Anaerobic digestion of food wastes for biogas production

Five types of food wastes were investigated as feedstock for a potential centralized anaerobic digester system in the area of Sacramento, California to produce biogas energy. The wastes were from a soup processing plant, a cafeteria, a commercial kitchen, a fish farm, and grease trap collection service. Digestibilities of the food wastes, individually and in mixtures, were conducted at mesophilic (35°C) and thermophilic (50°C) temperatures and at two food to microorganism ratios (F/M) of 0.5 and 1.0, for 28 days. A continuously fed mesophilic single-stage anaerobic digester was evaluated using a mixture of the five food wastes at organic loading rates of 0.5 to 1.0 g VS/L/d. In the batch digestion tests, fish and grease trap wastes required longer time to complete the digestion and had higher biogas yields than the other wastes. The continuously-fed digester required the addition of sodium hydroxide to maintain pH at proper levels in the digester. Alkalinity of about 2,500 mg CaCO3/L and pH above 7 was maintained by adding 0.2 g NaOH/g VS. The results of this study indicated that it was necessary to use the chemicals, such as NaOH, to control the pH of the single-stage anaerobic digester treating the food waste. For commercial applications, the cost of chemicals and proper management of additional salts in the digester effluent need to be carefully considered.