Household-scale anaerobic digestion of food waste—a community case study from Bozeman, Montana

Abstract

Introduction: One-third of food in the United States is wasted, creating significant environmental and social challenges which anaerobic digestion (AD) can address by converting that waste into a nutrient-rich digestate suitable for use as a biofertilizer. While AD is commonly used in municipal and industrial-scale operations for obtaining biogas, household-scale AD to generate of biofertilizer is a promising on-site option without the infrastructure and process complexities associated with large-scale systems. Methods: This community-based case study investigated the characteristics of food waste and digestate nutrient content from two household-scale AD designs. Digesters were deployed in 12 households (six households used commercial digesters and six used a smaller custom-built prototype) in Bozeman, Montana, USA. Food waste was collected from households, separated by categories, and nutrient content was determined. Results and discussion: Over 12 weeks of digester operation, the nutrient content of the digestate was measured. It changed during digester operation with N, P, K, S, pH, and conductivity all increasing (p < 0.05) over time. While food waste C:N ratios varied from 12.1 to 25.7, the C:N ratio of digestate was significantly lower (p < 0.001), with a mean C:N ratio of 6.6. Digestate ammonia levels remained low (< 60 mg l−1) and below inhibitory thresholds during the collection period, but the ratio of NH3-N to total N increased from 0.09 in the food waste to 0.25 in the digestate. Digestate C and N content did not differ between digester models, while different pH (p < 0.05) was observed with a final pH of 6.1 in the commercial small-scale digester and a pH of 5.3 in the custom-built prototype. A survey of participating households revealed that 64% of respondents gained a new awareness of the volume of food waste they generated, and 87% expressed a willingness to recommend biodigesters despite challenges related to convenience and using this technology in a cold climate. Participants perceived the custom-built digester as more user-friendly than the commercial one. This case study highlights the potential of household-scale AD systems to generate biofertilizer, emphasizing the importance of user-friendliness and design tailored to household needs, generated feedstock, and environmental conditions.