The textile industry is one of the world’s most influential and rapidly developing sectors [45]. These textile industries have a global market share of around $2000 and offer employment to ~120 million people across the globe. In India textile industry is one of the oldest industries and the second largest producer of textile apparel which shares around 5% of total global exports and contributes to 27% of Nationals Gross Domestic Product (GDP) [46]. Unfortunately, this sector employs a significant amount of water for its various wet processing activities and generates vast amounts of highly hazardous wastewater with a wide range of characteristics [47,48,49]. It is estimated that, on average, to create 8000 kg of textile fabric each day, a typical textile industry uses 1.6 million liters of groundwater, of which 30–40% is utilized in the dyeing process, 60–70% in the washing stage and 10–50% of unwanted dyes are discharged into water resource together with the produced effluent [50,51]. A typical textile industry is estimated to produce 5.2–6.6 MLD of textile wastewater through different unit operations [52]. The wastewater from these industries is generally characterized by high COD, color, and total dissolved and suspended solids, which make its treatment difficult [53,54,55]. Although the makeup of textile effluents varies considerably depending on the methods employed and the kind of fibers used, it generally includes a variety of unutilized organic and chemical components, such as dye waste, color residues, acids, alkalis, starch, various kinds of surfactants, cleaning solvents, inorganic salts [56]. When these complex wastewaters comprising the toxic cocktail of numerous types of dyes and chemical reaches the water bodies, it threatens the ecological balance of the aquatic ecosystem. Hence, they should be adequately treated before releasing them into the environment.

Dyes are compounds that have the property to absorb light radiance in the visible spectra (400–700 nm). The chromophoric group in the dye structure is responsible for the selective absorption of the incident light, and reflected light provides specific color to the dyes [57]. There are mainly two types of dyes such as natural and synthetic dyes, used in tie and dye sectors. Natural dyes are mainly derived from plants, animals, microbes, and minerals. They are considered less toxic to nature and can easily be degraded microbially. Some important sources of natural dyes are henna (Lawsonia inermis L.), Indigo (Indigofera tinctoria), Turmeric (Curcuma longa), Safflower (Carthamus tinctorius), Saffron (Crocus sativus) and Pomegranate rind (Punica granatum) [58]. However, due to rapid growth and market demand, the industry’s demand for synthetic dyes continuously increases. Chandanshive et al. [5] reported that around 7 × 10⁷ tons/year of synthetic dyes are produced worldwide for the textile industry. ~10% of the dyes are discharged into the waste stream. The dyes are characterized based on chromophore structure, which provides specific color to the dye. The most common chromophores are azo group (N=N), nitro (-NO₂), nitroso (-N=O), and carbonyl (-C=S) [47]. These chromophores absorb the incident electromagnetic wave due to the excitation of electrons to the higher orbit. The auxochrome group is also an important structure present in the dye structure, which is responsible for the fixation of dye to the fabrics. Various auxochromes exist, such as -COOH, -OH, NH₂, NR₂, etc. [57]. The textile industry fabrics are divided into two major categories: natural fabrics (cotton, hemp, wool, silk) and synthetic fabrics (nylon, polyester, polypropylene). Table 1 shows the list of industries using various dyes for specific fabrics.

The qualities of the textile wastewater emitted vary by industry depending upon the machinery, processing unit and the nature of fabric and dyes required for desirable fabric production [62]. However, most textile industries comprise a range of wet processes such as de-sizing, scouring, bleaching, mercerization, dyeing, and washing [59]. The water footprint is very high in these wet processes, leading to a huge volume of effluent discharge. The steps are commonly known as the pre-treatment range units and are required to remove the starch and other impurities (like salts, enzymes, dust, etc.) from fabrics and provide white color to the fabric material. The main purpose of the de-sizing unit is to detach the starch from the fabric through hydrolysis or oxidation. This leads to the direct discharge of enzymes, starch, and hydrogen peroxide into the waste stream, making effluent rich in organic content.

Similarly, the scouring and bleaching processes are required to remove the cotton wax and natural color substance from the fabric surface. These processes include using hot alkali, detergent, organic solvent, and hypochlorite (bleaching agent). Afterwards, mercerization must provide luster, increase strength, and improve dye uptake capacity [63]. The dyeing unit is the central process where a wide range of dyes or pigments are applied to the fabric to provide desirable shade. As discussed in the previous section, different dyes are used according to the fabric. In addition, various chemicals such as surfactants, salts, metals, sulfide, and organic chemicals might also be added to specific dyes to enhance the dye binding to the fabric. These chemicals and dyes are the key contaminants in the generated dye effluent. The primary metals responsible for environmental degradation are zinc, chromium, iron, lead, and mercury. Table 2 shows the reported physicochemical characteristic of actual textile effluent in different textile industries worldwide. Literature reported that the high chemical oxygen demand (COD), biochemical oxygen demand (BOD), TDS, and color of textile effluent were due to the presence of a large number of chemicals, salts, starch, fabric residue, and complex dyes [64,65,66]. Moreover, the effluent also has high pH and temperature. The range of organic content and other parameters in the effluent varies throughout the year as per the market demand for fabric.