Agricultural wastes, Animal waste, Methane
A microbiological and economic study of methane production from dairy cattle waste was performed. The profit potential of producing methane and other vendable products from dairy cattle wastes was studied using a computerized cost model. The unit gas cost ($/cu. ft. methane) was determined for refractory volatile solids (VS) concentrations between 52% and 28% (W/W). Reaction rate constants (RKO) between 5.92 x 109 and 1.24 x 1011 were used. Retention time (RT) was varied between 1 and 10 days. Total solids (TS) concentration was varied between 8% and 14%. Analyses were performed with and without a fertilizer plant option for upgrading digester effluent solids. Unit gas cost (UGC) decreased as RKO increased and as the refractory VS concentration decreased when determined without the fertilizer option. UGC decreased at short retention times as RKO increased when the fertilizer option was included. The unit gas costs were always above $8.00 per M. cu. ft. CH4 without a fertilizer plant, and were consistently lower than the current intrastate market price of $3.18 per M. cu. ft. CH4 when a fertilizer plant was incorporated into the system. Microbiological studies were conducted using a multistage multistream digester. The design consisted of a 1,700 liter central digester with a working volume of 1,200 liters and 10, 50 liter satellite digesters with a working volume of 40 liters each. The digester design allowed for the automatic addition of substrate to the central digester once per hour and three times per hour to the satellites. The digester was operated at 55┬░C and 10% TS with a 6 day RT in the central digester and 2 days RT in the satellites. Manure from a commercial dairy was utilized for substrate. Methane production was directly related to the type of cattle feed ration. It ranged between 1.27 and 0.3 liters CH4 per liter of reactor fluid per day at a 6 day RT. Alkalinity, volatile fatty acids (VFA) and ammonia concentrations were related to methane production. VFA concentrations were lower and methane production slightly higher in the satellite digesters. Analysis of the digester effluent for fertilizer value was investigated by drying for 10 days on a as and drying bed at an initial depth of 10 cm. Total nitrogen, phosphorous (as P2O5) and potassium (as K2O) concentrations were: 1.8%, 1.1% and 7.2% for undigested manure; 4.5%, 2.3%, and 9.1% for 6 day RT effluent; 2.0%, 1.1% and 7.5% for 8 day RT effluent. Our economic studies indicate that digester operating conditions should include a 3-5 day RT, 10-12% TS, minimal changes in feed ration and recovery of solids for upgrading to fertilizer.
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Master of Science (M.S.)
College of Arts and Sciences
Length of Campus-only Access
Masters Thesis (Open Access)
Scholla, Michael H., "Methane Production from Dairy Cattle Waste" (1981). Retrospective Theses and Dissertations. 592.