*3.4. Microplastics Entrapped in Sewage Sludge*

It has been found that during the warmest months (May-September) the MP concentrations in dry sludge (28–39 MPs/g) were higher than those detected the rest of the months (12–22 MPs/g) (Figure 2c). These values are expressed by dry weight considering the sludge moisture of the different samples analysed (78–86% *w*/*w*). In the sludge samples there was a mean concentration of 24.0 ± 8.6 MPs/g dry sludge, value similar to those reported in literature for urban WWTPs [31,64,67,70,72]. According to Figure 2c, it can be observed that MP percentage retained in sludge varies between 47% and 100% with a mean value of 79%. These percentages are in agreement with those reported by different authors (8–92%) [16,28,32,76]. The removal of MPs in previous stages, i.e., during pre-treatment processes, can achieve notable values of elimination, for example, Murphy et al. [44] found that 45% of MPs that arrive at WWTP can be removed in grit and grease system. These percentages have been calculated based on the number of microplastics detected in the influent and the final effluent of the WWTP taking into account the daily flow (Table S5) in each sampling point. In addition, a trend between temperature and MPs retained in sewage sludge was observed, i.e., temperatures seem to favour the entrapping of MPs.

Physical and chemical properties of MPs retained in sludge samples were also analysed and results are summarised in Figure 7. Most MPs found in sludge are fragments and fibres (57 ± 18% and 33 ± 11%, respectively). Foams represent the 9%, but it should be noted that this specific shape was only detected in three samples (September, October and November) with percentages of 2%, 26% and 81%, respectively. The majority of the published works reported a higher abundance of fibres than fragments, with higher percentages than those found here (50–84%) [16,17,28,33,40,44,70,72,76]. However, it is remarkable that other works are in accordance with the results obtained in this case study, i.e., reported a higher proportion of fragments with respect to fibres [19,34,77].

As can be seen in Figure 7, no notable differences in abundance of MPs regarding chemical composition were found. The most predominant polymers in sludge samples were PET (36 ± 4%), followed by PS (25 ± 4%), PA (20 ± 4%) and PVC (9 ± 3%), in accordance with other studies, i.e., Kazour et al. [72] reported relative abundances of PS (25%), PET (20%), PA (10%) and PVC (5%) of the same order of magnitude than those found here. This agrees with the fact that the abundances of these polymers decreased throughout the wastewater treatment processes, as above commented. The high density of these polymers favours their sedimentation, being more easily entrapped in sludge. Regarding colour, around 82% of MPs found in sludge were white and black, as occurred in the wastewater samples.

**Figure 7.** Abundance of microplastics in sludge samples according to (**a**) the shape and (**b**) chemical composition.

#### *3.5. Release of MPs to the Environment*

Several studies reported the environmental incidence of MPs emitted to the environment by WWTPs. As far as we know, until now, there have been seven works published that analysed the incidence of MPs in Spanish WWTPs [10,17,45,66,78–80]. The values found in the present work (average values of 16.1 ± 3.3 MPs/L and 1.1 ± 0.3 MPs/L in influent and effluent, respectively) were within the ranges reported by the previous works (between 2.7 MPs/L and 645 MPs/L in influent and 0.31 MPs/L and 16 MPs/L in effluent).

Considering the MP concentration detected in the influents (between 11.4 and 23.8 MPs/L) and the volume of wastewater that arrives at the WWTP (an average value between 4089 m3/day and 5570 m3/day) (Table S5), it can be estimated that between 5.57·10<sup>7</sup> and 1.27·10<sup>8</sup> microplastics enter into the facility each day. Since the removal efficiency of the studied facility is 92.9 ± 2.1%, approximately between 2.50·10<sup>6</sup> and 6.98·10<sup>6</sup> microplastics per day are emitted to the environment. For example, Edo et al. (2020) [17], who analysed a WWTP five times larger than that studied here, estimated that around 3·10<sup>8</sup> microplastics per day are release into the Henares River (Madrid), even though the WWTP reduce the MP concentration by 93%. This highlights the importance of WWTPs as source of MPs released into the environment.

In dry sludge samples, an average value of 24.0 ± 8.6 MPs/g is found, value lower than those values reported by other authors that analysed sewage sludge samples in the same country (Spain) (between 50 and 165 MPs/g) [17,22,78]. Considering that the MP concentration detected in the sludge (between 12.0 and 39.4 MPs per gram of dry sludge) and the kg of sludge generated in the WWTP (values between 1764 kg and 3976 kg) (Table S6), it can be estimated that around between and 4.23·107 and 9.54·10<sup>7</sup> microplastics are entrapped in sludge. Thus, the subsequent management of the sludge is a determinant step to avoid the release of these MPs to the environment.
