# Stabilized Sewage Sludge as Fertilizer: Risks Related to the Presence of Microplastics
## 3 Results and Discussion
### The Number of Microplastics
Among macroplastics, plastic films were predominant, while microplastics were primarily composed of films and fragments. It is important to note, however, that the studied soil had not been exposed to materials likely to contain synthetic polymers. The authors emphasized that microplastic concentrations are likely to be significantly higher in areas where plastic-based agricultural practices are employed [22]. Quantitative analyses of microplastics (MPs) in soil revealed an increased abundance of MPs in soils fertilized with sewage sludge compost compared to unfertilized soils. The results showed 545.9 and 87.6 particles per kilogram in soils following one year of compost application, whereas only 5.00 particles per kilogram were detected in soils without compost treatment [23]. A comparative analysis was conducted to assess microplastic (MP) content across different agricultural fields subjected to various fertilization practices. The results indicated 1025 particles/kg in fields treated with inorganic fertilizers, 1000 particles/kg in fields receiving organic fertilizers, and 800 particles/kg in fields covered with plastic mulch. The most commonly observed colors among the microplastic particles were blue (37.71%) and pink (23.87%). Although direct comparison of MP quantities is challenging due to differences in sample matrices, the color distribution was comparable; in both cases, colored particles were predominant, accounting for more than 60% of the identified microplastics [24]. Differences in the number and size of microplastic particles observed across different months may result from various environmental and anthropogenic factors, such as precipitation variability, temperature fluctuations, seasonal agricultural activities, or changes in the sources of plastic pollution over time. For example, increased rainfall during certain months can lead to enhanced surface runoff, transporting larger amounts of plastic into the studied area. Additionally, seasonal patterns of plastic use (e.g., agricultural films) can affect both the quantity and degradation state of microplastic particles. The relative standard deviation (RSD) and coefficient of variation (CV) were calculated to assess the variability in microplastic concentrations within each sampling month. For the June samples, the mean concentration was 220.0 particles per 100 g of dry sludge, with a standard deviation of 75.66, resulting in a relative standard deviation (RSD) or coefficient of variation (CV) of 0.344 (34.4%). In contrast, the July samples showed a mean concentration of 183.33 particles per 100 g, with a standard deviation of 17.56 and an RSD/CV of 0.096 (9.6%). These results indicate greater variability in the June samples compared to July. In the case of the June samples, the dominant particle size ranges were 1500-2000 \(\upmu\)m, 2000-2500 \(\upmu\)m, and above 4000 \(\upmu\)m (Figure 3). For the July samples, the vast majority of particles exceeded 4000 \(\upmu\)m in size, followed by the 2000-2500 \(\upmu\)m range (Figure 3). The most common size of microplastic particles found in organic fertilizer was in the range of 1-3 mm, accounting for 55% of all identified particles [19]. The substantial presence of microplastics in organic fertilizers raises concerns about their potential ecological impacts. The study suggests that microplastic contamination in agricultural environments may lead to unintended consequences, such as the uptake of microplastics by plants and their subsequent accumulation in the food chain. Previous studies have demonstrated that the size distribution of microplastics in sewage sludge varies depending on the sampling location within the treatment plant. Reported particle sizes typically range from 0.002-0.1 mm up to 3-5 mm [20]. Among these, the most dominant size class across various sludge samples was found to be 500-1000 \(\upmu\)m, suggesting a consistent prevalence of mid-sized particles within sludge matrices [25].