Thermal modelling of the completely stirred anaerobic reactor treating pig manure at low range of mesophilic conditions

•Operation parameters of medium or small scale farm biogas plants are site-specific.•The developed model can be used to decide the optimal digesting temperature and OLR.•Co-digestion is helpful to increase Np,max at ambient temperature (Ta) <10 °C.•Np,max is achieved at 26 °C and OLR of 4.6–5.4 kgODM/m3 d under Ta of 10–20 °C.

Most of Chinese middle size agricultural biogas plants run at the lower range of mesophilic conditions and low organic loading rates (OLRs) which result in the low biogas production. How to obtain an economically viable operation mode is a challenge for Chinese farm biogas plants. In this study, the performance of completely stirred anaerobic reactors treating pig manure was studied at 20, 28 and 38 °C. A thermal mathematic model was accordingly developed to decide the optimum digesting temperature and OLRs considering ambient temperature of 20, 10 and 0 °C. The regression surface model can fit well on the experimental data when the ambient temperature was around 10–20 °C, at which maximum net energy production (Np,max) can be achieved when the digesters run at OLR of 4.6–5.4 kgODM/m3 d with temperature of above 26 °C. Co-digestion on the pig farm was suggested in winter in order to increase the Np.