Recycling sludge carbon sources via different iron-based activated PDS into denitrification systems for nitrogen removal: focusing on efficacy, community structure and molecular mechanism.

Advanced oxidation processes (AOPs) effectively solubilized organic matter from sludge, generating a liquid phase with substantial recovery potential. Because organic composition and concentration were governed by oxidation intensity, elucidating this relationship was essential for optimizing downstream resource recovery. This study systematically compared the cracking solution generated from sewage sludge utilizing different AOPs (US/Fe(II)/PDS vs US/Fe1/PDS vs US/Fe-C/PDS). The SCOD were 673 mg/L, 556.8 mg/L, and 676 mg/L in US/Fe(II)/PDS, US/Fe1/PDS and US/Fe-C/PDS systems, respectively, including high concentrations of short-chain volatile fatty acid, proteins and polysaccharides (PS). Correspondingly, there demonstrated superior NO3--N, NH4+-N and total nitrogen removal efficiencies of 13.6%, 50%, and 50%, respectively, primarily attributed to the optimal oxidation capacity and abundant organic carbon in US/Fe-C/PDS system. Additionally, no significant difference was observed between US/Fe-C/PDS system and control group (CH3COONa) during denitrification, suggesting cracking solution in sludge had strong application potential as a carbon source. Sequencing results revealed stable bacterial communities across all systems, implying that the cracking solution had negligible influence on the structure of core denitrifying taxa. A robust nitrogen-cycling function was maintained, accompanied by up-regulate of genes (napAB, nirS/K, norBC and nosZ) associated with PS-sustained-release carbon source metabolism and denitrification in US/Fe-C/PDS system. These results suggested that PS-sustained-release carbon source driving efficient nitrogen removal and promoting sludge resource recycling in US/Fe-C/PDS system.