Microplastics discharged from urban drainage system: Prominent contribution of sewer overflow pollution

Urban drainage system is an important channel for terrigenous microplastics (<5 mm in size) to migrate to urban water bodies, especially the input load caused by overflow pollution in wet weather. Investigating how they transport and discharge is essential to better understand the occurrence and variability of microplastics in different water ecosystems. This study evaluated the abundance and distribution characteristics of microplastics in the drainage systems of typical coastal cities in China. The impacts of meteorological conditions and land use were explored. In particular, the prominent contribution of drainage sewer overflow pollution during storm events were investigated. The results showed that the microplastics abundance in daily sewage discharge from different drainage plots ranged between 13.6 and 30.8 items/L, with fibers as the dominant type of microplastics. Sewer overflow discharge can greatly aggravate microplastic abundance to 83.1 ± 40.2 items/L. Road runoff and sewer sediment scouring were the main pollution sources. Systematic estimates based on detailed data showed that the average microplastics emitted per capita per day in household wastewater was 3461.5 items. A quantitative estimation method was proposed to show that the annual emissions load of microplastics via urban drainage system in this research area was 5.83×10¹⁰ items/km², of which the proportion of emissions in wet weather accounted for about 60%. This research provides the first full-process of assessment and source apportionment of the microplastic distribution characteristics in old drainage system. The occurrence of storm events is an important marker of increased microplastic abundance in urban rivers, with a view to urgent need for interception of surface runoff and purification of sewer overflow pollution.