Impact of cellulosic fibre microplastics on aerobic granular sludge in dyeing wastewater treatment

Aerobic granular sludge(AGS), characterized by high biological retention capacity and environmental stress resistance, has emerged as a frontier technology for dyeing wastewater treatment. However, the impact of cellulosic fibre microplastics(CFMPs), a typical contaminant in the dyeing industry, on the pollutant removal efficiency of AGS and the underlying mechanisms remain unclear. This study systematically investigated the gradient effects of CFMPs at 0-50 mg/L on AGS for the first time, revealing differential mechanisms based on the interaction between extracellular polymeric substances(EPS) and microbial communities. The results showed that the effect of CFMPs on AGS was concentration-dependent: the system remained stable with no significant change in pollutant removal efficiency at ≤5 mg/L. When the concentration exceeded 20 mg/L, AGS exhibited decreased removal capacities for chemical oxygen demand(COD), total nitrogen(TN), and total phosphorus(TP), with removal rates dropping to 81.6%,60.6%, and 81.3% respectively at 50 mg/L. Mechanistic analysis indicated that high-concentration CFMPs induced defensive stress in AGS through physical interference, leading to 32%-58% increase in EPS content compared to the control group. The imbalance between protein(PN) and polysaccharide(PS) in EPS exacerbated granular structure loosening. Meanwhile, the microbial community structure was significantly reconstructed, with reduced abundances of functional bacteria such as Proteobacteria and Bacteroidota, directly hindering metabolic pathways. This study for the first time confirmed the “low-concentration compatibility-high-concentration antagonism” effect of CFMPs on AGS,provided concentration control thresholds and microbial regulation strategies for the treatment of dyeing wastewater containing CFMPs, and improves the theoretical system of microplastic pollution control.