Competition for substrate between sulfate reducing and methane producing bacteria, and the inhibitory effects of sulfide produced from microbial sulfate reduction were investigated in this study for the selection of a proper anaerobic treatment process using laboratory anaerobic contact units. Substrates used were both synthetic waste with various COD/SO42− ratios, and industrial wastes. Sulfate reducers and methane producers were very competitive at COD/SO42− ratio of 1.7 to 2.7. As the ratio increased/methane producers predominated, and sulfate reducers were predominating as the ratio decreased. Inhibitory concentrations were 2000 mg/l sulfate (160 to 200 mg/l sulfide) for sulfate reducers and 1200 mg/l sulfate (120 to 140 mg/l sulfide) for methane producers with synthetic substrate. But sulfide inhibitory concentrations were varied with hydraulic or organic loading rates and substrate used. As loading rates increased/the inhibitory effect was greater. A sulfide concentration of 240mg/l did not inhibit methane producers for sea-food waste treatment.

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