A review on the characteristics of microplastics in wastewater treatment plants: A source for toxic chemicals

Wastewater treatment plants (WWTPs) are known to be one of the main and most important sources of microplastics discharge into the environment. Although the occurrence and distribution of microplastics (MPs) in various environments have been extensively investigated, their characteristics in certain environmental matrices remain almost unknown. This paper aims to review strategies for sampling, sample pretreatment, and characterization of MPs detected in WWTPs and sewage sludge. In addition, this paper will review the efficiency of WWTPs in the removal of microplastic particles as well as their fate and implications as a source of toxic chemicals. According to literature, microplastics in water samples are mostly comprised of secondary ones (fibers and fragments) with the main polymers composed of polyethylene terephthalate (PET), polystyrene (PS), polypropylene (PP), and polyethylene (PE). A significant proportion of particles are removed during the initial stages of primary and secondary treatment, while membrane bioreactor (MBR) is found to be the most efficient tertiary treatment technology. The removal rate is affected by the characteristics of MPs such as size, shape, and density. The majority of microplastics eliminated during wastewater treatment are prone to be found in sewage sludge. Thus, billions of MPs are released into the environment via sewage sludge applications each year. Microplastics can also act as a specific layer that adsorbs and transports distinct pathogenic/bacterial taxa assemblages and promotes the persistence of antibiotic resistance genes in wastewater treatment plants. Therefore, the current study provides an overview of recent research regarding MPs occurrence and removal in WWTPs and sewage sludge. In this review paper, the removal and the discharge of microplastics from WWTPs considering the properties of microplastics and treatment methods as well as microplastic interactions with other pollutants are studied.