# Long-Term Occurrence and Fate of Microlastics in WWTPs: A Case Study in Southwest Europe
## 3 Results and Discussion
### Characterization of MPs by Size, Shape and Colour  
As explained before, the sampling procedure allowed the MP classification by size. According to Figure 3, on average, in influent samples MP \(\geq\) 500 \(\upmu\)m only accounted around 30% of total MPs, whereas 56% and 80% represented MPs higher than 250 \(\upmu\)m and 100 \(\upmu\)m, respectively. This indicates a major percentage of small MPs than usual in the influent since most WWTPs it has been reported MPs abundance with a size greater than 500 \(\upmu\)m above 70% [18, 45, 61, 62, 63]. The variations in the percentages of MPs found in each range of size are noticeable thorough the treatment processes, i.e., the percentage of those MPs with a size greater than 500 \(\upmu\)m decreased from 30% in the influent to 24% in the secondary effluent. At the same time, the percentage of the smallest particles (20-100 \(\upmu\)m) increased from 20% in the influent to 23% in the secondary effluent. It should be noted that, after pre-treatment and secondary treatment, the MPs most easily eliminated were those larger than 500 \(\upmu\)m (57%) and those with a size between 250-500 \(\upmu\)m (52%), as can be seen in Table S2. This means that the grit and grease system and the secondary treatment removed the bigger MPs with higher efficiency than the smaller ones. Important variations in the percentages of the middle sizes were not detected and the sizes distribution in the final effluent is similar to the secondary one. In the final effluent samples, the vast majority of MPs were smaller than 500 \(\upmu\)m, around 76%, whereas a quarter of the microplastics were smaller than 100 \(\upmu\)m (Table S2). These results agree with other previous studies, which reported that most of the MPs in the final effluent were smaller than 500 \(\upmu\)m. However, the percentage of MPs smaller than 100 \(\upmu\)m in the effluents is usually over 60%, percentage higher than those found in this work [19, 31, 33, 34, 52, 64, 65, 66, 67]. Table S2 shows that, after tertiary treatment, the most easily eliminated MPs were, both, those larger than 500 \(\upmu\)m and those with a size between 100-250 \(\upmu\)m (approximately 30%). In addition, considering the temperature, it can be observed that in the warmest months (May-September) the MPs with a size higher of 250 \(\upmu\)m presented abundances of 60-70%, while during those months with lower temperature (November, February-April), it is observed that the MPs with sizes less than 250 \(\upmu\)m presented abundances of 60%. It has been reported that MP degradation are determined by the combined effect of different parameters, including temperature. Specifically, [PERSON] et al. [68] concluded that photolysis combined to low temperatures leads to plastic brittleness, which is in accordance with results commented above, since the coldest months showed a greater proportion of MPs smaller than 250 \(\upmu\)m. Finally, it can be observed that the overall microplastic removal efficiency was higher in MPs larger than 500 \(\upmu\)m (70%) compared to the rest of the sizes. The morphological characteristics of MPs found in wastewater samples can be classified into five different types: fragments, fibres, microspheres or pellets, films and foams, as can be observed in Figure 4. Fragments exhibit irregular and opaque shapes, whereas fibres show a high length-width ratio. Pellets have spherical form, foams are fluffy particles and, finally, films have a relatively flat surface. Figure 3: Size variation of microplastics in influent, secondary effluent, and final effluent samples during the period studied. Figure 5 shows the percentages of MP classified by shape found in the different points of sampling: influent, secondary effluent and final effluent. It can be observed that, in all samples, fibres and fragments constitute practically the totality of MPs (above 98%). According to the literature, fibres and fragments are the most predominant particle found in wastewater with a mean percentage of 56% and 34%, respectively [18, 41, 52, 57, 69, 70, 71].