# Stabilized Sewage Sludge as Fertilizer: Risks Related to the Presence of Microplastics
## 3 Results and Discussion
### The Number of Microplastics  
The samples exhibited a varied form of microplastics (Figure 1). The average fragment content in the June samples was 553 particles per 100 g of dry weight, while for fibers, this value was 327 particles per 100 g of dry weight, resulting in a total of 880 particles per 100 g of dry weight. Figure 1: Confocal microscope image of microplastic samples. In the July samples (Figure 2), the average fragment content was 507 per 100 g of dry weight, and for fibers, this value was 227, resulting in an average of 734 particles per 100 g of dry fertilizer. The June samples had, on average, 17% more microplastic content compared to the July samples. In the June samples, fragments accounted for 64% of the total microplastic content, while in the July samples, the proportion of fragments was 69%. Research on microplastics in fertilizers is scarce, with the majority focusing on the analysis of MPs in sewage sludge and soil, where they eventually accumulate. For comparison, earlier studies on organic fertilizers have reported microplastic concentrations averaging 325 \(\pm\) 511 particles per kilogram, with microplastics detected in 80.4% of the samples. The highest levels were recorded in Beijing (758 particles/kg) and in fertilizers with complex organic matter, averaging 386 particles per kilogram. The dominant types were white or transparent particles (75.9%), primarily film-like fragments (39%) [19]. In the case of the samples analyzed in this study, significantly higher levels of contamination were observed, with differences reaching over an order of magnitude. In the June samples, the number of microplastic particles exceeded 8000 per kilogram of fertilizer, while in the July samples, the average particle count was slightly below this value. Similar to previous studies, fragments were the dominant shape. However, in contrast to the literature data, the color distribution differed: in our samples from both June and July, colored fragments were predominant. The microplastic (MP) content in dewatered sludge and filter cake was reported to be 36.3 \(\pm\) 5.7 and 46.3 \(\pm\) 6.2 particles per gram of dry weight, respectively [20]. These values are lower compared to the fertilizers analyzed in this study, which contained, on average, 80% stabilized sewage sludge. Despite the lower concentrations, the data highlight the risk of secondary contamination through the application of materials such as fertilizers. Researchers analyzed activated sludge and reported a concentration of 113 \(\pm\) 57 MPs per gram of dry sludge [13]. The study highlighted the potential for wastewater treatment plants to emit microplastics into agroecosystems. This value is significantly higher than that of the fertilizer analyzed in our study. Additionally, the sewage sludge used for fertilizer production [14] was also examined, and in this case, the samples showed a lower  
Figure 2: Number of microplastics in the June and July samples per 100 g of dry weight (categorized by shape and color). concentration of microplastics. The results of the conducted analyses indicate a relatively high concentration of microplastics in the examined samples of fertilizer derived from stabilized sewage sludge. For comparison, other studies have reported the presence of approximately 113 microplastic particles per gram of sewage sludge, which represents a higher level of contamination compared to our findings [13]. In contrast, some studies have reported significantly lower concentrations--in the range of 370 to 950 particles per kilogram--indicating a markedly lower microplastic content than that found in the samples analyzed in this study [21]. Such considerable discrepancies may result from differences in sludge stabilization technologies, types of wastewater, environmental conditions, and the detection and isolation methods used for microplastic analysis. Researchers analyzed microplastics in agricultural soils, and the study reported an average of 0.34 \(\pm\) 0.36 particles of macroplastics and microplastics per kilogram of dry soil.