Effect of sulphate addition on volatile fatty acid and ethanol degradation in an anaerobic hybrid reactor. II: Microbial interactions and toxic effects: Microbial interactions and toxic effects

The microbial interactions occurring in an anaerobic hybrid reactor which was seeded with non-sulphate-adapted sludge and subsequently exposed to sulphate over a prolonged period of 423 days were studied. The population changes upon inoculation of the reactor with sulphate-adapted sludge from a full-scale treatment plant were also analysed. Addition of sulphate (4000 mg l) to the reactor influent resulted in a complete inhibition of syntrophic propionate degradation and severe inhibition of acetoclastic methanogenesis. No propionate or acetate-utilising sulphate-reducing bacteria (SRB) could be detected in the sulphate-fed reactor sludge. Toxicity tests carried out on sludge samples from a control reactor showed that acetoclastic methanogenesis was the most sensitive step in sulphide inhibition, with 50% inhibition observed at concentrations of 220-980 mg total sulphide l (69-150 mg free H2S l) over the pH range 6.5-8.0. The propionate-utilising syntrophs present in the sludge appeared to be irreversibly inhibited by sulphide and were also inhibited by acetate concentrations as low as 300 mg l. The inoculation of fullscale sulphate-adapted sludge resulted in the establishment of propionate and hydrogen-utilising SRB in the biofilm section, but not in the granular sludge bed section, of the anaerobic hybrid reactor. (C) 1998 Elsevier Science Ltd. All rights reserved.