The performance of a bench-scale horizontal-flow anaerobic immobilized sludge (HAIS) reactor treating a synthetic substrate under increasing influent sulfate (SO42-) concentrations was investigated. The synthetic substrate was composed of glucose, methanol and ammonium acetate, besides bicarbonate and trace metal nutritional solution. The reactor was filled with polyurethane foam cubic matrices for biomass immobilization. Influent chemical oxygen demand (COD) was kept almost constant along the experiments (2162 ± 250 mg.l−1) while influent sulfate concentration was increased from 9 to 501 mg.l−1, resulting in COD/SO42- ratio of 249, 66, 19.7 and 4.3 in the four experimental phases tested. After a short start-up period, no significant variations in COD removal efficiency were observed for the COD to sulfate ratios evaluated. Although sulfate reduction was expected to interfere with the methanogenic process, detrimental effects on the overall reactor performance were not observed. Moreover, monitoring parameters indicated that the reactor achieved stable operational conditions immediately after short transient periods following the increase of influent sulfate concentrations. Microscopic analysis of the sludge within the reactor permitted to observe the presence of both methanogenic and sulfate reducing bacteria, indicating that syntrophism and not competition seemed to be the predominant relationship between these two groups under the COD to sulfate ratios assayed.

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