Biological Energy Recovery Using Dairy Cow Waste

Laboratory research which applied anaerobic digestion to dairy manure was concerned with optimizing gas production for potential energy recovery. The digesters were operated successfully at loading rates of 0.14 lb VS/cu ft/day to 0.56 lb VS/cu ft/day (2.3 kg/m³/day to 9.2 kg/m³/day) which corresponded to SRT values of 7.5 to 30 days. Calculated heat balances for a 1,270-cu ft (36 m³) digester indicate that winter is the critical time for which the system must be economical. Operation at a high loading rate and short SRT are recommended for maximizing energy recovery. Estimated energy surpluses for a 60-cow herd increased from 116,000 Btu/day to 722,000 Btu/day (35,000 kg.cal/day to 199,000 kg.cal/day) for the loading rates examined. Operation of the prototype digester is being directed by the laboratory findings. The prototype digester, including primary and secondary zones, has a fluid volume of 3,600 cu ft (100 m³).