Microplastics in Wastewater Treatment Plants: Characteristics, Occurrence and Removal Technologies

Pollution of the aquatic environment with microplastics has recently been recognised as a new environmental threat considering their negative impact on the ecosystem. Due to the low density and small particle size of microplastics, they are easily discharged into sewage systems and wastewater treatment plants. Thus, wastewater treatment plants are considered major sources of microplastic pollution in aquatic and terrestrial environments. Therefore, there is an urgent need for an in-depth understanding of the occurrence, behaviour, and fate of microplastics in wastewater treatment plants before they are discharged into natural water bodies. This paper comprehensively reviews the current state of knowledge on the characteristics and removal of microplastics in a series of wastewater treatment plants by comparing their removal efficiency in different unit processes, both during pretreatment, biological treatment, and tertiary treatment. The study found varying efficiencies in wastewater treatment technologies, with the first stage of treatment removing between 16.5 and 98.4% of microplastics, while during biological treatment the overall efficiency of microplastics removal ranges from 78.1 to 99.9% (membrane bioreactor). Nevertheless, given the large volumes of wastewater continuously discharged to receiving bodies, even tertiary treatment plants can be a significant source of microplastics in surface waters. The largest fraction of MPs removed in conventional wastewater treatment plants is trapped in the sludge. Among the critical treatment technologies, microplastic quantitative analysis showed that membrane bioreactors and filter-based treatment technologies have the highest microplastic removal efficiency. Based on a review of the existing literature, it was concluded that existing wastewater treatment plants are ineffective in removing microplastics completely, and there is a risk that they could be discharged into surrounding water sources.