Stabilizing the Anaerobic Digestion of Food Waste for Biomethane Production

Anaerobic digestion (AD) is a viable method for conversion of food waste and other organic materials into methane-rich biogas. However, when used at high organic loading rates, using only food waste can lead to an unstable process. Process instability is indicated by frequent changes in pH, and increase in ammonia and hydrogen sulfide concentration. These uncontrolled changes combined with over-production of organic acids can inhibit biogas production and ultimately lead to digester failure. Therefore, certain co-substrates produced as wastes in the regional food sector were tested as stabilizing agents for food waste digestion with an objective of achieving stable non-manure based digestion. The substrates tested were acid whey, bread, manure, caffeinated drink, paper napkins and apple pomace. The biogas production was increased by 12% in reactors containing bread, by 10% with acid whey, and by 12% when the co-substrate was caffeinated drink. The reactors containing paper and manure showed decreased biogas production by 6% and 12% respectively, but these changes are relatively small and thus not considered inhibitory. Co-digestion with apple pomace was found to be inhibitory and resulted in digester failure. This initial study has demonstrated that the stability of AD systems may be improved by strategically combining available food waste feedstocks.