Reviewing textile wastewater produced by industries: characteristics, environmental impacts, and treatment strategies Open Access

Various scientific fields have made immense progress in the 21st century. Extensive research has been carried out on diverse topics in the fields of Environmental Science and Engineering research in the last 20 years. Large amounts of wastewater are generated daily from the textile, cosmetics, paper, rubber, leather, and printing industries (Younis et al. 2021). It is difficult to treat the toxic and complex textile wastewater produced by industries. Currently, water contamination caused by the wastewater released from the textile industry poses a threat to economic growth. As highly water-soluble and toxic substances (microbial pathogens and organic dyes) are present in the discharged water, the wastewater directly contaminates the natural ecosystem and reduces the availability of clean and fresh water that can be utilized for drinking. The complex and stable structure of the dyes makes the degradation of dyes (present in waste water and other complex substrates) difficult. The mineralization of dyes, presence of organic compounds, and toxicity of the wastewater released from textile and dye manufacturing industries negatively affect the environment. Therefore, it is important to gain practical knowledge and develop methods to effectively treat textile wastewater to save the environment (Holkar et al. 2016).

In recent years, extensive research has been carried out in the field of treating dyeing and weaving wastewater (Figure 1). To date, considerable efforts have been made to remove organic dyes/pollutants from wastewater using various methods (chemical, physical, and biological). It has been reported that it is difficult to remove color following traditional treatment methods (e.g., ozonation-, bleaching-, hydrogen peroxide/ultraviolet (UV)-, and electrochemistry-based) as most textile dyes have complex aromatic molecular structures that make their degradation difficult (Akbari et al. 2002). These dyes are stable in the presence of light and oxidants. They can also withstand conditions of aerobic digestion. Therefore, it is important to develop a green and sustainable method to effectively treat textile wastewater. Environmental scientists and engineers have focused on developing economically viable treatment methods.

To date, various review articles on the properties and applications of the most important approaches used for removing dyes from wastewater have been reported (Brillas & Martínez-Huitle 2015). However, the introduction of synthetic fibers has led to the emergence of some current markets that need to use other dye categories, becoming a recent major environmental problem. The aim of this review article is to provide extensive knowledge on the recent methods (such as chemical, physical, biological, and eco-friendly) that are used to remove dyes from wastewater produced by the textile industry. The latest and exhaustive data on the treatment processes used to treat textile wastewater have been presented here.

It is important to characterize textile wastewater to develop effective treatment methods and process flow. Various raw materials, such as cotton, synthetic fibers, and wool, are used in the textile industry. Wastewater is primarily produced during the execution of four steps: pretreatment, dyeing, printing, and functional finishing (Figure 2 presents the possible contaminants and the nature of effluent discharged at each step of the industrial process). The percentage of a definite parameter for characterization of textile wastewater included chemical oxygen demand (COD), pH, color, suspended solids, biochemical oxygen demand (BOD₅), N-NH_x, total phosphate (TP), total Kjeldahl nitrogen (TKN), conductivity, metals, total oxygen demand (TOC), Cl⁻, total dissolved solid (TDS), grease, alkalinity, surfactants, hardness, volatile suspended solid (VSS), sulfide, N-NO_x, total solids, turbidity, dissolved organic carbon (DOC), absorbable organic halogen (AOX), total carbon (TC), Org. N (Bisschops & Spanjers 2003). Composite textile wastewater is primarily characterized by analyzing BOD, COD, suspended solids (SS), and dissolved solids (DS) (Al-Kdasi et al. 2004). The classic characteristics of the conventional textile industry wastewater are presented in Table 1. Data analysis reveals that the COD value corresponding to mixed wastewater is significantly high.

Notably, the major contaminants present in textile wastewater are produced during the processes of dyeing and finishing. Currently, aromatic hydrocarbons and heterocyclic dyes are commonly used in the textile industry (Holkar et al. 2016). A dye molecule consists of two parts: the dye group and the dye auxiliary pigment (Liang et al. 2014). When the dye molecules are exposed to light, the structure containing double bonds (C = C) oscillates to absorb light and produce visible colors (Akbari et al. 2002). Dye at low concentrations can also exhibit highly intense color (Nigam et al. 2000; Liang et al. 2014). The complex and stable structures exist not only in textile wastewater but also in any kind of complex substances. Dyes can be classified into various categories based on their characteristics. They are primarily classified as ionic and non-ionic dyes (Robinson et al. 2001). Ionic dyes are direct, reactive, and acidic dyes. Non-ionic dyes remain dispersed as they do not ionize in a water-borne medium (Deive et al. 2010). Methyl orange, acid red-B, rhodamine-B, Prussian red, alizarin red, Congo red, orange green, rose Bengal, and basic yellow 28 are examples of ionic dyes (Islam et al. 2019). Textile dyes are classified as acidic, alkaline, direct, disperse, active, sulfur, or reducing dyes (Table 2) (Akbari et al. 2002). Acidic dyes are negatively charged, and alkaline dyes are positively charged. The dyes are active if anionic dyes are used in the textile industry, medium if metal ions are present, reduced if they are derived from natural indigo, and disperse if non-ionic (Brillas & Martínez-Huitle 2015). Direct dyes are the most popular class of dyes, as they are easy to use, exhibit a wide range of colors, and are economically friendly. The structures of most direct dyes contain di-azo and tri-azo moieties. The maximum range of colors can be observed for the azo dye (percentage of dyes belonging to this class: 60–70%) (Deive et al. 2010).

In addition to harmful dyes, wastewater produced by the textile industry also contains various pigments, heavy metals, sulfates, oils, surfactants, and chlorides (Wei et al. 2020) (Figure 2). These contaminants can adversely affect aquatic life and water quality. Heavy metals have often been used during the process of dye fixation and also in dyes. It has been reported that the metal units present in dyes help impart color so that the dues can be used as textile colorants. Textile wastewater contains trace amounts of metals such as Cu, Cr, As, and Zn, which harm the environment (Table 2) (Mirghani et al. 2008; Mani et al. 2019). These metals can bind with organic dyes and/or fibers (Khan & Malik 2014). The major contaminants present in textile wastewater are high suspended substances, COD, acidity, heat, color, and other soluble substances (Al-Kdasi et al. 2004). In general, textile wastewater exhibits intense color and is characterized by high BOD/COD values and high saline loading (Holkar et al. 2016). The BOD/COD ratio for composite textile wastewater is approximately 0.25. This indicates that wastewater contains a large amount of non-biodegradable organic substances. (Al-Kdasi et al. 2004). As reported by Paździor et al. (2019), industrial textile wastewater treatment methods are studied using various effluents generated during the execution of different processes within the dye-house. These effluents are collected under conditions of equilibrium or in neutralization pools. The pollutions at a lower concentration may be present in the effluents.

It has been established that wastewater significantly pollutes the environment (Abdel-Karim et al. 2021). Wastewater can pollute the surface water, groundwater, soil, and air. Numerous textile and dyeing factories are found in developing countries, where wastewater is often poorly treated (Khan & Malik 2014). Textile wastewater is hazardous to the environment as it contains carcinogenic, toxic, mutagenic, and difficult-to-degrade compounds (Hubadillah et al. 2020). It has been reported that approximately 2,000 different types of chemicals (dye, transfer agents, etc.) find their use in the textile industry (Khan & Malik 2014). Dyes are one of the main contaminants present in wastewater released by the textile industry (Nor et al. 2021). Since the discovery of the first synthetic dye in 1,856, more than 10,000 different types of textile dyes (estimated annual output: 8 × 10⁵ metric tons) have been commercialized worldwide. Approximately half of these dyes fall under the category of are azo dyes (Deive et al. 2010). A large number of these toxic dyes eventually enter the waterways, causing serious environmental problems (Nabil et al. 2014). It has been reported that the textile industry utilizes large amounts of water during the process of textile processing. The percentage of toxic and hazardous dyes present in the wastewater system ranges from 5 to 10% (Prasad & Aikat 2014). An estimated 280,000 tons of textile dyes are discharged (annually) worldwide through industrial wastewater (Zainith et al. 2016). Approximately 10–15% of the dyes are discharged into the environment during various substrates staining. The substrates include synthetic fibers, natural textile fibers, plastics, leather, paper, mineral oils, waxes, specific types of food items, and cosmetics (Bae & Freeman 2007). It is extremely difficult to handle textile wastewater as it is characterized by high content variability and color strength. The color of these dyes can potentially change the extent of turbidity causes, COD value, pH value, and temperature of the water body (Verma et al. 2012). It is estimated that approximately 2% of the dyes are directly released into the aqueous effluent and 10% of the dyes are therefore lost during the process of coloring (Mani et al. 2019).

The maximum amount of harm to the environment is caused when sunlight is absorbed and reflected by the water system containing dyes. The absorption of light changes the algal photosynthetic activity, altering the food chain (Mani et al. 2019). The discharge of these harmful substances into the soil environment and aquatic system results in low light transmittance and low oxygen consumption. This can negatively influence the process of photosynthesis and aquatic life (Holkar et al. 2016). Apart from exerting negative aesthetic effects, these dyes can harm organisms as they exert carcinogenic and mutagenic effects (Das & Mishra 2017). It was estimated that out of 3,200 azo dyes used, 130 dyes could be used to produce carcinogenic aromatic amines following the processes of reduction and degradation (Bae & Freeman 2007). Contact with azo dyes can result in skin, lung, and gastrointestinal problems. These dyes can enter the body through the digestive system and destroy the roots of hemoglobin and DNA substances. The substances can induce cancer in humans and animals (Islam et al. 2019). The contact to leukemia with multiple colors affecting the circulatory, respiratory disease, allergic reactions, neurobehavioral, and immune suppression disorders. Carcinoma of the kidneys, liver, and urinary bladder has been reported in textile workers (Islam et al. 2019). Results from experimental studies conducted on animal models by Raj et al. (2012) indicate that the main category of textile dyes, i.e., azo dyes, is directly associated with human bladder cancer, splenetic sarcomas, and hepatocarcinoma (the major cause of chromosome aberration in mammalian cells).

Mathur et al. (2006) assessed the mutation-causing ability of textile dyes from Pali (Rajasthan) by conducting an Ames bioassay. In their study, a total of seven dyes were conducted for their mutagenicity by Ames assay, using strain TA 100 of Salmonella typhimurium (Mathur et al. 2006). The results indicated that only one dye, Violet, exhibited no mutational activity. The use of the remaining six dyes resulted in mutation (Mathur et al. 2006). It has also been reported that bioassays are sensitive and reliable methods that can be conducted to determine the toxicity of industrial wastewater. Hence, they can be used to assess the efficiency of emerging tools (Rosa et al. 2001). The relative sensitivity of biological assays toward textile wastewater is arranged in descending order: plant enzymes > bacteria > algae ≈ daphnids ≈ plant biomass ≈ germination rate > fish. Significant effects on genetic toxicity were not observed (Rosa et al. 2001). The aquatic toxicity of a series of unique direct dyes containing benzidine congeners, 2,2′-dimethyl-5,5′-dipropoxybenzidine, and 5,5′-dipropoxybenzidine, and the toxicity of a commercial dye (C.I. Direct Blue 218) were assessed by conducting acute toxicity studies in the presence of Daphnia magna (Bae & Freeman 2007). The results revealed that C.I. Direct Blue 218 was highly toxic toward daphnids. It was more toxic than the unmetallized direct dyes. In addition, the results also revealed that the assay conducted with D. magna could be effectively used to assess the aquatic toxicity of dyes (Bae & Freeman 2007). Villegas-Navarro et al. (1999) used the crustacean Daphnia magna as a sensor organism and 50% lethal concentrations (LC₅₀) as the standard for measuring the toxicity of textile effluents (treated and non-treated). The results indicated that all the five textile industries could produce toxic non-treated water (ATU: 2.1–25.4). The treated water was also toxic (ATU: 1.5–7.2). This suggested that the treatment plants and methods used by the five textile industries to remove toxic water were not highly efficient (Villegas-Navarro et al. 1999). Sharma et al. (2007) used Swiss Albino rats to assess the toxicity of the wastewater generated from the textile industry. Table 3 presents information on the toxicity of some commonly used azo dyes.

The discharge of untreated or semi-treated colored sewage into the nearby water affects the extent of penetration of light and oxygen, and this ultimately affects the aquatic ecosystem. These toxic and harmful contaminants must be removed from textile wastewater to minimize the extent of pollution caused (or avoid causing pollution) when wastewater mixes with river water or is reused for other industrial or agricultural processes (Abdel-Karim et al. 2021). Hence, different textile wastewater treatment processes are discussed in the subsequent sections of this manuscript.

In the past two years, different treatment techniques have been studied to realize the sustainable degradation of textile wastewater (Figure 3). More than 7 × 10⁵ tons of dyes are synthesized annually worldwide. The structures of the dyes used in the textile industry are changed continuously to meet the color shade requirements and realize colorfastness (Watari et al. 2021). The annual global market growth rate has been reported to be 8.13%. Hence, it is estimated that large amounts of effluents consisting of approximately 280,000 tons of refractory textile dyes will be produced by the textile industry (Zhou et al. 2021). The choice of appropriate treatment method depends on the production process and the chemicals used in the textile plant. The choice is also influenced by the composition of wastewater, discharge rule, location, business costs, operational costs, availability of land, selection, and availability of reuse/recycling of treated effluents, process, and expertise (Jegatheesan et al. 2016). The cost of treating river water that can be used for drinking should be reduced, and drinking water should be colorless and devoid of toxic compounds. Thus, numerous treatment processes (such as physical, chemical, and biological) have been developed to treat wastewater before the wastewater is released into river bodies. A combination of these treatment methods has also been used to treat textile wastewater in an economically effective manner. It has been reported that these techniques can be effectively used for textile wastewater treatment (Holkar et al. 2016).

Physical treatment methods involve the removal of substances from wastewater by exploiting commonly occurring forces (e.g., electrical attractive, gravitational, and/or van der Waals forces) or physical barriers. The use of these methods does not cause a change in the chemical structure of the substances present in water (Mani et al. 2019). Occasionally, the physical state gets changed, or the coagulation of discharge substances takes place. The characteristics of some of these technologies have been explained in detail in this manuscript.

It is environmentally important to remove dyes from colored effluents (especially from the effluents produced by the textile industries). The process of adsorption is one of the diverse methods that has been favorably used to treat dye-containing wastewater. A large number of materials, such as activated carbon (the most commonly used and expensive adsorbent), polymeric resins, and numerous economical adsorbents (agricultural and industrial by-products, such as peat, bentonite, chitin, silica, other clays, and fly ash) have been used as appropriate sorbents to realize the decolorization of industrial wastewater (Suteu et al. 2009). Activated carbon is the most well-known adsorbent. It is usually produced following the processes of physical or chemical activation. Although activated carbon can be effectively used to adsorb dyes, its application is limited as it is expensive. Hence, there is an increasing demand for the production of an adsorbent that is as efficient as activated carbon but cheaper than activated carbon. Several economically viable treatment methods for the treatment of dyes have been reported. The execution of these methods requires the use of various adsorbents such as rice husk, cotton, bark, hair, coal, perlite, sewage sludge-based activated carbon, apple pomace, wheat straw, banana peel, orange peel, organobentonite, pearl millet husk, peat particles, wood, fly ash, and coal (Khan et al. 2004). The removal ability of activated carbon processed from coir pith was studied in the presence of three strikingly applied reactive dyes in the textile industry (Santhy & Selvapathy 2006). It was reported that the activated carbon obtained from the coir pith not only effectively removed the color but also significantly reduced the COD levels of textile effluents. Recently, Suleman et al. (2021) used castor biomass-based biochar to realize the adsorption of safranin. The results revealed that the biochar could be used to realize a high adsorption capacity (4.98 mg/L). Approximately 99.6% of the safranin dye could be removed (99.6%). Oke & Mohan (2022) reported that textile sludge-based activated carbon could be used as an adsorbent to adsorb reactive yellow 145, methylene blue, and reactive red 198. The adsorption capacities were recorded to be 75.1 mg/L, 101.8 mg/L, and 76.6 mg/L, respectively. It is noteworthy that it is difficult to remove activated carbon from the solution, and thus, it can be released into the environment along with the processed sludge used to treat wastewater. This can result in secondary contamination (Singh & Arora 2011). Recently, the adsorption of titanium dioxide (TiO₂) is a highly regarded advanced photocatalyst that constitutes a new growing branch of activated carbon composites to improve the adsorption rate and discoloration ability, causing serious consideration and support around the world (Foo & Hameed 2010). Liu et al. fabricated a highly active TiO₂/activated carbon photocatalyst following a hydrothermal method. This system could be readily isolated from the treatment system and could be used to effectively adsorb methyl orange (Liu et al. 2007). Activated carbon present in the TiO₂/activated carbon catalysts can potentially act as organic-molecule-adsorbing centers. Subsequently, the adsorbed molecules get transferred to the decomposition center and the TiO₂ units present on the surface of activated carbon get illuminated. This can be attributed to the concentration differences (Figure 4). It is important to understand the conditions affecting the adsorption capability of such compounds. It is believed that the adsorption ability is influenced by various factors such as the hardness of water and time of processing (Siddique et al. 2017).

The membrane filtration technique used to treat wastewater has attracted immense attention from people working in industries (Othman et al. 2021). This technology is capable of producing stable water without chemical consumption and relatively low energy demand. What is more, one of the most common methods is filtration technology. Large amounts of coloring wastewater containing inorganic salts are produced by textile industries. The filtration technology can be used to filter and recycle pigment-rich wastewater systems. This technique can also be used to bleach and mercerize wastewater (Verma et al. 2012). The process of membrane filtration proceeds over several steps. Low-molecular-weight compounds and a few divalent salts are degraded (or broken into smaller units) during the process of nanofiltration (NF). The process of ultrafiltration (UF) can be used to remove particles and macromolecules from the wastewater system, and the process of microfiltration (MF) can be used to remove suspended matter (Barredo-Damas et al. 2012).

The process of NF is being increasingly used to manage color emissions (Table 4). Tang & Chen (2002) followed the process of NF to treat textile wastewater to make the water reusable. The results revealed that high fluxes were generated under conditions of low pressures (≤500 kPa). The average dye removal rate achieved under these conditions was 98%, and the NaCl removal rate was less than 14%. Loose membranes, used to execute the NF process, are used in the art-of-the-state NF technology to achieve efficient fractionation of dyes and monovalent salts (i.e., NaCl). This is because the loose surface structure of the membranes facilitates the passage of salts (Ye et al. 2018). A decrease in the penetration flux is observed during membrane processes, and this can be attributed to the accumulation of molecules on the surface of the membranes. Noël et al. (2000) have studied the efficiency of two types of membranes (NF45 and BQ01) using a direct dye solution. They conducted their experiments under conditions of an electric field to address the problem of fouling.

Recently, the process of UF has been used to recover high-molecular-weight and unsolvable dyes (such as indigo and disperse dyes) (Liu et al. 1994). The membrane used to execute the UF process is characterized by a porous structure. The process is characterized by moderate separation properties. The dimension of the aperture of the membrane lies in the range of 2–200 nm. Thus, these membranes are smaller than large micromembranes but larger than small nanofiltration membranes (Othman et al. 2021). A low pressure-driven UF membrane was advanced using α-aminophosphonate-modified montmorillonite (MMT), cellulose acetate (CA), and Ag-TiO₂ nanoparticles to treat textile wastewater (Abdel-Karim et al. 2021). Jiang et al. (2018) reported that the process of UF could be effectively used to realize dye/salt fractionation during the process of textile wastewater treatment. Barredo-Damas et al. (2010) reported that the color removal efficiency varied between 82 and 98% when ceramic UF membranes were used.

The dimensions of the aperture of the MF membrane lie in the range of 0.2–0.5 μm. This membrane is primarily used to remove particulate suspensions and colloidal dyes from the load-running dye baths and out-of-date rinsing bath (Dasgupta et al. 2015). MF supports unconsumed auxiliary chemicals, dissolved organic pollutants, ions, and other soluble pollutions to pass through the membrane together with the permeability (Couto et al. 2017). Amaral et al. (2014) studied the use and operating parameters of the MF system to recover insoluble indigo blue present in the cotton yarn dye bath to realize pigment reuse and water washing. The results suggested that the MF process can be potentially used to recover insoluble indigo blue presents in synthetic wastewater systems (Amaral et al. 2014). It has been recently reported that coal is a good carbon material that can be used as a carbon source for the fabrication of asymmetric microfiltration carbon membranes characterized by high porosity, controllable aperture, and narrow pore size (Song et al. 2006). A tubular carbon microfiltration membrane was fabricated by mixing mineral coal (average particle size: 100 μm) with a solution of phenolic resin and organic additive (Tahri et al. 2013). The fabricated membrane exhibits interesting properties in terms of mechanical and chemical tolerance. In addition, high permeability flux and removal efficiency could be achieved. The extents of retention of COD values and salinity were 50 and 30%, respectively. Turbidity and color could be removed efficiently (Tahri et al. 2013).

The reverse osmosis (RO) technique can be used to effectively remove mineral salts and organic compounds (Barredo-Damas et al. 2012). According to De Jager et al. (2014), RO exerts a significant impact on the residual salt and color. RO membranes can remove inorganic ions and various combinations of organic molecules more efficiently than NF membranes. The infiltrates are usually colorless and are characterized by low conductivity. These membranes should operate under conditions of high pressures and are potential alternatives to NF membranes. These can be used to recover wastewater from dyebath effluents (Kurt et al. 2012). Vishnu et al. (2008) proposed a series of stained wastewater treatment methods, including NF and RO. They reported that these processes were economically and physically friendly. The processes could be effectively used to treat wastewater and recover salt. The wastewater treated using these techniques can be reused. The industry-specific RO treatment performance depends on the nature of the effluent and the pretreatment processes. Therefore, the outgoing water characteristics need to be evaluated before adopting specific RO methods. De Jager et al. (2014) reported the use of a pilot-scale membrane bioreactor and a subsequent RO process. Although the effluent treated using the membrane bioreactor satisfied the discharge standard, the residual color and conducting wastewater needed to be separated following the RO process. Ebrahim et al. (2018) used the RO technology to remove the acidic blue dye from industrial wastewater under various performing conditions (applied pressure: 5–10 bar; initial concentration of dye: 25–100 mg/L) to meet the concentration criteria laid down by the factory. The processes were conducted at a constant pH at room temperature. The results suggested that the removal efficiency increased during 90 min of the operation as the pressure raised to 98% and the original dye concentration (Ebrahim et al. 2018).

Various chemicals are used during the execution of various processes to accelerate the process of disinfecting wastewater during purification. These chemical processes involve various chemical reactions that are labeled as chemical unit processes. These processes accompany various biological and physical processes (Mani et al. 2019). Conventional chemical methods (coagulation and flocculation), electrochemical techniques, and AOPs are some of the methods and techniques that are commonly used to treat textile wastewater.

The coagulation/flocculation (CF) process is commonly used to destabilize particles or colloids. This process can be effectively used for dye removal (Huang et al. 2014). The CF process is widely used to remove dyes as it is a cost-efficient process that is easy to operate (Riera-Torres et al. 2010). Golob et al. (2005) followed the CF method to purify residual dyebaths obtained when a cotton/polyamide blend was dyed black. During the CF process, selected coagulants play critical roles and help remove target contaminants. Various categories of coagulants (such as inorganic coagulants, inorganic-organic dual coagulants, and synthetic polymer flocculants) are commercially available (Chen et al. 2010). Inorganic coagulants, such as iron and aluminum salts, are widely used in the field of textile wastewater treatment (Liang et al. 2014). Abbas et al. (2021) reported that iron chloride (2.72 g/L) could be used to remove 91.89% of color attributable to the use of dyes. Recently, Jalal et al. (2021) have reported the use of aluminum-based coagulants to treat textile wastewater. The maximum color reduction achieved was 98%. However, iron and aluminum salts, used as conventional coagulants, negatively impact the environment and human health. Environmentally friendly substances that can be used as alternatives to these toxic coagulants have attracted the attention of scientists worldwide. Various biodegradable, non-toxic, and widely available compounds (Elkady et al. 2011) such as chitosan, Moringa oleifera seeds, tannins, and Jatropha curcas seeds are being increasingly studied as alternatives (Hameed et al. 2018).

AOPs are up-and-coming alternatives that can be used to obtain hydroxyl radicals (HO·). These processes can be used to effectively remove dyes and refractory contaminants (Güyer et al. 2016). In general, AOPs include a series of methods, such as ozonation, photocatalysis, Fenton reaction, and Fenton-like processes (Li et al. 2021a). Different types of AOPs, producing HO· are being increasingly studied to realize the de-coloration of textile effluents. The high reactivity of HO· can be attributed to the presence of unpaired electrons, and these radicals can help to oxidize stubborn organic matter (Jorfi et al. 2016). HO· is a nonselective and highly powerful oxidizing agent. The rate constant recorded for the reactions (involving HO·) associated with the removal of organic matter lies in the range of 109–1,010 M⁻¹ S⁻¹ (Eslami et al. 2013). In addition, other oxidants such as sulfate (SO₄⁻) radicals, permanganate (MnO₄⁻), hypochlorite (ClO⁻), chlorine dioxide (ClO₂), and ozone (O₃) are used during the process of textile wastewater treatment (Asghar et al. 2015). Khatri et al. (2018) followed various AOPs based on zero-valent aluminum (ZVAl) to treat textile wastewater (Figure 5). They reported that the maximum COD, color, and ammonia-nitrogen removal efficiencies achieved following the ZVAl-based AOPs were 97.9%, 94.4%, and 58.3%, respectively (conditions: ZVAl (1 g/L), Fe³⁺ (0.5 g/L), hydrogen peroxide (H₂O₂; 6.7 g/L), 3 h after contact time) (Khatri et al. 2018). Results obtained the following the process of external addition of tert-butyl indicated that in situ HO· and SO₄^·- are the primary oxidants that are responsible for the oxidation of contaminants (Khatri et al. 2018).

The removal of color has been widely achieved using H₂O₂, sodium hypochlorite (NaClO), and other chemical agents in the textile industry. Argun and Karastas et al. (Argun & Karatas 2011) used 2,000 mg/L of H₂O₂ to degrade synthetic dyes (concentration: 200 mg/L). However, the polishing operations associated with these chemicals are cost ineffective (Lin & Chen 1997). It has been recently reported that the process of ozonation can be a potential alternative to established methods that are used for decoloration. According to Hassaan & El Nemr (2017), the removal of COD, ozone is much less efficient. It was observed that the process of ozonation could not be used to reduce COD effectively. The results revealed that it could be used to realize one-step decolorization and partial oxidation to improve biodegradability. The increased BOD/COD ratio following ozonation could be potentially attributed to the increased biodegradability of toxic substances (Hassaan & El Nemr 2017). In addition, it has been suggested that O₃ should be used in combination with other technologies for effective wastewater treatment. According to Destaillats et al. (2000), 30% mineralization of methyl orange can be achieved in the presence of O₃. When the process is conducted in combination with the ultrasound-based treatment methods, the cumulative extent of mineralization achieved is >80% in the pH range of 5.5–6.5. According to Vecitis et al. (2010), the generation of the synergetic effect can be attributed to the fact that the decomposition of one O₃ molecule results in the production of two OH· radicals under conditions of sonolysis.

Recently, electrochemical oxidation methods (e.g., electrokinetic coagulation, electroflotation, electrodecantation, electrooxidation, etc.) have emerged as the primary textile wastewater purification methods (Brillas & Martínez-Huitle 2015). The primary reagents are electrons which are also labeled as the ‘clean reagents’. When electrons are used as the reagents, organic matters are generated, and secondary contaminants or by-products are not formed (Mohan et al. 2007). The electrochemical oxidation of C.I. acid orange 7 was performed on a boron-mixed diamond electrode (Fernandes et al. 2004). The electrochemical treatment of the outflow from the UASB reactor (containing acid orange 7) promoted the reduction of aromatic amines. Reduction reactions were facilitated even when the concentrations were low, and the electrolyte already present in the outflowing system was used (Fernandes et al. 2004). Sakalis et al. (2005) reported a new device that could be used for the electrochemical treatment of textile wastewater. The results revealed that the dye removal rate was 94.4% when the device was used to treat wastewater under conditions of neutral pH (Sakalis et al. 2005). In addition to the management parameters, the rate of pollutant degradation is also influenced by the anode materials. Naumczyk et al. (1996) demonstrated that several anode materials, such as graphite and precious metal anodes, can be successfully used for the mediated oxidation of organic contaminants. Sakalis et al. (2005) reported that the electrochemical treatment method is a relatively new method that can be used to achieve complete decoloration. The treatment methods can be conducted under conditions of medium pH. A low final temperature can be maintained and the COD and BOD₅ values can be significantly reduced. The formation of sludge can also be avoided under these conditions. Unfortunately, in most cases, high concentrations of supporting electrolytes, in particular NaCl, are required to obtain acceptable results. This results in the production of large amounts of free chlorine, hypochlorite anions, and polychlorinated aromatic products, which significantly harm the environment (Rehorek et al. 2002).

The process of biological degradation of dyes is a green technique that can be used for removing dyes from textile wastewater. The cost of operation is minimum, and the process can be conducted under conditions of an optimal operating time. Ali recommends the use of biomaterials such as algae, bacteria, fungi, and yeasts (that can decompose and absorb various synthetic dyes) to achieve biological degradation (Holkar et al. 2016). The potential of multiple microorganisms to decolorize and degrade these harmful dyes has been reported (Table 5). We have discussed the processes of microbial decolorization and enzyme degradation in the following sections.

As mentioned above, the biological wastewater treatment technique is technically attractive, environmentally friendly, and cost-effective (Das & Mishra 2017). Various bacteria can be used to effectively remove dyed from wastewater as they can effectively discolor various dyes under anaerobic or aerobic conditions (Mahmood et al. 2016). Attempts were made (as early as 1970) to identify bacteria that could degrade azo dyes. Three strains of bacteria were identified: Bacillus subtilis, Aeromonas hydrophila, and Bacillus cereus (Dave et al. 2015). Recently, Spagni et al. (2012) studied the applicability of immersed anaerobic membrane bioreactors in the field of decolorization of azo dye-containing wastewater. The results revealed that immersed anaerobic membrane bioreactors could be used to achieve a significantly high decolorization rate (>99%). It was also observed that P. aeruginosa could be used to remove commercially used tannery and textile dye. Navitan Fast blue S5R is an example of such a dye that can be removed from wastewater systems in the presence of glucose under aerobic conditions (Pandey et al. 2007). In addition, an anaerobic (facultative anaerobic bacterial culture)–aerobic sequence system was used to remove the color from a pilot-scale actual textile wastewater system and reduce COD of the wastewater system under study (Kapdan & Alparslan 2005).

The effect of microbial decolorization is influenced by the adaptability and activity of the chosen microorganisms (Nor et al. 2021). From this point of view, in microbial decolorization, the bacteria have the ability to decolorize, which is assumed to be associated with the production of a different enzyme (Nor et al. 2021). Aeromonas veronii GRI (KF964486) was isolated from domestic textile effluents after selective enrichment on azo dye to evaluate the biodegradation effectiveness of methyl orange (Mnif et al. 2016). It was observed that when the system was vaccinated with an initial light density of approximately 0.5, sucrose (0.25%), yeast extracts (0.125%), and Bacillus subtilis SPB1 biosurfactant (0.01%), bacteria could effectively decolorize azo dyes. The stirring stage was initiated approximately 24 h after the stage of static incubation (Nor et al. 2021). The researchers also reported that the removal of methyl orange could be potentially attributed to intracellular enzyme activities (Nor et al. 2021). Furthermore, the textile dye-decolorization effect achieved in the presence of microorganisms could be improved by producing biological surfactants. Mnif et al. (2015) reported that lipopeptides derived from Bacillus subtilis SPB1 maximize the decolorization efficiency and accelerate the decolorization rate when the optimal concentrations of biosurfactants are approximately 0.075%. It is essential to summarize the influence of each parameter associated with biodegradation to develop a more effective and faster bacterial degradation method. Table 6 summarizes the possible ranges of the staffing parameters to achieve better biodegradation effects (Mani et al. 2019).

Azo dyes are electron-deficient compounds containing the -N = N- chromophore group. These dyes may also contain several other electron-withdrawing groups in their skeletal structure. The production of electronic defects makes the compounds less susceptible to the degradation process. Bacteria can effectively degrade dyes as diverse, and well-constructed enzyme systems are present in these organisms (Sarkar et al. 2017). It has been reported that azo dye-decolorizing microorganisms produce various enzymes, such as azo reductase, laccase, peroxidases, NADH-DCIP reductase, tyrosinase, MG reductase, and aminopyrine N-demethylase. Azoreductases, laccases, and peroxidases are the major enzymes that are responsible for the decolorization of azo dyes (Imran et al. 2015).

Most of the organic matter in sewage systems is non-biodegradable. This results in inefficient biotreatment. Chemical treatment technologies, such as flocculation and coagulation, can be used to effectively remove color. However, large amounts of harmful residues that require further treatment are produced when these techniques are used. This makes these technologies cost-ineffective (Buscio et al. 2015). Hence, a combination of various treatment methods has been suggested to effectively treat wastewater.

The CF and NF methods and a combination of the two methods (CF–NF) have been used to treat wastewater systems containing synthetic dyes (Liang et al. 2014). Liang et al. (2014) reported that the CF process could be used to achieve almost 90% dye removal efficiency under conditions of optimal dosage (polyaluminum chloride (PAC)/polydiallyldimethyl ammonium chloride (PDDA) = 400/200 ppm; pH of the mixed dye wastewater >3). In addition, they found that the CF and NF methods could complement each other's strengths. The problems faced when each method was used individually could be addressed using the CF–NF method. Riera-Torres et al. (2010) reported that NF removed except 40 and 80% of color for the five dyes, while CF reached 85–95% color removal rates of the four dyes except polyurethane resin for RB5. The color removal efficiency for RB5 reached 90%. When the combination techniques were used, the efficiency reached >98% for all dyes. Aouni et al. (2009) reported that the color removal efficiency was >99% when the NF technique was used following electrocoagulation.

López-López et al. (2016) improved the efficiency of AOPs by introducing CF as a pretreatment method (to reduce the turbidity observed in the textile effluents). The experiments were conducted with five different concentrations of industrial coagulants. The coagulants (FLOCUSOL-PA/18) were used to reduce turbidity (approximately 99% of the turbidity could be removed). In addition, the color removal rate for all AOPs was nearly 100%. Ozone combinations are the most widely used advanced oxidation methods before biological treatment to improve biodegradability and color removal efficiency. It has been reported that an increase in the BOD/COD ratio (following ozonation) can be attributed to the increased biodegradability of toxic substances (Sevimli & Sarikaya 2002). According to Ledakowicz et al. (Ledakowicz & Gonera 1999), AOPs should be conducted before subjecting the water systems to conditions of degradation. The results revealed that the combination of O₃ and UV radiation processes or the O₃/UV/H₂O₂ process were the highly efficient AOPs. The AOPs suppressed only 10% of the microbial growth (during the subsequent biodegradation process), while untreated wastewater exhibited 47% inhibition.

Microbial fuel cells (MFCs) have received immense attention as they can be efficiently used for power generation. These can also be used to conduct sustainable wastewater treatment methods (Li et al. 2021b). Electrochemically active bacteria (present at the anode) can produce electrons and reach the cathode via an external circuit. The formed protons and electrons bind with oxygen (in the cathode chamber) to produce water. Recently, MFCs have been used to treat textile wastewater. Logroño et al. (2017) designed an air-exposed single-chamber MFC with microalgal biocathodes to treat real-dye textile wastewater and generate bioelectricity. They reported high COD removal efficiencies (92–98%) using MFCs. Wu et al. (2020) developed an innovative device that combined dual MFC for the continuous removal of Victoria Blue R and power production. Analysis of the results revealed that when artificial wastewater containing 1,000 mg/L of Victoria Blue R was continuously injected into the system, the Victoria Blue R removal rate reached 98.7%.

Although physico–chemical methods are primarily used to treat wastewater, these methods lack versatility. The methods are cost-ineffective and less effective. Various wastewater components interfere with the treatment process, limiting the practical applications of these methods. The microbial decolorization method is economical and environmentally friendly. It is being increasingly used to treat textile wastewater. It is difficult to decolorize systems that contain complex and synthetic dyes. In summary, each method has its advantages and disadvantages (Table 7).

Further studies should be conducted to develop ways of treating textile wastewater (Holkar et al. 2016). Combinations of various processes (such as AOP and biological combination processes) should be considered. The cost of the wastewater treatment method should be borne in mind while treating large amounts of wastewater. When the integrated processes (chemical and biological oxidation) were used for wastewater treatment, the costs increased when the reagent doses were increased at the Fenton reaction stage (Rodrigues et al. 2014). According to Holkar et al. (2016) and Rodrigues et al. (2014), mineralization rates should be minimized during the pretreatment or post-treatment phase to reduce the usage and production of unnecessary compounds and energy consumption. This will eventually help reduce the operation cost of the treatment processes.

Textile wastewater contains color and is characterized by high TDS, BOD, COD, and total suspended solids (TSS). The textile wastewater system also contains mixtures of various dyes. Hence, it is difficult to treat textile wastewater. The discharge of colored and untreated (or partially treated) sewage into the nearby water bodies hinders the penetration of light and oxygen. This eventually affects the aquatic ecosystem negatively. Hence, it is important to treat textile wastewater. Various methods (such as physical and chemical) are commonly used to treat textile wastewater. However, the operational costs are high. Biological treatment processes are considered to be the best alternatives to these methods. These processes are eco-friendly and cost-effective. Most of the organic matter present in textile wastewater is non-biodegradable. Hence, it is difficult to degrade these compounds following biotreatment methods. Different combinations of treatment methods that can be used to treat wastewater produced by the textile industry have been suggested. Combinations of CF–NF, AOPs, and MFCs have been used to treat textile wastewater. In general, in the textile industry, choosing the most effective and cheapest treatment processes or their combinations based on the dyestuffs and dyeing methods utilized during the production.

All relevant data are included in the paper or its Supplementary Information.