## 3 Results and Discussions
### Effect of Solid Concentration on MP Retention  
Different concentrations of sludge (1, 5, 10, 25, 50, and 100 g) were mixed with 500 mL of filtered distilled water, resulting in TSS concentrations between 0.06 and 5.33 g/L, a range that included the values usually employed in activated sludge systems. The retention effectiveness vs. TSS concentration and MP concentration in the settled sludge following agitation-settling studies is shown in Figure 5 (the bars indicate MP concentrations and the red lines indicate percentages of retained MPs). As can be observed in Figure 5, at the lowest TSS concentration (0.06 g/L), the percentage of MPs retained in the settled sludge was only 13.8%. This value increased sharply with TSS concentration until reaching a value of 61% for a TSS concentration of 1.5 g TSS/L. Using higher sludge concentrations did not significantly improve MP retention (62% for 5.3 g TSS/L). Therefore, sludge saturation is observed, where it is not capable of capturing any more MPs, regardless of whether the amount of sludge is increased or not. The heterogeneity in terms of MP concentration retained in different types of sludge, depending on the type of WWTP and the treatments, prevents the establishment of a clear, significant correlation for MP retention in sewage sludge [50; 52]. Figure 6 illustrates how the quantity of MPs retained per mass of TSS increases with the MP concentration in the supernatant. If this process was compared to an adsorption process, according to [PERSON]'s classification, this curve could be considered the first part of an S-shaped isotherm with one inflection point [53]. The S curve usually indicates the presence of mesopores and appears when the adsorbate-adsorbent attraction is moderate and there is a strong competition for substrate sites [53; 54; 55]. However, more phenomena than just sorption processes are involved. The retention of MPs in sewage sludge happens due to the interactions between MPs and flocs during the agitation phase and subsequently in the static sedimentation phase. Intense agitation would be counterproductive, as flocs might disintegrate, causing them to shrink and release more MPs. Consequently, slow agitation was used for the experiments to allow the flocs to aggregate with one another or with suspended solids or MPs. This way, MPs can be dragged by the flocs or, depending on their size, even be encapsulated within the flocs. Fibres, which are usually longer than other shapes, can come into contact with multiple flocs simultaneously, increasing the settling rate. Larger flocs are preferable for MP retention because, if the size of the floc is too small, the removal of MPs can be inhibited [27]. Figure 5: MP concentration in the settled sludge after the agitation–settling experiments and retention effectiveness versus TSS concentration (the bars represent MP concentration, and the red line represents percentages of MPs retained). The percentage of retained MPs was calculated by subtracting the number of MPs that remained in the supernatant after settling from the number of MPs that were initially in the water–sludge mixture and dividing the result by the initial number of MPs. As shown in Figure 7a, the MPs retained in the settled sludge were mainly fibres and fragments, with similar percentages across all the experiments, i.e., average values of 60.7 \(\pm\) 3.4% and 36.3 \(\pm\) 4.3%, respectively. Compared to the shape distribution obtained in the initial sewage sludge, the percentage of fibres slightly decreased (from 64% to 61%), while the percentage of fragments slightly increased (from 34% to 36%). Other authors have observed that fragments have a greater tendency to be trapped by the sludge [1; 7; 45; 50]. However, in the present study, it seems that there is not a significant difference between the percentage of fibres and fragments found in the initial and settled sludges.