Single-stage C-N-P removal and ultra-low net sludge production driven by cyclic oxygen exposure and side-stream acidogenesis in an intermittent-aeration IFAS-OSA system

Achieving simultaneous carbon (C), nitrogen (N), and phosphorus (P) removal and minimizing sludge production remain major challenges in wastewater treatment. This study provides the first demonstration of an intermittent aeration (IA) integrated fixed-film activated sludge (IFAS) process embedded within an oxic-settling-anaerobic (OSA) cycle capable of delivering efficient C-N-P removal with ultra-low net sludge yield. Operating the sludge holding tank (SHT) under mesophilic conditions or extended hydraulic retention time enhanced the generation of short- and medium-chain carboxylates up to one order of magnitude, supporting non-classical PAO activity and enabling stable P removal (up to 66%). The system consistently achieved > 95% COD removal across varying COD/N ratios and DO regimes. TN removal exceeded 95% at elevated COD/TN ratios (∼16), whereas lower ratios (∼10–11) reduced the efficiency. Microbial community analysis revealed the dominance of denitrifying genera such as Thauera and Hydrogenophaga , alongside PAO-like taxa including Acinetobacter , supporting the observed shortcut N removal and fermentation-driven P uptake. Under optimal conditions, N 2 O emissions (0.9% EF) were markedly below values typically reported for IA systems. Observed net sludge yields remained exceptionally low (0.09–0.20 gTSS/gCOD), significantly outperforming conventional and other OSA-based biological systems. The sludge exhibited good dewaterability (nCST 4–6 s·L/gTSS), while settleability deteriorated especially under mesophilic and extended anoxic exposure, reflecting known trade-offs of OSA operation. Overall, the IA IFAS–OSA configuration provides an extremely compact, efficient, and straightforwardly retrofittable solution for mainstream wastewater treatment.