**3. Textile wastewater treatment**

The textile industry uses a large amount of drinking water for the production of fibers. It is estimated that per kilogram of a textile material, 200 L of water is used, which leads to large volumes of wastewater [16]. This, together with the toxic effect of some colorants and their low biodegradability, has driven the search and implementation of technologies for the treatment and recycling of textile effluents. So, before the discharge of textile effluents to bodies of water, they must be treated either by a physical, chemical, biological process or a hybrid system.

### **3.1 Physical methods**

Coagulation-flocculation-based methods are efficient for decolorizing wastewater containing dispersed dyes but show low efficiencies with reactive and vat dyes [20]. Filtration techniques (ultrafiltration, nanofiltration, and reverse osmosis) have been used to recover and reuse water. However, the high costs of the membranes, possible fouling of the same, and the generation of waste containing water-insoluble dyes (for example, indigo dye) limit their large-scale application [21]. On the other hand, adsorption processes (based on activated carbon) have been efficient in removing colorants present in wastewater [22]. However, its price and difficulty to regenerate it make it difficult to apply it in treatment plants.

### **3.2 Chemical methods**

These are the degradation methods most used in the removal of colorants due to their easy application. In this category, we find the advanced oxidation processes (AOPs). These methods have the ability to degrade both the initial colorant and its by-products, either partially or totally under environmental conditions. Furthermore, they can be used in synergy with other methods [23].

Within AOP, the Fenton process (a combination of H2O2 and Fe(II) salts) is the most popular, which has been successfully applied in the degradation of soluble and insoluble dyes [24, 25]. Its main disadvantages are the generation of sludge due to the flocculation of the reagents with the dye and the cost of the reagents. However, the photo-Fenton process offers an improvement to the traditional process, so that in the presence of UV light (even sunlight can be used), it is possible to regenerate Fe(II), making the degradation process more efficient [26].

#### **3.3 Biological methods**

Biological processes, due to their cost, are the most used treatments in the removal of colorants present in industrial effluents [27, 28]. Based on oxygen requirements, biological methods are classified as aerobic, anaerobic, and anoxic or facultative, or a combination of these. Few studies have reported on the degradation in aerobic conditions, since in general long periods of acclimatization are required, and the process is sensitive to changes in the concentration of the dye [29]. On the other hand, anaerobic processes are efficient for the bleaching of textile effluents [28]. However, the aromatic amines generated are more toxic than the original compounds and are difficult to break down under anaerobic conditions. In addition, fungal cultures and enzymes have been used for the degradation of dyes [30, 31].
