**4. Conclusions**

In this work the annual occurrence and fate of microplastics have been evaluated in a WWTP site in Southwest Europe employed as a case study. Results showed that this WWTP has a high removal efficiency (89–95%) all along the period studied, reducing considerably the number of MPs in treated water in comparison to influent values. Specifically, most microplastics (88%) were eliminated in the secondary treatment stage, being entrapped into the sludge. It was also found that the concentration of MPs in the influent was slightly higher during the warmer months (April–September) (17.1–23.8 MPs/L) compared to the colder ones (October–March) (11.4–15.6 MPs/L). MPs more easily eliminated from the wastewater samples were those with sizes greater than 500 μm and fragments and fibres were the shapes most frequently detected in wastewater and sludge samples. In addition, it was found that PP and PE were the commonest polymers in wastewater samples, whereas in sludge samples the majority were PET, PS and PA, which is due to the fact that denser polymers tend to settle more easily during the treatment processes. Furthermore, the temperature seems to favour the retention of MP in sludge. Future works should be focus on improving the removal of MPs from wastewater and, especially, from sewage sludge in order to reduce the release of these MPs to the environment.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/app12042133/s1, Figure S1: Device employed for wastewater sampling. The photograph shows the filtration module, pressure indicator and flow meter as part of the device, Figure S2: (a) Example of FTIR spectra registered for polyethylene (PE) obtained from the analysis of a black fibre recovered from the secondary effluent sample of July; (b): Example of FTIR spectra registered for polypropylene (PP) obtained from the analysis of a white fragment recovered from influent sample of August; (c): Example of FTIR spectra registered for polystyrene (PS) obtained from the analysis of a red fibre recovered from the influent sample of September; (d): Example of FTIR spectra registered for polyamide (PA) obtained from the analysis of a white foam recovered from the secondary effluent sample of October; (e): Example of FTIR spectra registered for polyethylene terephthalate (PET) obtained from the analysis of a white fragment recovered from the final effluent sample of March; (f): Example of FTIR spectra registered for polyvinyl chloride (PVC) obtained from the analysis of a black fragment recovered from the sludge sample of October, Table S1: Volumes of wastewater (L) and amount of sludge (g) sampled during the period of the study, Table S2: Size and shape evolution of microplastics after each treatment (influent, secondary effluent and final effluent) and the overall removal efficiency of each type of microplastic, Table S3: Relationship between the colours and chemical composition found for each sampling point expressed in percentage, Table S4: Relationship between the shapes and chemical composition found for each sampling point expressed in percentage, Table S5: Summary of the concentrations of microplastics (MPs/L) in the influent, secondary treatment and final effluent during the period of study. Influent and effluent average flow values are also indicated, Table S6: Summary of the concentrations of microplastics (MPs/g) in dehydrated sludge during the period of study. Average mass flow values are also indicated.

**Author Contributions:** Conceptualization, A.M.-M. and D.S.; methodology, A.M.-M., R.M.-D. and D.S.; validation, A.L. (Amanda Laca), A.L. (Adriana Laca) and M.D.; formal analysis, A.M.-M., D.S. and A.L. (Amanda Laca); investigation, A.M.-M., R.M.-D. and D.S.; resources, A.M.-M., D.S., A.M.-M. and M.D.; data curation, D.S., A.L. (Amanda Laca) and A.L. (Adriana Laca); writing—original draft preparation, A.M.-M. and D.S.; writing—review and editing, A.M.-M. and D.S.; visualization, A.L. (Amanda Laca) and A.L. (Adriana Laca); supervision, A.L. (Amanda Laca), A.L. (Adriana Laca) and M.D.; project administration, M.D. and A.R.; funding acquisition, A.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** Foundation University of Oviedo, with Project FUO-395-19, has financed this work.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** Authors gratefully acknowledge the cooperation of ACCIONA Agua and ESAMUR (Regional Entity for Sanitation and Wastewater Treatment of the Region of Murcia) with wastewater samples collection, especially to the workers of the WWTP of Caravaca de la Cruz.

**Conflicts of Interest:** The authors declare no conflict of interest.
