Proceedings of the 2022 Mississippi Water Resources Conference

Year: 2022 Authors: Gao Z., Cizdziel J.

Microplastics (MPs) are a diverse suite of contaminants commonly found in wastewater. However, wastewater treatment plants (WWTPs) were not designed to remove MPs. Further, MP pollution has not been examined in wastewater stabilization ponds (WSPs), which serve rural and small communities worldwide, and in WWTPs that serve universities where sudden and drastic changes in on-campus populations occur. Here, we characterized MPs (~45 μm-5 mm) in a WSP serving ~500 houses and in an adjacent lake as well as in a modern oxidation ditch WWTP at the University of Mississippi. In the pond, putative MPs were most abundant in duckweed (Lemna minor) and sludge (75 ± 22 and 12.8 ±3.1 particles/g, respectively: ±1 standard deviation (SD), n = 6, dry weight). In the water, average concentrations (particles/L ± 1 SD, n = 6) were highest in the WSP (4.1 ± 0.6), followed by WSP effluent (3.9 ± 0.5) and the lake (2.6 ± 0.6). Over 20 types of MPs were identified, with the distribution varying somewhat between the water, sludge, and duckweed. Polyester and polyethylene were the predominant types, followed by polyethylene terephthalate, polyacrylate, PVC, and polystyrene. Morphologies consisted of fibers (62-71%), fragments (28-37%), and beads (1-6%). High-density polymers were more frequently found in sludge. Potential sources include synthetic textiles from laundry and plastics washed down household drains. Overall, with ~786,000 MPs/day released from the pond and with duckweed a source of food for waterflow, we demonstrate that WSPs can be point sources of MPs to both aquatic and terrestrial ecosystems. At the University WWTP, we observed the highest abundance of MPs after a football game (62.3 ± 7.6 particles/L, average ± 1SD, n=3), followed by 46.3 ± 9.5 particles/L sampled the prior week, and 22.8 ± 4.5 particles/L during a period with little on-campus activity. Over 90% of the MPs were removed in the primary treatment. MPs were most abundant in the oxidation ditch biological unit (1962 particles/L) as the MPs accumulate in the activated sludge. Concentrations of MPs in secondary clarifier and final effluent were <4 particles/L during higher-flow periods and between 16-39 particles/L during lower flow periods. MPs were mainly composed of polyester, acrylic paint, PVC, polyethylene, polypropylene and polyurethane. Most MPs in the effluent were fibers (61%), fragments (21%), and films (13%), with beads and foams contributing the rest (5%).