Determination of the microplastics emission in the effluent of a municipal waste water treatment plant using Raman microspectroscopy

Samples from the secondary clarifier effluent of a waste water treatment plant (serving 98500 inhabitants) were analyzed to determine the microplastics (MP) emission. The samples were collected using a stainless steel centrifugal pump and filtered through a 10 μm stainless steel cartridge filter. Microplastics particles (MPPs) and microplastics fibers (MPFs) were recovered by chemical and physical sample purification. To remove natural organic matter, the samples were first subjected to oxidative treatment with H₂O₂ and NaClO. Inorganic materials were subsequently removed by density separation in ZnCl₂ (ρ = 1.9 g/cm³) using a centrifuge. Special centrifuge tubes were developed for this purpose. Sample analysis was performed on a Si filter by Raman micro-spectroscopy. Particles with a diameter (d_p) ≥ 10 μm were analyzed. The results were differentiated by dry and wet weather samples. On average, 5900 MPPs m^-3 were identified in the effluent on wet weather days compared to 3000 MPPs m^-3 on dry weather days. Most of the MPPs detected were in the 30 μm < d_p < 100 μm size range. The MPFs ranged between 100 μm and 1000 μm in length. While most of the MPFs were of PET origin, the MPPs consisted mainly of PET, PP, PE and PS.