Enhancing low-temperature nitrogen removal in municipal wastewater through sludge fermentation products and hydraulic...

Low-temperature (15 °C) municipal wastewater treatment is constrained by electron-donor deficiency and slowed microbial kinetics. Within an integrated floc-biofilm sequencing batch reactor under anaerobic-aerobic-anoxic mode, this study investigated the combined effects of sludge fermentation products (SFP) addition and hydraulic retention time (HRT) modulation on total inorganic nitrogen (TIN) removal. A 2 × 2 factorial experiment with two-way analysis of variance revealed that, under conventional HRT, SFP addition resulted in a non-significant decrease in TIN removal from 68.2% to 61.0% (F = 0.08, P = 0.783). Conversely, HRT extension interacted positively with SFP (F = 92.85, P < 0.001), eliminating this suppression and raising TIN removal to 80.5%. Long-term operation achieved 81.5% TIN removal and a 31.0% aerobic nitrite accumulation rate. SFP served as an electron donor for denitrification, while nitrite-oxidizing bacteria showed greater sensitivity to SFP than ammonia-oxidizing bacteria. HRT extension (aerobic: 6 to 7 h, anoxic: 8 to 9 h) alleviated cold-induced kinetic limitations and enhanced substrate conversion. Crucially, polyhydroxyalkanoate (PHA)-associated endogenous carbon metabolism was enhanced after HRT extension, with PHA peak, storage, and consumption increasing 2.7-, 3.1-, and 3.2-fold, respectively, relative to baseline (no SFP, no HRT extension). Furthermore, community analysis revealed spatial niche differentiation. The biofilm was enriched in cold-tolerant Candidatus Brocadia fulgida associated with anammox, while heterotrophic denitrification and carbon metabolism predominated in the flocs. These results support a carbon-kinetic dual-factor strategy for advanced nitrogen removal in cold-region municipal wastewater treatment.