Enhancing the decomposition of extracellular polymeric substances and the recovery of short-chain fatty acids from waste activated sludge: Analysis of the performance and mechanism of co-treatment by free nitrous acid and calcium peroxide.

At present, the bioproduction of short-chain fatty acids (SCFAs) from waste activated sludge (WAS) has attracted worldwide attention due to the demand of carbon neutrality during waste treatment. Calcium peroxide (CaO) has been reported to be an effective method for the solubilization of WAS and the accumulation of SCFAs, but the high reagent cost limits its industrial application. Therefore, free nitrous acid (FNA) was introduced into the WAS pretreatment system to assist with CaO for enhancing the disruption of extracellular polymeric substances (EPS) and the subsequent acidogenesis process. The results showed that FNA and CaO synergistically enhanced EPS decomposition and the release of biodegradable organic compounds during pretreatment. The highest soluble chemical oxygen demand (3.1- and 2.6-fold higher compared to individual pretreatments at the same concentrations) after pretreatment and the highest SCFAs accumulation (2.0- and 6.4-fold compared to individual pretreatments at the same concentrations) after a 2-day fermentation period was observed in the FNA + CaO (0.15 g/g VSS) co-treated group. Therefore, the FNA + CaO (0.15 g/g VSS) co-treatment was determined to be the optimal strategy for ensuring the disintegration of the EPS matrix and enhancing the accumulation of SCFAs in pretreated sludge during anaerobic digestion.