Co-digestion of mushroom compost with switchgrass using solid-state anaerobic digester

Abstract

Spent mushroom compost (SMC) already broken down into smaller particles by fungal action is an ideal material for producing biogas. Two cycles of five solid-state anaerobic digesters (SS-ADs) with different mix-ratio of SMC and switchgrass (SG) were operated at feedstock-to-effluent ratio of 2 at a temperature 35 ± 2°C. The total solids concentration of the digester was kept at ∼17%. Initial biogas production observed during the start-up of the digester confirmed the presence of readily available extractives for digestion. In the first cycle, the highest methane yield was observed in SMC 0 (0% SMC + 100% SG) of 28.82 l/kg VS/d and the lowest yield was observed in SMC 4 (100% SMC + 0% SG) as 10.32 l/kg VS/d. The substrate containing 100% SG (SMC 0) recorded the highest cumulative biogas yield of 295.43 l/kg VS in 63 days. The digesters with higher SMC fraction showed lower methane production, low pH value and high volatile fatty acids content upon decomposition. The SS-ADs having SMC/SG of 50 : 50 showed more than 2 times methane production in comparison with SS-ADs having SMC as sole substrate. An estimation of volumetric productivity also established a linear relationship with the SMC/SG ratio.