Technical Cooperation Officer, Overseas Development Administration, London2 THE PRODUCTION OF BIOGAS FROM CATTLE SLURRY, THE EFFECTS OF CONCENTRATION OF TOTAL SOLIDS AND ANIMAL DIET1

Two pairs of young bulls were fed contrasting diets based on grass / molasses or chopped sugar cane. The faeces and urine from each pair were collected daily and diluted to 1, 3, 5 and 8 % total solids (TS). The resulting 8 slurry mixtures were used as substrate for eight 200 1 drum anaerobic. digesters. Biogas production increase, 1inearly with increasing total solids concentration to a maximum of 0.63 m /m digester volume/day. This can be attributed to a greater concentration of organic matter being3 available for degradation to methane and carbon dioxide. The efficiency of gas production however, decreased with increasing TS concentration yielding up to 60 % more gas/kg TS at 1 % TS than at 8%. There was a significant difference in the gas production fro. the slurries derived from animals fed the two contrasting diets, with the sugar cane based diet giving consistently higher yields. Physical conditions of pH and organic solids concentration would appear to favour the molasses/grass diet. It is suggested that the higher C:N ratio is the reason for the superiority of the sugar cane diet Key words: Biogas, anaerobic digestion, total solids, diets. The majority of work on the factors affecting biogas production has been done on high rate, completely mixed digesters. For developing tropical countries simple plug flow, unmixed digesters operating at ambient temperatures are more appropriate i