Feasibility and mechanism exploration of enhancing short chain fatty acid production assisted by nitrate photolysis during sludge fermentation

Seeking for the eco-friendly and low-cost pretreatment technology has become the prerequisite for the efficient release of the embedded organics in waste activated sludge (WAS), further for the higher production of short chain fatty acids (SCFAs) via anaerobic fermentation. Nitrate, which can produce reactive oxygen species (ROS) and reactive nitrogen species (RNS) during photolysis, may have the positive effect on WAS disintegration and organics degradation, while its performance and mechanism for WAS treatment is still unknown. This study investigated the feasibility of the nitrate photolysis on WAS disintegration and SCFAs production, and elucidated the underlying mechanism. Results showed that the highest acidification rate (26.6 h⁻¹) and short-chain fatty acid (SCFAs) production (5370.6 mg COD/L) were achieved at 8 d in the nitrate photolysis group (Nitrate-UV) during the anaerobic fermentation, significantly promoted by 2.3 and 12.7 folds than that of Control (raw WAS). Acetic acid (HAc) and propionic acid (HPr) accounted for 89.1 % and 90.1 % in Nitrate-UV and Nitrate group (sole nitrate addition), which were much higher than that obtained in UV and Control. HO^• and O₂^•− were the main contributors compared with NO₂^•, ONOO^– and NO^• on the release and bioconversion of the soluble organics. Microbial community and metagenomics revealed the promotion of nitrate photolysis on anaerobic fermentation bacteria (AFB) and nitrate reducing bacteria (NRB), as well as the gene abundance related to the metabolic pathways of glycolysis, amino acid, acetate and nitrogen metabolism, which jointly devoted to the SCFAs enhancement during the anaerobic fermentation.