Where do they go? A review of the wastewater treatment process and its impact on the fate of microplastics

Wastewater treatment plants (WWTPs) are facilities designed to sanitise wastewater (WW) from different sources. A WW treatment process combines physical, chemical and biological reactions divided into 3 or 4 stages, according to the effluent standards of each site. WWTPs are intended to remove certain pollutants from the water stream, but not microplastics (MPs), plastic particles ranging from 5 mm to 1 µm. WWTPs work as source and destination for MPs. This review provides an insight into the stages included in WW treatment processes according to different technologies employed, alongside a critical evaluation of their influence on MPs. Relevant information and knowledge gaps have been identified. The removal of MPs depends on a variety of factors than just the type of incoming water or WWTP location. The most significant removal of MPs from WW occurs during the preliminary and primary stages. MPs removal efficiency fluctuates depending on the systems operating during the secondary and tertiary phases. As the comparison of information between studies is complicated due to the units and terms used to report and label the collected WW, we recommend that (1) kg/day units are applied if mass balance is of interest, and number of particles per L/day, considering the flow rate data from the sampling dates or annual average flow; (2) to further explain the selection criteria to classify MPs based on their size; and (3) to reach a consensus to name and identify the sampling points, such as the raw WW and effluent from the next stages.