Relationship between methanogenesis and sulfate reduction in anaerobic digestion of municipal sewage sludge was investigated. The density of methanogenic bacteria in the sludge was estimated to be at the order of 104-106 cells/ml. The density of sulfate reducing bacteria was at the order of 105 colony forming units/ml, while the concentration of sulfate in the sludge was low (< 0.2 mM). Addition of sulfate to the sludge markedly enhanced sulfate reduction without significantly affecting methanogenesis. In the sludge supplemented with sulfate, both methanogenesis and sulfate reduction were significantly enhanced upon the addition of H2. In the presence of exogenous H2, inhibition of methanogenesis or that of sulfate reduction resulted in enhancement of sulfate reduction or that of methanogenesis, respectively. The addition of acetate markedly enhanced methanogenesis but did not affect sulfate reduction, and the addition of propionate markedly enhanced both methanogenesis and sulfate reduction. Degradation of propionate essentially depended on sulfate reduction, and acetate accumulated in response to the propionate degradation when methanogenesis was inhibited. In conclusion, in the sludge, acetate was used only in methanogenesis, and H2 was used in both methanogenesis and sulfate reduction. Sulfate reduction degraded propionate to acetate and enhanced electron flow to methanogenesis.Methanogenesis and sulfate reduction are terminal steps in the anaerobic degradation of organic matter. These reactions are known to compete with each other for electron donors, i.e., H2 and acetate, in various environments. In the presence of sulfate at available levels, sulfate reduction generally dominates over methanogenesis due to kinetic and thermodynamic properties (1,5,6,9,10). Beside this, an inhibitory effect of sulfide, the end-product of sulfate reduction, on anaerobic biogas production is known. Thus, sulfate reduction is generally thought

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