Performance of hortizontal-flow anaerobic immobilized sludge (HAIS) reactor treating synthetic substrate subjected to decreasing COD to sulfate ratios

The performance of a bench-scale horizontal-flow anaerobic immobilized slud ge (HAIS) reactor treating a synthetic substrate under increasing influent sulfate (SO42-) concentrations was investigated. The synthetic substrate wa s composed of glucose, methanol and ammonium acetate, besides bicarbonate a nd trace metal nutritional solution. The reactor was filled with polyuretha ne 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 r atios evaluated. Although sulfate reduction was expected to interfere with the methanogenic process, detrimental effects on the overall reactor perfor mance were not observed. Moreover, monitoring parameters indicated that the reactor achieved stable operational conditions immediately after short tra nsient periods following the increase of influent sulfate concentrations. M icroscopic analysis of the sludge within the reactor permitted to observe t he presence of both methanogenic and sulfate reducing bacteria, indicating that syntrophism and not competition seemed to be the predominant relations hip between these two groups under the COD to sulfate ratios assayed. (C) 1 999 Published by Elsevier Science Ltd on behalf of the IAWQ. All rights res erved.