Dual effects of high mixed-microplastic loading on aerobic granular sludge: from initial stress to adaptive granulation

Microplastic (MP) accumulation in wastewater treatment systems challenges the development of cleaner and more resilient biological treatment processes. This study investigated whether high loading of mixed microplastics (100 mg/L) could reshape aerobic granular sludge (AGS) formation in sequencing batch reactors. MP exposure initially suppressed microbial activity, as reflected by a lower specific oxygen uptake rate, but prolonged exposure progressively promoted granulation and structural stabilization. Compared with the control, complete granulation in the MP-exposed reactor was achieved 11 days earlier, with higher biomass retention, better settleability, and lower integrity coefficient. Morphological observations showed that mixed microplastics acted as colonizable surfaces and nucleation sites for microbial attachment, whereas filamentous microorganisms provided a structural scaffold for granule assembly. Microplastic exposure also enhanced extracellular polymeric substances secretion, particularly tightly bound extracellular polymeric substances (EPS) and protein-rich fractions, and markedly increased intracellular c-di-GMP levels, indicating that EPS remodeling and signal-regulated aggregation contributed to adaptive granulation. High-throughput sequencing further revealed that mixed microplastics reshaped the microbial community toward taxa associated with EPS production, nutrient transformation, and granule stability. As a result, the AGS system maintained stable total organic carbon and total nitrogen removal. These findings demonstrate that, under sustained exposure, mixed microplastics can shift from process stressors to granulation-promoting particulates, providing a new perspective for cleaner water systems that integrate contaminant control, stable treatment performance, and microplastic mitigation.