Low-sludge partial nitrification enabled by carbon-nitrogen decoupling and encapsulated biofillers.

Excessive sludge production and unstable suppression of nitrite-oxidizing bacteria (NOB) are key obstacles to sustainable mainstream partial nitrification (PN) in conventional activated sludge systems. This study demonstrates how a mainstream PN reactor equipped with encapsulated biofillers, operated under carbon-nitrogen decoupling, can achieve stable nitritation with minimal sludge production for low C/N domestic wastewater. Hydrolysis-acidification and denitrification (HA-DN) was used as a front-end carbon management module, redirecting readily biodegradable organics to denitrification, removing 72.9% of influent SCOD and producing an extremely carbon-limited influent (C/N ≈ 0.8-1.2) for the PN stage. Within the PN reactor, the encapsulated biofiller created a high-density, long-SRT autotrophic niche whose internal oxygen gradient selectively enriched ammonia-oxidizing bacteria (AOB), completely washed out NOB, and maintained a nitrite accumulation rate >98% even under increased ammonium loading. Under this combined high-DO and carbon-starved regime, residual heterotrophic bacteria (HB) were confined to a low-abundance, maintenance-oriented guild and shifted toward endogenous respiration rather than net growth. Consequently, the observed sludge yield in the PN stage was as low as 0.013 kgMLSS/kgCOD, representing a >93% reduction compared with typical CAS operation. Thus, carbon-nitrogen decoupling, realized through an encapsulated, carbon-starved PN niche, provides a generalizable strategy to achieve stable mainstream partial nitrification with minimal sludge production in low C/N domestic wastewater treatment.