Novel insights into sulfide-enhanced nitritation with aerobic granular sludge

Sulfide inhibition offers a potential strategy for achieving nitritation in wastewater treatment, yet its long-term feasibility and synergy with microbial stratification in continuous-flow aerobic granular sludge (AGS) systems remain unexplored. This study experimentally demonstrates that sulfide stabilizes nitritation in continuous-flow AGS reactors. Sustained high-rate nitritation was achieved under elevated sulfide loading (20-80 mg S/L) without residual ammonium control, with nitrite accumulation efficiency (NAE) > 85% and ammonium oxidation rate (AOR) > 1.46 kg N/(m³·d). This performance challenges the paradigm that ammonium-based suppression is essential for suppressing nitrite-oxidizing bacteria (NOB) in granules. Sulfide appeared to directly inhibited NOB while concurrently enriching sulfide-oxidizing bacteria (SOB, primarily Thiobacillus). Correspondingly, the metabolism of these SOB is likely to have intensified oxygen consumption, exacerbating oxygen limitation in granule interiors and further restricting NOB. Granular structure enabled spatial proximity between SOB and ammonia-oxidizing bacteria (AOB), allowing SOB to potentially act as a biological shield by rapidly oxidizing sulfide and reducing toxicity to AOB. Microbial exposure experience critically modulated this response, as prior sulfide exposure shortened AOB inhibition lag by selecting for metabolically active SOB consortia and inducing physiological adaptations. Despite sulfide fluctuations (20-80 mg S/L), granules maintained structural integrity (sludge volume index at 3 min (SVI₃) <29 mL/g) and settling function, with increased extracellular polymeric substances (especially proteins) enhancing stability. This work demonstrates sulfide as an enabling agent-not a disruptor-for stable nitritation in AGS, facilitating integration with anammox or denitritation for energy-efficient nitrogen removal from sulfide-containing wastewaters.