Influence of wastewater treatment plant discharges on microplastic concentrations in surface water

The abundance of microplastic particles in the marine environment is well documented, but less is known about microplastics in the freshwater environment. Wastewater treatment plants (WWTPs) may not effectively remove microplastics allowing for their release to the freshwater environment. To investigate concentration of microplastic in fresh water and the impact of WWTP effluent, samples were collected upstream and downstream of four major municipal WWTPs on the Raritan River, NJ. Microplastics were categorized into three quantitative categories (500-2000 μm, 250-500 μm, 125-250 μm), and one semi-quantitative category (63-125 μm). Then, microplastics were classified as primary (manufactured in small size) or secondary (derived from larger plastics) based on morphology. The concentration of microplastics in the 125-250 and 250-500 μm size categories significantly increased downstream of WWTP. The smaller size classes, often not quantified in microplastic studies, were in high relative abundance across sampling sites. While primary microplastics significantly increased downstream of WWTP, secondary microplastic was the dominant type in the quantitative size categories (66-88%). A moderate correlation between microplastic and distance downstream was observed. These results have implications for understanding the fate and transport of microplastics in the freshwater environment.