Anaerobic treatment of PCP in fluidized-bed GAC bioreactors

A fluidized-bed granular activated carbon (GAC) bioreactor was evaluated for anaerobically treating wastewater containing pentachlorophenol (PCP). PCP removal efficiencies of greater than 99.9% were achieved at PCP loadings as high as 4 g/kg of GAC per day and influent PCP concentrations as high as 1333 mg/litre. In addition to PCP, ethanol was fed to the bioreactors as the primary substrate. The chemical oxygen demand (COD) loading to the reactor was increased several times as high as 63 g/kg of GAC per day using reactor empty bed contact times as low as 9.3 h in order to exhaust the adsorptive capacity of the GAC for PCP, thus allowing for the development of a PCP-degrading biofilm on the carbon. A major proportion of the influent COD, ranging from 70 to 90%, was converted to methane, which constituted more than 85% of the product gas stream. Due to incomplete mineralization, nearly all PCP was stoichiometrically converted to 3- and 4-chlorophenols. Low levels of 2,4,6- and 3,4,5-trichlorophenols, 2,4- and 3,4-dichlorophenols and phenol were observed in the reactor. The primary biotransformation product of PCP, 4-chlorophenol, inhibited the methanogenic culture at concentrations as low as 116 mg/litre. Toxicity was minimized by controlling the influent PCP concentration, the primary source of the inhibitory products.