Microplastics in biosolids: A review of ecological implications and methods for identification, enumeration, and characterization

Biosolids, or treated sludge, are by-products of the wastewater treatment processes and are commonly used in agricultural applications to enrich soil nutrients. However, it contains microplastics, plastic particles with a diameter below 1 mm. Microplastics exist and accumulate in the environment, which can have major impacts on the ecosystem. Despite their abundance in the environment, there are to date no standardized methods for their enumeration and characterization. A literature review was conducted focusing on the occurrence of microplastics at wastewater treatment plants, particularly in the solid waste stream, and their influence on the soil ecosystem where biosolids is applied. We found a conflicting evidence to which extent microplastics negatively impact the ecosystem. Some reported either a direct negative impact of microplastics or because of microplastic interaction with other soil contaminants. Meanwhile, other studies showed no effect or at certain amount of microplastics on the ecosystem. We also found that microplastics size, shape, type, concentration, and exposure time play a critical role in their ecological impacts. However, currently, there is no unified approach for microplastics identification and characterization in solid waste resulting in a various and incomparable data. Therefore, utilizing standardized methods for microplastics analysis must be considered as the initial step to better understand the impact of microplastics onto the environment. We suggest a method's scaling comparison as a practical approach to select and develop techniques based on cost, time, data obtained, accuracy, and sensitivity criteria. Further research into the ecotoxicity of microplastics and continuous monitoring of biosolid applications are also necessary.