Unraveling the behavior of nitrite on promoting short-chain fatty acids accumulation from waste activated sludge by peracetic acid pretreatment: Extracellular polymeric substance decomposition and underlying mechanism

Peracetic acid (PAA) is an emerging oxidant for waste activated sludge (WAS) treatment due to its strong oxidization and few toxic byproducts. Nitrite which can be in-situ recovered from WAS fermentation liquor, its protonated form (free nitrous acid, FNA) is regarded as the cost-effective inactivator. The stubborn extracellular polymeric substance (EPS) is the rate-limiting step for energy/resource recovery from WAS. This work found that the co-pretreatment of PAA and FNA can effectively promote short-chain fatty acids (SCFAs) production during anaerobic fermentation. Higher PAA dosage (100 mg/g VSS, FP4WAS) in co-pretreatment was beneficial for organics release (1976.9 mg COD/L), remarkably increased by 10.3- 96.5 % than that of low PAA dosage (25- 75 mg/g VSS), and promoted by 105.1 % and 62.1 % than FNA (FWAS)/PAA (100 mg/g VSS, P4WAS)-pretreated WAS. Effective release of soluble organics contributed to the SCFAs accumulation (7679 +/- 86 mg COD/L, 4 d), enhanced by 200.0 % and 19.0 % than FWAS and P4WAS, respectively. Acetic (HAc) and propionic acid (HPr) peaked at 6344.7 mg COD/L in FP4WAS (accounted for 82.6 %), which increased by 10.6- 899.0% than other groups. Moreover, OH center dot and O2 center dot -were detected in co-pretreatment, may play the synchronous effect with the crucial intermediates of NO center dot, NO2 center dot and ONOO-/ONOOH on EPS decomposition.

Automated summary

This study found that the co-pretreatment of PAA and FNA can effectively promote the production of short-chain fatty acids during anaerobic fermentation of waste activated sludge. Higher dosage of PAA was beneficial for organics release and significantly increased the release compared to lower dosage of PAA. The effective release of soluble organics contributed to the accumulation of SCFAs, with acetic acid and propionic acid being the major components. OH· and O2·- were detected in the co-pretreatment and may play a role in the decomposition of EPS along with other intermediates.