Thermophilic methane production from cattle waste

Abstract

Methane production from waste of cattle fed a finishing diet was investigated, using four 3-l-working volume anaerobic digestors at 60.degree. C. At 55.degree. C a start-up culture in which waste was the only source of bacteria was generated within 8 days and readily adapted to 60.degree. C, where efficiency of methanogenesis was greater. Increasing the temperature from 60.degree. to 65.degree. C tended to drastically lower efficiency. When feed concentrations of volatile solids (VS, organic matter) were increased in steps of 2% after holding for 1 mo. at a given concentration, the maximum concentrations for efficient fermentation were 8.2, 10.0, 11.6 and 11.6% for the retention times (RT) of 3, 6, 9 and 12 days, respectively. The VS destructions for these and lower feed concentrations were 31 to 37, 36 to 40, 47 to 49 and 51 to 53% for the 3-, 6-, 9- and 12-day RT digestors, respectively, and the corresponding methane production rates were .apprx. 0.16, 0.18, 0.20 and 0.22 l/day per g of VS in the feed. Gas contained 52-57% methane. At the above RT and feed concentrations, alkalinity rose to 5000-7700 mg of CaCO3/l (pH to 7.5-7.8), NH3 plus NH4+ to 64-90 mM, and total volatile acids to 850-2050 mg/l as acetate. The 3-day RT digestor was quite stable up to 8.2% feed VS and at this feed concentration produced methane at the very high rate of 4.5 l/day per l of digestor. Increasing the percentage of feed VS beyond those values indicated above resulted in greatly decreased organic matter destruction and methane production; variable decrease in pH; and increased alkalinity, NH3 and total volatile acid concentrations, with propionate being the first to accumulate in large amounts. In a 2nd experiment with another lot of waste, the results were similar. Loading rates can be much higher than those previously thought useful for maximizing methanogenesis from cattle waste.