Enhanced volatile fatty acids production in thermal hydrolysis pretreatment with constant pH regulation: Effects evaluation and mechanism elucidation

Thermal hydrolysis pretreatment (THP) is a promising technology, and this technology has gained significant attention in sludge fermentation pretreatment. However, the operational conditions favoring VFAs production by improving hydrolysis and acidogenesis in subsequent process remain unclear. In this study, we investigated and elucidated the crucial role of maintaining a constant pH value after THP in influencing the chemical and microbial components. Results indicate that the highest VFAs production and acetic acid proportion (9847.5 mg COD L- 1 and 45.5 %) were achieved under pH of 11, which was much higher than that derived from acidic pH condition. Mechanism investigations revealed that the alkaline and acidic pH conditions all had a positive effect on hydrolysis and solubilization, especially at pH=11. Different functional microbes involved in VFAs production and hydrolysis were predominant under alkaline and acidic pH conditions, contributing to enhanced VFAs production. At the genetic level, the functional genes participating in fermentation substrate metabolisms and VFAs biosynthesis showed improvement under the alkaline pH condition after THP. This enhancement likely played a role in the efficient production of VFAs. Overall, the study suggests that implementing a constant pH control strategy with an alkaline pH condition after THP can effectively improve VFAs accumulation.