## 2 Principal Textile Colored WW Characteristics and Composition
### Coagulation-Flocculation Technologies and Its Performances in Colored Textile WWs Treatment  
The scientific literature clearly indicates that colored colloids from textile effluents cannot be separated via simple gravitational means, and some chemicals (e.g., lime, ferrous and ferric salts, aluminum salts, various polymers containing these metallic ions, such as poly aluminum chloride (PAC), poly aluminum sulphate (PAS), or cationic, anionic, and nonionic organic polymers/co-polymers (polyelectrolytes) based on poly acrylamide, polyacrylic acid, polyimides, other macromolecular structures, such as processed hybrid materials of inorganic, organic, and mixed/hybrid origins [60; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82; 83; 84; 85; 86; 87; 88; 89; 90; 91; 92; 93; 94; 95; 96] are added to cause the fine solids to separate via settlement or/and filtration. These chemicals destabilize colloidal suspended small particles (e.g., dyes, clay, iron, heavy metals, organic solids, and oil in WW) and emulsions entrapping solids (coagulation) and/or lead to the agglomeration of these particles to flocs large enough to settle (flocculation) or highly improve further filtration/biofiltration [60; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82; 83; 84; 85; 86; 87; 88; 89; 90; 91; 92; 96; 97; 98; 99; 100; 101; 102; 103; 104]. Anionic, nonionic, and cationic polymers can be used for the flocculation process. If an over dosage with coagulants and/or flocculations was applied, it is possible to intervene in a particle charge reversal due to the adsorption of excess ions, and thus stable colloid particles are presented [60; 92; 96; 105; 106; 107; 108; 109; 110; 111; 112; 113]. The scientific literature has reported _four major coagulation-flocculation mechanisms_ to explain the agglomeration of colloidal particles at higher particle size dimensions and, consequently, the ease of separation via gravity sedimentation or filtration [60; 92; 93; 94; 95; 96; 97; 98; 99; 100; 101; 102; 103; 104]:  
_(i) double layer compression_: The colloids present in WW are commonly charged and often described in terms of an electrical double layer. The charge serves to attract opposite counterions from the surrounding aqueous medium and, thus, it forms a layer adjacent to the colloid surface, which leads to ion exchange properties (Figure 4a). The stability of colloids and suspended solids in water is maintained because the small, charged particles repel each other. If they come sufficiently close, the repulsive forces of the surface charge are balanced by the attractive forces of counterions, and aggregation can occur with the formation of larger and settle flocs. One destabilization mode occurs when there is a high concentration of electrolyte in the water (acting as a coagulant), representing a source of counterions that accumulate around the solid surface and reduce the thickness of its  
Figure 3: Coagulation–flocculation treatment step for colored colloids separation from WWs. double layer. The space charge density of ions in water rapidly decreases with distance from the particle surface, and the potential exponentially declines as a function of distance (DLVO model: Derjaguin-Lindau-Verney-Overbeek). For the concentration of monovalent electrolyte (NaCl) in water of 0.1 M and 10\({}^{-3}\) M, the double layer thickness is of 11 A and 101 A, and, in the case of certain salts, small concentrations may be enough for particle aggregation. The double layer thickness decreases markedly with increasing counterion valence (Hardy rule). Therefore, the interaction energy (\(E_{T}\)) is defined as follows:  
\[E_{T}=E_{R}+E_{A}\leq 0 \tag{1}\]  
where \(E_{A}\) is the van der Waals attraction energy, and \(E_{R}\) is the coulombic repulsive energy of the double layer.