The fate of microplastics in wastewater treatment plants: An overview of source and remediation technologies

Microplastics (MPs) in everyday consumer products have paved the way for high MPs concentrations in wastewater. Wastewater treatment plants (WWTP)s are the significant focal points for MP pollution, as millions of MP particles with varying characteristics enter WWTPs through the sewage systems of urban areas. Although the units in the WWTPs are not primarily designed to target MPs specifically, researchers have reported a large amount of microplastic removal from various units of WWTPs. The scientific community has remarkably studied various conventional and advanced treatment methods to remediate MPs and nanoplastics (NPs) from wastewater. This review aims to provide comprehensive knowledge about the source of MPs and NPs in wastewater, their composition, toxic effects, and remediation from treatment units of the WWTPs. The WWTPs comprise several physical, chemical, and biological treatment processes broadly categorized into three key stages: primary, secondary, and tertiary. The interaction of MPs and NPs with these processes have also been reviewed. Literature indicates that the WWTPs can separate more than 90% of MPs from wastewater. However, despite low concentrations of MPs in the treated effluent, large discharge volumes contribute to millions of MPs/NPs into the environment. Also, the separated MPs/NPs re-enter the environment through sludge applied on land. The paper also discusses the application of various advanced wastewater treatment technologies and their efficacies in remediating microplastics. Finally, the paper also highlights the research gaps where additional exploration is required, providing a new perspective on developing policies for controlling microplastic pollution.