Starting-up low temperature dry anaerobic digestion of cow feces and wheat straw

Psychrophilic dry anaerobic digestion (PDAD) of animal manures and agriculture residues is of high interest in cold-climate regions. This paper reports the results of a start-up experiment (113 days) of PDAD of cow feces and wheat straw mixture (at two total solids (TS) of 18 and 21%) in laboratory scale sequence batch reactor (SBR) at 20 °C. An average specific methane yield (SMY) of 96.1 ± 5 L of CH4 per kg of volatile solid (VS) corrected to standard pressure and temperature (101.3 kPa and 273 K) (NL CH4 kg−1 VS) has been achieved for a feed with TS of 18% along with an organic loading rate (OLR) 4.0 g total chemical oxygen demand (TCOD) kg−1 inoculum day−1 and a treatment cycle length (TCL) of 21 days. An average SMY of 149.9 ± 14 NL CH4 kg−1 VS with a maximum daily CH4 production rate of 7.2 ± 0.7 NL CH4 kg−1 VS day−1 have been obtained for a feed with total solid of 21% along with an average daily inoculum OLR of 4.2 g TCOD kg−1 inoculum day−1 and TCL of 21 days. The rapid decrease in volatile fatty acids concentration after 7 days of treatment and their low concentration thereafter indicated that hydrolysis was the reaction limiting step. The results indicate that PDAD of cow feces and wheat straw is feasible at feed TS of 21%.