Understanding the retention of microplastics in wastewater treatment plants: Insights from tracer tests and numerical modeling

This study integrates full-scale tracer-test experiments and microplastics (MPs) tracer transport modeling to determine the role of flow patterns in the retention of MPs within wastewater treatment plants (WWTPs). Unlike previous studies that determined MP content through discrete sampling without controlling their entrance, this study tracked nile red-stained acrylic fiber tracer-MPs across a disinfection tank in a WWTP. The full-scale tracer test showed that ∼70 % of the input tracers was retained in the disinfection tank 20 min after the minimum Hydraulic Retention Time (HRT, 30 min) (total duration: 50 min), but dropped to 39 % 4 h after the minimum HRT (total duration: 4.5 h), indicating the existing tank was not effective in retaining MPs and resulted in substantial amount of MPs being carried over with the effluent. An advection-dispersion-based transport model was developed using the velocity profiles obtained from COMSOL to predict MP concentration throughout the disinfection tank. The model simulation results showed the tracers' retention descended from 19 % at the end of the rapid disposal phase (duration: 70 min) to 8 % at the gradual disposal phase (duration: 4.5 h), which corresponded with the trend observed in the full-scale tracer tests. Due to the poor retention of MPs during the longer period, retrofitting options to improve MP retaining in the tank were evaluated using the transport model. The simulation showed that introducing baffle walls could improve MP retention by 33 %-65 % at the gradual disposal phase, while altering the spacing between baffle walls would reduce the MP retention by 17 %-18 % at the gradual disposal phase. This study unveiled the impacts of flow patterns on the MP fate and transport across WWTPs and formulated the retrofitting solutions for improved MP retention in treatment units without compromising overall wastewater treatment efficiency.