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Saxena, A., and Gupta, S. (2020). "Bioefficacies of microbes for mitigation of Azo dyes in textile industry effluent: A review," BioRes. 15(4), 9858-9881.


In recent years, India has emerged as a promising industrial hub. It has a cluster of textile, dyeing, and printing industries. The adjoining rivers/water bodies receive mostly untreated discharge from these industries. Textile industrial effluent contains various contaminants (dyes, heavy metals, toxicants, and other organic/inorganic dissolved solids) that alter the physico-chemical properties of adjoining land and waterbodies in which it is discharged, thereby degrading the water quality and subsequently affecting the landscapes in the vicinity. This ultimately affects the flora and fauna of the locale and has adverse effects on human health. Out of the total dyes (approximately 10,000 dyes) exploited in the textile dyeing and printing units, azo dyes possess a complex structure and are synthetic in origin. They contribute nearly 70% to the total effluent discharge. Biological processes are based on the ability of inhabiting indigenous microorganisms in these contaminated environments to tolerate, resist, decolorize/degrade, and mitigate the recalcitrant compounds. Exploring microbes with higher efficacy of azo dye degradation can reduce the amount of chemical discharged from the process. The present review explores the potential of microbial diversity for the development of an effective bioremediation approach. The review also includes the impact of azo dyes on the flora and fauna, as well as conventional and microbe-assisted nanoparticle technology for treatment of the textile wastewater targeting the degradation of dye contaminants.

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Bioefficacies of Microbes for Mitigation of Azo Dyes in Textile Industry Effluent: A review

Ambika Saxena, and Sarika Gupta*

In recent years, India has emerged as a promising industrial hub. It has a cluster of textile, dyeing, and printing industries. The adjoining rivers/water bodies receive mostly untreated discharge from these industries. Textile industrial effluent contains various contaminants (dyes, heavy metals, toxicants, and other organic/inorganic dissolved solids) that alter the physico-chemical properties of adjoining land and waterbodies in which it is discharged, thereby degrading the water quality and subsequently affecting the landscapes in the vicinity. This ultimately affects the flora and fauna of the locale and has adverse effects on human health. Out of the total dyes (approximately 10,000 dyes) exploited in the textile dyeing and printing units, azo dyes possess a complex structure and are synthetic in origin. They contribute nearly 70% to the total effluent discharge. Biological processes are based on the ability of inhabiting indigenous microorganisms in these contaminated environments to tolerate, resist, decolorize/degrade, and mitigate the recalcitrant compounds. Exploring microbes with higher efficacy of azo dye degradation can reduce the amount of chemical discharged from the process. The present review explores the potential of microbial diversity for the development of an effective bioremediation approach. The review also includes the impact of azo dyes on the flora and fauna, as well as conventional and microbe-assisted nanoparticle technology for treatment of the textile wastewater targeting the degradation of dye contaminants.

Keywords: Bio-remediation; Azo dye; Nanoparticle; Textile effluent; Dye degradation

Contact information: Department of Bioscience and Biotechnology, Banasthali Vidyapith, Banasthali, Tonk-y304022, Rajasthan, India; *Corresponding author:


Dye pollution results via effluent discharge from industries such as leather, food, paper printing, pharmaceutical, textile, etc. (Carmen and Daniela 2012). These may amount to lethal levels, causing a variety of ecological damage under different environmental conditions. A significant amount of mainly untreated textile dye effluent (7×10tons annually) is released into various waterbodies worldwide adjoining the textile dyeing and printing units, thereby changing its physico-chemical properties (APHA 1998; Hossen et al. 2019). Subsequently, the contaminated water takes the solvate (contaminants) to the fields in the vicinity and its consumers, adversely affecting quality of the agricultural produce, animal, and human health, causing chemosis, contact dermatitis, exophthalmose, lacrimation, permanent blindness, skin irritation, etc. (Sudha et al. 2014; Sarkar et al. 2017; Parmar and Shukla 2018).

Remediation of such hazardous wastes is considered to be one of the most critical environmental challenges. Compared to conventional treatment, technologies with a lack of specificity for large volumes of wastes, microorganism-based bioremediation is gaining importance, as it has been shown to have high efficiency in mitigating, detoxifying, and degrading these contaminants (Sarkar et al. 2017; Tang et al. 2019). An understanding of the role of native microbes in complex biogeochemical reactions adds great significance for the development of microorganism-based remediation strategies. In recent years, considerable efforts have been made to explore the microbial community structure and their functional diversity in various sites contaminated with industrial effluents (Dafale et al. 2010; Yang et al. 2016). The results have indicated that these environments harbor a large number of diverse microorganisms that may have great potential for the bioremediation of these environments containing large amounts of industrial effluents.

Recently, nanotechnology utilizing nano-sized particles has become evident as a feasible alternative to the conventional methods because it is robust, readily accessible, and has a large surface area heterogeneous catalyst support (Modi et al. 2015; Rajput et al. 2017). These nano-sized particles can greatly enhance the contact between the reactants and the catalyst by increasing the exposed surface area of the active component (Cruz et al. 2019). Due to repeated discharge, the concentration of the dye is increasing in the environment; hence it is important to identify microbes with higher dye degradation capacity and to give effective bioremediation options for textile wastewater, such as bio-augmentation, microbial degradation, and natural attenuation to influence biostimulation either alone or with microbe-assisted nanoparticles (Ozkan et al. 2018). The translational efficacy has been explored to a limited extent, and microbe-assisted nanoparticles remain an enigma in environmental bioremediation at the industrial scale (Modi et al. 2015).

Role of Azo Dyes in Textile Dyeing Industry

Azo dyes (Acid, Disperse, Direct, Pigments, etc.)(namely Acid Red 183, Disperse Yellow 1/3, Disperse Orange 3/37/76, Basic Red 9, Basic Violet 14, Direct Black 38, Biebrich Scarlet, Methyl Red Sodium, tartrazine, carmoisine, p-di-methylaminobenzene, Sudan 1, etc.) constitute a major portion of the dyes used in textile industries. These are also the ones raising the biggest concern due to their mutagenic and carcinogenic nature. They link the aromatic structures with the help of one or more azo bond (-N=N-), and the cleavage of this bond biologically or chemically often releases more mutagenic and toxic end products. Azo dyes have a more intense color than anthraquinone dyes and are also relatively cheap to produce, which has resulted in their dominance in the market usability. Azo dyes form the majority of dyes being discharged into effluents. Most of the residual dyes are highly toxic by acting as a carcinogen posing a potential threat to all living organisms (Table 1).

Contaminants/Toxicants Associated with Textile Industrial Effluents

Textile industries alone discharge a wide variety of toxicants, including biodegradable organic matter, suspended solids, toxic organic compounds (phenol), synthetic dyes (such as azo, anthraquinone, phthalocyanine, and triarylmethane), heavy metal, and their conjugates. Approximately, 10 to 15% of synthetic dyes are dissipated throughout various processes in the textile dyeing and printing industry (Baban et al. 2003; Sudha et al. 2014). The pre-dominant metals associated with dyes are Pb (lead), Hg (mercury), Cr (VI) (chromium), Cd (cadmium), and As (arsenic), which are considered as highly toxic and primarily associated with textile effluents (Singh et al. 2017). High concentrations of these pollutants in the effluent are of solemn concern (Banat et al. 1996).

Table 1. Absorbance Maxima of Various Groups of Azo Dyes Facilitating Their Quantitative Indexing

Impact of Azo Dye Contamination on Flora and Fauna

Azo dyes bio-accumulate in the environment, causing growth reduction, neurosensory damage, metabolic stress, and death of fauna and growth reduction, less productivity, and necrosis in flora. They have also been reported to be carcinogenic and mutagenic in nature. Asses et al. (2018) showed that the toxicity of dyes decreases after microbial treatment through phytotoxicity and micro toxicity tests. Singh et al. (2017) reported toxicity reduction of dyes through microbial remediation. A similar study was conducted by Lade et al. (2015) on the degraded metabolites of dye RB172 through acute and phytotoxicity, and the same findings were reported. Lobiuc et al. (2018) assessed CR toxicity towards Lemna minor and reported reduction in root growth, total frond surface and fresh mass reduced from 5 ppm dye concentration, whereas above 2500 ppm concentration, complete plant growth was inhibited. Khandare et al. (2013) demonstrated the metabolism fate of Direct Red 5B by P. grandiflora, P. putida, and their consortium with the help of GC–MS analysis. Gita et al. (2018) reported that the specific growth rate decreased with the increase in concentration of Optilan Red; maximum percentage inhibition was 66.6% and 79.4% for total chlorophyll and carotenoid, respectively (at 50 ppm). Khatun (2017) revealed severe histopathological effects of silk dye waste effluent on the tissues of both intestines and stomachs of Swiss albino mice. Atrophy of musculature, degeneration of mucosal epithelial cells characterized by nuclear pyknosis, cytoplasmic vacuolization, and nuclear fragmentation were reported along with damage in the Brunner’s gland and the crypts of Lieberkuhn. A study performed on mung bean seed germination demonstrated reduction in radicle-plumule growth and percent seed germination (Khan and Malik 2017). Desai (2017) demonstrated phytotoxicity studies on mung seeds and reported that good germination and shoot, root length of the plants were observed for degraded dye metabolite exposed seeds after comparing with the control using Klebsiella sp. and Staphylococcus sp. Laxmi and Nikam (2015) demonstrated toxicity reduction of the metabolites formed after dye degradation, as the growth of seedlings and seed germination percentage of Guizotia abyssinica were at par with water and the decolorized dye sample (using A. flavus). Rani et al. (2014) reported that seeds of Triticum aestivum inoculated with textile dye solution of Malachite Green treated with A. niger and P. chrysosporium show germination, while uninoculated solution hindered germination. Rajeswari et al. (2014) used Lysinibacillus sphaericus and Stenotrophomonas maltophilia treated solutions on Triticum aestivum and the human embryonic kidney cell line (HEK 293) for evaluation of phytotoxicity and cytotoxicity.

Another study illustrated toxicity reduction using a phytotoxicity assay on the seeds of Phaseolus mungo and Sorghum vulgare, as the seeds were more sensitive towards the dye in comparison to its by-product (Kalyani et al. 2008). Sharma et al. (2007) conducted serum biochemical and haematological studies on Swiss albino rats and stated that the values of white blood cells (WBC), red blood cells (RBC), packed cell volume (PCV), haemoglobin (Hb), and mean corpuscular hemoglobin concentration (MCHC) significantly decreased in wastewater-exposed animals (12 to 46%) with respect to control animals (potable water). Further, reduction in RBC size (13 to 27%) and the shape modification (poikilocytosis) was observed. The serum biochemical parameters alanine transaminase (ALT), aspartate aminotransferase (AST), creatinine, urea, and bilirubin significantly increased (5 to 97%), while cholesterol, glucose, total protein, albumin, and globulin contents decreased (8 to 53%). Sponza and Isik (2004) published that Daphnia magna tests and anaerobic toxicity assays (ATA) respiration/inhibition showed reduction in toxicity of C.I Direct Red 28. The LC50 of dyes revealed toxicity of blue>yellow>red>orange dye (Sani et al. 2018). Another study reported that the Procion Red MX-5B dye solution after treatment with A. niger showed an increase in toxicity, thereby retarding the growth of Lactuca sativa seeds by 43% and mortality to 100% in A. salina larvae (Almeida and Corso 2014). Zhang et al. (2012) evaluated the toxicity of effluent samples from sewage treatment plants (STPs) using bioassays with zebrafish, which indicated high acute toxicity and genotoxicity. Przystaś et al. (2012) conducted zootoxicity and phytotoxicity tests with Daphnia magna and Lemna minor, respectively, of degraded by-products. The degradation of brilliant green correlated with the decrease of zootoxicity (D. magna) and phytotoxicity (L. minor) (Table 2).

Table 2. Toxicity Analysis of Azo Dyes on Flora and Fauna



Various studies have been conducted worldwide on effluent discharge, its charac-teristics, and treatment. Hasan and Miah (2014) investigated the impact of textile mill effluent on surface water and reported that concentrations of electrical conductivity (EC), biological oxygen demand (BOD), total dissolved solids (TDS), Na+, Cl, NH4+, NO3, HCO3– , SO42- , PO43-, and toxic metals (Cd, Cr, and Pb) of the collected effluent samples exceeded the standard levels and were unsuitable for drinking, domestic purposes, or irrigation purposes. Starovoitova and Odido (2014) reported that the compounds categorized as carcinogenic to human beings were used in the industry as metal/ complex/chrome/mordants dye. However, in India, it has been reported that the textile effluents collected from Sanganer (Jaipur) had higher temperature, pH, EC, total suspended solids (TSS), total dissolved solids (TDS), chloride content, and hardness compared to the limits prescribed by World Health Organization (WHO) guidelines for textile industrial effluent (Sharma et al. 2013; Jaishree and Khan 2014; Rahi et al. 2018). Satija and Bhatnagar (2017) published that the wastewater collected from dyeing and printing industries revealed slightly alkaline pH (7.7 to 13.02); and significant TDS (3337.25 to 1494.6 µS/cm); TSS (22.20 to 5.8 NTU); and cations and anions (Ca+2: 427.6 to 175, Mg+2: 174.4 to 77.8, Cl: 2028 to 1039, F: 16.8 to 9.2, SO4-2: 304.6 to 182.8, CO3-2: 144.2 to 53.6, and HCO3-2: 408.8 to 180.2) that was higher than the desired limits. Another study evaluated physico-chemical parameters, such as pH, color, total hardness, COD, BOD, TSS, TDS, turbidity, chlorides, sulphides, silica, calcium, iron, oil, and grease, of the effluent and confirmed that all the parameters studied were above guideline permissible limits, excluding calcium, sulphide, and iron, in the context of the water quality standards of the Bureau of Indian Standard (BIS), Central Pollution Control Board (CPCB), and National Environment Quality Standards (NEQS) (Mahawar and Akhtar 2015; Patel et al. 2015; Sriram and Reetha 2015; Elango et al. 2017).

Various microorganisms have been explored throughout these years for azo dye degradation, namely bacteria, fungi, and algae. Ajaz et al. (2019) reported that Alcaligenes aquatilis decolorizes 82% of Synazol red 6HBN in 4 days at 37 °C, pH 7, under static conditions with yeast extract and saw dust as nitrogen and carbon sources. Another study confirmed that indigenous Bacillus cereus AZ27, Alcaligenes faecalis AZ26, and Bacillus sp. AZ28 have the potential to mitigate various dyes with more than 25% of degradation optimized using Novacron Super Black G (NSB-G) (Hossen et al. 2019). Another study illustrated that the bacterial consortium comprising of Sphingomonas paucimobilisRhizobium radiobacter, and Bacillus subtilis removes heavy metals from industrial wastewater and decolorizes Methyl Orange (MO) and CR textile azo dyes better than their corresponding single cultures, correlating to synergistic activity of different metabolites of bacterial cultures and protection of cells from toxic pollutants that was provided by Ca-alginate matrices (Allam 2017). Lade et al. (2015) used single bacterium Providencia rettgeri strain HSL1 that completely decolorized 50 mgL−1 of dye C.I. Reactive Blue 172 (RB172) in 20 h at 30±0.2 °C under microaerophilic conditions and showed considerable reduction in total organic carbon (TOC) (52%) and chemical oxygen demand (85%) contents, which correlated with nicotinamide adenine dinucleotide-dichlorophenol indophenols (NADH-DCIP) reductase (88%) and azoreductase (159%) activity. Singh et al. (2017) demonstrated that bacterial strains of Enterobacter asburiae and E. cloacae, used as consortium, efficiently decolorized (up to 98%) at pH 1.67, 32°C within 10 min under aerobic condition. Khan and Malik (2018) reported that Arthrobacter soli BS5 degrades textile dye reactive black 5 with maximum degradation of 98% at pH 5 to 9, 37°C after 120 h of incubation. Another study published that Klebsiella sp. and Staphylococcus sp. decolorizes 200 ppm of Direct Red 2B dye by 98.8% and 98.7%, respectively (Desai 2017). Lalnunhlimi and Krishnaswamy (2016) demonstrated that bacterial consortium isolated from soil samples of a saline environment decolorizes 200 mg/L of Direct Blue 151 (DB151) and Direct Red 31 (DR 31) by 97.6% and 95.2%, respectively, within 5 days, which further improved supplementation of sucrose and yeast extract that were used as carbon and nitrogen sources. Vimala et al. (2015) isolated bacteria as Pseudomonas sp., Citrobacter sp., Escherichia coli, and Micrococcus sp. and reported that plasmid was present in all isolates. Micrococcus sp. was reported to work well in an adapted environment, while E. coli showed the same decolorization potential in adapted and non-adapted environments. Singh et al. (2014) identified Staphylococcus hominis as a potential degrader of Acid Orange dye up to 600 mg/L at pH 7.0, 35°C, after 60 h of incubation with yeast extract and glucose supplementation (Table 3). Khandare et al. (2013) utilized bacterial and plants consortium of Pseudomonas putida and Portulaca grandiflora and reported complete degradation of a sulfonated diazo dye. Enzymes involved were reported as 2,6-dichlorophenol indophenol reductase, riboflavin reductase, lignin peroxidase, and tyrosinase in P. grandiflora and veratryl alcohol oxidase, laccase, and 2,6-dichlorophenol indophenol reductase in P. putida.

Table 3. Degradation of Various Dyes using Bacteria

Al-Tohamy et al. (2020) illustrated that the decolorizing ability of yeast Sterigmatomyces halophilus on Reactive Black 5 depends on NADH-dichlorophenol indophenol (NADH-DCIP) reductase and lignin peroxidase (LiP). Asses et al. (2018) used Aspergillus niger for biodegradation of Congo red (CR), an azo dye. The decolorization rate reached 97% on inoculation of 2 g mycelia and 200 mg/L of dye in 6 days at pH 5, 28 °C, and 120 to 150 rpm, which correlated with manganese peroxidase and lignin peroxidase production. A study reported Pseudomonas sp. as having high efficacy in the mitigation of azo dyes and Pseudomonas sp., Micrococcus sp., and Bacillus sp. in removing heavy metals ranging up to 350 to 550 µg/mL (Islam et al. 2017). Cheng et al. (2016) screened white-rot fungi for their azo dyes degradation capacity using Biebrich Scarlet (C.I. 26905), Direct Blue 71 (C.I. 34140), Orange G (C.I. 16230), and Ponceau 2R (C.I. 16450). Coriolopsis sp. strain arf5 was identified as a microbe that completely degraded all four dyes in the shortest time interval when supplemented with an additional carbon source (glucose) and nitrogen-limiting conditions. At the same time, Yang et al. (2016) isolated freshwater fungal strains from submerged woods and used them for the degradation of seven synthetic dyes. Another study used indigenous bacterial and fungal isolate for degradation of red and green textile dyes at 10, 50, 100, and 150 mg/L. Pseudomonas sp. demonstrated higher dye decolorization (67% for red and 73% for green dye) in comparison to Aspergillus sp. (59% for red dye and 70% for green dye) in three days (Fatima and Alamgir 2015). Almeida and Corso (2014) reported that Aspergillus terreus and A. niger decolorizes 30% of Procion Red MX-5B dye in 3 h of biosorption (100%). The ultraviolet (UV)–visible (VIS) spectroscopy analysis indicated the removal of the dye molecules occurred without statistically significant molecular changes. Laxmi and Nikam (2015) isolated A. flavus, A. niger, Fusarium oxysporium, and Penicillium notatum and used them for the decolorization of azo textile dyes. The study reported that A. niger decolorized basic fuchsin (81.85%)>Nigrosin (77.47%)>Malachite green (72.77%) >dye mixture (33.08%), while Phanerochaete chrysosporium decolorized Nigrosin (90.15%)> basic fuchsin (89.8%)>Malachite green (83.25%)>mixture (78.4%) under shaking condition (Rani et al. 2014) (Table 4).

Table 4. Degradation of Various Dyes using Fungi

Ishchi and Sibi (2020) illustrated that microalgae Chlorella vulgaris have azo dye degrading capacity using Reactive Black 5, Direct Blue 71, and Disperse Red 1. The decolorizing results were shown to depend on initial dye concentration and different pH for different dyes. Tang et al. (2019) reported that Disperse Red 3B decolonization occurred better in the consortium of Chlorella sorokiniana XJK and Aspergillus sp. XJ-2(98.09%) with respect to their single system, while the removal rate of TP (total phosphorus) 83.9%, COD (chemical oxygen demand) 93.9%, and ammonia nitrogen 87.6% under the optimized conditions were achieved due to lignin peroxidase and manganese peroxidase enzyme activities. Hernandez-Zamora et al. (2015) showed that through biodegradation and biosorption processes, Chlorella vulgaris removed 83 and 58% of congo red dye at concentrations of 5 and 25 mg L−1, respectively. Khataee et al. (2010) used Xanthophyta alga, Vaucheria sp. to degrade Malachite green and concluded that degradation is inversely proportional to initial dye concentration and directly proportional to pH, temperature, and algal biomass. El-Sheekh et al (2009) investigated the decolorizing potential of Elkatothrix viridis, Chlorella vulgaris, Nostoc linckia, Lyngbya lagerlerimi, Oscillatoria rubescens, and Volvox aureus using methyl red, orange II, basic cationic, G-Red (FN-3G), and basic fuchsin and concluded that decolorizing using C. vulgaris or N. Linckia with G-Red or methyl red, respectively, induced the algal azo dye reductase enzyme by 72 and 71% at the same order (Table 5).

Table 5. Degradation of Various Dyes using Algae

Ajaz et al. (2019) used high-performance liquid chromatography (HPLC), thin layer chromatography (TLC), gas chromatography–mass spectroscopy (GC-MS), and Fourier transform infrared spectroscopy (FTIR) analysis to confirm the cleavage of azo bond. The bacterially treated FTIR sample showed an absence of peaks at the 1532 cm-1 and 1612 cm-1 wavelengths, demonstrating the breakdown of the azo bond. Ayed et al. (2011), Ahmed et al. (2016), and Hossen et al. (2019) investigated dye decolorization using UV-VIS spectrophotometry and FTIR analysis. The statistically significant difference in UV-VIS absorbance spectra and the FTIR spectrum of the decolorized dye from those of the parent dye revealed that the dye was mitigated by the bacterial isolates. Tang et al. (2019) reported that UV-VIS spectrophotometry, FTIR, and GC-MS analysis revealed that the colored functional groups of Dispersed Red 3B were broken down into less toxic small molecular compounds. Asses et al. (2018) characterized degraded metabolites using liquid chromatography – tandem mass spectrometry (LC-MS/MS) of Congo Red mainly by peroxidases activities. Yang et al. (2016) reported that biotransformation occurred after fungal biodegradation of synthetic dyes as it formed new absorbance peaks (Table 6).

Khandare et al. (2013) and Singh et al. (2017) used UV-VIS and FTIR analysis for dye degradation analysis. Laxmi and Nikam (2015) confirmed that dye decolorization occurred through degradation using UV-VIS spectrophotometric and high-performance thin layer chromatography (HPTLC) analysis.

Table 6. Degradation Analysis of Dyes Using High Throughput Spectral Scan


Conventional Treatment (Primary, Secondary, Tertiary, and Pain Point After Tertiary Treatment as per CPCB Guidelines)

Textile industry is one of the largest industries in the world, and different fibers, such as cotton, silk, and wool, as well as synthetic fibers, are all pre-treated, colored, and after-treated using a large amount of water and other chemicals. The pollutants include various dyes, starches, and detergents that undergo various physio-chemical changes that consume dissolved oxygen from the receiving stream and destroy aquatic life. Such organics should be removed to prevent septic conditions and avoid rendering the water stream unsuitable for municipal, industrial, agricultural, and residential uses. Treatment of wastewater reduces the waste, prevents negative effects, and makes positive effects on its further usage. Effluent treatment plants (ETPs) treat the water that comes out of these industries. Parameters including pH, color, biological oxygen demand, chemical oxygen demand, oil and grease, total dissolved solids, total suspended solids, etc. are evaluated in compliance with the Central Pollution Control Board (CPCB). Preliminary treatment level comprises of physical separation of large-sized impurities like plastics, polythene bags, paper, wooden logs, etc. This is done either through clarification that uses a belt to remove large-sized impurities or sedimentation that uses gravity for the separation.

Further treatment of the effluent is characterized into the following categories: Primary, Secondary, and Tertiary. Primary treatment is a physio-chemical method used to remove suspended solids and treat parameters such as pH, oil, and grease using coagulation, chemical precipitation, and oxidation. Sodium hydroxide, sodium carbonate, and calcium carbonate are used to treat the pH of acidic effluent, while sulphuric acid or hydrochloric acid are used to treat the pH of alkaline effluent. Alum (Al2(SO4)) is used as a chemical coagulant, further, a chemical flocculent is added to aid precipitation by bringing fine particles together to form large masses.

Secondary treatment involves biological treatment of the effluent to remove organic and inorganic impurities using microbes, i.e. bacteria or fungi. Mainly aerobic treatment is performed, i.e. in the presence of oxygen. Nitrifying bacteria are used to convert the compounds into other by-products.

Tertiary treatment finally processes the water to meet the disposable guidelines for further reusing, recycling, or disposing into the environment. It removes the remaining impurities such as inorganic compounds, bacteria, parasites, etc. Alum is further added to remove any additional particle by grouping them so that they are being removed at the last stage. Chlorine is added to disinfect the treated wastewater from bacteria, fungi, parasites, etc. Sodium bisulphate is then added to remove excessive chlorine. The wastewater is then centrifuged before being discharged through the outlet into the environment (Fig.1).

Fig. 1. Flowchart depicting the effluent flow inside an ETP through different treatment and its release through outlet after treatment

Bioremediation Approach (Microbial)

Biological treatments fundamentally rely on the ability of microbe to transform the contaminants by using them as sources of energy, carbon, and other minerals that are essential for their growth. Microbial-based enzymatic treatment is preferred for the degradation of the xenobiotic and recalcitrant azo dyes from the textile effluent because of the following advantages: (1) environmentally benevolent, (2) economic factors, (3) produces less sludge, (4) yielding end products that are non-toxic or have complete mineralization, and (5) requiring less water consumption as compared to the physicochemical methods. Thus, the challenge is to find microorganisms endowed with potential to degrade all azo dyes and at the same time thrive in the presence of salts, metals, other toxicants, and atypical conditions of textile effluents (Jamee and Siddique 2019).

Microbial nanoparticle approach

All the methods applied for wastewater treatment have different advantages and disadvantages. Nanoparticles, however, have gained attention due to their small size range (1 nm to 100 nm), large surface area, high adsorption properties, less resistance to diffusion and the fact that they show faster rates of equilibrium and increased chemical reactivity (Ahmad et al. 2015). Nanoparticle are divided into four functional classes that are used in water purification: carbonaceous nanomaterials, dendrimers, metal-containing nanoparticles and zeolites (Marimuthu et al. 2020). Nanotechnology-derived products reduce the level of toxic substances to sub-ppb levels and help attain higher water quality standards (Savage and Diallo 2005). Nanoparticle degrades or decolorizes azo dye either through absorption, photocatalytic degradation or their combined action. Nano-sized metal oxides are preferred adsorbents for the removal of water toxins as such materials are associated with the characteristics of simplicity, efficacy, versatility, and high surface reactivity (Zafar et al. 2019). Photocatalysis is a principal mechanism in dye effluent treatment; here the electrons are excited from the valence band to conduction band upon irradiation, resulting in electron-hole pair generation. The hydroxyl radical generated acts as a potent oxidizing agent and completely degrades the dye to nontoxic products.

Nano-scale size provides properties such as improved catalysis, adsorption, and high reactivity. These properties have been exploited in recent years in all the domains, including wastewater treatment. Cruz et al. (2019) reported that cobalt nanoparticles (CoNPs) removed the Remazol golden yellow RNL by almost 100% in 30 min. The X-ray diffraction (XRD) and Raman spectroscopy study showed presence of CoO and Co0, which was supported by thermogravimetric analysis coupled to mass spectrometry (TG-MS) analysis. The application of CoNPs to textile effluent resulted in 88% degradation of dyes and 32% reduction in COD. Foster et al. (2019) used bimetallic nanoparticles comprising of iron (Fe) and nickel (Ni,) at 1000 mg/L concentration, which showed high efficacy and consistent Orange G removal. Amabye and Hagos (2017) synthesized AgNPs using cell-free supernatant of an isolated bacterial strain AN-1 for the decolorization of dyes. Further characterization with scanning electron microscope (SEM) analysis revealed the spherical, polydisperse AgNPs of particle size ranging from 74.56 to 92.67 nm. Another study synthesized exopolysaccharide-stabilized AgNPs and characterized it using surface plasmon spectra using UV–VIS spectroscopy, XRD, TEM, SEM, atomic force microscopy (AFM), and Raman spectroscopy for MO and CR (Saravanan et al. 2017). Ramalingam et al. (2017) synthesized AgNPs using cell-free extract of Staphylococcus aureus. Roughly 62% of MO (2000µg/mL) was degraded after treatment with 200 µg/mL of AgNPs. Nadaf and Kanase (2016) reported that gold nanoparticles (AuNPs) synthesized using cell-free extract of Bacillus marisflavi showed outstanding catalytic activity in the decolorization of CR and methylene blue. Modi et al. (2015) focused on synthesis of AgNPs using Bacillus pumilus and verified that nano-based remediation was more efficient as compared to microbial remediation (Table 7).

Table 7. Degradation of Azo Dyes Using Microbe Nanoparticle Approach