1.1 Introduction
Textile industries (TIs) are spread globally, having a market size of ≈1 trillion dollars, and India contributes to ≈7% of the total world exports. Globally, TIs offer employment to ≈35 million workers and are the fifth largest source of foreign currency (Kaur et al., 2018; Tara et al., 2019; Kishor et al., 2020). India is the second largest exporter of dyes after China. But, unfortunately, TIs are a major source of environmental pollution because they release huge volumes of coloured wastewater into valuable water resources (Bener et al., 2019; Kishor et al., 2021a). Textile production is a complex process, which consists of sizing, desizing, bleaching, scouring, mercerizing, dyeing, printing, washing and finishing stages (Kadam et al., 2018; Sen et al., 2019; Kishor et al., 2021b). These stages use large volumes of freshwater and a large number of different chemicals (Kishor et al., 2020).
For example, TIs consume ≈1.6 million L of groundwater for the production of 8,000 kg of textile fabrics per day (Khan and Malik, 2017; Kishor et al., 2018) and ≈20% of wastewater is discharged into environment (Kishor et al., 2021a). Several chemicals such as acids, bases, surfactants, salts, dispersants, dyes and finishing agents are used at different stages of textile production (Bener et al., 2019; Kishor et al., 2021c). Among these chemicals, the dyes are the major source of environmental pollution (Chandanshive et al., 2020; Kishor et al., 2021c). Dyes are employed in textile, cosmetic, leather, printing, paper and medicine industries as a colouring agent (Haq et al., 2018; Kishor et al., 2021c). Besides, azo dyes are the largest used synthetic dye, about 70% of all dye production per year (Kaur et al., 2018; Kishor et al., 2020).
They contain at least one azo group (–N=N–) as a chromophore as well as sulphonic (SO3–) and hydroxyl (OH–) groups (Kishor et al., 2021a). TIWW is characterized by its intensive colour, high pH, temperature, BOD, COD, TSS, total nitrogen, total solids and toxic metals (Cao et al., 2019; Kishor et al., 2021b). TIWW causes serious threats in water and soil ecologies. In a water ecosystem, it reduces photosynthetic activity and dissolved oxygen (DO) content, leading to anoxic conditions, which ultimately affects fauna and flora. In soil ecologies, it reduces soil fertility due to the accumulation of recalcitrant pollutants and metals (Cao et al., 2019; Bener et al., 2019; Kishor et al., 2021b). TIWW is highly toxic to plants (Kishor et al., 2021b). It also causes severe threats to human beings (Sen et al., 2019; Kishor et al., 2021d). Hence, the treatment of TIWW is urgently needed for the protection of environment and public health.
Different physico-chemical, advanced treatment and biological methods have been reported for the treatment of TIWW (Kaur et al., 2018; Kishor et al., 2018; Ceretta et al., 2020). The physico-chemical methods are not feasible due to the production of sludge and high costs (Kishor et al., 2018; Cao et al., 2019). AOPs and biological process effectively degrade pollutants into non-toxic and inorganic compounds (Kishor et al., 2021a). The biological treatments use archaea, bacteria, fungi, yeasts, algae and plants to transform and degrade pollutants (Khandare and Govindwar, 2015; Kadam et al., 2018; Kishor et al., 2021c). These biological agents are used in bioreactors, with adequate agitation and aeration. These agents are also used in wetland treatment. Biological treatments are able to degrade, transform and detoxify pollutants into non-toxic and mineralized compounds (Garg et al., 2020; Kishor et al., 2021a). Bioprocesses have many limitations. They can degrade only biodegradable compounds and take long time for the complete degradation of pollutants. In addition, the biological agents may be inhibited/prevented by toxic compounds during the treatment process (Cao et al., 2019; Bener et al., 2019; Kishor et al., 2021a).
Besides, AOPs such as ozonation and photocatalytic, photo-Fenton, electrocatalytic and electrochemical oxidation can degrade wastewater pollutants. In AOPs, various oxidizing agents such as H2O2 and O3; many catalysts such as CdS, Fe2O3, TiO2, GaP, ZnO and ZnS; and also high-energy radiations such as UV light are utilized (Kaur et al., 2018; Bener et al., 2019; Kishor et al., 2021a). AOPs are well reported as effective and efficient, but may not be suitable due to high cost, incomplete mineralization and the toxic products generated (Kaur et al., 2018). Nowadays, a combination of AOPs and biological process is an alternative solution for the treatment of recalcitrant compounds. In a combination system, AOPs can break down complex recalcitrant and persistent pollutants into more easily biodegradable compounds...
