Upflow anaerobic sludge blanket-hollow centered packed bed (UASB-HCPB) reactor was developed with the aim to minimize operational problems in the anaerobic treatment of Palm Oil Mill Effluent (POME) under thermophilic conditions. The performance of UASB-HCPB reactor on POME treatment was investigated at 55 degrees C. Subsequent to start-up, the performance of the UASB-HCPB reactor was evaluated in terms of i) effect of hydraulic retention time (HRT); effect of organic loading rate (OLR); and effect of mixed liquor volatile suspended solid (MLVSS) concentration on thermophffic POME treatment. Start-up up of the UASB-HCPB reactor was completed in 36 days, removing 88% COD and 90% BOD respectively at an OLR of 28.12 g L-1 d(-1), producing biogas with 52% of methane. Results from the performance study of the UASB-HCPB reactor on thermophilic POME treatment indicated that HRT of 2 days, OLR of 27.65 g L-2 el and MLVSS concentration of 14.7 g was required to remove 90% of COD and BOD, 80% of suspended solid and at the same time produce 60% of methane. (C) 2014 Elsevier Ltd. All rights reserved.