A two-continuous mesophilic (37 +/- 2 degrees C) UASB system with step-feed was investigated as an attractive optimization strategy for enhancing COD and total phenols removal of the system and improving aerobic biodegradability of real coal gasification wastewater. Through the step-feed period, the maximum removal efficiencies of COD and total phenols reached 55-60% and 58-63% respectively in the system, at an influent flow distribution ratio of 0.2 and influent COD concentration of 2500 mg/L; the corresponding efficiencies were at low levels of 45-50% and 43-50% respectively at total HRT of 48 h during the single-feed period. The maximum specific methanogenic activity and substrate utilization rate were 592 +/- 16 mg COD-CH(4)/(gVSS d) and 89 +/- 12 mg phenol/(gVSS d) during the step-feed operation. After the anaerobic digestion with step-feed, the aerobic effluent COD concentration decreased from 270 +/- 9 to 215 +/- 10 mg/L. The results suggested that step-feed enhanced the degradation of refractory organics in the second reactor. (C) 2010 Elsevier Ltd. All rights reserved.