## 2 Microplastics and Microfibres in WWTPs
### Technologies for Microfiber Removal in WWTPs
The aeration and mixing is discontinued after the biological reactions are complete, the biomass settles (sludge) and the treated supernatant is removed [135]. After that, the reactor is filled again, and the process is repeated. [PERSON] and [PERSON] [97] reported 99% of microfiber removal by SBR systems, whereas 98% was described for this process by [PERSON] et al. [85]. Coagulation-floculation is a typical chemical treatment which is followed by a physical separation, usually a sedimentation. Coagulation consists of the destabilisation of colloidal particles, whereas flocculation implies the agglomeration of the destabilised particles in floccules by the addition of chemical reagents--namely, coagulants and flocculants [136, 137]. This process is frequently employed in drinking water treatment plants (DWTPs), although it is also used in WWTPs. For example, in a tertiary treatment consisting of a coagulation-floculation process and rapid sand filtration (RSF), 98.6% of MPF removal can be reached [104]. The implementation of this technology with advanced processes, such as ozone and membrane disc-filter, would increase the removal efficiency of microfibres in treated water to above 99.6% [104]. RSF is a widely used low-cost technology applied to wastewater treatment due to its rapid and efficient removal of contaminants [12]. Their main disadvantage regarding MP removal is the possible fragmentation of these micropollutants into smaller particles [56]. RSF has reported removal rates of MPFs that can reach 96% [94]. In general, according to the reviewed literature, the most effective technique for removing MPs in WWTPs is a membrane bioreactor (MBR), achieving efficiencies near to 100% [51, 67, 84]. However, depending on the characteristics of wastewater and operating conditions, efficiencies lower than 80% [106] and even of 25% have been reported [99]. MBR consists of a biological reactor with suspended solids removal by a perm-selective membrane, e.g., microfiltration or ultrafiltration [12]. Only the smallest particles can pass through the membranes, so high effluent quality can be achieved [138]. In comparison with other processes developed for wastewater treatment, MBR presents as main disadvantages high membrane costs, high energy demand, low flux and maintenance issues [139]. For microfibres, a high removal efficiency of 99% has been described for this system [51], although values lower than 90% [67; 96] and even as low as 54% [106] have also been found. Therefore, to eliminate MPFs from wastewater, the different processes or technologies cited above can be employed with different MPF removal efficiencies--for example, MBR (54-99%), RSF (58-96%), oxidation ditch (96.6-97.2%), SBR (98-99%), coagulation-floculation (98.6-99.8%), CAS (95-99.9%) and A\({}^{2}\)O (95-99.9%), with the last three being the techniques that show better performance regarding MPF removal. From an economics point of view and according to the literature [40], the technologies mentioned above can be classified based on their installation and maintenance cost as follows: low-cost (RSF, DAF, oxidation ditch and CAS), low-medium cost (SBR and A\({}^{2}\)O), medium cost (discfilter) and high cost (DM and MBR). Thus, considering the low and low-medium cost, the technologies with the best performance in removing MPs and microfibres are conventional activated sludge (CAS), oxidation ditch and sequencing batch reactor (SBR). Optimising these technologies and/or complementing them with other processes is still a challenge for avoiding the release of large quantities of MPs, and especially microfibres, from WWTPs to the environment.