The hydrogen content in the biogas and the COD removal percentage was maintained in the range of 68e70% and 64e66% at all HRTs of 32e8 h during study-state operation. At all range of HRT, a high POME conversion of over 91.4% was attained except for the runs at a lower HRT of 8 h. This indicates efficient utilization of the substrate by the hydrogen producing bacterial population in the reactor