Fate and Modeling of Pentachlorophenol Degradation in a Laboratory-Scale Anaerobic Sludge Digester Shyi-Tien Chen 1 ; C.-Y. Hsu 2 ; and P. M. Berthouex 3 Abstract: Anaerobic pentachlorophenol PCP reductive dechlorination, commonly conducted in various media and systems, deprives chlorides from the PCP’s phenolic ring and has a great potential as an effective and harmless means in PCP removal. To evaluate the potential use of the anaerobic sludge digesters to treat PCP, two laboratory-scale sludge digesters, one reactive with PCP and one a control no PCP, were operated in parallel in two modes: 1 a semicontinuous-fed mode to examine the available PCP congeners and sludge acclimation process over time; and 2 a batch-fed mode to show the effect of sludge acclimation to PCP. Two multiresponse models were also employed to determine the degradation rates and extent of the PCP and its by-products. The results of the semicontinuous-fed runs showed that it took 26 days to transform PCP to 3-monochlorophenol 3-MCP by following two major PCP degradation pathways observed in series. It appeared that PCP’s orthochlorine was first removed followed by the para lastly the meta. In the batch-fed run using the acclimation sludge, the results showed the immediate PCP dechlorination to lower chlorinated intermediates i.e., mono- and dichlorophenols via diverse pathways. Two multiresponse models, one comprised of seven parameters and the other of nine parameters, were deemed adequate in terms of describing the kinetics of the observed chlorophenols degradation. On the other hand, the results of the batch-fed run using unacclimated sludge showed a four-day delay before the degradation of PCP, in which a single PCP degradation pathway and the absence of 3-MCP suggested the lack of microbial ability to remove the meta chlorine of the PCP. DOI: 10.1061/ASCE0733-93722006132:7795 CE Database subject headings: Sludge; Parameters; Degradation; Anaerobic treatment. Background and Objectives Pentachlorophenol PCP, once a widely used wood preservative, is now a great concern due to its toxicity and widespread presence in the environment. Hydrophobicity of PCP allows its adsorption to soil and limits its bioavailability, and toxicity of PCP leads to the inhibition of microbes that degrade PCP. As a result, the USEPA 1995 reported that PCP “. . . has been found in at least 313 of 1,585 National Priorities List sites identified by the Envi- ronmental Protection Agency.” Many treatment alternatives were proposed to treat PCP bioti- cally and/or abiotically. For example, the chemical treatment of PCP-contaminated soil using heme and peroxide Chen et al. 1999 degraded 80% of 3.5 mM/kg soil PCP in several hours; however, a low percentage of PCP mineralization and a high per- centage of unknown substances bound to the treated soil hindered its applications. Other than chemical treatments, several PCP de- graders have been isolated and were able to perform PCP dechlo- rination Bouchard et al. 1996; Mohn and Kennedy 1992, yet no successful PCP remedial cases using pure culture have been re- ported so far. Though PCP is extremely toxic to microbes, anaerobic dechlo- rination process reduces the oxidized carbon atoms of PCP, gen- erates less toxic and predictable intermediates, and is deemed appropriate for treating PCP wastes. The process eventually re- moves the chlorines on chlorinated phenols from its phenolic ring and forms less chlorinated intermediates Boyd et al. 1983. There are many studies on the anaerobic PCP dechlorination, but the reported PCP reductive pathways were rather diverse. Table 1 gives some of the PCP reductive dechlorination pathways con- ducted in various anaerobic media, in which the incomplete and/or variable PCP dechlorination pathways are seen. Timing of experimental sampling i.e., transient intermediates degraded be- fore taking samples might have been the cause of the reported incomplete pathways due to not detecting missing some of the PCP transient intermediates at the time of their presence. In some cases the missing intermediates could be known through reason- ing. For example, in Table 1 Bryant et al. 1991 reported the observation of PCP degradation to 2,3,5,6-TeCP to 2,3,5- and 3,4,5-TCP to 3,5-DCP to 3-MCP from which it is reasonable to speculate the production of 2,3,4,5-TeCP during the reaction. Analogous examples could be seen in the studies of Mikesell and Boyd 1985 that detected no 2,3,4,5-TeCP and Mikesell and Boyd 1988 that detected no 3,4,5-TCP. On the other hand, the reported pathways might not be specific enough to know the hidden intermediates. For example, the observed 2,3,5-TCP in Hendricksen et al. 1992 might be derived from 2,3,4,5-TeCP or 1 Dept. of Safety, Health, and Environmental Engineering, National Kaohsiung First Univ. of Science and Technology, 2 Juoyue Rd., Nantsu, Kaohsiung 811, Taiwan corresponding auhtor. E-mail: shyitien@ ccms.nkfust.edu.tw 2 Dept. of Safety, Health, and Environmental Engineering, National Kaohsiung First Univ. of Science and Technology, 2 Juoyue Rd., Nantsu, Kaohsiung 811, Taiwan. 3 Dept. of Civil and Environmental Engineering, Univ. of Wisconsin– Madison, Madison, WI 53706-1498. Note. Discussion open until December 1, 2006. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and pos- sible publication on March 23, 2005; approved on September 27, 2005. This paper is part of the Journal of Environmental Engineering, Vol. 132, No. 7, July 1, 2006. ©ASCE, ISSN 0733-9372/2006/7-795–802/ $25.00.