The fate of microplastics in natural and engineered aquatic systems: a case study of unplanned indirect potable reuse

Our study used empirical modeling to predict the fate of microplastics (MPs) in natural and engineered water systems, focusing on the unplanned indirect potable reuse practiced in Auckland for water drawn from the Waikato River. The study estimated the contribution of wastewater treatment plants (WWTPs) and nonpoint sources (agricultural and urban runoffs) of MPs using a worst-case and a best-case scenario developed using the literature review. The contribution of nonpoint sources of MPs was integrated into WWTP discharges for simplifying calculations. The concentrations of MPs in WWTP influents were deduced from the corresponding population served by the WWTP and estimated per capita generation rates of MPs. Relevant removal efficiencies were applied for estimating effluent concentrations at different stages of wastewater treatment. The fate of MPs along the river was approximated using an empirical riverine transport model by classifying MPs in terms of particle sizes. Finally, the concentrations of MPs in drinking water were estimated by applying literature-based removal efficiencies at every stage of drinking water treatment. It was estimated that the smaller size MPs (10–50 μm) would be predominant in Auckland's drinking water. However, the total concentration of MPs in Auckland's drinking water was estimated to be < 30 ng/L, even in the worst-case scenario. The proposed model could serve as an initial blueprint for estimating MPs' fate in natural and engineered water systems worldwide, especially where the occurrence data are lacking or where extensive water sampling and analysis of MPs are not possible owing to resource constraints.