Continuous Lactic Acid Fermentation of Actual Food Waste Slurry: Effects of Reaction Temperature and Hydraulic Retention Time

Food waste (FW) valorization via lactic acid (LA) fermentation is a promising approach, yet most studies rely on surrogate substrates, limiting industrial applicability. This study investigated continuous LA fermentation of actual FW slurry from a Shenzhen treatment plant, focusing on the effects of hydraulic retention time (HRT: 1.33, 2.0, 3.0 days) and temperature (25, 30, 35, 40 °C) under open-culture conditions. Results showed that hydraulic retention time (HRT) and temperature jointly regulated fermentation performance and microbial communities. Short HRT (1.33 days) caused system instability with fluctuating LA concentrations, while extending the HRT to 2 days enhanced stability, achieving LA concentrations exceeding 14,000 mg/L across all temperatures. Further increasing the HRT to 3 days reduced LA yield and acidification rate, accompanied by acetic/propionic acid accumulation, especially at higher temperatures. Temperature affected the system’s hydrolysis rate, with higher temperatures promoting the hydrolysis of cellulose and hemicellulose. Metagenomic analysis revealed that Lactobacillaceae dominated (>90% of bacteria). The phosphoketolase pathway was the most abundant LA-producing route, and temperature and HRT affected carbohydrate-active enzyme profiles. Balancing efficiency and cost, an HRT of 2 days at 30–35 °C is recommended for industrial LA fermentation of actual FW.