Lowering Costs of Food Waste Codigestion for Renewable Biogas Production - An Innovative Combination of Preprocessing Technique and Strategy for Cake Solids Reduction
September 29, 2020
Energy Research and Development (500)
Electric Program Investment Charge - EPIC
Ganesh Rajagopalan, Ph.D., PE, BCEE; Bhargavi Subramanian, Ph.D.; Helia Safaee; Ryan Holloway, Ph.D., PE
This project demonstrated a new technology to lower the preprocessing cost of food wastes and a new strategy to add fats, oil, and grease or pre-processed food wastes for anaerobic digestion with wastewater sludge (codigestion). These two steps aim to lower dewatered cake requiring disposal, which, in turn, will encourage the diversion of food waste from landfills, and enhance production of digester gas, a renewable energy source. The new technology for the preprocessing of food wastes uses an organic waste separation technique known as an organic extrusion press for selective extrusion of organic materials from inorganic contaminants, based on the differences in their viscosities. The extraction efficiency of this technology is higher than that of conventional, size-based separation techniques predominantly practiced by the industry. Further, since the proposed technology requires minimal preprocessing of the municipal solid waste, it is more economical than typical source separation programs.
Results indicated that preprocessing of food wastes using the organic extrusion press resulted in 54 percent cost savings compared to conventional food wastes separation technique. Economic evaluation of a 100-million-gallons-per-day plant indicated that codigestion of food waste preprocessed using this technology resulted in $8.9 to $9.3 million worth of energy savings due to increased gas production. Codigestion with fats, oils, and grease increased gas production by up to 58 percent compared to the sludge-only digestion. The gas production for every pound of combined sludge and food waste added was 20 percent higher compared to a pound of only sludge fed to the digester. Strategic addition of the food waste improved the percent of solids of the dewatered cake by approximately 13 percent, lowering the net mass of cake requiring disposal by 11.5 percent. The estimated payback period for a 100-million-gallons-per-day wastewater treatment plant to implement codigestion ranged from 4.7 to 5.3 years.