Optimisation of biogas production from manure through serial digestion: Lab-scale and pilot-scale studies

In the present study, the possibility of optimizing biogas production from manure by serial digestion was investigated. In the lab-scale experiments, process performance and biogas production of serial digestion, two methanogenic continuously stirred tank reactors (CSTR) connected in series, was compared to a conventional one-step CSTR process. The one-step process was operated at 55 °C with 15 d HRT and 5 l working volume (control). For serial digestion, the total working volume of 5 l was distributed as 70/30%, 50/50%, 30/70% or 13/87% between the two methanogenic reactors, respectively. Results showed that serial digestion improved biogas production from manure compared to one-step process. Among the tested reactor configurations, best results were obtained when serial reactors were operated with 70/30% and 50/50% volume distribution. Serial digestion at 70/30% and 50/50% volume distribution produced 13–17.8% more biogas and methane and, contained low VFA and residual methane potential loss in the effluent compared to the one-step CSTR process. At 30/70% volume distribution, an increase in biogas production was also noticed but the process was very unstable with low methane production. At 13/87% volume distribution, no difference in biogas production was noticed and methane production was much lower than the one-step CSTR process. Pilot-scale experiments also showed that serial digestion with 77/23% volume distribution could improve biogas yields by 1.9–6.1% compared to one-step process. The study thus suggests that the biogas production from manure can be optimized through serial digestion with an optimal volume distribution of 70/30% or 50/50% as the operational fluctuations are typically high during full scale application. However, process temperature between the two methanogenic reactors should be as close as possible in order to derive the benefits of serial coupling.