## 2 Microplastics and Microfibres in WWTPs  
MPs originated in industrial and urban activities can be driven into the sewage system, arriving to wastewater treatment plants (WWTPs). Even though these facilities can remove even more than 90% of MPs from wastewater, still millions of MPs are discharged to the environment in treated water each day [12; 39; 40]. Although great variability of data has been reported, the MP concentration usually ranges between 6.10 \(\times\) 10\({}^{2}\) and 3.14 \(\times\) 10\({}^{4}\) particles/L in influent and 0.01 and 2.97 \(\times\) 10\({}^{2}\) particles/L in effluent [39; 41]. At the household level, microfibres can be originated in items of clothing and furnishing, whereas at the industrial level, microfibres come from the automotive sector and the construction and clothing industries, amongst others [42]. It is remarkable that the clothing industry generates around 42 million tonnes of microfibres annually [43]. Microfibres originated in laundry contributes 35% of the global release of primary MPs to the environment, and the vast majority of these microparticles found in wastewater come from household  
Figure 1: A possible route of entry of MPFs into living beings via WWTPs and DWTPs. chores [2]. For example, between \(1.1\times 10^{5}\) and \(1.3\times 10^{7}\) polyester and cotton fibres can be emitted in only one wash [44; 45]. Additionally, the use of garments also contributes to the emission of microfibres to the atmosphere due to wear and tear [46; 47]. Most of these microfibres are introduced into the sewage system by means of surface runoff, and they subsequently arrive at WWTPs [48]. For instance, in Paris it was reported that fallout deposits an average value of 106.2 microfibres\(\cdot\)m\({}^{-2}\) per day [49; 50]. In a conventional WWTP, wastewater treatment is carried out in different stages that can be classified as follows: pretreatment, primary treatment, secondary treatment and tertiary treatment. The pretreatment is a physical process that aims to remove large debris and/or residues contained in the wastewater, such as oil, grease, sand and solid wastes, in order to avoid clogging and other problems that would affect the correct performance of the facility [40]. This stage includes screening systems and grit and grease removal systems. Different works analysed the presence of MPs throughout WWTP operations. It has been reported that 45% of MPs are removed during the pretreatment process [48; 51]. After that, a primary clarifier is commonly used to eliminate suspended solids [40]. Different parameters, such as the structure of suspended solids, the concentration, the retention time and shape of the settling tanks, affect the sedimentation capacity of the solid particles [40; 52; 53; 54; 55]. It has been found that primary treatment together with pretreatment could reduce the concentration of MPs with respect to raw wastewater by 78-98% [40; 56]. Primary treatment can achieve different removal efficiencies with respect to pretreatment, depending on the specific characteristics of the settling tank, varying from 22% to 99% [48; 51; 52; 57]. Once primary sludge is separated from the wastewater, the effluent from the primary treatment undergoes a secondary treatment consisting of a biological treatment that usually takes place under aerobic conditions [40]. Therefore, the aeration system used to supply the necessary oxygen for the process may make some MPs pass into the atmosphere. After the biological treatment, a settle is employed to separate the treated water from the secondary sludge [40]. For example, a study developed in Spain reported a MP removal efficiency of 67% in secondary treatment relative to primary treatment [48], whereas there was a 28% removal efficiency in a study from China [57]. Finally, a tertiary treatment is sometimes employed. The processes carried out in this stage depend on different factors, such as legal requirements, water reuse, etc.