## 3 Results and Discussions  
Using the FTIR microscope (Figure 2) and the confocal microscope (Figure 3), photos of the separated plastics were captured for later statistical analysis. Based on of the photos acquired, a quantitative analysis of the separated MPs was carried out. The MPs were counted and divided according to color, shape, size, and quality. The largest number of MPs was found in the June samples, with about 2942 fragments per 100 g of dry-weight stabilized sewage sludge. The smallest number of MPs was found in the samples from May, which contained an average of 1746 particles per 100 g from dry matter of stabilized sewage sludge. This may be related to the increased use of disposable utensils, straws, cutler, and packaging during these months (Figure 4). Increased rates have also been found in dry sludge during the warmest months (May-September). Based on the arithmetic mean, standard deviation, and relative standard deviation, the coefficient of variation was estimated for samples from May, June, and July. Based on the results obtained regarding the amount of microplastics in the samples, arithmetic means and standard deviations were calculated. For the May samples, the mean was approximately 1746 fragments, with a standard deviation of 364. The coefficient of variation (CV) was 20.87%, indicating very little variability in the data in this group. Figure 2: FTIR microscope imaging of MPs extracted from stabilized sewage sludge (**A**–**D**). For the June samples, the average was 2942 fractions, and the standard deviation was 407. The coefficient of variation (CV) in this case was 13.83%, which also indicates a very low variability of the results. Average variability was observed in the July samples. The arithmetic mean was approximately 2341 fractions, and the standard deviation was 871. The coefficient of variation (CV) was 37.22%. All calculations indicate that for the May and June samples, the variability of the data was very low, while for the July samples, it was moderate. Figure 4: Number of MPs separated from stabilized sewage samples according to month. Figure 3: Confocal microscopy imaging of MPs separated from stabilized sewage sludge (**A–C**). Spatial and temporal variations in microplastics (MPs) abundance in sludge are associated with urban indicators, including population density, level of urbanization, economic income, and climate factors [28]. Black was the dominant color in the samples tested in May, June, and July (Figure 5). In May, black fragments constituted an average of 968 fractions, with fibers comprising 234 per 100 g from dry matter of stabilized sewage sludge. In June, the number of black fragments was 1306, and the number of fibers was 293. In July, black fragments constituted 816 and fibers 282. The number of colored fragments in May was on average 296, and the number of fibers was 247. The number of colored pieces in June was 894 and 449 fibers. Meanwhile, in July, the average number of colored pieces was 670, with 553 fibers. It can be seen that the colored fragments and fibers had a similar value, except for in June, when the colored fragments were almost twice as dominant. However, a trend is visible in the division order, i.e., black fragments > colored fragments > colored fibers > black fibers, in relation to each month. Color provides a qualitative indication of the age of the MPs present in the samples, as well as the origin of the plastic [29]. Regarding color, some authors note the fact that the majority of MPs are colorful, which shows that they are broken from colored plastic products [28]. On the contrary, others indicate that the sludge was dominated by both clear (white or transparent) and colored fibers [30]. [PERSON] [PERSON] (2021) [20] found that most MPs in sludge are white (80%), clear and black (70%), and red and blue (40%). Similar results are obtained by [PERSON] et al. (2023) [31], who observed white, black, brown, blue, red, green, and other colored fibers. Other colors include violet, yellow, gray, and orange.