## 2 Microplastics and Microfibres in WWTPs
### Technologies for Microfiber Removal in WWTPs  
Different processes (physical, chemical and biological) are employed in WWTPs to treat wastewater [12]. Although these facilities are not specifically designed to remove MPs from wastewater, high removal efficiencies are achieved (\(\geq\)90%) [48; 62; 63; 65; 67; 81; 83; 84; 97; 102; 107]. However, this is still insufficient due to the large number of MPs that are emitted from the WWTPs into the environment [41]. This may change in the coming years since the European Parliament has recently submitted a proposal to regulate the environmental problems associated with the presence of MPs in treated water and sewage sludge. In this context, the foreseeable approval of this proposal by the European Commission (that is being studied) would imply a regulation of the presence of these micropollutants in WWTPs [128]. According to Table 1, 17% of the reviewed studies have a MPF removal efficiency lower than 60%. In addition, 33%, 11% and 28% have reported a capacity of MPF elimination in the ranges 60-90%, 90-95% and 95-99%, respectively. Removal efficiency higher than 99% is found in just 11% of the studies. To determine which technologies are the best for removing MPFs from wastewater, all studies that have achieved at least 95% of MPF removal will be discussed hereafter. A conventional activated sludge system (CAS) is a process commonly applied in WWTPs. This system includes an aeration tank, which is used for biological degradation of organic matter, and a secondary clarifier (sedimentation tank), where the sludge is separated from the treated wastewater [129]. There are different studies where this technology is analysed to remove MPs, and specially microfibres, reporting removal efficiencies of MPFs between 95% and 99.9% [63; 81; 89; 100; 101; 109; 112; 117; 120]. Some WWTPs applied CAS together with advanced processes, such as biological active filter (BAF) [82, 84] or trickling filter, followed by granular sand filter [120]. The simultaneous operation of CAS and BAF reaches MPF removal efficiencies of 97-99.5% [82, 84], whereas 95% is obtained using CAS combined with a trickling filter and granular sand filter [120]. Well-established and common biological nutrient removal processes, such as anaerobic/oxic (A/O) and anaerobic/anoxic/aerobic (A\({}^{2}\)O) processes can be found in several WWTPs. In the anaerobic tanks, the phosphorus is released, while in the anoxic tank a denitrification takes place, and the organic matter is removed in the aerobic tank, with high cost-effectiveness and efficiency [130, 131, 132, 133]. A microfiber removal efficiency of 96.5% can be achieve using the A/O (anaerobic/oxic) process [78]. On the other hand, higher MPF removal efficiencies could be achieved using A\({}^{2}\)O technologies in WWTPs, such as 98.3% [97] or almost 100% [99]. The simultaneous combination of several processes should increase the removal efficiency, although [PERSON] et al. [77] reported that the implementation of A\({}^{2}\)O together with advanced denitrification, ultrafiltration (UF) and ozone technologies achieved values of 95.1%, whereas 96.3% was obtained together with membrane bioreactor (MBR). Oxidation ditch is a modified activated sludge biological treatment process that uses long solids retention times to remove biodegradable matter. Typically, this system consists of a ring, oval or horseshoe-shaped basin through which the wastewater flows. Flow to the oxidation ditch is aerated and mixed with return sludge from a secondary clarifier [134]. A WWTP that uses this process removes around 97-98% of MPFs using the oxidation ditch alone or together with an A\({}^{2}\)O system [52, 78, 91]. If the process consists of an anaerobic, anoxic and aerobic basin that use filled carrier, an MPF elimination of 99% can be achieved [97]. Sequencing batch reactors (SBR) are also a modified type of activated sludge process for wastewater treatment. Oxygen is bubbled through the mixture of wastewater and biomass.