The textile manufacturing process is a major contributor to water consumption, using several thousand cubic meters of water per day. The amount of water used in textile processes varies depending on the specific technique and chemicals utilized, but it is estimated to range from 50 to 240 liters of water for every kilogram of finished textile.
The wastewater produced during fabric production, which involves various operations such as desizing, bleaching, dyeing, printing, and washing, can contain colored and biodegradation-resistant pollutants. The World Bank has estimated that dyeing and finishing textiles produce approximately 17 to 20 percent of industrial wastewater.
The textile dyeing industry has increasingly used synthetic dyes because they are cost-effective and more stable than natural dyes in light, temperature, detergent, and microbial resistance. When coloring textiles, the dye does not fully stick to the fabric, which creates wastewater-containing dye. The wastewater deriving from textile wet processing factories encompasses various chemical elements, including inorganic finishing agents, surfactants, chlorine compounds, salts, total phosphate, polymers, and organic substances.
This wastewater harms the environment because it contains high levels of pollutants like suspended solids, chemical oxygen demand, biochemical oxygen demand, heat, acidity, basicity, and other soluble substances. Azo dyes, which make up the largest group of synthetic dyes, are particularly popular due to their economic stability. However, these dyes are toxic and persist in the environment, and their byproducts are mutagenic and carcinogenic.
Currently, around 10,000 different synthetic dyes are produced, with azo dyes being the most common and containing azo groups (-N=N-) in their structure. Certain dyes and chemicals in textiles can pose environmental and health risks. These substances are often complex and difficult to remove from wastewater, which has led to regulations mandating their removal before discharge.
In recent decades, developed countries have become increasingly conscious of the negative environmental impacts of industrial activities. As a response, regulations regarding wastewater treatment have become more stringent and there is now a greater emphasis on adopting eco-friendly methods and approaches. The objective of these measures is not only to reduce pollution but also to conserve water resources through inventive technologies that support water recovery and reuse.
Physiochemical Treatment
There are four different physiochemical techniques for decolorizing textile wastewater: Electrochemical, Flocculation, Coagulation, and Precipitation. The choice of method ultimately depends on the concentration and type of pollutants in the wastewater and the cost and regenerability of the treatment method. While coagulation-flocculation can be effective for wastewater containing disperse dyes, it is limited in its ability to treat reactive and vat dyes and generates a significant amount of sludge.
Conventional treatment methods for textile wastewater, such as chemical coagulation, adsorption processes, and membrane filtration, are expensive and produce large volumes of secondary pollutants. Biological methods are limited due to the non-biodegradability of dyes.
Electrochemical advanced oxidation processes (EAOPs), specifically electro-Fenton (EF) process, have shown promise in removing organic matter. Various EAOPs have been studied, but research has shown that the electro-Fenton (EF) process is a particularly effective and eco-friendly method for eliminating organic substances. EF process generates H2O2 and ·OH radicals in situ, eliminating the need for storing and transporting highly reactive H2O2.
The process does not produce secondary pollutants, and organic compounds are removed through two mechanisms in the EF process: Fenton’s reaction (as shown in equation 2), which occurs throughout the solution, and anodic oxidation, which takes place at the surface of the anode (as depicted in equation 4).
EF process uses high-oxygen-overvoltage anodes, and ferrous ions added into the system generate ·OH radicals. The process oxidizes organics to CO2, water, and inorganic ions and regenerates ferrous ions at the cathode reducing its further addition (as shown in equation 3).
- O2 + 2H+ + 2e− → H2O2 (1)
- Fe2+ + H2O2 → Fe3+ + OH− + ⋅OH (2)
- Fe3+ + e− → Fe2+ (3)
- M (H2O) → M(⋅OH) + H+ + e− (4)
To improve the efficiency of removing organic substances from wastewater in a parallel plate electrochemical reactor, Sparging Air (SA) was introduced. SA is common in industrial applications such as oxidation, hydrogenation, chlorination, and wastewater treatment.
SA columns or bubble columns are advantageous due to their simple geometry, lack of moving parts, and ease of manufacturing. These methods have proven effective in treating discharges from ink, pharmaceutical, and textile industries, removing suspended solids, TOC, effluent color, oily emulsions, and other substances.
Chemical coagulation is an alternative method to remove color from textile wastewater. This process involves using metal salts, polymers, and polyelectrolytes to break down emulsions and suspensions. Electrocoagulation (EC), on the other hand, uses metal plates as electrodes to create highly charged polymeric metal hydroxide species in the water.
It is mainly used to remove heavy metal ions from manufacturing wastewater sewage. Electrochemical methods are fast and effective in removing heavy metals from industrial wastewater. Membrane-based separation is also used to reduce coloration and BOD/COD levels through reverse osmosis, nanofiltration, ultrafiltration, or microfiltration techniques. The choice of membrane process depends on the quality of the final product.
The photovoltaic cells can supply the necessary electrical energy for EC and EF processes, which reduces the cost of energy and makes the process sustainable by lowering carbon emissions.
| Process | kWh/kg COD ($) |
| EC | 3.16 |
| ECSA | 2.33 |
| EF | 3.33 |
| EFSA | 3.67 |
Biological Treatment
In recent years, various physical and chemical methods have been developed for decolorizing azo dye. These methods can be expensive, produce much sludge, and require safe disposal. Physical methods such as adsorption and membrane filtration are time-consuming and require further treatment.
Bioremediation through microorganisms is a more cost-effective and eco-friendly approach that can naturally develop resistant strains to remove dyes from effluent. This method may transform toxic chemicals into less harmful ones, which has advantages and disadvantages depending on the specific treatment method used.
Microalgae-Based Biodegradation of Textile Effluent
Using microalgae to treat textile effluent is the most effective process, as it can transform, degrade, and adsorb dye in natural wastewater. Microalgae have a higher rate of degradation than bacteria and fungus, making them more efficient in eliminating particulate pollution in wastewater. Green microalgae found in freshwater and saltwater ecosystems are commonly used in treating dye effluents due to their high biosorption potential resulting from their large surface area on wastewater.
Algae absorb harmful metabolites in wastewater, such as PO43-, RCOO–, -NH2, and –OH, through electrostatic attraction. The decolourization mechanism of algae is different from that of fungi and bacteria. It involves three stages: conversion of algae biomass into carbon dioxide and water, conversion of chromophore material to non-chromophore material, and adsorption of chromophore by algal biomass. Studies show that algae produce the azoreductase enzyme for decolourization and utilize azo dyes as a carbon and nitrogen source for their growth.
Fungal-Based Biodegradation of Textile Effluent
Numerous efforts have been made on the use of fungus-based methods for treating dye in wastewater, with white-rot fungus being a commonly preferred organism due to its ability to degrade plant lignin and other polymers found in plant cell walls. However, other strains of fungus have also been found to be effective in decolorizing or biosorbing various types of dye.
Fungi can be classified into two types based on their mode of action and surrounding environment: live cells, which can decolorize or biosorb dye, and dead cells, which can only adsorb dye. Fungi are commonly used for biodegrading or bioremediating effluent from industrial processes like textile, pulp, and paper production by producing extracellular enzymes such as lignin peroxidase (Lip), manganese peroxidase (MnP), and laccase.
Different fungal cultures are often used for bioremediation due to their large biomass and unique characteristics like hyphal spectra and filamentous growth patterns that allow them to effectively degrade under specific conditions.
Bacterial-Based Biodegradation of Textile Effluent
The presence of a microbial population in dye effluent or wastewater can aid in transforming complex feed substances into simpler forms, improving the treatment process. Different bacterial species are effective in degrading and mineralising various dyes through aerobic, anaerobic, or combined processes.
Using bacterial cultures for degradation is advantageous due to their short growth period compared to other microorganisms, and genetic manipulation can further enhance their ability to degrade dyes. Microorganisms can also catabolize organic pollutants and act as pollution indicators for toxic compounds.
Azo dyes can be degraded rapidly by bacterial enzymes, with intermediate metabolites further broken down by other bacterial enzymes like hydroxylase and oxygenase. Microbial consortia are commonly used for textile effluent degradation due to the combined action of several enzymes. Various bacterial strains have been identified as capable of decolorizing different structurally distinct dyes.
Nano base treatment
Nanofiltration is a commonly used method for removing industrial waste due to its energy efficiency. Compared to traditional separation methods, nanofiltration offers benefits such as lower energy consumption and minimal harm to processed substances. Recent experiments have successfully removed red, black, and blue dyes from the solution using nanofiltration, with removal rates of 93.77%, 95.67%, and 97%, respectively.
Effluent from an integrated dyeing wastewater-treatment plant can be treated using a combination of Fenton oxidation and membrane bioreactor (MBR) processes. This treatment reduces the total organic carbon and color by 40% and 69% after 35 minutes of reaction when optimal Fenton oxidation conditions are met (initial pH 5, H2O2 dosage 17 mmol/L, and 1.7 mmol/L).
The MBR process further purifies the effluent, resulting in a COD and TOC removal efficiency of 86%. The final effluent meets the reuse criteria of urban recycling water standards. Due to discovering rarer contaminants and promulgating new water quality requirements, the textile sector must install appropriate advanced water treatment processes.
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