Acetate oxidation in a thermophilic anaerobic sewage-sludge digestor: the importance of non-aceticlastic methanogenesis from acetate

Acetate conversion to methane in a steady-state, thermophilic (60°C) anaerobic sewage-sludge digestor and in a thermophilic (60°C) acetate chemostat inoculated with anaerobic thermophilic sewage sludge, was investigated by use of radiotracer methodology. When the acetate pool in the sewage-sludge digestor was 1–2 mM, 4.1% of 2-labeled acetate was converted to CO2. However, when acetate was consumed to less than 1.0 mM, prior to isotopic examinations, this increased to 14.1%. Microscopic observations showed a shift in the acetate-degrading populations during start-up of the acetate-limited chemostat inoculated from the sewage-sludge digestor. Large numbers of Methanosarcina-aggregates were seen during the first 100–150 days of operation, while Methanosaeta-like rods were not observed. The Methanosarcina-aggregates disappeared concurrently with a decrease in the acetate concentration to approx. 0.4 mM, and the culture consisted mainly of a large number of autofluorescent, short rods together with fewer and longer, non-fluorescent, rods. Non-aceticlastic oxidation of acetate to methane was the mechanism of acetate conversion in the chemostat after 7 months of operation. Our results indicate that the concentration of acetate can influence the mechanism of acetate conversion during thermophilic anaerobic digestion of organic matter.