Pretreatment-promoted sludge fermentation liquor improves biological nitrogen removal: Molecular insight into the role of dissolved organic matter

[Display omitted] •THALK and EC-AOP outperformed OZN and EC for WAS solubilization and acidogenesis.•EEMF-PARAFAC and FT-ICR-MS are used to characterize DOM transformation in sludge.•THALK enhanced PHA-like DOM release that promoted WAS acidogenesis.•DOM in WAS fermentation liquor affected its effectiveness as BNR carbon source.•Lignin/CRAM-like structures and tannins DOM improved denitrification performance. Waste activated sludge (WAS) can be used as carbon sources to support biological nutrient removal (BNR). In this study, thermal-alkaline (THALK), ozonation (OZN), electrolysis (EC) and NaClO-promoted electrolysis (EC-AOP) were investigated to facilitate WAS solubilization and production of volatile fatty acids (VFAs). EEMF-PARAFAC and FT-ICR-MS were employed to characterize the transformation of dissolved organic matter (DOM) in WAS fermentation liquors at molecular level. THALK achieved the highest fluorescence intensity of C1 protein after pretreatment. Proteins and lipids were the dominant DOM in the pretreated WAS, while the DOM shifted towards substances with higher H/C and lower O/C after fermentation. The BNR results showed that THALK (100%) and EC-AOP (96.9%) outperformed other groups (78.9–90.3%) in terms of NO3-N removal, indicating the significant impact of DOM compositions. Overall, these results demonstrated that THALK and EC-AOP effectively enhanced release of VFAs and DOM, which subsequently improved NO3-N removal efficiency.