Effects of Electron Donors on Degradation of Pentachlorophenol in a Methanogenic Fluidized Bed Reactor

Abstract

Anaerobic reductive dechlorination of pentachlorophenol (PCP) and other chlorophenols was studied in a laboratory-scale fluidized-bed reactor (FBR). Complete dechlorination of PCP and subsequent phenol removal were achieved at 35 °C with a mixed culture from anaerobic sewage sludge, soil and compost, but only after reinoculation with enrichments fed monochlorophenols. The FBR was fed a mixture of glucose, butyric acid and methanol to provide carbon and electrons for the anaerobic culture. FBR tests conducted after 8 months of operation demonstrated that glucose and butyric acid supported chlorophenol dechlorination significantly better than methanol. With 200 mg l−1 chemical oxygen demand (COD) from glucose or butyric acid, more than 96% dechlorination and 68% mineralization of 7 µmol l−1 PCP were observed during continuous FBR operation. Twenty mg l−1 COD from glucose resulted in 84% dechlorination and 55% mineralization of the PCP. After 10 months the FBR performance further improved; during this late period PCP was also extensively mineralized when methanol was supplied as electron donor. When PCP was spike fed and electron donor was continuously supplied, a PCP removal rate of 86 µmole l−1 d−1 was measured (0.94 µmole gVSS−1 h−1). Mineralization was slower by about a factor of 10. Most of the FBR tests demonstrated initial removal of meta- chlorines from PCP. The major PCP dechlorination pathway was as follows: 2,3,4,6-tetrachlorophenol, 2,4,6-trichlorophenol, 2,4-dichlorophenol, 2-monochlorophenol or 4-monochlorophenol, and phenol, which also was removed.