Optimization of high solids batch anaerobic co-digestion of lignocellulosic biomass and cow dung under mesophilic temperature conditions

Anaerobic digestion is applied to revover energy from rice straw (RS) which is a lignocellulosic agricultural residue produced in huge quantities in Asia and Africa. Because of the high solids content of this feedstock, high solids anaerobic co-digestion in batch mode must be further investigated. In this study, optimal operating conditions for the anaerobic co-digestion of RS with cow dung (CD) in batch mode, with and without leachate recirculation, were assessed under mesophilic temperature conditions. Preliminary experiments carried out in 2 L batch reactors confirmed that the concentration of RS in the mixture of substrates S-0, in g VSRS/kg of mixture is an important parameter. Only batch reactors with the lowest S-0 values (29 g VSRS/kg) produced biogas and rest of the reactors followed a long lag phase. The use of digestate from a previous batch as an inoculum was investigated with S-0 values of 29 and 55 g VSRS/kg. Use of the digestate with S-0 of 29 g VSRS/kg improved both initial degradation kinetics and the methane yield measured after 60 days. However, at S-0 of 55 g VSRS/kg, the degradation kinetics were affected and after two months, 32 % of the biodegradable organic matter could not be eliminated. When leachate recirculation was performed in 6L leach-bed reactors (LBRs) with S-0 between 30 and 65 g VSRS/kg, the highest methane yield was recorded at the lowest S-0 value. It can be concluded that under batch mode, an RS concentration around 30 g VSRS/kg may be recommended for industrial applications.