Critical review of microplastic in membrane treatment plant: Removal efficiency, environmental risk assessment, membrane fouling, and MP release

Microplastics (MP) are emerging contaminants that are leading to great concern in the water and wastewater treatment sector. Membrane technologies have been applied for the direct or indirect removal of MP from aqueous matrices. Therefore, this review sought to compile studies on MP removal from water, wastewater, and leachate by membranes to fill gaps in knowledge about the occurrence of MP and their removal using these technologies. Furthermore, the environmental risks of raw wastewater and those treated by membrane systems were measured using the risk quotient (RQ) for the first time. Understanding the challenges of membrane fouling by MP and the release of these particles during membrane treatment processes were discussed in depth. The analyses demonstrated that ultrafiltration and membrane bioreactors are the most studied membrane processes focusing on MP removal. The removal of MP reported was generally > 85 % for all technologies studied. It was demonstrated that the environmental risk of MP in effluents was drastically reduced by the membrane processes. However, the different sizes and shapes of MP were reported as factors directly related to membrane fouling and the passage of these contaminants to the permeate side. Furthermore, membranes can release low concentrations of microplastic, although presenting negligible risks, studies should be carried out to understand the MP release mechanism and promote its mitigation. Thus, this work stands out for clarifying the state of knowledge and challenges, explaining the apparent contradictions reported in the literature, and identifying necessary future research that can contribute to enhancing research in the advancement of membrane technologies for MP removal applications and environmental risk reduction.