The anaerobic treatment of a sulfate-containing waste water using a UASB reactor was studied at 55 °C. As substrate, acetate and a mixture of acetate, propionate and butyrate were used. With acetate as substrate it was shown that sulfate reducers are capable of using acetate as substrate at 55 °C, and that, under the conditions applied, they even outcompete acetoclastic methanogens. Batch-activity measurements with the sludge revealed temperature optima for acetate, propionate and butyrate degradation of ± 56-59, < 40 and 52-54 °C respectively.
After switching the substrate to a mixture of acetate, propionate and butyrate, the reactor pH dropped from 8.3-8.6 to 7.6-7.9 and the methane production recovered. After the establishment of a pseudo-steady state situation the part of COD removed by methane production and sulfate reduction was ± 60 and 40 %. Results of batch activity experiments showed that the methanogenic activity dropped sharply at pH ≥ 8 and ≤ 6 causing a predominance of sulfate reducers at pH ≥ 8.