Spatiotemporal characteristics of microplastics in a university wastewater treatment plant: Influence of sudden on-campus population swings

Abstract

We studied the occurrence and characteristics (sizes, shapes, and polymer compositions) of microplastics (MPs) in a secondary wastewater treatment plant (WWTP) at the University of Mississippi both spatially and temporally. Putative MPs were isolated by sieving, matrix digestion, and density separation, and quantified using stereomicroscopy, with a subset of samples analyzed by Focal Plane Array (FPA) µ-FTIR imaging. In the influent, the highest MP concentration (particles/L ± 1 SD, n = 3) occurred after a football game on campus (62.3 ± 7.6) and the lowest (19.7 ± 2.1) during the summer with little activity on campus. Over 90% of the MPs were removed in the primary treatment. Downstream, MPs were most abundant in the closed loop reactor with concentrations as high as 1962 particles/L. Concentrations in secondary clarifier and final treated effluent were both < 4 particles/L during both normal flow (∼2000 m3/d) and high-flow (>2500 m3/d) periods, and between 16 and 39 particles/L during a low-flow (<1500 m3/d) period. This difference likely stems from changes in plant operations during the low-flow period to support the activated sludge, including longer process times. MPs were mainly composed of polyester, polyethylene, acrylates, polypropylene, polyurethane, polyvinyl chloride, and polystyrene. Fibers were most abundant throughout the system, averaging 61%, followed by fragments (21%), films (13%), and beads and foams (∼5%). Overall, we show that flow rates and treatment times can profoundly influence MPs concentrations in the treated effluent, and that the optimal wastewater treatment conditions also yield the best MP removal efficiency.