Enhancement of waste activated sludge hydrolysis and decomposition by combined free nitrous acid and sodium citrate pretreatment

Hydrolysis of waste activated sludge (WAS) is the limiting step in the anaerobic fermentation process. In view of this, this study evaluated the feasibility of a combination of free nitrous acid (FNA) and sodium citrate (SC) pretreatment to promote WAS hydrolysis and anaerobic fermentation. The results showed that the combined treatment of FNA and SC could promote the decomposition of WAS. Under the conditions of 2.504 mg/L FNA and 0.25 g/g total suspended solids of SC pretreatment for 5 h, the soluble chemical oxygen demand release rate (DDSCOD) was 1.86 and 1.42 times that of FNA and SC pretreatment alone. Meanwhile, the contribution rate of intracellular organic compounds reached 84.18 %. The maximum short-chain fatty acids yield was obtained at the 8th day of fermentation, which was 580.34 mg COD/g volatile suspended solids. The protein utilization rate of combined pretreatment was increased to 74.10 %, which was much higher than that of single treatment. Moreover, FNA-SC also changed the activities of key enzymes, increasing the activities of protease and & alpha;-glucosidase but inhibiting the activities of coenzyme F420. Microbial analysis showed that the combined pretreatment significantly changed the composition of microbial community and effectively enriched acidproducing bacteria (such as Petrimonas and Macellibacteroides).