## 2 Principal Textile Colored WW Characteristics and Composition
### Textile WW Treatment Processes for Dye(s) and Color Removal  
Equalization and sedimentation were considered viable as preliminary treatment operations in the primary mechanical treatment step of textile WWs until 1990; they were used for dye and color removal since no limits were imposed on their content in the treated effluent [3; 54]. The primary treatment step was usually followed by the secondary step based on specific biological-mechanical processes. After the approval of restrictive/limitative quality standards for treated effluent discharged to different aquatic receptors, technological treatment processes were introduced that were available and efficient techniques used for color and dye removal. The majority were based on discoloration and degradation processes [10; 55; 56; 57; 58; 59; 60], which are active in the _primary_ (physical-chemical-mechanical treatments based on precipitation, coagulation, flocculation, sedimentation, filtration, or air flotation, etc. ), _secondary_ (biological-mechanical treatments based on biodegradation, adsorption, redox and/or ionic exchange, sedimentation, and filtration), and _tertiary_ (advanced physical-chemical treatments based on advanced oxidation, precipitation, membrane processes, filtration on multiple adsorptive materials' layers, etc.) treatment steps but also in _sludge treatment_ (achieved via supervised tipping, chemical-mechanical conditioning, recycling, especially for sludge dehydration, or even incineration). Discoloration processes are especially used in removing colors from textile WWs, but the treated effluent can remain loaded with a significant content of organics expressed through high values of COD-Cr, BOD\({}_{5}\), and/or TOC, commonly exceeding the standard limits. For both color and dye removal from textile effluents, degradation processes are prominently used, which are processes involving the destruction of complex dye and auxiliary organic structures based on decomposition or chemically breaking down dye molecules into small molecular structures, i.e., degradation products such as carbon dioxide, water, simple minerals, and organic by-products (e.g., organic acids and alcohols, small organics with low molecular weights). All states of the European Community enforce strict legislative norms and measures concerning the presence of coloring and dyes in WW produced within the community and natural water resources; however, there is no official document that lists the different limits of numerous classes of dyes present in treated effluent, only a limitative sum of dye (<1 mg/L) and color (<10-50 HU) content that can be present in the treated effluent, which is dependent on a country's strategic policy and the norms imposed by environmental authorities/regulators on the compliance plan of certain textile companies. Moreover, in UK, a law that declares 'no synthetic chemicals should be discharged into the marine environment' [54], including synthetic dyes, is still active. In recent years, the E.U., Canada, USA and Australia have permitted environmental legislative authorities to specify the threshold concentration levels of different polluting species in treated WW, including dyes. Morocco and Turkey use the EU model, while Thailand adopted the USA system. In Pakistan, India, and Malaysia, effluent discharge limits are regulated by specific directives of their Central Pollution Control Board, but the limits for azo dyes are not specified, and other dye classes are considered separate groups, unrelated to the other physical-chemical characteristics of treated WW (e.g., the total dissolved solids (TDSs) content or color index) [58; 59; 60; 61; 62; 63; 64; 65; 66; 67]. The scientific literature reports various treatment schemes for textile WWs while referring to their initial compositions, but an important indicator when forming a decision is the \(\text{COD}_{\text{Cr}}/\text{BOD}_{\text{5}}\) ratio, which estimates the biodegradability of WW [58; 59; 70; 71; 72].