Enhanced nitrogen removal and sludge characteristic in aerobic SBR by simultaneous nitrification and denitrification bacteria

Four efficient heterotrophic nitrification and aerobic denitrification (HN-AD) strains were applied in sequencing batch reactors (SBRs) via two bioaugmentation strategies to enhance the nitrogen removal and sludge characteristic. Synthetic domestic wastewater with NH4-N concentrations of 30~50 mg L–1, was treated in three SBRs, with DO maintained at 4±0.5 mg L–1. Compared with the control (crude activated sludge), bioaugmentation improved TN removal by 7% averagely, increased nitration rate by 0.54 mg g–1 h–1, and reduced sludge volume index at 30 min (SVI30) by 3.5~14.7 mL g–1. The maximum TN removal efficiency reached 50.37% with effluent TN concentration of 14.64 mg L–1, meeting China’s Class 1A discharge standard (TN ≤ 15 mg L–1). SBR started by bacterial suspension without activated sludge exhibited high adaptability to low carbon/nitrogen ratio, achieving 35% TN removal at C/N = 3 (vs <10% in control), with <5% MLSS fluctuation versus 30% decline in control. Microbial community analysis revealed Saccharibacteria dominance (15.34% vs control’s 8.48%) coupled with 7.6% reduction in filamentous Saprospiraceae (12.78% to 5.18%), collectively explaining the enhanced nitrogen removal and sludge settleability. This study provides the first evidence of granular sludge formation via HN-AD bacterial coaggregation under low C/N conditions, offering a novel strategy for energy-efficient wastewater treatment.