Anaerobic Digestion of Food Industry Wastes: Effect of Codigestion on Methane Yield

Anaerobic treatability of two different wastes that represent the main refuse streams generated by a frozen food factory (fresh vegetable waste and precooked food waste) was assessed. Moreover, the sludge coming from agro-industrial wastewater treatment was codigested with the previously mentioned wastes. Batch tests were performed at different solids content both on the single wastes and on appropriate mixtures of them (also in order to simulate the seasonality of factory production). Both fresh vegetable and precooked food wastes strongly inhibited methanogenesis from unacclimated inoculum at 10% solids content (undiluted waste) and 5% solids content (eight- to nine-fold diluted waste), respectively. This was due to their high contents of potassium and lipids, respectively. The aerobic sludge from the wastewater treatment plant did not exert inhibitory effect up to 10% solids content (undiluted waste). Codigestion of the fresh vegetable waste and sludge (60 and 40% on wet basis) was more effective both in terms of rate and yield of methane production with respect to the single wastes (due to dilution and synergic effects). On the other hand, methanogenesis remained strongly inhibited from mixtures containing precooked food waste (at 25 and 45%, on wet basis). Methanogenesis inhibition could be overcome by a long acclimation period. The results showed that fill-and-draw digestion in a 0.5 L lab-scale reactor of the fresh vegetable waste and sludge mixture after start up with acclimated inoculum allowed higher methane yields (37% at high organic load and 57% at low organic load). Better results were obtained in a 1.7 L micropilot fill-anddraw reactor (yield of 67%) fed at higher frequency.