Colored Effluents Research Articles

Data-Based Modeling, Multi-Objective Optimization and Multi-Criteria Decision Making of a Catalytic Ozonation Process for Degradation of a Colored Effluent

In the catalytic ozonation process (COP), the reactions are complex, and it is very difficult to determine the effect of different operating parameters on the degradation rate of pollutants. Data-based modeling tools, such as the multilayer perceptron (MLP) neural network, can be useful in establishing the complex relationship of degradation efficiency with the operating variables. In this work, the COP of acid red 88 (AR88) with Fe3O4 nano catalyst was investigated in a semi-batch reactor and a MLP model was developed to predict the degradation efficiency (%DE) of AR88 in the range of 25 to 96%. The MLP model was trained using 78 experimental data having five input variables, namely, AR88 initial concentration, catalyst concentration, pH, inlet air flow rate and batch time (in the ranges of 150–400 mg L−1, 0.04–0.4 g L−1, 4.5–8.5, 0.5–1.90 mg min−1 and 5–30 min, respectively). Its optimal topology was obtained by changing the number of neurons in the hidden layer, the momentum and the learning rates to 7, 0.075 and 0.025, respectively. A high correlation coefficient (R2 > 0.98) was found between the experimental and predicted values by the MLP model. Simultaneous maximization of %DE and minimization of Fe3O4 concentration was carried out by multi-objective particle swarm optimization (MOPSO) and the Pareto-optimal solutions were successfully obtained. The trade-off was analyzed through multi-criteria decision making, and one Pareto-optimal solution was selected. The developed model and optimal points are useful for treatment of AR88 wastewater.

Comb-Like Cationic Chitosan-Based Flocculant with Enhanced H-Bond Hydration for Effectively Treating Saline Colored Effluents with High Salinity

Comb-Like Cationic Chitosan-Based Flocculant with Enhanced H-Bond Hydration for Effectively Treating Saline Colored Effluents with High Salinity

The synthesis of the P/N-type NdCoO3/g-C3N4 nano-heterojunction as a high-performance photocatalyst for the enhanced photocatalytic degradation of pollutants under visible-light irradiation

The treatment of industrial and domestic colored effluents and the use of photocatalysts have today attracted much attention among researchers. With the varied photocatalytic materials, the ones with narrow bandgap are thus of greater significance thanks to their acceptable performance in optical fields. The other obvious property of such materials is their lower costs because a large part of sunlight is in the visible region. In this work, the bare perovskite-type neodymium cobaltite and neodymium cobaltite/graphite carbon nitride (p/n-type NdCoO3/g-C3N4) was synthesized using sol-gel auto combustion. Different reductants were also employed during this fabrication for morphological engineerings, such as glucose (carbohydrate), L-valine (amino acid), and citric acid. As well, X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FTIR), and energy-dispersive X-ray spectroscopy (EDS) were utilized to prove the purity of the specimens, and then transmission electron microscopy (TEM) and field-emission scanning electron microscopy (FE-SEM) were applied to explore their morphology. The various properties of the product, such as the optical, magnetic, porosity, and surface features, were correspondingly checked by diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy, vibrating-sample magnetometry (VSM), and Brunauer-Emmett-Teller (BET) surface area analysis, respectively. Citric acid was thus selected as an optimal fuel because of the formation of finer particles and morphology. In addition, the NdCoO3 nanoparticles (NPs) were exploited as a catalyst for the degradation of anionic dyes (viz. erythrosine [Red No. 3], methyl orange [MO], and eriochrome black T [EBT]) and cationic ones (i.e., rhodamine B [RhB], acid red 14 [ACR14], and acid yellow 23 [AY23]) under visible-light irradiation. The study findings accordingly revealed that the heterostructures emerged at the interfaces between the p-type NdCoO3 and n-type g-C3N4, leading to the EBT decolorization, reached 95.8% after 120 min, and were higher than the photodegradation of the pristine NdCoO3 and g-C3N4.

Feasibility Study of Zero Discharge Concept in Sugar Industry After Anaerobic Treatment: Case Study of Solapur

Abstract: Purification and reutilisation of waste water from different industries is a challenge for a smarter and healthier environment. An important role in Indian economy is played by an agro based industry segment which is nothing but sugar industry. But the effluent from the industry is a proven fact as a threat for environment. Effective waste water management can result in smart and healthy city environment. The treated colored effluent from the industry is not preferred for reuse and recycle, though the other BOD, COD, TDS, TSS, MPN are in permissible limit. Root zone technique is one of the important approaches for, as the process is economical, easy in operation and maintenance. Pilot plant is constructed and executed using a typical species. The intake of treated effluent and the final effluent from root zone technique were collected and tested for various contact hours. To accommodate the massive urbanization it can be used to find smarter ways to reduce expenses, manage complexities that’s the treated waste water from industry can serve for following: 1) Gardening 2) Fire Fighting 3) Washing 4) Toilet Flushing etc. For a better and smart management of treated waste. Keywords: Agro based industry, sugar industry, colored, root zone technique, species, contact hours.

Removal of Toxic Dyestuffs from Aqueous Solution by Amphoteric Bioadsorbent

Background: Color effluents generated from the production industry of dyes and pigments and their use in different applications, such as textile, paper, leather tanning, and food industries, are high in color and contaminants that damage the aquatic life. It is estimated that about 105 of various commercial dyes and pigments amounted to 7×105 tons are produced annually worldwide. Ultimately, about 10-15% is wasted into the effluents of the textile industry. Chitin is abundant in nature, and it is a linear biopolymer containing acetamido and hydroxyl groups amenable to render it atmospheric by introducing amino and carboxyl groups, hence able to remove different classes of toxic organic dyes from colored effluents. Methods: Chitin was chemically modified to render it amphoteric via the introduction of carboxyl and amino groups. The amphoteric chitin has fully been characterized by FTIR, TGA-DTG, elemental analysis, SEM, and point of zero charges. Adsorption optimization for both anionic and cationic dyes was made by batch adsorption method, and the conditions obtained were used for studying the kinetics and thermodynamics of adsorption. Results: The results of dye removal proved that the adsorbent was proven effective in removing both anionic and cationic dyes (Acid Red 1 and methylene blue (MB)), at their respective optimum pHs (2 for acid and 8 for cationic dye). The equilibrium isotherm at room temperature fitted the Freundlich model for MB, and the maximum adsorption capacity was 98.2 mg/g using 50 mg/l of MB, whereas the equilibrium isotherm fitted the Freundlich and Langmuir model for AR1 and the maximum adsorption capacity was 128.2 mg/g. Kinetic results indicate that the adsorption is a two-step diffusion process for both dyes as indicated by the values of the initial adsorption factor (Ri) and follows the pseudo-second-order kinetics. Also, thermodynamic calculations suggest that the adsorption of AR1 on the amphoteric chitin is an endothermic process from 294 to 303 K. The result indicated that the mechanism of adsorption is chemisorption via an ion-exchange process. Also, recycling of the adsorbent was easy, and its reuse for dye removal was effective. Conclusion: New amphoteric chitin has successfully been synthesized and characterized. This resin material, which contains amino and carboxyl groups, is novel as such chemical modification of chitin hasn’t been reported. The amphoteric chitin has proven effective in decolorizing aqueous solution from anionic and cationic dyes. The adsorption behavior of amphoteric chitin is believed to follow chemical adsorption with an ion-exchange process. The recycling process for few cycles indicated that the loaded adsorbent could be regenerated by simple treatment and retested for removing anionic and cationic dyes without any loss in the adsorbability. Therefore, the study introduces a new and easy approach for the development of amphoteric adsorbent for application in the removal of different dyes from aqueous solutions.

Dye degradation potential of Acinetobacter baumannii strain VITVB against commercial azo dyes

The textile industry is an age-old industry in human civilization and to date, it contributes to a major portion of the economy in several nations. However, the generation of a huge volume of toxic colored effluent is a major environmental concern. Azo dyes are extensively used for textile dyeing and are thus found in the wastewater. In this article, bacterial bioremediation of azo dyes was studied. Acinetobacter baumannii strain VITVB, a gram-negative bacterium aerobically decolorized 500 mg L−1 of two textile azo dyes Reactive Blue 221 (RB-221) and Reactive Black 5 (RB-5) with 90% and 87% efficiency after 48 h under static condition. Dye degradation was further maximized by optimization studies. A. baumanii VITVB exerted maximum dye degradation within 24 h under optimized conditions of pH 9, temperature at 45 °C, 5% NaCl concentration, and supplementation of 1% starch (Carbon source) and 1% peptone (Nitrogen source) in growth media. Dye Degradation was then analyzed. Fourier transform infrared spectra of pure and degraded dyes revealed that the potential bacterial strain targeted the characteristic chromophore group (N = N) during the break down of dyes. High-performance liquid chromatography of pure and degraded dyes presented the emergence of different peaks at altered retention times, which also indicates the degradation of dyes. Gas Chromatography–Mass Spectrometry chromatogram revealed probable end products as diphenylmethane from RB-221 degradation and benzene,1,1'-Pentylidenebis from RB-5 degradation. Finally, phytotoxicity and biotoxicity assays implied the nontoxicity of the degraded metabolites of RB-221 and RB-5. Pure dyes inhibited seed germination, showed marked phenotypic changes and cellular damages in plants (Cicer arietinum and Tagetes erecta) while the degraded metabolites promoted a healthy seed germination rate and no marked cellular damages or phenotypic changes in plants. Artemia salina cysts also showed a similar pattern as major hatchability inhibition occurred with pure dyes compared to degraded dye metabolites.

Adsorption Characteristics of Activated Carbon for the Reclamation of Eosin Y and Indigo Carmine Colored Effluents and New Isotherm Model

The adsorption response of eosin Y and indigo carmine acid dyes on activated carbon as a function of system temperature for a fixed concentration was investigated at various temperatures via adsorption isotherms and their thermodynamic quantities such as enthalpy, entropy, and Gibbs free energy changes. The adsorption data were exploited to develop a new adsorption isotherm. The new isotherm was developed with the spirit of solid–liquid phase equilibrium and regular solution theory. The proposed model has four adjustable constants and correlates adsorption isotherm in terms of the system temperature and melting temperature of the dye. The effect of pH on the removal of acid dyes was reported. The pH variation was observed to affect the adsorption efficiency. The removal of eosin Y and indigo carmine decreased from 99.4% to 82.6% and 92.38% to 79.48%, respectively, when the pH of the solution varied from 2 to 12. The thermodynamic analysis of the process reveals that the process of the removal of acid dyes is exothermic and spontaneous. Moreover, the kinetics parameters of the batch process are reported.

Evaluation of an anaerobic/aerobic system for Reactive Black 5 removal: kinetic study

Colored effluents are one of the most important environmental pollutants in the world. This study focused on the removal of dye Reactive Black5 (RB5) using a combination of an anaerobic digester and an integrated fixed-film activated sludge (IFAS) reactor. The effects of hydraulic retention times (HRT), temperature and filling ratio in the anaerobic digester, the effect of hydraulic retention time and filling ratio in IFAS, and also the effect of initial dye concentration on color removal and chemical oxygen demand (COD) reduction efficiencies were investigated. The Maximum efficiency of color removal and COD reduction in the anaerobic digester at HRT of 2.5 days, 35 °C and 50% of filling ratio was 81% and 99%, respectively, and in IFAS at HRT of 6 h and 70% of filling ratio was 25% and 100%, respectively. According to the scanning electron microscope (SEM) photographs of the microbial community, there were morphological differences in the microorganisms of the two reactors. The Stover–Kincannon and Monod models were used to describe kinetic data. The results showed that Stover–Kincannon model follow the experimental data well. Maximum utilization rate constant and Saturation value constant of Stover–Kincannon model for dye and COD were determined as Umax = 0.15 g / (l day), KB = 0.165 g / (l day) and Umax = 11.31 g / (l day), KB = 11.31g (l day) respectively

Usage Potential of Sawdust as a Forest Industry Waste For Treatment of Colored Effluents

In the present study, the usage potential of sawdust as a forest industry waste was evaluated for the treatment of colored industrial effluents containing Fast Green FCF synthetic dye. The effects of various environmental variables such as pH, biosorbent amount, synthetic dye concentration and contact time on the dye removal ability of biosorbent material were studied in batch operating conditions. The kinetic and equilibrium studies were performed to elucidate the dye biosorption behavior of biosorbent. The synthetic dye biosorption capacity of biosorbent was highly affected by the environmental conditions. The kinetic and equilibrium studies showed that Elovich and Langmuir models were the best models for the elucidation of dye removal behavior of biosorbent. Based on Langmuir model, the maximum monolayer biosorption capacity under the optimum environmental conditions was estimated to be 26.834 mg g-1. The obtained results revealed that the forest industry waste biomass used has usage potential for the treatment of synthetic dye pollution in aquatic environments.

Dye removal by biosorption using cross-linked chitosan-based hydrogels

Synthetic dyes are an important class of recalcitrant organic compounds that are often found in the environment as a result of their wide industrial use. There are estimated to be more than 100,000 commercially available dyes. These substances are common contaminants, and many of them are known to be toxic or carcinogenic. Colored effluents from the industry is perceived by the public as an indication of the presence of a dangerous pollution. Even at very low concentrations, dyes are highly visible—an esthetic pollution—and modify the aquatic life and food chain, as a chemical contamination. Dye contamination of water is a major problem worldwide, and the treatment of wastewaters before their discharge into the environment has become a priority. Dyes are difficult to treat due to their complex aromatic structure and synthetic origin. In general, a combination of different physical, chemical and biological processes is often used to obtain the targeted water quality. Nonetheless, there is a need to develop new removal strategies and decolorization methods that are more effective, acceptable for industrial use and ecofriendly. Currently, there is increasing interest in the application of biological materials as effective adsorbents for dye removal. Among all the materials proposed, cross-linked chitosan-based hydrogels are the most popular biosorbents. These polymeric matrices are the object of numerous fundamental studies. In this review, after a brief description of the use of chitosan in wastewater treatment and the basic principles of chitosan-based hydrogels and biosorption, we focus on some of the work published over the past 5 years. Overall, these polymeric materials have demonstrated outstanding removal capabilities for some dyes. They might be promising biosorbents for environmental purposes.

Congo red dye diversely affects organisms of different trophic levels: a comparative study with microalgae, cladocerans, and zebrafish embryos.

Global consumption of synthetic dyes is roughly 7 × 105 tons per year, of which the textile industry expends about two-thirds. Consumption of synthetic dyes produces large volumes of wastewater discharged into aquatic ecosystems. Colored effluents produce toxic effects in the hydrobionts, reduce light penetration, and alter the photosynthetic activity, causing oxygen depletion, among other effects. Some dyes, such as Congo red (CR), are elaborated with benzidine, a known carcinogenic compound. Information regarding dye toxicity in aquatic ecosystems is scarce; therefore, our study was aimed at evaluating the toxicity of CR on a battery of bioassays: the microalga Pseudokirchneriella subcapitata, the cladocerans Daphnia magna and Ceriodaphnia rigaudi, and the zebrafish Danio rerio. P. subcapitata was the most sensitive species to CR (IC50, 3.11mgL-1); in exposed individuals, population growth was inhibited, but photosynthetic pigments and macromolecule concentrations were stimulated. D. magna was tolerant to high dye concentrations, the determined LC50 (322.9mgL-1) is not an environmentally relevant value, but for C. rigaudi, LC50 was significantly lower (62.92mgL-1). In zebrafish embryos, exposure to CR produced yolk sac edema, skeletal deformities, and stopped larvae hatching; lack of heart beating was the only observed lethal effect. CR affected organisms of different trophic levels diversely. Particularly, the effects observed in microalgae confirm the vulnerability of primary producers to dye-polluted wastewaters, because dyes produced toxic effects and interfered with photosynthesis. Different cladoceran species displayed different acute effects; thus, species sensitivity must also be considered when toxicity of dyes is assessed. Inhibition of fish larvae hatching is a significant effect not previously reported that warns about the toxicity of dyes in fish population dynamics. Synthetic azo colorants should be considered as emerging pollutants because they are discharged into the aquatic environment and are not currently included in the environmental regulation of several countries.

Landfill leachate treatment by sequential combination of activated persulfate and Fenton oxidation

This work assesses the feasibility of sequential persulfate and Fenton oxidation for the decolorization and mineralization of landfill leachate (5600 mg L−1 TOC; pH0: 8.6) in a continuous batch-recirculation system. Firstly, it was analyzed the role of the operational conditions upon the persulfate activation evaluating the effects of electrolysis, ilmenite (FeTiO3) as a source of Fe(II) and UV-LED (at 365 nm). The studied variables include current density (j) (50–200 mA cm−2), persulfate dose (46.8–234 mM) and mineral concentration (500–1500 mg L−1). The increase in j enhanced the hypochlorite generation and PS conversion to SO4− and, consequently, decolorization efficiency increasing the penetration of light through the solution and the photoreduction of Fe(III) to Fe(II) in the FeTiO3 surface. The combined electrolysis/FeTiO3/UV-LED showed synergetic effect compared to the individual processes, achieving mineralization around 53% under the optimum operating conditions (1 g L−1 of FeTiO3, using 234 mM of PS at 200 mA cm−2 under UV-LED radiation). The subsequent Fenton oxidation once the pH decreased up to around 3, led to overall mineralization above 90% after 480 min, confirming the suitability of this combined treatment to deal with recalcitrant and highly colored effluents.

Methylene blue removal by carbonized textile sludge-based adsorbent.

Colored effluent and a large amount of sludge are major pollutant sources derived from textile industry activity. In this research, the idea for converting textile sludge into a potential adsorbent was conducted through a carbonization process in order to solve the colored effluent problem. Textile sludge was carbonized at a temperature ranging from 400 to 800 °C in the absence of oxygen. Maximum adsorption capacity of carbonized sludge for methylene blue removal reached 60.30 mg/g when the sludge was carbonized at 600 °C with specific surface area of 138.9 m2/g and no significant alteration was observed until 800 °C. Experimental research by using a real wastewater also showed that there was almost no disruption during adsorption of methylene blue into surface of carbonized sludge. While reactivation process revealed that the regeneration of carbonized sludge was applicable by secondary heating at the same carbonization temperature. Furthermore, the application of this research demonstrated that the carbonized textile sludge was a good adsorbent for methylene blue removal and had a capability to be reactivated.

Use of Brazilian Kaolin as a Potential Low-cost Adsorbent for the Removal of Malachite Green from Colored Effluents

This study investigated the potential of Brazilian kaolin as a low-cost adsorbent for the removal of Malachite Green (MG) from colored effluents. The morphology, chemical structure and surface properties of the adsorbent were investigated by characterization techniques such as X-ray diffraction, N2 adsorption-desorption isotherms, Fourier transform infrared spectroscopy, X-ray fluorescence spectrometry, scanning electron microscopy, thermogravimetric analysis and particle size distribution. A possible technological application of raw kaolin is the MG removal from aqueous media, which was investigated using batch adsorption experiments. The adsorption kinetics was studied using the pseudo-first order, pseudo-second order and Elovich models. The adsorption isotherms were studied using the Langmuir, Freundlich and Sips models. The Elovich model was the more adequate to represent the adsorption kinetic, while the equilibrium was well represented by the Langmuir model. The maximum adsorption capacity, at pH of 6.3 and temperature of 25oC, was 128 mg g-1, and this satisfactory result may be associated with some adsorbent properties. Therefore, the results revealed that raw kaolin can be utilized as a promising low-cost adsorbent to remove MG from colored effluents.

Chitosan hydrogels for targeted dye and protein adsorption

ABSTRACTThere is an urgent requirement to devise and develop highly‐effective materials for the purification of industrial wastewater. Colored effluents are particularly problematic due to their toxicity in plants, aquatic organisms, and animals, dictating that polymeric adsorbents are highly sought for their capture. Chitosan hydrogels are a cost‐effective substrate for the adsorption of dye molecules as they contain a biopolymeric gelator that can form hydrogels with a polymer content as low as 3% by mass. Such materials are also highly suited for use within a biomedical context as carrier vehicles for the encapsulation and delivery of protein macromolecules, due to their hydrophilic nature. We disclose the capability chitosan‐based hydrogels to noncovalently adsorb both reactive and disperse dye molecules, and a model anionic protein from aqueous solution. The materials generated offer both a platform for dye removal from industrial wastewater, and for the encapsulation and pH‐mediated release of protein macromolecules. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44846.

Adsorption Characteristics of Activated Carbon for the Reclamation of Colored Effluents Containing Orange G and New Solid–Liquid Phase Equilibrium Model

Adsorption characteristics of activated carbon for the removal of orange G, an acid dye from aqueous solutions, have been investigated. The effect of initial dye concentration, temperature, and pH on the removal of orange G was investigated. The removal of orange G decreased from 95% to 82%, when the pH was increased from 2 to 12. The study of the effect of temperature and associated thermodynamic properties revealed the process of removal of dye is exothermic and spontaneous. Kinetic and mass transfer studies of the process were also carried out. The experimental data were used to develop a new solid–liquid phase equilibrium isotherm model. The average absolute relative deviation of the experimental data from the proposed model equation was 5.68%. In order to scale up the batch process for the removal of orange G, the concentration versus time data were analyzed to obtain mass transfer coefficient. The liquid-phase volumetric mass transfer coefficient (kLapL) was found to decrease from 0.008057 to 0.0028...

Mutagenicity, cytotoxicity and phytotoxicity evaluation of biodegraded textile effluent by fungal ligninolytic enzymes.

Colored effluents from the textile industry have led to severe environmental pollution, and this has emerged as a global issue. The feasibility of ligninolytic enzymes for the detoxification and degradation of textile wastewater was investigated. Ganoderma lucidum crude ligninolytic enzymes extract (MnP 717.7, LiP 576.3, and Laccase 323.2 IU/mL) was produced using solid-state culture using wheat bran as substrate. The biodegradation treatment efficiency was evaluated on the basis of degradation and detoxification of textile effluents. Standard bioassays were employed for mutagenicity, cytotoxicity and phytotoxicity evaluation before and after biodegradation. The degradation of Masood Textile, Kalash Textile, Khyber Textile and Sitara Textile effluents was achieved up to 87.29%, 80.17%, 77.31% and 69.04%, respectively. The biochemical oxygen demand, chemical oxygen demand, total suspended solids and total organic carbon were improved considerably as a result of biodegradation of textile effluents, which were beyond the permissible limits established by the National Environmental Quality Standards before treatment. The cytotoxicity (Allium cepa, hemolytic, Daphnia magna and brine shrimp), mutagenicity (Ames TA98 and TA100) and phytotoxicity (Triticum aestivum) tests revealed that biodegradation significantly (P < 0.05) detoxifies the toxic agents in wastewater. Results revealed that biodegradation could possibly be used for remediation of textile effluents. However, detoxification monitoring is crucial and should always be used to evaluate the bio-efficiency of a treatment technique.

Effect of gamma-irradiated textile effluent on plant growth

Introduction Textile industry produces large quantities of highly colored effluents, which are generally toxic and resistant to biodegradation. The present study was carried out to find out the efficiency of using gamma radiation to degrade or decontaminate combined textile effluent and its potential application as irrigation water on spleen amaranth.

Decolorization of direct red and direct blue dyes used in handmade paper making by ozonation treatment

Handmade paper utilizes direct dyes for imparting it bright colors to be used in wedding cards, fancy, decorative arts, etc. The effluent of handmade paper industry faces problem of highly colored effluent. The present study aims at decolorization of direct red and direct blue dyes, used abundantly in the industry, with the help of ozonation treatment. The batch studies have been conducted on decolourization of synthetic samples of these dyes at various initial concentrations, temperature, and pH. The effect of these parameters has been studied on decolorization efficiency. The maximum decolorization is achieved at pH of 10 and temperature of 313 K. The equilibrium time of decolorization with ozonation treatment for direct red and direct blue dye for 50 ppm initial concentration was 6 and 4 min, respectively. The kinetics of decolorization was found to be pseudo-first-order type.

Destruction of azo dyes by anaerobic–aerobic sequential biological treatment: a review

Dyes are synthetic organic compounds widely used in various industries such as, textile, leather, plastic, food, pharmaceutical, and paints manufacturing industries. Coloured effluents are highly toxic to the aquatic life and mutagenic to humans. Wastewater containing dyes has become an important issue demanding serious attention. Among the synthetic dyes, azo dyes are the largest and most widely used dyes and account for more than half of the annually produced dyes. The biodegradation of azo dyes is difficult due to their complex structure and synthetic nature. Several treatments have been proposed for efficient azo dye removal, most of them presenting some limitations such as generation of waste sludge, high operational costs, poor efficiency, and incomplete mineralization. Biological treatment is a cost-effective and eco-friendly process for dye degradation. Sequential anaerobic–aerobic biological treatment is considered as one of the most cost-effective methods for the complete mineralization of azo dyes. The anaerobic stage yields decolourization through reductive cleavage of the dye’s azo linkages, resulting in the formation of generally colourless but potentially hazardous aromatic amines. The aerobic stage involves degradation of the aromatic amines. It is the most logical step for removing the azo dyes from the wastewater. Several factors can influence the microbial activity and consequently the efficacy and effectiveness of the complete biodegradation processes. This paper summarizes the results of biological decolourization of azo dyes using various types of reactors, elaborates biochemical mechanisms involved, and discusses influence of various operational parameters on decolourization based on reports published in the last decade.