Response characteristics of partial denitrification system under N-methylpyrrolidone exposure: EPS regulation,...

N-methylpyrrolidone (NMP), a widely used solvent in lithium-ion battery production, shows potential as a carbon source for denitrification. Its use in nitrogen removal may achieve simultaneous pollutant removal and resource recovery. However, its effectiveness and microbial adaptation mechanisms in partial denitrification (PD) systems remain unclear. In this study, PD system was operated for 60 days under NMP concentrations of 0 (R1), 50 (R2), 400 (R3), and 1000 mg/L (R4). Nitrite accumulation, sludge properties, and microbial metabolism were analyzed to evaluate system performance and adaptation. Results demonstrated that 50 mg/L NMP stabilized nitrate-to-nitrite transformation ratio (NTR) at 82.74 ± 4.17%, while 1000 mg/L NMP reduced NTR to 21%. Electron transfer system activity (ETSA) decreased from 6.03 to 4.20 μg/(mg·h) a decline of 30.35%. Reactive oxygen species (ROS)(R2: 200 ± 4%; R3: 159 ± 2%; R4: 88 ± 0.8%)stimulated extracellular polymeric substances (EPS) secretion, raising total EPS (R1: 88.56 ± 0.85; R2: 124.77 ± 0.74; R3: 141.56 ± 2.52; R4: 147.50 ± 3.34 mg/g VSS)and the protein/polysaccharide (PN/PS) ratio(R1: 5.59 ± 0.18; R2: 6.46 ± 0.72; R3: 6.59 ± 0.03; R4: 7.31 ± 0.06). At 50 mg/L NMP, enhanced EPS adhesiveness and sludge hydrophobicity promoted granulation and mitigated stress; at 1000 mg/L NMP, electron limitation and oxidative stress decreased NRE and NTR. High-throughput sequencing revealed that NMP reduced community diversity and drove enrichment of tolerant genera organic matter conversion (e.g., Paracoccus). This study clarifies concentration-dependent effect of NMP in PD systems, providing theoretical guidance for treatment of NMP-containing wastewater.