In this thesis, complex samples from key utilities e.g., the sandtrap, the liquid and the dewatered sewage sludge of an Austrian wastewater treatment plant with a mixed sewer system is investigated for microplastics occurrence. An extensive literature research concerning the status and legal basis, microplastic in wastewater treatment plants, as well as sampling, preparation und measurement is conducted. A problem for this specific sampling target is especially the inconsistent analysis approach and presentation of results. While there is no legal foundation for the sampling of sewage sludge, the norm ÖNORM S 2127 is used for the sampling of the sandtrap material. Derived from the study of available research in the literature, the samples are treated with the Fenton¿s reagent to eliminate the organic matrix. The sandtrap material is further prepared with a density separation with potassium carbonate in two steps to remove the inorganic matrix. The solutions are then filtered and measured, especially the Whatman Anodisc filter is experienced effective. To analyze the filters, microscopes, a Fourier transformation infrared spectroscopy, a Raman spectroscopy and a LDIR spectroscopy are used, and microplastic is evidenced.