Performance evaluation of a completely stirred anaerobic reactor treating pig manure at a low range of mesophilic conditions

• The biogas process can run stably at 20 °C at extremely low OLR after long-term acclimation of bacteria.
• A biogas plant running at 28 °C seems as efficient as that operated at 38 °C at low OLR of 1.3 g ODM L−1 d−1.
• Lower temperature operation is inadvisable for the commercial biogas plant running at rather high OLR.
• The estimated sludge yield at 28 °C is higher than that at 38 °C.

Many Chinese biogas plants run in the lower range of mesophilic conditions. This study evaluated the performance of a completely stirred anaerobic reactor treating pig manure at different temperatures (20, 28 and 38 °C). The start-up phase of the reactor at 20 °C was very long and extremely poor performance was observed with increasing organic loading rate (OLR). At an OLR of 4.3 g ODM L−1 d−1, methane production at 28 °C was comparable (3% less) with that at 38 °C, but the risk of acidification was high at 28 °C. At low OLR (1.3 g ODM L−1 d−1), the biogas process appeared stable at 28 °C and gave same methane yields as compared to the reactor operating at 38 °C. The estimated sludge yield at 28 °C was 0.065 g VSS g−1 CODremoved, which was higher than that at 38 °C (0.016 g VSS g−1 CODremoved).