An anaerobic upflow anaerobic sludge blanket (UASB) and an aerobic completely stirred tank (CSTR) reactor system with partially granulated sludge were used sequentially to treat trichlorotoluene (TCT). During methanogenic operation of the UASB reactor molasses was supplied as the carbon and energy source at 2900-3300 mg/l. The anaerobic biodegradation of TCT was investigated by decreasing the hydraulic retention time (HRT) and increasing the chemical oxygen demand (COD) and TCT loadings from 6 to 225 g/l per day and from 0.3 to 11.5 g/l per day, respectively. Influent TCT was transformed to toluene and volatile fatty acid (VIA) under anaerobic conditions while the remaining TCT and toluene was biodegraded and ultimately mineralized to CO2 and H2O under aerobic conditions. Increasing the volumetric loading rate of TCT and COD from 0.3 and 6 g/l per day to 3.37 and 67.5 g/l per day, respectively, resulted in higher COD conversion (79 and 94%) rates and methane production (2400 and 18,000 ml per day). The corresponding VIA concentrations varied between 80 and 400 mg/l. COD and TCT removal efficiencies of 60 and 68% were achieved when the UASB reactor was operating at TCT and COD loading rates of 11.5 and 225 g/l per day, respectively. The maximum TCT removal efficiency was 90 at a TCT loading rate of 3.5 g/l per day in UASB reactor. The maximum methane percentage was 65% at a TCT loading rate of 11. 5 g/l per day. The maximum specific methanogenic activity (SMA) was 0.92 g CH4-COD/g volatile suspended solids (VSS) per day while the maximum specific substrate utilization rate (SSUR) was 0.8 g COD/g VSS per day. It can be suggested that TCT, the breakdown products and the COD were ultimately mineralized in the combined UASB/CSTR reactor. More than 95% COD and TCT removal efficiencies were obtained in combined UASB/CSTR system at an initial TCT concentration of 150 mg/l at TCT and COD loading rates of 11.5 g TCT/l per day and 225 g COD/l per day, respectively. Methanothrix, Methanosarcina, and Methanococcus spp. were identified in UASB reactor. (C) 2003 Elsevier Ltd. All rights reserved.