The influence of sulphate on substrate utilization in a thermophilic sewage sludge digestor

Summary Bacterial sulphate reduction and the interaction between sulphate reduction and methane production was studied in an unadapted and sulphate-adapted thermophilic anaerobic sludge digestor. Addition of sulphate to a concentration of 5 mm (100 times the background level) did not influence gas production or volatile fatty acid concentration compared to the control digestor. When sulphate reduction was not limited by the sulphate concentration, the sulphate-adapted digestor had a sulphate reduction rate of 910 mol l^–1 day compared with 17 mol l^–1 day in the control digestor. The results indicate that the potential for sulphate reduction is low in a thermophilic sewage sludge digestor receiving a low sulphate concentration. Counts of sulphate-reducing bacteria and methanogens showed that sulphate-reducing bacteria were found only in significant numbers in the sulphate-adapted digestor and only with H₂/CO₂ as substrate. Only low numbers of acetate-utilizing sulphate-reducing bacteria were found in both digestors. When using radio-labelled acetate, the relative percentage of 2-labelled acetate converted to CO₂ was two to four times higher in the sulphate-adapted digestor compared to the control digestor. These results suggest that oxidation of acetate seems to play a larger role in the sulphate-adapted digestor.Offprint requests to: B. K. Ahring