Psychrophilic dry anaerobic digestion of high solids content dairy manure: Long-term operation

This is the first study on successful long-term psychrophilic (20°C) dry anaerobic digestion (PDAD) of cow feces with wheat straw at 27% total solids (TS) in feed. Experimental results demonstrated the feasibility of cow feces and wheat straw PDAD in long-term operation of a laboratory-scale sequencing batch reactor. An average specific methane yield (SMY) of 182.9 ±16.9 NL CH4 kg-1 VS fed during 12 successive cycles (273 days) was obtained for feed TS of 27% at an organic loading rate 3.0 g TCOD kg-1 inoculum d-1 and treatment cycle length (TCL) of 21 days. A maximum SMY of 219.2 ±18 NL CH4 kg-1 VS fed with a maximum CH4 production rate of 10.2 ±0.8 NL CH4 kg-1 VS d-1 was achieved. The low levels of volatile fatty acid concentrations in the bioreactor indicated that hydrolysis was the reaction-limiting step. The SMY, methane production rate, and their reproducibility during 12 successive cycles indicate that PDAD of cow feces and straw (27% TS in feed) is as efficient as mesophilic DAD.