Effect of temperature increase from 55 to 65°C on performance and microbial population dynamics of an anaerobic reactor treating cattle manure

The effect of a temperature increase from 55 to 65 °C on process performance and microbial population dynamics were investigated in thermophilic, lab-scale, continuously stirred tank reactors. The reactors had a working volume of 3 l and were fed with cattle manure at an organic loading rate of 3 g VS/l reactor volume/d. The hydraulic retention time in the reactors was 15 days. A stable reactor performance was obtained for periods of three retention times both at 55°C and 65 °C. At 65°C methane yield stabilized at approximately 165 ml/g VS/d compared to 200 ml/g VS/d at 55°C. Simultaneously, the level of total volatile fatty acids, VFA, increased from being below 0.3 g/l to 1.8–2.4 g acetate/l. The specific methanogenic activities (SMA) of biomass from the reactors were measured with acetate, propionate, butyrate, hydrogen, formate and glucose. At 65°C, a decreased activity was found for glucose-, acetate-, butyrate- and formate-utilizers and no significant activity was measured with propionate. Only the hydrogen-consuming methanogens showed an enhanced activity at 65°C. Numbers of cultivable methanogens, estimated by the most probable number (MPN) method, were significantly lower on glucose, acetate and butyrate at the increased operational temperature, while the numbers of hydrogenotrophic methanogens remained unchanged. No viable propionate-degrading bacteria were enriched at 65°C. Use of ribosomal oligonucleotide probes showed that an increase in temperature resulted in a decreased contribution of the rRNA of the domain bacteria from 74–79 to 57–62% of the universal probe, while the rRNA of the domain archaea, increased from 18–23 to 34–36%.