pH regulation of the first phase could enhance the energy recovery from two-phase anaerobic digestion of food waste.

The effect of pH regulation in phase I on hydrolysis and acidogenesis rate, metabolites production, microbial community, and the overall energy recovery efficiency during two-phase anaerobic digestion (AD) of food waste (FW) was investigated. pH strongly affected the acidogenesis rate and the yield of the fermentation products. The highest acidogenesis efficiency (60.4%) and total volatile fatty acids (VFA)/ethanol concentration (12.4 g/L) were obtained at pH 8 during phase I. Microbial community analysis revealed that Clostridium IV was enriched at pH 8, relating to the accumulation of butyrate. Also, Clostridium sensu stricto played a crucial role in hydrogen production and was abundant at pH 6, resulting in the highest hydrogen yield (212.2 ml/g VS). In phase II, the highest cumulative methane yield (412.6 ml/g VS) was obtained at pH 8. By considering the hydrogen and methane production stages, the highest energy yield (22.8 kJ/g VS, corresponding to a 76.4% recovery efficiency) was generated at pH 8, which indicates that pH 8 was optimal for energy recovery during two-phase AD of FW. Overall, the results demonstrated the possibility of increasing the energy recovery from FW by regulating the pH in the hydrolysis/acidogenesis phase based on the two-phase AD system. PRACTITIONER POINTS: pH 8 was suitable for hydrolysis, acidogenesis, and methanogenesis. High hydrogen yields were obtained at pH 5-8 (about 200 ml/d). Clostridium sensu stricto might have played a crucial role in hydrogen production. High methane production (about 400 ml/g VS) was obtained at pH 7-9. pH 8 was optimal for energy recovery from FW with an efficiency of 76.4% (22.8 kJ/g VS).