Methane production from napier grass by two-stage anaerobic digestion

Abstract

Digestion of Napier grass was investigated in two-stage anaerobic reactors. The reactor volume was 4 and 5 litres for the acidogenic and methanogenic reactors, respectively. The acidogenic reactors were fed once daily at 100, 150, 200, 250, and 300 mL/day with a slurry of Napier grass. The organic loading rates (OLRs) were 0.60, 0.90, 1.20, 1.50 and 1.80 kg chemical oxygen demand (COD)/m3.day. Mixed ruminal microorganisms from cows were used as the inoculum. The pH was adjusted to 7.5 for all methanogenic reactors at the start-up period. The reactors were operated at 30°C. The maximum methane (CH4) yield was observed at an OLR of 1.20 kg COD/m3.day. The average pH in the acidogenic and methanogenic reactors was 5.3 and 7.4, respectively. At the maximum CH4 yield OLR, the CH4 production was 148 litres at STP/kg dry Napier grass added to the reactor, which indicated that 6.76 kg of dry Napier grass is needed to produce 1 m3 of pure CH4.