PH: S0273-1223(97)00513-1

~ Pergamon Waf. Sci. Tech. Vol. 36, No. 6-7, pp. 107-115, 1997.© 1997 IAWQ. Published by Elsevier Science Ltd

All rights reserved. Printed in Great Britain0273-1223/97 $17'00 + 0'00

ANAEROBIC/AEROBICBIODEGRADATION OFPENTACHLOROPHENOL USING GACFLUIDIZED BED REACTORS:OPTIMIZATION OF THE EMPTY BEDCONTACT TIME

Gregory J. Wilson*, Amid P. Khodadoust*,Makram T. Suidan1* and Richard C. Brenner**

*Department ofCivil and Environmental Engineering, University ofCincinnati,Cincinnati, OH 45221-0071, USAt National Risk Management Research Laboratory, u.s. Environmental ProtectionAgency, 26 WM.L. King Drive, Cincinnati, OH 45268, USA

ABSTRACT

An integrated reactor system has been developed to remediate pentachlorophenol (PCP) containing wastesusing sequential anaerobic and aerobic biodegradation. Anaerobically, PCP was degraded to predominatelyequimolar concentrations (>99%) of monochlorophenol (MCP) in two GAC fluidized bed reactors at EmptyBed Contact Times (EBCTs) ranging from 18.6 to 1.15 hours. However, at lower EBCTs, MCPconcentrations decreased to less than 10% of the influent PCP concentration suggesting mineralization. Theoptimal EBCT was determined to be 2.3 hours based on PCP conversion to MCPs and stable reactoroperation. Decreasing the EBCT fourfold did not inhibit degradation of PCP and its intermediates, thusallowing removal of PCP at much lower detention time and resulting in a significant cost advantage.

Analytical grade PCP was fed via syringe pumps into two fluidized bed reactors at influent concentrations of100 mgll and 200 mgll, respectively. Acting as the primary substrate, ethanol was also fed into the reactors atconcentrations of 697 and 1388 mgll. Effluent PCP and chlorinated phenolic compounds were analyzedweekly to evaluate reactor performance. Biodegradation pathways were also identified. 3-chlorophenol (CP)was the predominant MCP and varied simultaneou~ly with 3,5-dichlorophenol (DCP) concentrations.Likewise, 4-CP concentrations varied simultaneously with 3,4-DCP concentrations.

A second stage aerobic GAC fluidized bed reactor was added after the anaerobic reactor to completelymineralize the remaining MCP and phenols. Data show no presence of phenol and MCP in the effluent or onthe GAC. Overall, the chemical oxygen demand (COD) fed to the system was reduced from 75 gld in theinfluent to less than 1.5 gld in the effluent. © 1997 IAWQ. Published by Elsevier Science Ltd

KEYWORDS

PCP; anaerobic; EBCT; fluidized bed; activated carbon; chlorophenols; aerobic.

1 Corresponding author, msuidan@boss.cee.uc.edu

107

108

INTRODUCTION

G. J. WILSON et al.

Pentachlorophenol is widely used in the wood preserving and agriculture industry as a fungicide, pesticideand herbicide. A suspected carcinogen, PCP has received scrutiny in recent years .. RCRA (ResourceConservation and Recovery Act) and Superfund sites contain soils contaminated wIth PCP, creosote,polycyclic aromatic hydrocarbons (PAHs), other hydrocarbons, and heavy metals such as copper, ~hromium,arsenic, and zinc. Approximately 15% of the Records of Decision (ROD) by the U.S. EnvIronmentalProtection Agency involve wood preserving waste. Further, CERCLA (Comprehensive EnvironmentalResponse, Compensation, and Liability Act) mandates removal of PCP contaminated soil withconcentrations greater than 1 ppm.

The fIrst step in developing a comprehensive extraction process is analyzing the physical and chemicalinteractions of a pollutant with different soil fractions. Several researchers have identifIed adsorptionparameters for PCP in different soils through batch studies (Bell and Tsezos, 1986; Boyd et ai., 1988). Usingthis information, along with other factors such as soil pH, soil particle size distribution, soil moisture,pesticide solubility and contact time in a solvent, technologies have been developed to remove wastecontaining pesticide from contaminated soils including supercritical C02 extraction, steam flushing, solventwashing, and soil washing. Khodadoust et ai (1994) have demonstrated PCP can effectively be removedwith an ethanol/water blend through soil washing. An important fInd since ethanol is used as the primarysubstrate in an anaerobic GAC fluidized bed reactor. A separation process may also be included to recoverpart of the ethanol for reuse, further reducing the cost of operation. When used as part of an integratedsystem, anaerobic/aerobic biodegradation of PCP has been shown to be an effective treatment process.

Treating wash fluids in an anaerobic GAC fluidized bed reactor requires quantifIcation of numerousoperation parameters, including mass loading rate, hydraulic loading rate, and empty bed contact time. Aninitial definition of these parameters was attempted by Khodadoust et ai (in press). Operating under constantPCP and ethanol mass loading rates, their experimental data showed PCP was not totally mineralized butconverted to MCP. Concluding MCP toxicity was inhibitory to reactor performance, control of influent PCPconcentration became an essential parameter. After failed attempts at aggressive influent PCP concentrationsof 650 and 1350 mg/l with long hydraulic detention times (>18 hrs) in two anaerobic GAC fluidized bedreactors, 100 and 200 mg/l influent PCP concentrations were used. By holding the influent PCP and ethanolconcentration constant, the mass and hydraulic loading rates were simultaneously doubled to evaluate theeffect of a 50% reduction in EBCT.

Noting MCP dominated the effluent, a second stage aerobic reactor was proposed to mineralize theremaining aromatic chlorinated phenols. Since activated carbon provided an excellent attachment surface forbiomass in the anaerobic reactors, an aerobic GAC fluidized bed reactor was suggested.

Both anaerobic and aerobic biodegradation pathways for PCP and other CPs have been widely reported byinvestigators over the past 15 years (Boyd et ai., 1988; Hrudey et ai., 1987; Nicholson et ai., 1992).Chlorinated phenols are anaerobically biodegraded through reductive dechlorination. Researchers havedegraded PCP and other chlorinated compounds with both acclimated and unacclimated cultures obtainedfrom sediments and sewage in batch tests (Boyd et ai., 1983, 1984; Bryant et ai., 1991; Mikesell et ai.,1986). They have shown biodegradation pathways are influenced by the culture's sources and acclimationprocess. However, variation exists in reaction rates and pathways presumably due to differences in thecultures. Still, important knowledge is obtained from these studies.

The purpose of this study is to define operating parameters at constant PCP and ethanol influentconcentrations, establish aerobic degradation of MCP and phenol, and provide insight into thebiodegradation pathways for PCP.

Anaerobic/aerobic biodegradation of pentachlorophenol

METHODOLOGY

109

The anaerobic granular activated carbon (GAC) fluidized-bed reactor (FBR) was used to study PCPdegradation. The reactor has a total volume of 10 L including the recycle loop and consists of a waterjacketed main column, an influent header, and an effluent header. The inner jacket of the main column is a96.5-cm long Plexiglas tube with a 1O.2-cm internal diameter. The Plexiglas influent header is packed withmarbles to uniformly distribute the fluidizing liquid and prevent GAC from entering the recycle line. ThePlexiglas effluent header contains an effluent sampling port and a gas collection port. A side arm within thereactor extends below the recycle inlet allowing for carbon replacement, if necessary. The aerobic reactorhas the same features and dimensions as the anaerobic reactor except that a bubbleless aeration membrane(Membran Corp., Minneapolis MN) was placed in the pressure side of the recycle line.

Three feed solutions were introduced into the suction side of the recycle loop. The feed solutions were fedseparately to minimize any external biological activity. The synthetic pollutant stream consisted of PCP(99%, Aldrich Chemical Co., Milwaukee,WI) and ethanol (95%, USP Grade, Midwest Grain Products,Weston, MO). The nutrient solutions for the reactors provided the essential vitamins and salts needed formicrobial growth (Fox et ai, 1984). The inorganic buffer solutions contained sodium hydroxide, sodiumcarbonate, sodium sulfide, and some ethanol. In order to maintain constant daily mass loadings, ethanol notplaced in the syringe was added to the buffer solution. Throughout this study, the composition of the buffersolution was varied to maintain reactor pH at 7.2. Each feed reservoir was equipped with a constant-speedpump controlled through a programmable timer. The PCP solution was dissolved in ethanol and fed into thesuction side of the recycle loop using a Model 11 high precision syringe infusion pump (Harvard Apparatus,Inc., South Natick, MA) with a 10- mL fixed needle syringe (Hamilton Company, Reno, Nevada) via 1/16•in. A316 stainless steel tubing. Initially, 1 kg of 16 x 20 U.S. Mesh F400 GAC (Calgon Corporation,Pittsburgh, PA) was placed in each of the anaerobic reactors. The recycle rate was set to achieve a 30%expansion from the initial carbon height. To maintain mesophilic conditions, a constant temperature bathprovided 35°C water circulating through the outer jacket.

The aerobic reactor was charged with 500 g of 16 x 20 U.S. Mesh F400 GAC (Calgon Corporation,Pittsburgh, PA). Effluent from the anaerobic reactor was the only feed solution to the aerobic reactor. Notemperature control was used for the aerobic reactor.

Daily monitoring of the reactor included gas volume production, buffer and nutrient flow rates, syringe flowrates, and pH. Gas production was measured using wet tip gas meters (Environmental & Water ResearchEngineering, Nashville, TN). The pH was measured offline using an Orion Model 720A pH meter (OrionResearch Co., Boston, MA). Weekly offline samples were analyzed for effluent concentration of PCP and itsdegradation products, gas composition, chemical oxygen demand (COD), chlorides, volatile fatty acids(VFAs), and alcohols. The effluent concentrations of PCP, tetra-, tri-, and DCPs were analyzed on a HewlettPackard 5890 Series II gas chromatograph (GC) (Hewlett Packard, Palo Alto, CA) using an electron capturedetector (ECD) with tribromophenol as the internal standard. The 2-, 3-, and 4-CPs and phenol wereanalyzed on the same GC using a flame ionization detector (FID) with an internal standard of p-cresol. Bothinternal standard methods were developed inhouse. A Hewlett Packard 5890 Series II GC (Hewlett Packard,Palo Alto, CA) equipped with a thermal conductivity detector was used to determine the percentage ofcarbon dioxide, oxygen, nitrogen, and methane in the product gas. CODs were determined using the Hachlow range digestion vials (0-150 mgIL) and a Hach COD reactor Model 45600 (Hach Co., Loveland, CO).The digested vials were measured for transmittance on a Bausch & Lomb Spectronic 7? Spect~ophotom~ter(Bausch and Lomb, U.S.A.). Chloride concentrations in effluent samples were determmed usmg an OnonModel 9617BN chloride selective electrode. VFA and alcohol concentrations were analyzed via HewlettPackard 5890 Series II GCIFID (Hewlett Packard, Palo Alto, CA). Carbon extractions were conductedperiodically using the procedure outlined by Fox et ai (1984).

110

RESULTS

G. J. WILSON el al.

Anaerobic GAC Fluidized Bed Reactor Results. Initially, two identical anaerobic GAC fluidized bed reactorswere operated in parallel and received the same PCP and ethanol mass loading rate, but the hydraulicloading rate of reactor A was double that of reactor B. The mass and hydraulic loading rates were increasedsimultaneously in both reactors, and thus, the EBCT was reduced while maintaining a constant influent PCPconcentration of 100 and 200 mg/l in reactors A and B, respectively. Table 1 summarizes the operatingconditions for both reactors A and B during all phases of this study.

Table 1. Operation parameters for anaerobic reactors A and B

Reactor A Reactor B

Operation Days of PCP Ethanol Flow Rate EBCT Flow Rate EBCTPhase Operation Loading Loading (Vd) (hr) (Vd) (hr)

(gld) (gld)

I 480-606 0.6 4.28 6 9.30 3 18.6

II 607-824 1.2 8.33 12 4.65 6 9.30

ill 825-999 2.4 16.66 24 2.32 12 4.65

IV 1000-1340 4.8 33.32 48 1.17 24 2.32

After reaching steady state at each EBCT, the loading rate was increased in 10% increments every four days.As a result, after 40 days, each reactor was operating at a new EBCT. Operation at the new EBCT continueduntil steady state conditions were achieved. Reactors were considered to be operating at steady state whenPCP was consistently converted to MCP without large fluctuations among effluent chlorinated phenolconcentrations.

1~ TP•••• t P....Dip•••• Jn I P•••• IV

..:l liP I I I - J.n•••,pcP;:::; . . .• Em..., MCP.o n1I I I I.. Em•••,PIl•••1El h,.. I I I a Eln•••, PCP

~ 10' I I i

~ 1&1 I-Ii ~I.....: ~.~.t: "., ••=1&] .~. ~ ~'t~ .. "'" .. •~ "---1fW' ~ INA" "j tt!o 4- •~ 1&3 o~ ~. t a orJ' a II oii(ft. dfC%a>

lP~o a a IC£::IjJ 6 CD D DaD D C D a

10'4 a a OfZJO oL fZJ a I000 L-.~~o a 16~ ltIf a a a

a ro ';00 i"oI &' 1.....oc1O-"-....u....-..........................-o-Lcb>.o....o...JL...o............~....................................,.........-'-L..o...............J

500 600 700 800 900 1000 1100 1200 1300 1400nay.

Figure 1. PCP and PCP intennediate concentrations for reactor A.

Figure 1 shows reactor A converted more than 99% of the influent PCP to MCP through out the first threephases. Shortly after the transition into phase IV was completed, total MCP formation declined by an orderof magnitude below the influent PCP molar concentration. The MCP decrease was equal to the phenolincrease suggesting the conversion of MCP to phenol. 99.6% of the influent PCP was converted either toMCP or phenol. However, phenol did not rise when the MCP concentrations decreased the second time. Thebehavior of reactor B was similar to reactor A. MCP concentrations accounted for more than 99% of theinfluent PCP mol~ co?~e~tration thro~ghout the first three phases. Figure 2 shows that during phase IV,phenol concentrations IOltIally rose whIle MCP concentrations fell. However, MCP concentrations leveled

Anaerobic/aerobic biodegradation of pentachlorophenol 111

off as phenol concentrations continued to decrease indicating mineralization of the MCPs. Effluent PCPconcentrations rose at least an order of magnitude from phase I to phase IV but greater than 99% conversionof influent PCP to other chlorinated phenolic compounds was maintained.

lIP

SUP

0 101IIIIwi u,0:c 10"1•....II 10"2~~•0 10"5tJ

10'4

..... '1 P ..... II P..... III P..... IV

- l.n•••tPCP• .fn•••tMCP.... am•••tp....olD Em...t PCP

Days

Figure 2. PCP and PCP intermediate concentrations for reactor B.

..

•

•,..••

P..... IV• 3-CPo 4-CP... 3,4DCP• 3;5 Dcr

•A."""":•••

P...... P..... II

During phase I, nearly equal amounts of 4-CP and 3-CP were found in the effluent as seen in Figure 3.However, in phase II, 3-CP was the predominant species and continued into phase III, eventually accountingfor nearly all of the total MCP conversion. 4-CP concentrations decreased until day 935; its detection on theGAC between day 935 and 1047 was not possible due to masking by the 3-CP peale. When 3-CPconcentrations began to decrease after day 1047, 4-CP reappeared. However, 3-CP levels returned toprevious standings resulting in the masking phenomena once again. 3,4-DCP was the predominant DCPspecies throughout the first three phases, declining as the EBCT decreased. 3,5-DCPs and other DCPs werepresent during phases II and III in lower concentrations. 3,5-DCP concentrations rose during phase IV untilreaching concentrations similar to 3,4-DCP. Similar to reactor A, 4-CP was the most common MCP in phaseI but declined during later phases as 3-CP increased in reactor B. At an EBCT of 4.65 hours, 4-CP levelsbecame undetectable due to 3-CP masking. However, late in phase IV, MCP concentrations decreased by anorder of magnitude enabling 4-CP detection once again. The relationship between MCP and DCP is shownin Figure 4. 4-CP decreased at the same rate as 3,4-DCP. At the end of phase IV, the 3-CP decline coincidedwith a decrease in 3,5-DCP concentrations as well. The 3,4-DCP fraction was greatest in phase I anddeclined to 50% of the total DCPs in phase IV. As the suggested mineralization occurred in phase IV, 3,5•DCP declined dramatically leaving 3,4-DCP as the dominant species.

101

10'4 L........-......l"'----...L.....-.........:...i....o-~....:..... ..........--..........--....&-.o..........~.....-....J

500 600 700 800 '00 1000 UOO 1100 1300 1400Day.

Figure 3. DCP and MCP concentrations for reactor A.

112 G. J. Wn..SON et al.

10'

10'4

PIlaMIV3-CP~-ep

3.~DCP3,5 DCP

1~00 600 700 800 900 10001100120013001400150016001700

nays

Figure 4. DCP and MCP concentrations for reactor B.

p ..... m P..... IV--Ian•••t• 1M...,+ E~Ir."do.

o M •••t 0.",o EJ:tr."d•• o.

.. .. ..

P..... II

..

P..... I

2.00e+ 0 ..

1.20e+-o=1.00e+.;~ 8.00e+•-•=6.00e+....•~

Dec OOcc o Dec

Figure 5. Cumulative mass balance for reactor A.

..... I P..... II Pill••• m Pill••• IV

-- I.ftue.t.. Eln••• t + EJ:tr.cdoo Klftaeat 0.'"D EJ:tr.cdoD O.

Figures 5 and 6 show the cumulative PCP and intermediate chlorinated phenolic compound mass in theinfluent, effluent, and on the carbon. During the first three phases, the influent and effluent slopes are similar

Anaerobic/aerobic biodegradation of pentachlorophenol 113

indicating the conversion of PCP to MCP. Carbon extracts taken during the initial phases for both reactorsindicate carbon adsorption was more prevalent than during the last phase. Overall, amounts of chlorinatedcompound on the carbon decreased through out all phases indicating bioregeneration of the carbon. By theend of phases IV, biodegradation was the main removal mechanism in both reactors. Very little PCP wasadsorbed onto the carbon compared to the other chlorinated compounds, rising only during loading rateincreases and reactor upsets and, subsequently, quickly declining. By day 1400, little mass accumulation inthe effluent of reactor B results in a flat slope. Further evidence of conversion can be seen from the carbonextraction data where relatively small amounts of any chlorinated phenolic compounds appeared on thecarbon.

COD performance in reactors A and B were similar, therefore only the findings from reactor B arepresented. The influent COD loadings include the PCP, ethanol, and trace vitamins present in the nutrientsolution. Noting PCP and ethanol COD ratios are 0.69 and 2.09, influent COD composition is weightedmore heavily by ethanol in terms of a high COD ratio and daily mass loading. The mass of COD in theeffluent has increased steadily from 0.5 to 6.5 gld through the first four phases but then decreased to 2.2 gldwhen concentrations of MCP decreased supporting the preposition that MCP was mineralized. In addition,its COD equivalent of methane in the gas phase when added to the effluent COD accounts for all of thereduced compound leaving the reactor. The remaining balance is attributed to biomass production and 8•12% effluent CO2 gas composition.

As an important measure of acidogenesis and methanogenesis, volatile fatty acids (VFAs) and alcohols wereanalyzed for in the effluent of both reactors. Influent ethanol concentrations were 694 and 1388 mg/l inreactors A and B, respectively. Sharp rises in acetate and ethanol concentrations occurred during periods ofincreased loading rates and reactor upsets, but quickly returned to steady state levels. Reactor upsetsoccurred when biomass periodically blocked the recycle line temporarily collapsing the GAC medium.Propionate and methanol remained low throughout the study.

Aerobic GAC Fluidized Bed Reactor Results. The objective of aerobic reactor operation was to demonstratethat effluent an containing MCP and phenol from the anaerobic reactor could be mineralized. After 90 daysof operation, MCP and phenol were not observed in the reactor effluent or the GAC extracts. Additionally,analysis of the aerobic reactor revealed that PCP present in the anaerobic reactor effluent adsorbed on thecarbon. Overall, COD loading was reduced from 75 to less than 1.5 gld and TOC loading was no greaterthan 0.12 gld.

DISCUSSION

Variations in reactor performance can be attributed to either the lowering of the BBCT or acclimation of theculture. In order to evaluate the principle condition, a comparison is made between phases of each reactorwith equal BBCT and phases with the same loading rate. Simultaneous changes among chlorinated phenolconcentrations which occur in phases with equal mass loadings in both reactors suggest that acclimationdominates. However, changes in chlorinated phenolic concentrations occurring in phases with the sameBBCT are attributed to BBCT.

During phase I, each anaerobic reactor exhibited similar stoichiometric conversion of PCP to MCP with 4•CP being the dominant MCP in the effluent. However, 4-CP concentrations decreased with a correspondingrise in 3-CP concentrations in phase II for both reactors. The trend continued into phase IV, indicating achanging culture in both reactors. After nearly three years of operation, MCP concentrations de~lined atdifferent BBCT in each reactor. 4-CP and 3,4-DCP concentrations decreased at the same rate whIle 3-CPand 3 5-DCP concentrations rose in both reactors during the same three phases. These data show preferentialdechl~rination in the ortho position thus providing further evidence of an acclimating culture. Mi~esell andBoyd (1986) have observed similar results in batch tests using PC:P a~cli.mate~ cultures: Smce TCPconcentrations were low in both reactors throughout most of the study, IdentIfymg bIOdegradatIon pathwayswas difficult. However, during phase IV, higher 3,4,5-,2,4,5-, and 2,3,5-TCP concentrations were noticed inthe effluent and on the carbon in both reactors indicating a possible pathway for 3,4-DCP and 3,5-DCP. At

114 G. J. WILSON eral.

the same EBCTs effluent PCP concentrations in each reactor were similar, increasing as the EBCTdeclined. In other'words, at shorter hydraulic detention times, a larger fraction of the chlorinated phenoliccompounds analyzed from the effluent was PCP. High PCP concentrations and varying MCP conc.entrationsin reactor A during phase IV exhibited possible unstable operating conditions suggesting ~ operatmg ~BCTof 2.3 hours. Hence, decreasing the EBCT resulted in higher effluent PCP concentratIons, but did notnecessarily affect the mineralization of MCP. This is an important finding which allows PCP degradation atlow EBCT providing a significant cost advantage. Results from the aerobic reactor suggest any remainingMCPs and phenols from the anaerobic reactor are eliminated.

CONCLUSIONS

PCP was stoichiometrically converted by greater than 99% to MCP in two anaerobic GAC fluidized-bedreactors at EBCTs ranging from 18.6 to 4.65 hours with 3-CP eventually dominating. An EBCT of 2.3 hourswas determined to be optimal based on PCP conversion to MCPs and stable operation. Decreasing the EBCffourfold did not inhibit degradation of PCP and its intermediates, thus allowing removal of PCP at muchlower detention times and providing a significant cost advantage. A relationship between concentrationvariations of 3,5-DCP and 3-CP as well as 3,4-DCP and 4-CP was established. COD, volatile fatty acids, andalcohol results indicate stable reactor performance throughout the entire study. Aerobically, the remainingMCP and phenol from reactor B were completely removed.

ACKNOWLEDGMENTS

This research was made possible through Cooperative Agreement CR-821029 from the U.S. EnvironmentalProtection Agency. The findings and conclusions expressed in this paper are solely those of the authors anddo not necessarily reflect the views of the Agency. Gregory Wilson and Amid Khodadoust are graduateresearch assistants and Makram T. Suidan is a professor in the Department of Civil and EnvironmentalEngineering at the University of Cincinnati. Richard C. Brenner is a senior research engineer, BiosystemsEngineering Section, National Risk Management Research Laboratory, U.S. EPA, Cincinnati, Ohio.

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Anaerobic/aerobic biodegradation of pentachlorophenol 115

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