Microplastic removal by aerated grit chambers versus settling tanks of a municipal wastewater treatment plant

Abstract

This study investigates microplastic (MP) removal by flotation, and by settling units of a municipal wastewater treatment plant (WWTP) that consisted of coarse screens, fine screens, aerated grit chambers (AGCs), and the conventional activated sludge (CAS) system. AGCs operate with rigorous mixing and short hydraulic residence times (HRT) that cause AGCs to remove MPs moderately, while settling tanks operate with long HRTs and steady flow conditions that favor sedimentation of MPs. MP characteristics such as flat shape (e.g., films), large size (1-5 mm), and low density support MP removal by flotation, while MPs with large 3D structures (1-5 mm) and high material density settle quickly. As the highest rates of MP removal by relevant units, AGCs removed 59% of influent concentration of large size films (1-5 mm), while settling tanks removed around 90% of the influent concentration of fragments. The observed MP removal rates by the WWTP followed the order of fragments > films > fibers as 90% > 85% > 70%, respectively. For improvement of MP removal by WWTPs, addition of gravity-based settling units, installation of inclined-plate and/or tube-settlers into existing settling tanks, and use of coagulants in settling process are among the feasible options.