## 3 Results and Discussions
### Effect of Adding Coagulant/Flocculant Agents on MP Retention  
To analyse the possibility of improving the capacity of the sludge to retain MPs using chemical agents, the experiment with a sludge concentration equivalent to 0.63 g TSS/L was repeated under the same conditions, except for the addition of a coagulant or a flocculant. Specifically, as explained in Section 2.4, FeCl\({}_{3}\) and PAM were assayed separately, with final concentrations of 1000 and 0.1 ppm, respectively. These concentrations were chosen as a first approach so that they fall within the ranges reported in the literature and are low enough to be economically feasible [23; 25; 26; 27]. As shown in Figure 10, the percentage of MPs retained in the settled sludge notably improved with the addition of chemicals, i.e., from 27.7%, without chemicals, to 81.6 \(\pm\) 0.7% and 69.6 \(\pm\) 0.3%, with FeCl\({}_{3}\) and PAM, respectively. These findings are consistent with previous studies [25; 26; 27]. Figure 9: (**a**) Sketch of the main mechanisms of floc rupture; (**b**) sketch of possible ways of MP entrapment in sludge flocs. As before, MPs retained in the settled sludge were characterized by their shape and size. When a coagulant was employed, the percentages of fibres and fragments were 69% and 28%, respectively. In addition, when a flocculant was used, the percentages of fibres and fragments were 77.0% and 21.0%, respectively (Figure 11a). Compared to the analogous experiment without the addition of chemicals, fibres and fragments continued to be the most abundant shapes, together representing 97-98% in all three cases. However, when the percentages of fibres and fragments were analysed separately, some differences could be observed. Specifically, compared to the percentage of fibres in the absence of chemicals (60%), an increase of 9% was observed with the addition of a coagulant, and 16% with a flocculant. On the other hand, the percentage of fragments decreased with the addition of both chemicals (by 8% with the coagulant and 15% with the flocculant) compared to the experiment without chemicals (36%). The observed improvement in MP retention primarily occurred for fibres, with both FeCl3 and non-ionic PAM (Table S3). A lower zeta potential value and the development of larger flocs (such as the \"pangea\" shape) favoured by the flocculation process likely contributed to the increased percentage of fibres. Because of the \"bridging effect\" between the MPs and the flocculant or coagulant, medium- and large-sized fibres can interact with several flocs, increasing their sedimentation. Another notable finding is that, with the coagulant, the percentage of microspheres trapped in the settled sludge improved, reaching a percentage of 7% (compared to 2% without chemicals) (Table S3). Figure 10: Comparison of efficiencies of MP retention in the settled sludges (0.63 g TSS/L) with and without the addition of chemicals (the bars represent MP concentrations in the settled sludge, and the red line represents percentages of retained MPs). The percentages of MPs of different sizes found in the sludge settled using coagulants and flocculants are shown in Figure 11b. No important differences were observed in the size distribution compared to the sludge settled without chemicals. Other studies have found that the use of flocculants particularly favours the removal of smaller MPs [63,64]. However, in this case, the improvement was observed across all sizes to a similar degree (Table S2). Regarding the chemical composition of the MPs entrapped by the settled sludge, significant variations were observed when coagulant and flocculant agents were used. When non-ionic PAM was used, the percentage of fibres was higher (77%), corresponding to an increase in RA, PEST, and PEI. However, when FeCl\({}_{3}\) was added, the percentage of fragments was higher (28%) than with PAM, which was associated to a higher presence of PE-MPs.