## 4 Conclusions  
The present work analysed the MPs contained in thickened secondary sludge from a local WWTP. In all samples, fibres were the most common type, followed by fragments. MPs with a size between 500 and 250 \(\upmu\)m were the most abundant, and RA and PE were the most common polymers. MPs recovered from these sludge samples were then used to study the effect of TSS concentration and the addition of chemical agents (FeCl\({}_{3}\)/PAM) on the effectiveness of MP entrapment in sludge flocs. Agitation-settling experiments showed that when TSS concentration was higher the MP removal efficacy increased. Higher amounts of TSS during the agitation step promotes more collisions and encourages the formation of MP-TSS aggregates through hydrophobic and electrostatic interactions, which settle more easily than the MPs. Maximum removal efficacies of around 60% were achieved  
Figure 11: Comparison of the abundance of MP shapes (**a**) and sizes (**b**) in sludge samples, with the application of coagulant or flocculant agents. for TSS concentrations above 1.5 g/L. Furthermore, the use of flocculants and coagulants improved the removal efficacy from 28% to 70% and 82%, respectively. MPs typically have a negative surface charge, and iron acts as a cationic bridge facilitating their retention in flocs whereas PAM creates hydrophobic bridges. The characterization of the MPs settled with and without adding chemicals showed that both flocculants and coagulants preferentially benefited the capture of fibres. The results found here demonstrate the potential of taking advantage of the synergistic effect that the presence of a certain amount of solids and the addition of chemicals exert on the trapping of MPs in the flocs. In particular, the high removal efficiencies obtained for concentrations of TSS similar to those used in activated sludge systems suggest the interest of adding modest quantities of chemicals before the secondary sedimentation step, in order to improve the removal of MPs from water and, at the same time, enhance the performance of the settling and subsequent DAF processes. It is necessary to take into account that the quantity of samples analysed represents a limitation that could compromise the direct applicability of the conclusions drawn here to other sludges and wastewater treatment facility types. This drawback should be addressed in future studies by examining more samples and incorporating a greater range of treatment plants in order to confirm and broaden the findings. In addition, further research is necessary to investigate in depth the most suitable chemicals, the minimum effective doses, and the economic feasibility of this approach to improve MP removal from treated waters, as an alternative to costly tertiary treatments. The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/app15073557/s1](https://www.mdpi.com/article/10.3390/app15073557/s1)]([https://www.mdpi.com/article/10.3390/app15073557/s1](https://www.mdpi.com/article/10.3390/app15073557/s1)), Table S1: MPs present in the sludge used in jar test experiments (beakers filled with 500 mL of distilled water); Figure S1: comparison of the abundance of native MPs in thickened secondary sludges: shape (a) and size (b) (samples before the treatments); Figure S2: % chemical composition of MPs analysed in secondary sewage sludges; Figure S3: \(\mu\)FTIR spectra of all 14 MP types analysed in this study; Table S2: main characteristics of MPs recovered in the supernatant samples; Table S3: comparing MPs in settled sludges according to their sizes and shapes. Conceptualization, C.C., D.S., and A.L. ; methodology, C.C. and A.L. ; formal analysis, A.L. ; investigation, C.C. ; data curation, C.C. and A.L. ; writing--original draft preparation, C.C. ; writing--review and editing, A.L. and M.D. ; visualization, C.C. and D.S. ; supervision, M.D. All authors have read and agreed to the published version of the manuscript.