Abstract
<title>Abstract</title> <p>Tailwater from sewage treatment plants is an important source of anthropogenic microplastic pollution in receiving water bodies. This study investigates the spatial and temporal distribution, morphological characteristics, and ecological risks of microplastics in the Fuyang River affected by it, and explores the tailwater driving mechanism. The results show that the discharge outlet of the sewage treatment plant is a hotspot of persistent pollution: the peak abundance of microplastics reaches 3600 items/m³ in the dry season, and the affected area extends for about 1 km along the river. The concentration in the downstream still remains 2200-2600 items/m³; in the wet season, tailwater and urban surface runoff cooperate, causing the concentration of downstream urban areas to significantly increase by 32%, forming a dual stress of "tailwater base + runoff increment". Among the pollutants, fibre forms (>68%) and PE/PP polymers dominate, indicating that domestic sources dominate; however,the risk of highly toxic PVC at the discharge outlet accounts for 99.2%, with an ecotoxicity coefficient of 1241.29, triggering local high risks of Level IV-V (potential ecological risk index PERI>300). The pollution load index of the entire basin continues to exceed the standard (PLIzone=1.18), and the cumulative effect of microplastics is obvious. Research shows that tailwater reshapes the spatial differentiation and ecological risk pattern of river microplastics through continuous input and release of highly toxic components. It is urgent to develop source prevention and control strategies for industrial-source polymers.</p>