Anaerobic Co-Digestion of Cow Dung with Processing Waste of Oil Sardine and Threadfin Bream for Biogas Production

Abstract- Biogas typically refers to gas produced by the biological breakdown of organic matter by anaerobic digestion, particularly methane that has use as bio-fuel. The main objectives of the present project were to work out suitable conditions for maximum methane yield by co-digestion of fish waste of either oil sardine (Sardinella longiceps) or Japanese threadfin bream (Nemipterus japonicus), with cow dung, and to study the influence of fat content of fish on gas production.

Different combinations of digestion slurry were prepared using waste of either of the fish species and cow dung in the ratios 1:1, 1:1.2 and 1:1.5, by weight, at a constant solid content of 8%. Digestion was carried out at a constant temperature of 37oC in laboratory scale digestion units with provision for removal of carbon dioxide and collection of methane gas. Solids were digested to the extent of 60-64%. Gas production commenced after 5-6 days of digestion, maximum production rate was observed in 9-11 days and the production was stopped in about two weeks. Both total methane production and methane yield were highest for the fish waste- cow dung ratio 1:1.2, possibly because of more suitable conditions such as the C: N ratio provided by this ratio, followed by the ratio 1:1.5 and least for the ratio 1:1, irrespective of the fish used.

The total gas productions were 59.5 ml and 38.5 ml, and gas yields were 13.87 ml/ g and 9.22 ml/ g, for sardine-dung mixture and threadfin bream- dung mixture, respectively. The data suggest that sardine-dung mixture is far superior to threadfin bream- dung mixture with respect to gas production, possibly because of the higher fat content of the former. The yield rates are comparable with observations of other workers.

A slight decrease in pH of the digestion slurry was noticed in all the combinations tested possibly because of accumulation of fatty acids. However, the pH variation from around 7.4 to around 6.6 was within the pH recommended by other workers for optimum digestion. The initial anaerobic bacterial load of the digestion slurry was in the narrow range 2- 3×106/ g for all the combinations of slurry tested. The counts of all the treatments were reduced during the digestion process to again a narrow range of 2.8- 4.0×105/ g. It could be presumed that even though considerable numbers of anaerobic bacteria were still present in the slurry even after stoppage of gas production the methanogenic organisms could not further act to produce gas on account of the unfavourable conditions set in the slurry.

From the study the waste of oil sardine mixed with cow dung in the ratio 1:1.2 and 8% solid content was found to be the most suitable combination of slurry for production of methane gas.