## 2 Microplastics and Microfibres in WWTPs
### Microfibres Retained in Sludge  
Sludge is a waste generated from different physical, biological and chemical processes employed for wastewater treatment. Certainly, sewage sludge is the main residue originated in the WWTPs. Sludge is principally generated during primary treatment (primary clarifier) and secondary treatment (secondary clarifier), and it is called \"primary sludge\" and \"secondary sludge\" [140]. Sometimes, when advanced wastewater treatment processes are employed, tertiary sludge can also be generated during tertiary treatment. For sludge management, primary and secondary sludge (and also tertiary sludge in cases in which it has been generated) are combined, obtaining a so called \"mixed sludge\". The mixed sludge can be managed in different ways (anaerobic digestion, dewatering, incineration, etc. ), depending on its final fate [140; 141]. Depending on the sludge origin, its characteristics are different with respect to its composition and physical properties. The composition of the sludge is a key parameter for the management of this residue. During wastewater treatment processes, many pollutants, such as heavy metals and organic contaminants, are retained in the sludge [140; 141]. Specifically, it is well known that most of the MPs removed during wastewater treatment are entrapped in the sludge [12; 40]. Again, fibres are the most common MPs found in sewage sludge, mainly coming from textile handling and the washing of clothes [141]. The different processes used in the sludge line can alter the amount and characteristics of the MPs entrapped in it. For example, [PERSON] et al. [58] analysed the occurrence of MPs after sludge treatment by means of anaerobic digestion, thermal drying and lime stabilisation. These authors found that after lime stabilisation MPs were smaller compared with the previous two stages due to the combination of basic pH, high temperature and mechanical mixing that produce the fragmentation of MPs. It was reported that fibres are more resistant to fragmentation and degradation in sludge treatment processes than other MP forms [125]. Different devices (belt filter press, plate and frame filter press, decanter centrifuge, etc.) can be employed for sludge dewatering. [PERSON] et al. [142] analysed the effect of dewatering processes on the MPs entrapped in sludge. These authors reported that low-density MPs tend to remain in the aqueous phase during centrifugation, whereas the inorganic conditioners added during filtration are notably difficult to remove based on their MP analysis. In both cases, it was found that the number of MPs was underestimated due to methodological issues. The large amount of sewage sludge produced globally makes necessary the proper management of this organic waste. Sludge management methods can substantially differ in each region of the world. In the European Union, the annual production of sewage sludge is around 10 million tons (expressed as dry solids) [143]. Sludge management in the EU is regulated and, as happens in all cases of waste management, prevention and reuse are the most preferred options [144]. The most common use of sludge is in agricultural soils as fertiliser, since sludge is rich in organic matter and nutrients (nitrogen, phosphorus and potassium). However, as was commented above, organic and inorganic pollutants can be accumulated in sludge, which entails potential risks when sludge is applied to soils [145; 146]. It has been estimated that 44,000-300,000 (Europe) and 63,000-430,000 (North America) MP tons are scattered on agricultural soils each year by sewage sludge application [147; 148]. Specifically, microfibres are the most abundant MP particle (\(\geq\)50%), so a large amount of MPFs are being added to the soil, which entails potential harmful effects on fauna and flora [39; 149]. Sludge can also be incinated to generate energy; in this case, MPs entrapped are degraded, but it is important to consider that this management option contributes to greenhouse gases (GHG) emission [150].