Two anaerobic hybrid AHRs (AHR), mesophilic (35 degrees C) and thermophilic (55 degrees C) were operated with coal wastewater at different hydraulic retention times (HRT) ranging from 3-0.5 to 3.12-0.6 d with organic loading rates (OLR) of 1.12-6.72 g L-1 d(-1). Synthetic coal wastewater with an average chemical oxygen demand (COD) of 2240 mg L-1 and phenolics concentration of 752 mg L-1 was used as substrate. At each HRT, the thermophilic AHR gave a better performance, measured in terms of phenolics/COD removal and gas production. The specific methane yield was also higher for thermophilic AHR at each HRT compared to mesophilic one. The volatile fatty acid concentration in the effluent increased with the lowering of HRT. The Stover-Kincannon model was applicable at both temperatures and showed higher substrate utilization in thermophilic AHR. Energy economic study of the AHRs revealed that 11,938 MJ d(-1) more energy can be generated using thermophilic AHR than mesophilic. (C) 2012 Elsevier Ltd. All rights reserved.