Aeration-Phase Calcium Addition for Stabilizing Aerobic Granular Sludge in Treating Municipal Wastewater with Low-Strength Municipal Wastewater

Aerobic granular sludge (AGS) is a promising technology for wastewater treatment due to its excellent settling and pollutant removal capabilities. However, its long granulation period and instability under real wastewater conditions hinder full-scale application. This study evaluated calcium addition at the influent (R1) and aeration stage (R2) under real wastewater, focusing on granulation, oxygen transfer, EPS production, microbial activity, and economic benefits. Aeration-phase dosing (R2) significantly improved granule formation (>200 μm, 33.73% vs 13.41%), compactness, and inorganic content. It enhanced LB-EPS and TB-EPS secretion, calcium retention (2.51% vs 0.91%), and hydroxyapatite (HAP) formation, resulting in stronger granules (SVI 30 /SVI 5 > 0.9). R2 showed higher oxygen transfer efficiency ( K L a = 1.21 vs 0.81 s –1 ), promoted heterotrophic activity ( R H/T = 65.10% vs 62%), upregulated AOB genes (amoA, amoB, amoC), and suppressed NOB genes (nxrA). Increased nirK and norB expression indicated partial nitrification-denitrification. Economically, R2 reduced calcium use by 63.6% and shortened granulation by ∼30 days. These findings demonstrate that aeration-phase calcium addition enhances EPS–mineral synergy, microbial performance, and structural stability, offering a cost-effective and scalable strategy for AGS applications.