Treatment of simulated wastewater containing 40 mg/l of 4-chlorophenol (4-CP) was carried out in an upflow anaerobic sludge blanket (UASB) reactor under methanogenic condition. The performance of this test UASB reactor was evaluated in terms of 4-CP removal. Hydraulic retention time (HRT) and substrate:co-substrate ratio for the 4-CP removal was optimized by varying the influent flow rate (13-34.7 ml/min) and sodium acetate concentration (2-5 g/l), respectively. A control UASB reactor, which was not exposed to 4-CP was also operated under similar conditions. Organic loading rate (OLR) was varied in the range of 2-5.3 kg/m(3)/d and 1.7-4.2 kg/m(3)/d, respectively, for HRT and substrate:co-substrate ratio studies, respectively, The optimum HRT and substrate: co-substrate ratio for the removal of 4-CP was 12 h and 1:75, respectively. Removal of 4-CP achieved at optimum HRT and substrate:co-substrate ratio was 88.3 +/- 0.7%. Removal of 4-CP occurred through dehalogenation and caused increase in chloride ion concentration in the effluent by 0.23-0.27 mg/mg 4-CP removed. The ring cleavage test showed the ortho mode of ring cleavage of 4-CP. Change in the elemental composition of the anaerobic biomass of UASB reactors was observed during the study period. Concentration of Ca2+ increased in the biomass and this could be attributed to the biosoftening. Specific methanogenic activity of the sludge of control and test UASB reactor was 0.832 g CH4 COD/g VSS d and 0.694 g CH4 COD/g VSS d, respectively. (C) 2007 Elsevier Ltd. All rights reserved.