## 2 Principal Textile Colored WW Characteristics and Composition
### Textile WW Treatment Processes for Dye(s) and Color Removal
The chemical processes used for dye removal from dye-containing effluents are often more expensive than physical and biological treatments (except electrochemical techniques) and require chemicals, specific equipment, and electricity and can produce toxic by-products (secondary metabolites) that entail additional disposal and treatment problems [3; 10; 12; 72; 73]. Figure 2: Alternative treatment processes applied to colored textile WW. (* BAS—biochemical oxygen demand and suspended solids). The representative chemical treatments are as follows:  
(i) advanced oxidation processes (AOPs) operating in the presence of UV light and/or oxidizing agents (hydroxyl radicals, persulphate radicals, ozone, hydrogen peroxide, and other oxidizing agents in association or not with various catalysts) under specific conditions of temperature and pressure as stand-alone or hybrid technologies can be used [74; 75; 76; 77; 78; 79];  
(ii) coagulation processes operating under vigorous mixing conditions for the charge neutralization of fine particles in WW followed by flocculation under gentle mixing for fine solid agglomeration, and further floc separation via sedimentation can be applied. They are natural and synthetic coagulants/flocculants of inorganic and organic natures; recently, however, increasing interest has been on the development of hybrid materials (of inorganic-inorganic, inorganic-organic, organic-natural, inorganic-natural, or inorganic-organic-natural origin) [60; 80; 81; 82; 93; 94];  
(iii) electrochemical treatments can be used that operate in electrochemical cells/reactors with two metal electrodes connected at a direct current source in which the coagulant is in situ generated at the anode and hydrogen gas evolves at the cathode [83; 84; 85]. In addition, electro-Fenton and anode oxidation are considered electrochemical treatments that enable the removal of dyes in two stages based on a combination of oxidation and coagulation processes named usually electrochemical AOPs [86; 87; 88];  
(iv) ion exchange treatments developed based on the strong interactions between the functional groups of ion exchange resins and charged dyes are highly efficient, low-cost methods but can only be used for low and medium concentrations of contaminants [10; 12]. The physical processes used for the removal of dye from colored effluents involve mass transfer processes and are low-cost treatments with high efficiency (85-99%), simple designs, easy operating units, fewer chemical requirements, and no inhibitory effect due to the presence of toxic species. Such physical processes are adsorption, membrane filtration as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), which are all known to be membrane separation processes that remove contaminants from WW; the difference between these separation processes especially relating to pores size, which can be 0.1-10 \(\upmu\)m (MF), 0.001-0.1 \(\upmu\)m (UF), and 0.5-2.0 nm (NF) [89; 90; 91]. In the case of certain textile effluents, the scientific literature reports that WW treatments can be commonly based on the following three steps: [10; 61; 62; 63]: (i) a coarse prefiltration of mixed textile effluents to remove any fibrous matter; (ii) the chemical treatment of the effluent with a 10% lime slurry into a first tank to yield a pH of 11.30, after which an iron (II) sulphate solution can be added into a second tank, forming a precipitate of iron (II) hydroxide, and finally, the addition of polyelectrolyte to aggregate the floc particles into a third tank; and (iii) S/L separation via a faster and more effective particles settlement in the settlement tank; the sludge is automatically pumped to a sludge tank using a vacuum extraction device, and the supernatant passes through two further tanks where the pH is adjusted to values between 7.5 and 8.5 by injecting carbon dioxide under pressure or, in some instances, concentrated hydrochloric acid. The treated effluent is further pumped to the biological plant for treatment [61; 62; 63].