Sub-thermophilic anaerobic digestion: generation of favorable substrates for methanogenesis.

Sub-thermophilic temperatures (41-49 °C) have been reported to improve anaerobic digestion, but the reason remains unclear. In this study, sub-thermophilic conditions were employed to promote the anaerobic digestion of cattle manure. The results showed that the methane production at 45 °C was 28.5 % higher than that at 37 °C. Under the sub-thermophilic temperatures, hydrolytic-acidogenic bacteria were enriched to increase the production of ethanol, acetate and butyrate and decrease the production of propionate, which provided available substrates for methanogenesis. It might compensate for the decrease in the key enzyme activity of methanogens at 45 °C. Also, the increase in ethanol helped the establishment of direct interspecies electron transfer and the activation of electron bifurcation mechanisms to maintain the survival of methanogens under the unfavorable temperature. Furthermore, the sub-thermophilic temperature also accelerated humification to improve digestate maturity. The study indicated that the two-phase anaerobic reactors might operate independently at their respective optimal temperatures.