Anaerobic Digestion and Energy Recovery from Food Waste

This treatability study evaluated the technical and start-up considerations of biogas production and energy recovery from digestion of food wastes in the absence of waste activated sludge (i.e., separate food waste digestion rather than co-digestion), aiming to support design of full-scale anaerobic digesters at Department of Defense (DOD) installations. Project objectives included characterizing the composition and variability of multiple food wastes from Mitchell Hall at the U.S. Air Force Academy (USAFA), quantifying food waste digestibility and energy yield, identifying potential nutrient limitations, and determining appropriate specific energy loading rates (SELR) for these wastes. The SELR is a measure of energy loading relative to the reactor biomass, and is an innovative approach to characterizing digester capacity and stability. The treatability study consisted of three phases. In Phase I, food waste was collected from 15 different meals at USAFA's Mitchell Hall, as well as a sample of material from the grease trap. These 16 samples were analyzed for moisture, protein, fat, ash, and carbohydrate content, as well as for volatile solids (VS), chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), and total phosphorus. Selected samples were also analyzed for trace metals. Phase II was a test of the biological methane potential (BMP) of each of the 16 wastes. The characteristics of the food and grease trap wastes were examined for correlations with the BMPs of the wastes. In Phase III, food wastes were digested in bench-scale, semi-continuous reactors and monitored using an online respirometer capable of continuously monitoring gas flow rate and gas composition. The digesters were operated under varying conditions including composition (e.g., food waste with and without grease trap waste), SRTs between 10 and 100 days, organic loading rates between 2 and 10 g COD/L/day, feed VS ranging from 4 to 15 percent, and with nutrient addition. The results of the bench-scale analyses were used to calibrate a preliminary economic and digestion model, and to guide the design of the pilot system. Pilot testing will be conducted in 2012.