Alginate-Aided Encapsulation: A Novel and Facile Approach to Strengthen the Resilience of Aerobic Granular Sludge under Ethylene Glycol-Induced C/N Stress

C/N imbalanced and toxic wastewaters destabilize biological treatment processes by disrupting microbial structure and function. Ethylene glycol (EG), commonly encountered in de-icing-related effluents, represents a relevant model stressor for examining such instability. This study redefines aerobic granular sludge (AGS) recovery pathways under EG-induced C/N imbalance through alginate-aided microbial encapsulation. Under progressively increasing EG concentrations (120 – 2,000 mg/L), AGS maintained COD removal exceeding 90% and nearly complete simultaneous nitrification-denitrification (SND) at concentrations up to 800 mg/L, but granule destabilization and biomass washout occurred beyond 1,200 mg/L. Alginate-aided encapsulation actively confined microbial fragments within a transient protective hydrogel scaffold, transforming spontaneous fragmentation into a guided reaggregation process that restored structural cohesion and metabolic stability. Optical coherence tomography (OCT) imaging confirmed structural restabilization, consistent with restored secretion of extracellular polymeric substances (EPS), whereas microsensor-based oxygen profiling revealed improved diffusivity within encapsulated biomass. Although conventional nitrifiers, Nitrosomonas and Nitrospira , became undetectable post-encapsulation, stress-tolerant genera, including Comamonas and Pseudomonas, sustained nitrogen transformation with >85% total nitrogen removal, while Pedobacter potentially facilitated EPS-mediated reassembly. Together, this work establishes alginate-aided encapsulation as a facile, retrofittable intervention that converts passive AGS recovery into a controllable bioprocess, strengthening structural resilience and treatment stability under organic loading stress. • AGS tolerated initial EG stress but destabilized beyond 1,200 mg/L. • Alginate-aided encapsulation enabled rapid biomass reaggregation and recovery. • Transient hydrogel matrix enhanced cohesion and oxygen transfer. • Microbial community shifted toward stress-tolerant, nitrogen-transforming genera. • Establishes a controllable bioprocess framework for AGS post-stress restoration.