Occurrence, Fate, And Transport Of Microplastics From Wastewater Treatment Plants To Receiving Waters

Abstract

Environmental pollution due to the widespread presence of microplastics (MPs) is an emerging global problem. Wastewater treatment plants (WWTPs) are considered a significant point source for MPs. The first aim of this study was to develop extraction and detection methods of MPs from complex wastewater and sludge samples in the Detroit WWTP to understand the occurrence and fate of MPs in WWTPs. The second aim was to understand the spatial and temporal distribution of MPs originating from the Detroit WWTP discharge point into the Detroit River by utilizing the MIKE 21 Flow Model FM to simulate the movement of MPs in the river. Wet peroxide oxidation and density separation techniques were developed to extract MPs from wastewater and sludge samples. In addition, Nile red staining coupled with fluorescent microscopy, ImageJ, and Fourier- transform infrared spectroscopy (FTIR) methods were developed to analyze MPs' quantity, shape, size, and polymer type. The study findings identified that (0.29 × 1012) MPs are discharged into the Detroit River daily, with most MPs in the size range of < 100 um. In addition, the average daily MPs released were calculated as (0.40× 1012) and (0.05× 1012) MPs/day for primary and secondary dry sludge, respectively, with most MPs in the size range of < 500 um. The dominant polymer type of MPs was polyethylene, and the overall removal efficiency of MPs in Detroit WWTP was estimated as 76%. Regarding the modeling of MPs movement, for all simulated scenarios by MIKE 21 Flow Model, the results demonstrated that most MPs left the study area of the Detroit River and some sedimented on the riverbed. Furthermore, the findings showed that the density and size of MPs affect their fate and transport in the Detroit River, as the suggested hypothesis. This study provided new knowledge of the occurrence and fate of MPs in large wastewater treatment plants such as the Detroit WWTP. Moreover, interested policymakers and researchers can use our findings and modeling work of this study to develop microplastic pollution control and mitigation strategies for WWTPs and receiving water.