## 3 Results and Discussions  
### Native MPs in the Sewage Sludge  
Figure 2a shows a micrograph of the sewage sludge used for the experiments. The flocs observed are generally irregular in shape (Figure 2b), which may be due to the presence of filamentous organisms that provide networks for the floc structure [32]. EPS are a fundamental component of activated sludge flocs, and they are present in both the internal and external layers, playing a fundamental role in the properties of the sludge, such as flocculation, sedimentation, and subsequent dehydration [33,34]. As can be seen, some MPs are incorporated into the sludge flocs, primarily through the EPS [35]. The presence of MPs influences the buoyancy and settling speed of the flocs [33,36]. To determine the size of the flocs (before the experiments), 150 sludge flocs from the sludge sample were measured, obtaining an average value of 486 \(\pm\) 267 \(\upmu\)m (Figure 3), with a size distribution ranging from 51 to 1811 \(\upmu\)m. Specifically, it was found that most of the flocs were within the 300-600 \(\upmu\)m range, consistent with previous studies [37]. The size of the flocs depends on the specific treatment the sludge undergoes and is highly influenced by physical parameters such as rotation speed, hydrodynamic properties, etc. [38]. It is noteworthy that most of the flocs have similar sizes to those commonly reported for MPs found in wastewater (250-500 \(\upmu\)m) [17,29], and it is likely that this facilitates chemical interactions between them, such as Van der Waals forces. As shown in Figure 2, it can be noted that smaller MPs (essentially in the form of fragments) can become trapped inside the flocs, while, in particular, MP fibres can protrude and simultaneously be linked to more than one floc. Figure 2: (a) Stereomicroscope micrograph of sludge flocs and (b) sketch illustrating how sludge flocs contain the MPs. The concentrations of native MPs were analysed in the sewage sludge used for the experiments, and they ranged between 433 and 587 MPs/L, with an average value of 530 \(\pm\) 63 MPs/L. As can be seen in Figure 4, the differences observed between months were minimal, allowing for comparisons between experiments carried out with sludge samples taken on different dates. These values fall within the range of MP concentrations found for thickened secondary sludge samples, i.e., 138-8120 MPs/L [39-44]. The MPs found in the sludge samples were characterized according to their shape, size, and chemical composition (Figure S1). The sum of fibres and fragments accounted for 64.3 \(\pm\) 8.7% and 34.0 \(\pm\) 9.0% of the total MPs, respectively, whereas films and microbeads represented only 1.0 \(\pm\) 0.6% and 0.7 \(\pm\) 0.8%, respectively (Figure S1a). These results  
Figure 4: MP concentrations in the analysed samples of secondary sludge. Figure 3: Floc size distribution in the sludge samples. are consistent with the data reported in most published studies [1; 2; 45; 46]. In particular, values exceeding 96% of the sum of fibres and fragments were found by [7; 47]. The size distributions for the analysed MPs were as follows: 20.8 \(\pm\) 2.5% (>500 \(\upmu\)m), 42.2 \(\pm\) 3.3% (500-250 \(\upmu\)m), 27.0 \(\pm\) 2.2% (250-100 \(\upmu\)m), and 10.0 \(\pm\) 2.1% (100-20 \(\upmu\)m) (Figure S1b). The majority (69%) of all identified MPs were within the 500-100 \(\upmu\)m range, in accordance with previous studies [1; 48]. MP fibres found in wastewater and sludge are typically derived from textiles, such as synthetic apparel and textiles like acrylic, RA, PEST and PA. Small fibres are released during the washing of synthetic materials and wind up in the wastewater. However, MP fragments are mainly produced when bigger plastic particles (such as disposable bags and packaging) break down into smaller pieces as the result of biological, chemical, and mechanical processes [1; 3; 36].