Microplastics profile in sludge from a university wastewater treatment plant and the influence of chemical digestions on Nile red stained microplastics

Abstract

Many of the microplastics (MPs) entering wastewater treatment plants (WWTPs) become entrained in the waste sludge. Here, we assessed the effects of five common digestion methods on the degradation and fluorescence intensity of Nile-red stained MPs. Crumb rubber (CR), cellulose acetate (CA) and polyamide (PA) dissolved in an acid-based digestion; Polyethylene terephthalate (PET), polylactic acid (PLA) and CA degraded in alkali solutions; and CR was destroyed by 30 % H2O2. Fenton’s reagent resulted in little to no damage to the polymers and removed > 95 % of the organic matter, making it the best choice for MP analyses. We then applied Fenton’s reagent to extract MPs in the primary sludge and dewatered sludge from a modern WWTP at the University of Mississippi. There is limited research on MPs in university WWTPs, where the wastewater inflows and composition vary greatly depending on activities and populations on campus. Abundances of putative MPs ranged from 13.2 ± 3.8–380.0 ± 46.4 particles/g (dry weight), with higher levels in the primary sludge and dewatered sludge with students on campus. The majority of MPs were fibers < 500 µm. Over 20 polymers were identified by using µ-FTIR, with polyester (26.4 %), nylon (14.9 %), and polyethylene (12.2 %) most abundant. There was little change in the composition of polymers in the sludge over time. Overall, we found that Fenton’s reagent coupled with µ-FTIR is an effective approach to characterize MPs in WWTP sludge, and that sludge could serve as a source of MPs to the environment depending on its disposal.