Impact of organic loading rates on the performance of variable temperature biodigesters

Psychrophilic anaerobic digestion has often been examined with constant or minor temperature variations, with less focus on the greater temperature variations that characterize millions of ambient temperature digesters operating in temperate climates. This research examined the impact of organic loading rate (OLR) on digester performance under varying temperature conditions to improve management of ambient temperature digesters in temperate climates. Six lab-scale digesters (3 L) paired into three treatments with high (1.3 kg VS/m3/day), medium (0.8 kg VS/m3/day) and low (0.3 kg VS/m3/day) OLRs were assessed under a temperature regime that simulated an annual cycle of a temperate climate. Hydraulic retention times of the three treatments were 43, 71, and 188 days. After beginning the experiment at 27 °C for 14 days, the temperature was decreased from 27 to 10 °C over the course of 72 days, and held at 10 °C for 38 days before returning to 27 °C in the same manner. Digester performance of all treatments declined as temperature decreased, with pH decreasing from 7.8 to 7.2–5.5 and biogas production decreasing by 77–94%. As temperatures began to rise, the low OLR digesters recovered and began to produce biogas at levels similar to those before the temperature decrease (100–270 mL/day), but digesters with high and medium OLRs remained failed, with pH less than 6.5 and biogas production less than 100 mL/day. Laboratory results were used to inform the operation of an ambient temperature field-scale digester (∼1 m3) fed with dairy manure. Low OLRs successfully maintained a viable microbial community that recovered during warmer periods for two successive years. Results highlight the effectiveness of low and/or variable OLRs for the management of ambient temperature digesters in temperate climates.