For the control and improvement of the anaerobic digestion processes, it is important to understand the microbial interactions related to the decomposition of organic matter in the anaerobic treatment processes. Methanogensis and sulfidogensis which are the terminal processes of anaerobic degradation of organic compounds have the potential importance in anaerobic waste treatments. The aims of the present study are estimation in situ of the utilization rates of sulfate reducers and methanogens and clarification of the interactions between these bacteria. In order to estimate availabilities of substrates for methanogens and sulfate reducers, chloroform and molybdate were used as specific inhibitors of each group of bacteria. It was shown that methanogens and sulfate reducers used about 70% and 30% of organic acids in anaerobic sludges respectively. Sulfate reducers improve the anaerobic environment for methanogenesis by supplying sulfide which rapidly decreased the oxidation-reduction potential. The effect of sulfate and sulfide on methanogensis were also studied. Under the concentration of 4mM sulfate and 2mM sulfide the methane producing activity was increased and sulfide production rate had the maximum value with 15mM sulfate. The investigation on physiological chracteristics of sulfate reducing bacteria was carried out by using pure culture method. Sulfate reducers which were cultured in liquid media with lactate as the carbon source used lactate by anaerobic respiration and fermentation of lactate into acetate. It is therefore evident that methanogens and sulfate reducing bacteria have complex relationship each other in the form of competition and syntrophism according to the substrates present in the anaerobic sludge.
