No abstract

Anaerobic digestion is applied to recover 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 VS RS /kg of mixture is an important parameter. Only batch reactors with the lowest S 0 values (29 g VS RS /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 VS RS /kg. Use of the digestate with S 0 of 29 g VS RS /kg improved both initial degradation kinetics and the methane yield measured after 60 days. However, at S 0 of 55 g VS RS /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 VS RS /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 VS RS /kg may be recommended for industrial applications.