**3. Results and Discussion**

### *3.1. Occurrence and Evolution of MPs*

Nowadays, most of the studies dealing with the occurrence of MPs in WWTPs have been focused on wastewater and sludge samples collected over short periods, i.e., days or weeks [16,32,37]. In this work, the occurrence and evolution of microplastics in a WWTP have been examined, over a 12-month period (Figure 2). Figure 2a summarised the MP concentration in the different sampling points in the WWTP analysed during the study.

**Figure 2.** (**a**) Microplastic concentration (MPs/L) in the WWTP at the four sampling points and overall removal efficiency for one year (May 2020–April 2021), (**b**) rainfall (L/m2) and temperature (◦C) recorded in Caravaca de la Cruz over the sampling period (Source: State Meteorological Agency [38]), (**c**) Microplastic concentration in sludge expressed per dry weight (MPs/g) and percentages of MPs retained in the sludge with respect to the MPs removed during the treatment in the WWTP during the period studied.

The WWTP receives in the influent, after screening systems, a mean concentration of 16.1 ± 3.3 MPs/L. This value is in accordance with other studies reporting similar MP concentrations in the influent of urban WWTP, for example, between 12–16 MPs/L in China [39–41], 12.2 MPs/L in Thailand [42], 15.1 MPs/L in Sweden [43] and 15.7 MPs/L in Scotland [44]. Nevertheless, it should be noted that other works reported MP concentration in influent samples much higher [45] or slightly lower [46] than those found in this work. This can be since the number of MPs in wastewater can be affected by different factors such as population served, lifestyle, climate and seasonal conditions [47].

Considering Figure 2, it can be observed that during the warmest months, from April to September, the MP concentration in influents is, in general, slightly higher compared to the coldest, i.e., January to March. This is probably due to the higher evaporation of water that concentrates microplastics in the aqueous stream. This is in agreement with previous studies, carried out in Spain [10]. It may be due to the fragmentation of (micro)plastics by greater solar irradiation and, to the increase of MP concentration by evaporation of water. On the contrary, Ben-David et al. [48] studied a WWTP in a city located in the north of Israel that reported higher values of MP concentration in the rainy winter season, which was associated with a greater use of washing machines or a greater contribution from land runoff. In this case, there is not any clear correlation between rainfall and the MP concentrations found in influent.

After pre-treatment, the secondary treatment consists of a biological reactor together with a double settling tank. So far, there is no literature data reported on the effect of a double decanter for MP elimination. In general, secondary effluent shows a notable decrease in MP concentration in comparison with those in influent (an average value of 1.90 ± 0.38 MPs/L), which means a removal efficiency (grit and grease removal + biological treatment) higher than of 88%. Hidayaturrahman and Lee [49] analysed the influence of grit and grease and secondary treatment in three WWTPs with MP removals between 75–93%. Similar results were obtained by Ruan et al. [50], who found elimination efficiencies of 87%, whereas Yang et al. [41], after secondary treatment, obtained a removal efficiency of 72%.

It is clear that WWTPs with tertiary treatments have been reported to be more efficient in eliminating MPs than systems that present only a secondary treatment [51]. For example, Magni et al. [19] found a removal efficiency of 64% after the secondary treatment and 84% after the tertiary. In addition, Ziajahromi et al. [52] indicated that, after the secondary treatment, the removal efficiency of MPs was 66%, whereas, after a tertiary treatment, was 87%. Regarding the tertiary treatment applied in this WWTP, that consists of a coagulationflocculation, a lamellar settler, a RSF and an UV disinfection, the removal efficiency of MPs of around 41% was achieved, which increased the overall removal efficiency until values of around 93% and entails an emission of 1.13 MPs/L in the effluent. In those effluent samples, during the warmest months (April to September) the MP concentration was higher compared to the coldest ones (January to March) with ranges of 0.77–1.58 MPs/L (1.21 ± 0.31) and 0.59–1.31 MPs/L (0.87 ± 0.38), respectively. These results are in accordance with those reported by Jiang et al. [53].

Although coagulation-flocculation is a typical process found in drinking water treatment plants (DWTPs) [54,55], it is also commonly employed in WWTPs. For example, Hidayaturrahman and Lee [49] reported removal efficiencies of MPs between 50–82% by means of a coagulation-flocculation process.

The effect of RSF in the MP elimination has been analysed in previous works with a wide variety of results. For example, in a WWTP located in Finland, MPs were reduced from 0.7 to 0.02 MPs/L, which means an efficiency of 97% [30]. In another study carried out in two German WWTPs, the use of a sand filter achieved a noteworthy MP removal (above 99%) [56], whereas Magni et al. [19] described a MP elimination by a RSF of only 50%.

The overall MP removal efficiency of the WWTP analysed in this work was between 89% and 95%, with an average value of 92.9 ± 2.1% and it is remarkable that no noticeable variation between months was detected, so rainfall and temperature does not seem to affect MP elimination. The removal efficiencies found in the facility analysed in the present work were within the range reported in different European WWTPs (72–98%) [3,57–60]. A wide variation can be found depending on the treatment technology used and the operating conditions in the WWTP [61], the origin and type of wastewater [20], as well as the sampling and identification methods used in the process, population density and regional development [40].
