Thermophilic methane production from dairy cattle waste

Methane production from waste of dairy cattle fed diets containing 72% roughage (dry matter basis) was investigated in anaerobic reactors at 60°C with retention times (RT) set at 3, 6 and 9 days. Influent concentrations of volatile solids (VS) were increased in steps of 2% from 4 to 14% VS or until the reactor failed due to excess substrate. Gas production was measured and samples of effluent taken for six consecutive days after allowing three volume turnovers for each increment of VS. The maximum concentration of VS in the substrate for highest volumetric fermentation efficiency was 8–10, 10–12 and 12 for RT of 3, 6 and 9 days, respectively. Destruction of VS for these and lower substrate concentrations was 19–24, 26–27 and 30–33% for 3-, 6- and 9-day RT reactors, respectively. The corresponding methane production rates were 0·09-0·12, 0·11-0·14 and 0·11-0·16 liters/day/g of VS in substrate. The gas contained 51–56% methane with the rest mainly carbon dioxide. Increasing the percentage of feed VS beyond the values indicated resulted in greatly decreased VS destruction and methane production.

The dairy cattle waste contained the following constituents as percent of dry matter: Volatile solids (VS) 85, Na 0·37, K 3·8, P 1·0, Si 2, non-ammonia N expressed as crude protein (CP) 18, NH₃N 0·3, cell wall constituents 46, acid detergent fiber 32, hemicellulose 13, cellulose 21, lignin 10, acetate 3·1, lactate 6·6 and ether extract 4. Detailed analyses for organic acids indicated that the efficiency of methanogenesis was, in general, good until increasing organic loading rates caused total organic acid concentration to reach 120–150 mm. As loading rates increased, propionate was the first acid to increase but did not exceed levels higher than 27 mm. Acetate was the second acid to increase and reached levels as high as 150 mm at high reactor loading rates. Butyrate was low or absent initially but was produced at high loading rates, reaching levels of 30–40 mm. Inhibition of methane did not appear to be associated with a change in any specific acid. Isobutyrate and isovalerate plus 2-methylbutyrate, indicative of protein catabolism, accumulated in amounts as high as 5 and 7 mm, respectively, when 10–14% of VS were fed. Little n-valerate, n-caproate, formate or succinate accumulated.