Response of a sewage sludge mesophilic anaerobic digester to short and long-term thermophilic temperature fluctuations

Temperature has a great influence over the anaerobic digestion process, affecting both biochemical and physical reactions. The present study analyses the capability of the anaerobic biomass to stand different temperature operational conditions. Initially anaerobic biodegradability tests were done to evaluate the capability of mesophilic and thermophilic anaerobic biomass to adapt to different temperature conditions. Afterwards, the impact of the temperature fluctuation from mesophilic to thermophilic conditions was evaluated in a continuous stirred digester. Finally, the recovery of a volatile fatty acid inhibited digester was analysed. The results of the anaerobic biodegradability tests indicated that thermophilic biomass present in the mesophilic inoculum was developed when the inoculum was subjected to 55 °C; meanwhile, when the thermophilic biomass was subjected to 37 °C some thermophilic bacteria remained active. When a continuous stirred digester was subjected to daily temperature fluctuations between 37 °C and 45 °C during 1, 3, 6 and 12 h and between 37 °C and 55 °C during 1 h, neither the organic matter removal nor the biogas yields improved or decreased. In contrast, when the exposure time at 55 °C was 2 and 24 h, the anaerobic digestion process was adversely affected; the volatile fatty acid concentration increased and the process yields dropped. In both cases, methanogens were the most sensitive microorganisms to temperature fluctuation. The recovery stage took 22 days and it was developed in two periods: starving and feed re-establishing. Propionate was found to be a key indicator to monitor the digester recovery.