products �Fathepure and Vogel 1991�. There-

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appearance of cDCE, VC, and ethylene from the reactor indicatethat anaerobic biodegradation pathways were functional in thesystem.

All the intermediate compounds such as TCE, cDCE, VC, andethylene appeared in the effluent; however, both TCE and VCwere not present for long. The transient accumulation of TCE,cDCE, and VC in the reactor may be attributed to step increasesin PCE loading, stimulating transient accumulation of anaerobicand aerobic chloroethene-metabolizing biomass within the reactoruntil steady-state conditions were reestablished. Since cDCE wasthe main biodegradation intermediate accumulating in the system,it is possible that the cDCE dechlorinaters were not concentratedenough to sustain cDCE mineralization. However, the presence ofVC and ethane indicates that cDCE-dechlorinating bacteria wereactive, at least to a limited extent. Under the operating conditionsof this reactor, several factors could explain the apparent lowconcentration of cDCE-dechlorinating biomass. These factors arerelated to the biomass present in both the anaerobic and theaerobic layers of the biofilm. On the one hand, under anaerobicconditions many researchers have observed that PCE degradationresults in the accumulation of intermediate compounds such ascDCE and VC. These less chlorinated ethenes have a slow deg-radation rate under anaerobic conditions, and the cDCE accumu-lation observed in this experiment could be due to this reason. Onthe other hand, the limited aeration provided to the reactor couldhave hampered the growth of aerobic organisms which can useless chlorinated ethenes as a source of carbon and energy, thuslimiting their catabolic action on cDCE. Poor cDCE removalcould also be due to cytotoxicity caused by aerobic cometabolism�Clapp et al. 1999�.

Previous researchers have proved that during aerobic come-tabolism bacteria could grow on different substrates such as meth-ane, ethylene, and toluene, and synthesize oxygenases which candegrade less chlorinated ethenes such as TCE, cDCE, and VC�Arp et al. 2001; Koziollek et al. 1999; Coleman et al. 2002�.However, it has also been shown that aerobic cometabolism ofchloroethenes is constrained by several factors such as: competi-tive inhibition between growth and cometabolic substrates;reductant consumption during chloroethene oxidation;and product toxicity exerted by transient intermediates ofchloroethene oxidation, with an overall consequence of a decreasein the cometabolism rate or inactivation of the cells �Alvarez-Cohen and Speitel 2001�. Simultaneous production of methaneand ethylene in this reactor could have induced the synthesis of

Fig. 2. VOCs profiles in upper compartment of MABR duringacclimation period �influent PCE concentration at top�. VOCconcentrations are in aqueous phase: �1� indicates use of30%O2:70%N2 gas; �2� indicates use of pure oxygen.

methane and ethylene monooxygenases, respectively. Methane

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and ethene monooxygenases catalyze the cometabolic oxidationof TCE and cDCE and VC �Arp et al. 2001; Koziollek et al. 1999;Coleman et al. 2002�.

In this study, supplying pure oxygen resulted in the almostcomplete removal of cDCE �99%�, with an increase in the Cl−

released in the effluent. Such a result indicated that oxygen pro-moted the growth of aerobic bacteria capable of degrading thecDCE by either direct metabolism or cometabolism. It is possiblethat the layered biofilm had provided microniches with high aero-bic activity where cDCE degradation was occurring. Similar ob-servations have been made by previous researchers who reportedthat a membrane-bound methanotrophic biofilm could create highmethane–low TCE zones inside the biofilm where active growthoccurs without being affected by TCE transformation product tox-icity �Clapp et al. 1999�. However, this previous study focused onthe aerobic degradation of TCE, whereas PCE cannot be biode-graded aerobically. Under the operating conditions of this study itwas hoped that degradation of the less chlorinated compoundswould occur simultaneously in both the aerobic and the anaerobiclayers of the biofilm.

Effect of Increase in Glucose Concentrationand Oxygen Partial Pressure

On Day 35, when the reactor was being fed 10 mg L−1 of PCE, itwas observed that cDCE started accumulating in the system. Itwas thought that this accumulation was due to insufficient glucosein the feed, and consequently limited activity of the aerobic layerof the biofilm. Although the initial glucose supply in our reactorwas 1.25 g COD L−1, a further increase to 2.5 g COD L−1 waseffected; consequently, cDCE started decreasing following the in-crease in glucose concentration �Fig. 2�. However, later in the runat higher PCE concentrations this excess glucose did not preventan accumulation of either PCE or cDCE in the effluent, althoughboth eventually disappeared over time; this degradation also re-quired more oxygen. This result shows that although glucose wasnecessary to sustain degradation, an excess of glucose did notstop the appearance of intermediates at high feed concentrationsdespite the fact that no glucose was detected in the effluent �re-sults not shown�. This result agrees with the findings in our batchculture work where excess glucose had no impact on the extent ofdegradation �Ohandja and Stuckey 2006�. This result also agreeswith the results of Prakash and Gupta �2000� who observed thatan increase in the influent COD from 2,000 to 4,000 mg L−1 didnot result in any improvement in PCE removal in an upflowanaerobic sludge blanket reactor �UASBR�. Since an increase inglucose concentration did not improve PCE degradation, it wasdecided to increase the oxygen concentration in the system.

Initially �Day 0�, oxygen was provided to the biofilm using air.Soon after startup DO levels in the reactor decreased, PCE deg-radation started to occur, and intermediate biodegradation prod-ucts accumulated in the system. A change to an oxygen rich gas�30% O2 and 70% N2� on Day 106 produced a change in thedegradation of intermediates since cDCE started decreasing rap-idly �Fig. 2�. A supply of pure oxygen on Day 128 brought aboutthe biodegradation of PCE. Upon addition of pure oxygen, anincrease in DO was noticed in the bulk liquid �data not shown�,suggesting that the aerobic bacteria could not take up all the oxy-gen. However, the presence of oxygen in the bulk liquid seemsnot to have affected anaerobic degradation since no PCE wasdetected in the effluent, and methane continued to be produced,probably due to biofilm discontinuities and poor mass transfer of

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other researchers who observed PCE biodegradation in the pres-ence of low oxygen concentrations �Hirl and Irvine 1997�. How-ever, in this study the oxygen level decreased and reached zero inthe bulk liquid a few days after the oxygen content of the gas waschanged.

An increase in Cl− production �results not shown� concomitantwith cDCE disappearance from the effluent after addition of pureoxygen was undoubtedly due to an increase in the aerobic biofilmlayer activity. This result highlights the importance of oxygenconcentration in the dechlorination of less chlorinated ethenes,and agrees with previous findings on the aerobic biodegradabilityof low chlorinated compounds �Clapp et al. 1999�. Under theoperational conditions of this reactor, the presence of ethyleneand methane could boost the growth of aerobic chloroethene de-graders and induce the synthesis of oxygenases such as methaneand alkene monooxygenases responsible for aerobic chloroethenecometabolism.

Accumulation of VFAs

Fig. 3 shows the VFA profile in the effluent during the experi-ment; in the main it can be seen that very few VFAs accumulatedin the system. In general, VFAs accumulate due to either cellimpairment, especially by inhibition, or low activity of one ormore of the bacterial groups involved in the fermentation process.Acetate was the dominant VFA present, but did not accumulate inhigh concentrations. Transient accumulation of VFAs during astep change in PCE loading rate rapidly settled down after a fewdays, as shown in Fig. 3, and the small accumulation of VFAsmay be because the reactor was a hybrid reactor. VFAs are pro-duced during the anaerobic degradation of glucose, and are alsoreported to act as electron donors during the reductive dechlori-nation of PCE �Lu et al. 2002�, as well as a carbon source foraerobic growth. Therefore, both glucose and the VFAs may havebeen completely removed from the system due to their simulta-neous metabolism by the various microorganisms present in thebiofilm.

Soluble COD Removal

Since both aerobic and anaerobic conditions were present in thereactor, it was expected that COD removal would be very high;however, COD removal was only 85–93% �Fig. 4�. As the con-centration of PCE increased in the system, COD removal slowlydecreased from 93 to 85%. Nevertheless, these results are notvery different from COD removal efficiencies reported in the lit-

Fig. 3. VFAs profiles during acclimation period �influent PCEconcentration at top�

erature �Prakash and Gupta 2000�. As shown in Fig. 3, there was

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little accumulation of VFAs, and the COD due to PCE was insig-nificant, therefore the moderate COD removal observed in thissystem may have been due to the presence of soluble microbialproducts �SMP-Aquino and Stuckey 2004� in the reactor effluent.Aerobic cometabolism of less chlorinated compounds can alsoyield very toxic compounds such as epoxides, which can reactwith other bacterial material like RNA, DNA, and proteins�Yeager et al. 2001�, leading to cell lysis. If the intracellular ma-terials are released into the bulk liquid, they could account formost of the effluent COD.

Methane Production

Fig. 5 shows the methane content of the gas in the two compart-ments of the reactor. The cumulative production of methane couldnot be calculated since the reactor was also supplied with oxygen.With time, methane production increased indicating that either theanaerobic layer of the biofilm was growing and could sustainincreased methanogenesis, or the oxygen partial pressure was de-creasing in the lower compartment of the reactor due to thebuildup of methane and nitrogen. Increments in PCE concentra-

Fig. 4. Soluble COD removal during acclimation period �influentPCE concentration at top�: �1� indicates use of 30%O2:70%N2 gas;�2� indicates use of pure oxygen

Table 1. Reactor at Steady State during Acclimation Phase

Influent PCE��mol L−1�

Influent COD�glucose��g L−1�

Effluent PCE��mol L−1�

TCE��mol L−1�

30.1 1.25–2.5 NDa NDa

NDb NDb

60.2 2.5 NDa NDa

NDb NDb

90.4 2.5 NDa NDa

NDb NDb

150.5 2.5 NDa NDa

NDb NDb

241 2.5 NDa NDa

NDb NDb

301 2.5 NDa NDa

NDb NDb

421.4 2.5 NDa NDa

NDb NDb

Note: ND=nondetected. The limits of detections for PCE, TCE, cDCE, VaUpper compartment.b

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tion initially resulted in decreased methanogenesis, probably dueto the fact that high concentrations of PCE affected the growth ofmethanogens. However, methane production recovered a fewdays after each increment of PCE, and degradation of high con-centrations of PCE did not permanently inhibit methane produc-tion, contrary to a hypothesis by previous researchers �DiStephano et al. 1991�. Since oxygen was also supplied to thereactor, it is likely that methanotrophic bacteria may have grownin the biofilm and been involved in scavenging the methane pro-duced in the anaerobic layer. Nevertheless, the excess of methanein the system indicates that the population of methanotrophspresent in the reactor could not metabolize all the methane. Table1 summarizes the steady-state data during the acclimation period.

Effect of Hydraulic Retention Time

When the reactor was able to degrade around 70 mg L−1 of PCE,the HRT was gradually reduced from its original value of 48 h to36, 24, 12, 9, and 6 h �see Figs. 6 and 7�, and the “steady state”data attained at each HRT is summarized in Table 2. When thereactor was subjected to an HRT of 6 h, it did not adapt well. Theoverall effect of decreasing the HRT was similar to an increment

Fig. 5. Methane production in both reactor compartments duringacclimation period �influent PCE concentration at top�

cDCE��mol L−1�

VC��mol L−1�

Ethylene��mol L−1�

COD removal�%�

13.34±6.2 NDa 1.2±0.4a 93.2±3.6

NDb NDb 28±14b

NDa 1.4±1.4a 1.93±0.8a 90.2±0.8

NDb NDb 16.8±5.6b

NDa NDa 3.5±0.4a 91.4±4.2

NDb NDb 25.2±2.8b

30.9±37.03a NDa 5.4±0.4a 85.9±3.3

NDb NDb 47.6±11.2b

30.39±6.2a NDa 10.8±0.4a 85.6±7.9

NDb NDb 64.4±2.8b

NDa NDa 12.0±2.3a 85.4±4.4

NDb NDb 86.8±16.8b

24.7±6.2a 4.2±8.4a 12.7±0.4a 84.9±6.7

NDb 4.8±1.6b 92.4±2.8b

d ethylene were: 0.07, 0.3, 0.013, 0.16, and 1.8 �mol L−1, respectively.

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in PCE concentrations during the acclimation phase. However,these effects were milder than during the acclimation phase indi-cating that the biomass washout resulting from a decrease in theHRT did not have a significant effect on MABR stability, andbiomass concentrations in the effluent were low ��20 mg L−1�.These observations agree with the observation that MABRs canimmobilize high concentrations of biomass �Stephenson et al.2000�. Some intermediate PCE dechlorination products were de-tected in the effluent �Fig. 6�, and these started increasing overtime. After a period of 19 days at an HRT of 6 h, cDCE concen-trations were around 12 mg L−1 in the effluent, and it was decidedto increase the HRT back to 9 h. The reactor then reestablished itsstability, with very low concentrations of intermediate PCEdechlorination products in the effluent, as shown in Fig. 6.

With each decrease in HRT, there was an initial appearance ofPCE and some intermediate dechlorination products in the efflu-ent, as shown in Figs. 6 and 7, a decrease in COD removal, and aslight increase in VFAs �Fig. 8� as well as a decrease in methaneproduction. However, these perturbations stabilized after a fewdays. Decreasing the HRT resulted in an increase in the loadingrate of both the influent PCE and COD. Ethylene concentrationsremained fairly constant during the 36, 24, and 12 h HRT periods;however, the values of ethylene recorded �less than 3 mg L−1�could not account for the amount of PCE degraded. This suggeststhat ethene oxidizers degraded ethylene as it was produced. At anHRT of 9 h the reactor achieved complete removal of the biodeg-radation products. Since PCE is only readily biodegradable underanaerobic conditions, while TCE, cDCE, and VC are degradedboth aerobically and anaerobically, occurrence of these com-pounds �primarily cDCE� in the effluent during a change in reac-tor loading was not unexpected, and could have indicated reducedmetabolic activity due to inhibition of not only the anaerobic

Fig. 7. VOC profiles in lower compartment of reactor during studyon HRT �HRTs at top�. VOC concentrations are in gas phase

Fig. 6. VOC profiles in upper compartment of MABR during studyon HRT �HRTs at top�. VOC concentrations are in aqueous phase

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biomass in the reactor, but also the aerobic layer. This reductioncould have been due to the biomass present at the time not beingable to mineralize the high organic loading of influent PCE, andhence having to grow rapidly over time in order to degrade thePCE. However, it could have also been a result of mass transferlimitations of both the substrate and oxygen.

Volumetric Rates of Degradation

The MABR achieved a “volumetric rate of degradation” of247 mmol of PCE h−1 m−3. This dechlorination rate is higher thatthe rates observed in an anaerobic fixed bed of 3.7 mmole of PCEh−1 m−3 �De Bruin et al. 1992�, or a recent figure of 142 mmole ofPCE h−1 m−3 in small test tubes �Wang and Cutwright 2005�.Gerritse et al. �1997� used two reactors with a total volume of2.5 L to achieve a volumetric dechlorination rate of 56.32 mmolof PCE h−1 m−3. This volumetric removal rate is, to ourknowledge, the highest achieved compared to those reported inliterature. The high rate observed by Gerritse et al. �1997� wasprobably due to the fact that the anaerobic and aerobic dechlori-nation steps were spatially separated, thereby alleviating the ef-fect of aerobic cometabolism toxicity, whereas the MABR over-came the effect of aerobic cytotoxicity without spatiallyseparating the anaerobic and aerobic degradation sites. Moreover,their oxic chemostat used traditional aeration, which resulted instripping of volatile organics into the environment. In contrast,our work has developed an innovative reactor which combinedboth degradation pathways into the same unit, thereby reducingthe reactor size and cost, while achieving very high degradationrates.

Chloride Mass Balance

A mass balance was carried out on Cl− entering and leaving thereactor and is shown in Tables 3 and 4. It can be seen in bothtables that during steady state around 72–82% of the PCE fed intothe reactor could be accounted for based on chloride production.It is probable that some of the chlorinated ethenes were adsorbedonto the biofilm or that aerobic intermediates of low-chlorinatedcompounds such as trichloroethanol, dichloroacetyl, and chloro-acetaldehyde were produced in the system. These compoundscontain one or more atoms of chlorine which are not released intothe liquid, and could account for the failure to account for all theCl− in the mass balance. Unfortunately, these intermediates werenot analyzed for in this work because their analysis was very time

Fig. 8. VFAs profiles during study on HRT �HRTs at top�

consuming. Nevertheless, the chloride mass balance observed in

F ENVIRONMENTAL ENGINEERING © ASCE / JANUARY 2007 / 25

this work compares well with the 80% recovery observed byClapp et al. �1999� during the degradation of TCE in an MABR.Most researchers studying the reductive dechlorination of PCEonly carry out a mass balance based on the conversion of chlori-nated ethenes into less chlorinated ones.

Migration of VOCs into Lower Compartment of MABR

As shown in Fig. 7, some of the intermediate biodegradationcompounds migrated to the lower compartment of the reactor.Migration of PCE, its biodegradation intermediates and othergases such as methane to the lower compartment continuedthroughout the experiment. This observation indicated that thebiofilm did not act as a complete barrier to the migration ofVOCs, but was capable of reducing their migration into the gas

Table 2. Reactor at Steady State during HRT Study

HRT�h�

Effluent PCE��mol L−1�

TCE��mol L−1�

cDC��mol

48 NDa NDa 30.9±6

NDb NDb ND

36 8.9±1a NDa 18.5±3

NDb NDb ND

24 NDa NDa 216.1±1

NDb NDb ND

12 NDa NDa 185.1±2

NDb NDb ND

9 NDa NDa 246.92±

NDb NDb 21.9±2

Note: ND=nondetected. The limits of detections for PCE, TCE, cDCE, VaUpper compartment.bLower compartment.

Table 3. Chloride Mass Balance at Steady State during Acclimation Pha

Influent PCE�mg L−1�

Theoretical influent Cl−

concentration�mg L−1�

Theoretical Cl− releasedfrom PCE�mg L−1�

5 44 4.2

10 44 8.6

15 44 12.8

25 44 21.4

40 44 34.2

50 44 42.8

70 44 60

Table 4. Chloride Mass Balance at Steady State during HRT Study

HRT�h�

Theoretical influent Cl−

concentration�mg−1 L�

Theoretical Cl− releasedfrom PCE�mg L−1�

48 44 60

36 44 60

24 44 60

12 44 60

6 44 60

9 44 60aAt HRT 6, the reactor did not reach a “steady state,” so results are not s

phase. PCE and its biodegradation intermediates continued to mi-

26 / JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / JANUARY

grate to the lower compartment of the reactor throughout the ex-periment until a “steady state” was reached. At this point onlyethylene and/or very low VOC concentrations were observed inthe lower compartment. Since the reactor was dead ended, theless chlorinated biodegradation intermediates were not strippedinto the environment, but eventually migrated back into the uppercompartment of the reactor to be degraded. As a result, at steadystate only ethylene and/or low concentrations of VOCs werefound in the liquid effluent as the main biodegradation products,proving that biodegradation of PCE was occurring in the reactor.

Conclusions1. This study has demonstrated the applicability of a novel

VC��mol L−1�

Ethylene��mol L−1�

COD removal�%�

NDa 10.80±0.4a 85.6±7.9

NDb 82.11±3.6b

NDa 7.7±0.4a 90.2±0.8

NDb 21.42±7.1b

NDa 7.33±0.4a 91.4±4.2

NDb 32.13±3.6b

NDa 7.33±0.4a 85.9±3.3

NDb 60.7±14.3b

NDa 6.6±2a 88.3±3.0

NDb 60.7±10.7b

d ethylene were: 0.07, 0.3, 0.013, 0.16, and 1.8 �mol L−1, respectively.

Theoretical effluent Cl−

concentration�mg L−1�

Measured effluent Cl−

concentration�mg L−1�

Cl− recovery�%�

48.2 34.7±1.0 72.0±3.0

52.6 40.5±1.1 77.0±2.0

56.8 43.3±0.1 76.2±0.1

65.4 53.4±0.8 81.7±1.2

78.2 61.9±2.5 79.2±3.2

86.8 68.5±6.0 78.9±6.9

104 69.4±1.8 72.2±1.9

eoretical effluent Cl−

concentration�mg L−1�

Measured effluent Cl−

concentration�mg L−1�

Cl− recovery�%�

104 69.4±1.8 72.2±1.9

104 68.6±1.9 71.5±2.01

104 69.2±1.9 72.0±2.0

104 72.0±2.0 76.1±2.0

104 NAa NAa

104 77.8±6.8 81.0±7.0

EL−1�

.1a

b

0.9a

b

79.0a

b

4.7a

b

117.3a

.9b

C, an

se

Th

hown.

MABR for treatment of wastewaters containing PCE and

2007

high concentrations of a cometabolite �glucose� using bothaerobic and anaerobic biofilms. The volumetric rate of PCEremoval �247 mmol of PCE h−1 m−3� is the highest so farreported in the literature;

2. It appears that PCE degradation occurs in this reactor se-quentially, involving more than one trophic group of bacte-ria. Anaerobic degradation of PCE occurred in the anaerobiclayer of the biofilm, whereas aerobic degradation of PCEdechlorination products occurred in the aerobic layer of thebiofilm. Methanogens and anaerobic dechlorinators producedmethane and ethylene, respectively; the presence of thesetwo substrates, coupled with an oxygen supply to the biofilm,encouraged the growth of methane and ethylene oxidizersand induced synthesis of methane and/or ethylene monooxy-genase responsible for aerobic cometabolism of less chlori-nates ethenes;

3. The lower compartment of the reactor acts as a buffer bytrapping some of the excess VOCs during step changes ofPCE in the influent. These VOCs backdiffuse into the liquidphase of the MABR to be biodegraded. However, furtherstudies are needed to understand the effect of the partial pres-sure of all the gases in the lower compartment of the reactor,and their mass flows, in order to better understand PCE bio-degradation in the MABR;

4. Decreases in the HRT down to 9 h allowed the reactor toachieve high PCE mineralisation rates, and demonstrated therobustness of this reactor design; and

5. This study used a high influent COD influent in order tosustain biological activity of both aerobic and anaerobic bac-teria. Further studies may need to investigate the impact oflower COD concentrations on the biodegradation of PCE.

Acknowledgments

D. G. Ohandja would like to thank the Association of Common-wealth Universities �ACU�-United Kingdom for the award of ascholarship.

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