7. Methane Production from Napier Grass by Co-digestion with Cow Dung

Methane could substitute for fossil-fuel-derived energy and reduce environmental impacts including global warming. Grass can be transformed into energy by anaerobic digestion. The objective of this study was to investigate the co-digestion of napier grass with cow dung. Digestion of napier grass at a cutting interval of 60 days was investigated in single-stage, semi-continuous anaerobic reactors. Four reactors were operated at 30 °C with 5-day feeding. The first two reactors were fed with a slurry of napier grass alone at 10 % (napier grass:water = 10:90) and 20 % (napier grass:water = 20:80) by fresh weight. The other two reactors were fed with a mixture of napier grass and cow dung at separate concentrations of 10 % (napier grass:cow dung:water = 5:5:90) and 20 % (napier grass:cow dung:water = 10:10:80), respectively. Mixed ruminal microorganisms of approximately 8.5 g mixed liquor volatile suspended solids per litre were used as the inoculum. Each reactor working volume was 5 L and the feeding rate was 625 ml per 5 days resulting in a hydraulic retention time of 40 days. The pH was initially adjusted to be neutral in all reactors and the reactors functioned without any further pH control. The results showed that co-digestion of the mixture of napier grass and cow dung gave a higher yield than that of napier grass alone. The highest methane yield was obtained from the reactor that contained the 20 % mixture of napier grass and cow dung (napier grass: cow dung:water = 10:10:80) with 143 L at STP per kg chemical oxygen demand (COD) added and 169 L at STP per kg total volatile solids (TVS) added. The pH of the reactor was just over 7.