Fate and Modeling of Pentachlorophenol Degradation in a Laboratory-Scale Anaerobic Sludge Digester

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 $26\text{days}$ 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.