Spatiotemporally targeted electron-donor impulse dosing sustains carbon-efficient partial nitritation-partial denitrification-anammox nitrogen removal.

Single-stage partial nitritation-anammox (PN/A) is constrained by stoichiometric nitrate accumulation, which limits theoretical nitrogen removal efficiency to approximately 89%. Bulk or continuously supplied electron-donor dosing can enhance nitrate reduction, but reactor-wide donor exposure may increase donor demand, intensify heterotrophic competition, and limit control over donor delivery to specific nitrate-reducing niches in single-stage PN/A systems. In this study, spatiotemporally targeted electron-donor impulse dosing (STID) using a separate donor inlet and short pulses was implemented and evaluated over 156 days of continuous operation in a single-stage airlift internal circulation partitioned bioreactor. Across all 56 sodium acetate (NaAc)-dosing days, total nitrogen removal efficiency averaged 94.4 ± 5.9%, compared with 61.8 ± 11.6% across the 100 PN/A-only days. Metagenomic analyses showed enrichment of partial denitrifiers, especially Thauera, and increased abundance of nitrate-reduction genes (narG/H/I and napA/B), while the PN/A-associated genera Nitrosomonas and Ca. Brocadia were maintained. Abundances of amoA/B/C and hzs/hdh indicated the genetic potential for ammonia oxidation and anammox, while independent activity assays showed that ammonia-oxidizing and anammox activities were retained. Across all 56 dosing days, cumulative external donor input was 1.20-1.23 g NaAc per g N removed, equivalent to 0.94-0.96 g chemical oxygen demand-equivalent per g N removed. Overall, STID repeatedly improved nitrogen removal across three dosing periods, supporting its use as an operating strategy for continuous single-stage PN/A.