BACKGROUND: In anaerobic wastewater treatment processes, the presence of sulfate-reducing bacteria (SRB) produces H2S. Many techniques are being used to remove H2S from biogas to obtain H2S-free biogas but none of those are cost effective or efficient enough to remove the H2S completely. The objective of the present study was to introduce some changes/modifications to the process parameters of the wastewater treatment operation to eliminate SRB from the system. RESULTS: The growth of SRB was found to be completely suppressed under thermophilic conditions (55 degrees C) but not at 37 degrees C. H2S-free biogas containing 56.5% methane was obtained at 55 degrees C after 180 days of treatment. The effect of higher concentrations of volatile fatty acids (VFAs) on the growth of SRB and methanogens at 37 degrees C and 55 degrees C were also studied. At higher VFA concentrations, SRB outgrew the methanogens at 37 degrees C but at 55 degrees C the situation was found to be reversed. For continuous operation at 55 degrees C and low dilution rate (0.0075 h(-1)), SRB was suppressed and biogas having 29% methane but free of H2S was obtained. CONCLUSION: Operating the reactor at high temperature (550C) and low hydraulic retention time (HRT) can result in the production of H2S-free biogas, with a high concentration of methane. (C) 2008 Society of Chemical Industry.