Effectiveness of microplastics removal in wastewater treatment plants: A critical analysis of wastewater treatment processes

Microplastics (MPs) are among the contaminants that have been of considerable concern in the last decade. One of the most significant contributors to MPs pollution in the environment is the effluent from wastewater treatment plants (WWTP). Previous studies showed that high MPs removal could be achieved in WWTPs. However, the parameters affecting MPs removal performances have not yet been analyzed for actual WWTPs. This review critically evaluates the physical, chemical, and biological treatment processes implemented in the actual WWTPs to remove MPs and summarizes the parameters affecting the removal. This work shows that applying physical, chemical and biological methods is promising: each can be implemented in WWTPs separately, and when combined, higher MPs removal rates are possible. The main parameters that affect the MPs removal are the initial MPs load to the WWTPs and the retention time of MPs in the operational units. MP removal is mainly observed via the physical sedimentation process, which leads MPs to accumulate in wastewater sludge; hence treatment parameters and possibilities of extracting MPs from sludge should be considered to prevent release of MPs from the WWTPs to the environment. The main limitation of MP dissapearance estimation in WWTPs is the lack of a standard MP analysis procedure, which prevents a clear comparison between MP species identification, characterization, and separation. More parameters could be linked to the MPs removal if more consistent and standardized data were obtained from the WWTPs.