New aeration strategies in the aerobic granulation process under osmotic stress

Recent studies suggest that reduced aeration and elevated osmotic pressure can each favor aerobic granulation and bioresource recovery, but their combined impact remains poorly quantified. Here, four aerobic granular sludge (AGS) reactors were operated under controlled combinations of salinity and aeration intensity to assess granule formation and stability, biomass retention, organic and nitrogen removal, and extracellular bioproduct accumulation. In salt-free systems, reduced aeration with low air upflow velocity (0.66 cm s⁻¹; ~3 mg dissolved oxygen, DO, L⁻¹) enhanced biomass retention (up to 4.0 g mixed liquor volatile suspended solids, MLVSS, L⁻¹), minimized washout, and shortened start-up, yielding stable, dense granules within 16 days. Under saline conditions (5 g NaCl L⁻¹), aeration–salinity interactions governed structure and function: higher aeration supported early granule stabilization, whereas switching to reduced aeration after granulation improved functional resilience. Salinity combined with reduced aeration increased Extracellular Polymeric Substances (EPS) synthesis, particularly the tightly bound fraction, strengthening cohesion and resistance. Salt also stimulated alginate-like exopolysaccharide (ALE) production, and reduced aeration further promoted bioproduct accumulation while lowering recovery losses over time. Organic matter removal remained consistently high (>90%) across all reactors. In contrast, nitrogen removal was strongly controlled by aeration and salinity, with reduced aeration achieving the highest total nitrogen removal (up to 93%) and negligible NO 2 ⁻/NO 3 ⁻ accumulation, consistent with enhanced simultaneous nitrification–denitrification driven by intragranular redox stratification. Overall, aeration modulation, especially when coupled with moderate salinity, is a key lever to optimize granule stability, nitrogen removal, and bioresource recovery in AGS. • Biomass retention and growth declined in saline systems with reduced air upflow velocity • Gradually reducing aeration accelerated granulation only in salt-free systems • Granules with more organized morphology were produced when aeration was reduced • Under salt stress, reduced aeration stimulated EPS excretion and ALE recovery • The combination of reduced oxygenation and salt addition stimulated SND