High solids co-digestion of food and landscape waste and the potential for ammonia toxicity

•We evaluated co-digestion of food and landscape waste with a pilot-scale anaerobic dry digester.•We evaluated reactor performance at 35 °C under low and high organic loading rates.•Performance was stable under low organic loading rate, but declined under high organic loading rate.•Respirometry was employed to investigate potential inhibition due to ammonia.•Landscape waste was unsuitable in increasing the C:N ratio during codigestion.

A pilot-scale study was completed to determine the feasibility of high-solids anaerobic digestion (HSAD) of a mixture of food and landscape wastes at a university in central Pennsylvania (USA). HSAD was stable at low loadings (2 g COD/L-day), but developed inhibitory ammonia concentrations at high loadings (15 g COD/L-day). At low loadings, methane yields were 232 L CH4/kg COD fed and 229 L CH4/kg VS fed, and at high loadings yields were 211 L CH4/kg COD fed and 272 L CH4/kg VS fed. Based on characterization and biodegradability studies, food waste appears to be a good candidate for HSAD at low organic loading rates; however, the development of ammonia inhibition at high loading rates suggests that the C:N ratio is too low for use as a single substrate. The relatively low biodegradability of landscape waste as reported herein made it an unsuitable substrate to increase the C:N ratio. Codigestion of food waste with a substrate high in bioavailable carbon is recommended to increase the C:N ratio sufficiently to allow HSAD at loading rates of 15 g COD/L-day.