Effects of different treatment processes in four municipal wastewater treatment plants on the transport and fate of microplastics

The behavior of microplastics in wastewater treatment plants has been investigated, but specific effects of treatment process on microplastics' fate are still unclear due to varied analysis methods and regional differences. In this study, four wastewater treatment plants in Ningbo of southeastern China with different treatment processes were selected to investigate transport and fate of microplastics. Based on number of microplastic particles, fibers and fragments were the main microplastics types in wastewater, while synthetic cellulose represented the largest fraction. The dominance of fibers (76.7%–90.0%) and small particle sizes (<2.0 mm, 62.5%–81.5%) in effluents suggested that they escaped easily from the wastewater treatment plants. The abundance of microplastics particles decreased from 78.0 ± 2.9 items/L in influent to 6.0 ± 2.8 items/L in effluent for anaerobic-anoxic-oxic process, 100.0 ± 3.1 items/L to 4.3 ± 3.4 items/L for sequencing batch reactor activated sludge process, 105.0 ± 5.3 items/L to 3.5 ± 2.6 items/L for cyclic activated sludge technology, 65.0 ± 4.3 items/L to 3.0 ± 1.6 items/L for oxidation ditch process. The microplastics removal capacity of primary and secondary treatment processes for four wastewater treatment plants ranged from 83.7% to 96.3%. Application of different tertiary treatment processes (coagulation/flocculation, membrane related technology and disinfection) enhanced microplastics removal to achieve overall removal rate of 92.3%–96.7%. The removed microplastics from the wastewater treatment plants were mainly transferred to sludge (226.1 ± 95.7–896.0 ± 144.0 items/g dry weight). The biological treatment unit played an important role in microplastics removal with rates varying between 86.9%–95.2%, while tertiary treatment reduced daily microplastics emission 1.4 × 10⁸–2.3 × 10⁸ items/day. This study suggests that proper selection of wastewater treatment unit could significantly reduce the emission number of microplastics, which supports an efficient control strategy of microplastics in wastewater treatment plants.