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.