Sodium percarbonate effectively enhances medium-chain fatty acids production from waste activated sludge via facilitating electron acceptor formation, providing inorganic carbon and enriching functional microorganisms

Medium-chain fatty acids (MCFA) production from waste activated sludge (WAS) has been a promising approach for resource and energy recovery. However, the poor sludge biodegradability and existence of competitive mi-croorganisms result in inefficient electron donor utilization and low MCFA output. Therefore, a percarbonate (SPC)-based strategy is proposed to effectively enhance MCFA production. SPC pretreatment largely boosts substrate accessibility and organics transformation through the dual function of chemical oxidation and alkaline hydrolysis in the acidogenic fermentation stage, providing abundant electron acceptors directly available for functional microorganisms in the chain elongation (CE) stage. Consequently, MCFA production is significantly enhanced from 3522 mg COD/L of the control to 10,437 mg COD/L of 0.15 g SPC/g TSS, while decreased to 1898 mg COD/L at 0.25 g SPC/g TSS. Meanwhile, long-chain alcohols (LCA) production is apparently reduced by 68.3%, implying a decrease in the flow of electrons to byproducts. Importantly, SPC pretreatment with an appropriate level can provide inorganic carbon source for CE microbes to further increase the MCFA yield. Mechanistic studies suggest that SPC pretreatment with appropriate levels improves microbial metabolisms contributing to MCFA production and functional gene expression involved in CE pathways. Additionally, SPC enriches functional microorganisms from 19.25% to 59.75%, such as MCFA (i.e., Proteiniclasticum sp.) and butyrate (i.e., Romboutsia sp. and Alkaliphilus sp.) producers.