**3.3 Sample analysis**

Glass filters are then analyzed under the stereo microscope "Olympus" with 450× magnification for any suspected microplastics. Suspected particles are collected using metal forceps and placed in a glass slide to be analyzed using Leica CME 1000× compound microscope. Particles are then identified as microplastic based on the following criteria [32–33]: 1. the absence of organic and cellular structures, 2. the color is homogenous, and 3. not segmented and evenly thick. After that, the color and shape of microplastics were recorded and a picture was taken of each sample with the scaled optical glass inserted in the microscope lens. Later with the help of a calibration slide, the length of each particle was measured using IC Measure software. Part of the identified microplastics was isolated in a small glass bottle and was analyzed for polymer types using Bruker ATR-FTIR. To cover all possible polymer types, samples with different characteristics (shape and color) were used. The resulting spectrums were matched with referenced polymer spectra using the library in OPUS-spectroscopy software. Statistical analysis was performed on the data using Microsoft excel. First, using the Kolmogorov–Smirnov test for normal distribution. Then, one-way analysis of variance (ANOVA) was applied for microplastic abundance, size, and color considering p < 0.05 as statistically significant.
