Sulfide-driven partial denitrification serves as a remedial strategy for suboptimal partial nitrification-anammox performance.

The partial nitrification-anammox (PNA) process applied to low C/N municipal wastewater treatment is often constrained by unstable nitrite supply and nitrate accumulation. In this study, a coupled suboptimal partial nitrification, sulfide-driven partial denitrification, and anammox (sPN-SPDA) process was developed in a step-feed sequencing batch reactor, in which sulfide was introduced as an electron donor during the second feeding stage. A high nitrogen removal efficiency of 93.2 ± 1.2 % was achieved, with effluent total inorganic nitrogen reduced to as low as 5.1 ± 0.5 mg/L. Microbial community analysis demonstrated the enrichment of sulfide-driven partial denitrification (SPD)-related denitrifiers after sulfide addition, particularly Thauera (16.3 %), which supported the conversion of NO3--N to NO2--N. Metagenomic analysis showed increased abundances of nitrate-reduction genes napA and napB, consistent with improved nitrite availability for anammox. Overall, this study provides a promising strategy to alleviate nitrite limitation caused by suboptimal partial nitrification and to achieve advanced nitrogen removal from low C/N municipal wastewater.