Biogas production and digestate quality of diet-influenced food waste in anaerobic digestion

Emily Nungesser, Sarah Davis and Kimberley E. Miller, Voinovich School of Leadership and Public Affairs: Environmental Studies Program, Ohio University, Athens, OH

Background/Question/Methods
Anaerobic digestion (AD) is increasingly becoming a sustainable waste management strategy in the United States. Food waste (FW) composition varies with dietary choices and can affect the amount and quality of products from AD. Food waste is a high-quality feedstock for AD, a process that can divert greenhouse gases generated from biodegradation, generate fuel, and recover valuable nutrients, while reducing negative consequences of conventional waste management processes. The human diet that produces FW varies regionally and over time, but there is little known about the effect of diets on FW composition, potential energy generation from different dietary wastes, and secondary by-products from changing waste streams. The implications for changes in diet and subsequent wastes on AD performance is currently unknown. The objective of this research is to characterize the biogas production and digestate quality of anaerobically digested food wastes. Lab-scale AD vessels were used to compare food wastes from four popular, distinct diets: paleolithic, ketogenic, vegetarian, and omnivorous. Food waste compositions in each dietary treatment mimicked the proportional wastes for each diet. Replicated experimental AD vessels (N = 24) were measured for biogas production over 21 days. Carbon, nitrogen, and phosphorus nutrients were also measured in each substrate pre- and post-digestion.
Results/Conclusions
Results suggest that the AD of FW from the ketogenic diet, which eliminates carbohydrates and is higher in fat and protein, would result in biogas with 317% greater methane than the reference omnivorous diet. The digestate of the paleolithic diet contained 80% greater concentration of phosphorus. All alternative diets resulted in a digestate with lower nitrogen (-13% on average) than the omnivorous diet. Fats, as long as they are not found in concentrations that cause inhibition, produce high-quality biogas, which was the case for the ketogenic diet; while carbohydrate-rich diets, such as the vegetarian diet, resulted in nitrogen-rich effluents after AD, which produce high-quality fertilizer. As diets become popular, they have implications for the composition of FW, and the potential energy yield from AD systems. This research quantifies the relationship between the human diet and potential uses of food waste. The AD process generates methane-rich biogas as a renewable fuel, which can help reduce dependency on non-renewable fuels, and the remaining organic slurry can be further refined into a soil amendment that supplies nutrients for food crops, thereby closing the loop in the FW cycle.