Temperature effects on the trophic stages of perennial rye grass anaerobic digestion

Continuous Stirred Tank Reactors (CSTRs), operated in batch mode, were used to evaluate the feasibility of psychrophilic (low temperature) digestion of perennial rye grass in a long term experiment (150 days) for the first time. The reactors were operated in parallel at 3 different temperatures, 10, 15 and 37 °C. Hydrolysis, acidification and methanogenesis were assessed by VS degradation, by soluble chemical oxygen demand (SCOD) and volatile fatty acids (VFA) production, and by methane production, respectively. Hydrolysis was the rate-limiting step at all temperatures and the rates and extent of hydrolysis were considerably lower at 15 and 10 °C, than at 37 °C. The total VS degradation was 53%, 34% and 19% at 37, 15 and 10 °C, respectively. Acidification was not affected by temperature and VFA production and consumption was balanced in all cases, except at 10 °C. Methane yields were 0.215 m3 CH4 kg−1 VS−1 added, 0.160 m3 CH4 kg−1 VS−1 added and 0.125 m3 CH4 kg−1 VS−1 added at 37, 15 and 10 °C, respectively. Methanogenesis was not strongly affected at 15 °C but it became rate-limiting at 10 °C. Overall, the solid degradation and methane production performance under psychrophilic conditions was encouraging and greater than previously reported. Considering the non-acclimated, mesophilic nature of the inoculum, there are grounds to believe that low-temperature anaerobic digestion of grass could be feasible if coupled to efficient hydrolysis of the biomass.