Effectiveness of conventional municipal wastewater treatment plants in microplastics removal: Insights from multiple analytical techniques

Abstract

This study investigated microplastics (MPs) sized 10-5000 µm across stages of a conventional municipal wastewater treatment plant using multiple analytical techniques. Samples were collected via pumping and filtration, treated with the Fenton reaction for wet peroxidation, and separated by density separation. Analysis employed Focal Plane Array Micro-Fourier Transform Infrared Spectroscopy (FPA micro-FTIR), a widely used technique in MPs analysis, alongside the less common Laser Direct Infrared Spectroscopy (LDIR), providing complementary data on particle composition, shape, size, and colour. To enhance insights, spectroscopic methods were supplemented with Thermal Desorption Gas Chromatography-Mass Spectrometry (TD-GC/MS), calibrated for specific polymers, to quantify MPs by mass and assess removal efficiency. Wastewater treatment effectively reduced MPs. In influent samples, concentrations reached 72 MPs/L (FTIR), 2117 MPs/L (LDIR), and 177 µg/L (TD-GC/MS). Primary treatments removed 41 %-55 %, while the wastewater treatment plant effluent contained 1 MPs/L (FTIR), 93 MPs/L (LDIR), and 2 µg/L (TD-GC/MS), reflecting 96 %-99 % removal efficiency. Activated sludge showed concentrations of 123 MPs/L (FTIR), 10,800 MPs/L (LDIR), and 0.3 mg/g dry weight (TD-GC/MS), underscoring its role in MPs capture. However, sludge dewatering released significant MPs into centrifuge rejected water: 484 MPs/L (FTIR), 23,000 MPs/L (LDIR), and 1100 µg/L (TD-GC/MS). These results highlight the effectiveness of conventional treatments in MPs removal and the critical role of sludge in capturing these contaminants. However, sludge dewatering poses a risk of reintroducing MPs into the environment. Effective sludge management should prioritize nutrient recovery and biomass valorisation to mitigate these risks and minimise harmful environmental impacts.