Removal and generation of microplastics in wastewater treatment plants: A review

Abstract

Wastewater treatment plants (WWTPs) are the primary recipients of microplastics (MPs), prior to their discharge into natural waterbodies. The aim of this article is to summarize the generation process of MPs and the efficiency of their removal by treatment technologies currently adopted by WWTPs, as well as the influence of sludge treatment on the fragmentation of MPs. WWTPs mainly remove MPs by means of adhesion, sedimentation, and filtration. The average removal efficiency of MPs is less than 90% and is affected by the choice of wastewater treatment process and the properties of MPs (such as the size, density and morphology). The secretion of biological enzymes by microbes and the metabolites of biofilm may promote the hydrolysis of microbial carrier materials, resulting in the release of endogenous MPs under shear force. Furthermore, during wastewater pumping, disinfection and sludge thermal hydrolysis treatment, MPs may form sub-micron-scale secondary MPs through mechanical, ultraviolet, ozone and thermal degradation processes. Processes such as thermal drying of sludge and lime stabilization have been found to lead to the fragmentation of MPs increase the specific surface area and number of MPs, leading to an increase in the potential ecological threat posed by MPs in sludge. Currently, the level of MP pollution throughout the environment is continually increasing. This review will help improve wastewater and sludge treatment processes, effectively reducing the risk of MPs entering the natural water bodies and soil through WWTPs.