Wastewater and organic waste to bioenergy

Abstract

Throughout the world, wastewater and organic waste are increasingly being viewed as energy sources and the practice of converting them into bioenergy through conversion to biogas with anaerobic digestion is growing. This paper presents an overview of planning, research, and full-scale operations of both separate and codigestion of organic waste. Organic waste management methods are compared with respect to economic (life-cycle costs), environmental (equivalent carbon dioxide emissions), social, and operational impacts for a representative 100,000 population community. Management methods include using sewers or trucks to transport the organics to anaerobic digesters at a wastewater treatment plant, using a material recovery facility (MRF) to extract the organics from municipal solid waste for anaerobic digestion, composting the organic waste, or sending the organics to a landfill. Hauling the organics to anaerobic digesters had the lowest equivalent CO2 emissions, while using the sewer to convey organics had the lowest life-cycle cost. An example of codigestion of organic waste with wastewater sludge at the Des Moines Water Reclamation Facility (Iowa, USA) is described. The limits of organic loading rates for digestion of FOG (fats, oils, and grease) with wastewater sludge are presented based on research using 1,000-litre (L) pilot digesters. A specific energy loading rate (SELR) is proposed as an improved parameter for organic loading rates. The SELR is a measure of energy loading relative to the reactor biomass, and is an innovative approach to characterizing digester capacity and stability. Food wastes from the cafeteria at the U.S. Air Force Academy 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 biological methane potential (BMP) of several organic wastes were measured in lab-scale digesters. Organic wastes were digested with and without domestic wastewater sludge. Separate digestion of organic wastes was found to be nutrient (cobalt, nickel) deficient, where codigestion with wastewater sludge experienced no deficiencies. Codigestion could also handle a greater amount of FOG being fed to the digesters than separate digestion of food wastes.