A review of methods for mitigating microplastic contamination in biosolids from wastewater treatment plants before agricultural soil application

Wastewater treatment plants (WWTP) are recognized as major sources of microplastic (MP) particles in terrestrial environments, particularly in agricultural soils through biosolids application. While many reviews have focused on the distribution, detection, and mitigation of MPs in wastewater effluent to limit their discharge into oceans, our understanding of methods to mitigate biosolid contamination remains limited. This review focuses on methods for mitigating MPs contamination in biosolids at various intervention points, including sources, WWTP including the primary and secondary treatment stages where sludge is generated, and post-contamination. These methods are categorized as physical, physicochemical, and biological approaches, and their advantages and limitations are discussed. For instance, physicochemical methods, especially froth flotation, are cost-effective but are hindered by contaminants and reagents. Physical methods like microfibre filtration devices (MFD) are safe but their efficiency depends on the filter pore size and design. Biological methods, particularly microbial degradation, are limited by the varying efficiencies of microorganisms in breaking down MPs and the extended time required for their effective degradation. Other physical methods including dissolved air flotation, and ultrasonication already exist in WWTPs but may require retrofitting or optimization to enhance MP removal from biosolids. As each method inherently has limitations, the key to achieving MP-free biosolids, and thus preventing their release into agricultural soil, lies in integrating these methods through multi-coupling strategies.