Systematic Analysis of Factors That Affect Food-Waste Storage: Toward Maximizing Lactate Accumulation for Resource Recovery

In the U.S., several states have attempted to mitigate greenhouse-gas emissions by banning food wastes from landfills. As a result, U.S.-based companies are now providing decentralized food-waste management systems for supermarkets and restaurants, which include storage as a slurry. It is unclear, however, which storage conditions (factors) would affect the spontaneous microbial activity, resulting in a different fermentation product spectra, and how this would further affect post-treatment. Here, we performed two experiments to mimic: (1) storage and (2) subsequent anaerobic digestion. For the food-waste storage system, we designed a mixed-level fractional factorial analysis with 12 experimental combinations, including separating food waste into: carbohydrate-rich, lipid-rich, and protein-rich food waste. We found that all factors that we tested affected the fermentation outcome. We observed that relatively low pH levels of 3-4, which were achieved due to rapid lactate accumulation by microbial activity during storage, coincided with greater lactate production and a maximum chemical oxygen demand (COD) selectivity of 90%. Food-waste storage followed classical ensilage dynamics with homofermentation to lactate in combination with low pH preventing the subsequent breakdown of lacate into other carboxylic acids and hydrogen gas. The mechanistic understanding provides an opportunity to optimize lactate production, which is ideal for subsequent methane or chemical production.