Effect of temperature and retention time on methane production from beef cattle waste

The effect of temperature and retention time on the rate of methane production from waste of beef cattle fed a finishing diet was investigated by using continuously mixed 3-liter working volume anaerobic fermentors. The temperatures ranged from 30 to 65 degrees C with 5 degrees C increments between fermentors. The fermentors were fed once per day with 6% volatile solids (organic matter). Retention time for each temperature was varied from 18 to 2.5 days. After 3-volume turnovers, samples were obtained on 4 consecutive days. The highest methane production rate (liters/liter of fermentor per day) and methane yield at that rate (liters/gram of volatile solids) were 1.27 and 0.19 at 9 days and 30 degrees C, 1.60 and 0.16 at 6 days and 35 degrees C, 2.28 and 0.23 at 6 days and 40 degrees C, 2.42 and 0.24 at 6 days and 45 degrees C, 2.83 and 0.14 at 3 days and 50 degrees C, 2.75 and 0.14 at 3 days and 55 degrees C, 3.18 and 0.14 at 2.5 days and 60 degrees C, and 1.69 and 0.17 at 6 days and 65 degrees C. Volatile solids degradation at these retention times and temperatures was between 46 and 54%. The concentrations of volatile acids in the 30 to 55 degrees C fermentors were generally below 2,000 mg/liter, with the exception of the 3-day retention time. The 60 and 65 degrees C fermentors were usually above this level for all retention times. These studies indicate potential rates of methane production from the fermentation of untreated waste of beef cattle fed high-grain finishing diets. This information should serve as preliminary guidelines for various kinetic analyses and aid in economic evaluations of the potential feasibility of fermenting beef cattle waste to methane.