Colorants: Benefits, health impacts and environmental issues

Introduction

It is undeniable that water is the most essential resource on our planet, indispensable for humans, animals, plants and microorganisms. It covers three-quarters of the Earth's surface, is found underground, and is present in the atmosphere surrounding the Earth. Unfortunately, the planet is currently facing a drastic increase in water demand, a consequence of technological progress, population growth, and the expansion of agro-industrial activities, which are putting increasing pressure on freshwater reserves. In one century, global water consumption has increased more than tenfold, from approximately 400 billion m³ in 1900 to 7,000 billion m³ in 2001. Water withdrawals by industries represent more than 30% of the total volume withdrawn, and industrial treatments are responsible for half of the point-source discharges of organic pollution into the environment.

Among the major challenges related to water, the pollution of rivers, seas, groundwater and lakes, caused mainly by the discharge of untreated or insufficiently treated wastewater, seriously degrades ecosystems. The textile industry, in particular, uses 10,000 tonnes of various dyes marketed throughout the world, thus generating large quantities of effluents heavily contaminated by these dyes and their degradation products. These waters, often too colored, with very variable pH and a high chemical oxygen demand (COD), have a severe impact on the fauna and flora of the receiving ecosystems, and present toxic, carcinogenic or mutagenic risks for humans.

Faced with this alarming situation, the scientific community is working to develop innovative processes to treat these bio-recalcitrant pollutants.

Dyeing agents

Dyeing agents, whether natural or synthetic, are substances that add color to products, thus beautifying our environment. Textile dyes represent a category of organic compounds often considered as pollutants, which are found in wastewater, mainly due to chemical finishing processes in the textile industry [1,2]. This industry is characterized by a large number of small and medium-sized dyehouses, which use a wide range of textile dyes.

Classification of organic textile dyes

Nature and origin are the main criteria used to classify textile dyes, whether natural or synthetic.

Natural textile dyes

Natural textile dyes were mainly used in the treatment of textiles until 1856. Their use dates back to 2600 BC, with evidence of the use of dyes in China, extracted from plant and animal resources. It is also well documented that the Phoenicians used Tyrian purple, a dye obtained from certain species of crushed sea snails, as early as the 15th century BC. In addition, indigo dye, extracted from the indigo plant, was known and used since 3000 BC. Dyes derived from the madder plant were also used for wrapping and dyeing the clothing of Egyptian mummies, as well as for the fine textiles of the Incas in South America.

Synthetic dyes

Synthetic dyes were first discovered in 1856, with the invention of mauve, a dye derived from aniline, of a bright fuchsia color, synthesized by William Henry Perkin in the United Kingdom. Subsequently, some azo dyes were developed thanks to the diazotization reaction, discovered in 1858 by Gries in Germany [3].

These dyes are aromatic compounds produced by chemical synthesis, characterized by the presence of aromatic rings in their structure, containing delocalized electrons as well as various functional groups. Their color comes from the chromogenic structure, composed of chromophores (electron acceptors), while their ability to color is attributed to the auxochromic groups (electron donors).

The chromogen consists of an aromatic structure, usually based on benzene, naphthalene, or anthracene rings, to which conjugated chromophores with delocalized electrons are attached. Chromophore configurations include the azo group (-N=N-), the ethylene group (=C=C=), the methine group (-CH=), the carbonyl group (=C=O), the carbon-nitrogen group (C=NH; -CH=N-), the carbon-sulfur group (C=S=; ≡CS-SC≡), the nitro group (-NO₂; -NO-OH), and the nitroso group (-N=O; =N-OH). Auxochromes, on the other hand, are ionizable groups that allow dyes to bind to textile fibers. Common auxochromic groups include: -NH₂ (amino), -COOH (carboxyl), -SO₃H (sulfonate), and -OH (hydroxy) [1-3]. Five examples of textile dyes are shown in table 1.

Table 1: Main chromophore and auxochrome groups, classified by increasing intensity.

Textile dyes are mainly classified in two distinct ways:

Based on their application characteristics: This classification groups dyes under generic names such as acid, basic, direct, disperse, mordant, reactive, sulfur, pigment, vat dye, and insoluble azo.
Based on their chemical structure: In this classification, dyes are listed according to their chemical structure, with constitution numbers corresponding to categories such as nitro, azo, carotenoids, diphenylmethane, xanthene, acridine, quinoline, indamine, sulfur, amino and hydroxy ketone, anthraquinone, indigoid, phthalocyanine, inorganic pigment, etc.

Chemical classification

The classification of dyes according to their chemical structure is based on the nature of the chromophore group (Table 1).

Azo dyes

Compounds called "azo" are characterized by the presence of an azo functional group (-N=N-) that links two alkyl or aryl groups, identical or not (symmetrical and asymmetrical azo). These structures, generally based on the azobenzene skeleton (C6H5-N=N-C6H5), are aromatic or pseudo-aromatic systems linked by an azo chromophore group (-N=N-). The introduction of azo groups between two aromatic nuclei shifts the absorption spectrum of benzene to higher wavelengths, thus revealing the color (bathochromic effect). The simplest azo dye, azobenzene, is yellow-orange in color. The addition of amine or phenol groups also causes a bathochromic effect, as does the multiplication of azo groups, thus making it possible to obtain almost all the shades of the spectrum.

The presence of sulfonated, nitrated or halogenated substituents, which are acceptors or donors of n or π electrons that can be delocalized on the aromatic cycle(s), intensifies the resonance phenomenon. This makes it possible to influence both the color and the dyeing properties. In general, the more conjugated the π system of the molecule is, the greater the wavelength absorbed will be. However, the complexity of the molecules can reduce the intensity of the shades.

These azo dyes represent the most important family in terms of applications, constituting more than 50% of the world production of coloring materials, or 800,000 tons [4,7]. They are used in a wide range of fields, including textiles, printing, food, cosmetics and pharmaceuticals [8]. The textile industry remains the main market for these dyes [9].

Formula of azo dyes.

Anthraquinone dyes

After azo dyes, they represent the most important group of coloring materials. These dyes are generally recognized for offering excellent stability to light and chemical agents. The basic molecule of this type of dye is anthraquinone, which has a carbonyl chromophore group (>C=O) on a quinone nucleus, constituting the chromogen. These dyes are mainly used for dyeing polyester, acetate and cellulose triacetate fibers.

Formula of Anthraquinone dyes.

Triphenylmethane dyes

Triphenylmethanes are widely used in the paper and textile industries to dye materials such as nylon, wool, silk and cotton. Their intense coloring power is due to the extensive conjugation present in their cationic ion.

Formula of Triphenylmethane dyes.

Compounds of the triphenylmethane family are recognized for their genotoxicity towards bacterial cells and mammalian cells [10].

Indigoid dyes

Indigoid dyes are used as textile dyes, as additives in pharmaceuticals and confectionery, and in medical diagnostics [11,13]. They belong to a group of vat dyes similar to indigo in structure, used for coloring cellulose and protein fibers, as well as for printing on cotton. These dyes cover a wide range of colors, from orange to black. Although they have good light and water resistance, their durability is slightly lower than that of polycyclic dyes.

Indigo and thioindigo are typical examples of indigoid dyes. This range of dyes was greatly expanded in the early 20th century, although some of them were later replaced by other classes of dyes.

Formula of Indigoud dyes.

Nitro and nitrose dyes

Nitro dyes are still commonly used today, largely due to their very moderate cost, which is linked to the simplicity of their molecular structure. This is characterized by the presence of a nitro group (-NO2) in the ortho position relative to an electron-donating group, such as a hydroxyl or amino groups.

Formula of nitro and notrose dyes.

Dye classification

The dye classification of dyes is based on the nature of the auxochrome group (Table 1), which determines the type of bond between the dye and the substrate.

Acid or anionic dyes

These dyes are very soluble in water due to the presence of sulfonate or carboxylate groups. They are so called because they can be used to dye animal fibers (such as wool and silk) as well as certain modified acrylic fibers (such as nylon and polyamide) in a slightly acidic bath. The affinity between the dye and the fiber results from ionic bonds formed between the sulfonic acid part of the dye and the amino groups of the textile fibers.

Basic or cationic dyes

These salts of organic compounds, which contain amino or imino groups, have good solubility in water. The bonds are formed between the cationic sites of the dyes and the anionic sites of the fibers. These dyes have experienced a renewed interest with the emergence of acrylic fibers, on which they offer very bright and durable shades. They belong to different classes, such as azo dyes.

Vat dyes

Vat dyes are so named because they were originally solubilized in water using a vat, where they were first transformed into their co-derivatives by an alkaline reduction. The dyeing process concluded with the re-oxidation of the dye in situ, thus returning it to its initial insoluble form. Known for their excellent resistance to degradation agents, these dyes are still widely used today, such as indigo, which is commonly used to dye denim or jeans.

Formula of vat dyes.

Metal complex dyes

These dyes are organic compounds with groups close enough to form complexes by chelation with chromium, cobalt, calcium, tin or aluminum salts. They are also called mordants, because the fiber is usually treated with these salts before dyeing. During the dyeing process, an insoluble complex is formed within the pores of the fiber, which allows the dye to be fixed on the fiber.

Formula of metal complex dyes.

Reactive dyes

These dyes contain chromophore groups mainly from the azo, anthraquinone and phthalocyanine families. Their name is linked to the presence of a reactive chemical function, such as the triazine or vinylsulfone group, which ensures the formation of a strong covalent bond with the fibres. They are soluble in water and are increasingly used in the dyeing of cotton, wool and polyamides.

Developed or insoluble azo dyes

These dyes are formed directly on the fiber. In a first step, the textile support is impregnated with a solution of naphthol or coupler. The precursors of the molecule, small enough to diffuse into the pores and fibers, are then treated with a solution of diazonium salt, which triggers a coupler reaction leading to the immediate development of the azo dyes.

Developed or insoluble azo dyes.

Direct dyes

Direct dyes contain or can form positive or negative charges, which are electrostatically attracted to the charges of the fibers. They are distinguished by their affinity for cellulosic fibers without requiring the application of a mordant, a property linked to the planar structure of their molecules.

Direct dyes.

Mordant dyes

Mordant dyes generally contain a functional ligand capable of reacting strongly with aluminum, chromium, cobalt, copper, nickel or iron salts, thus forming various colored complexes with the textile.

Mordant dyes.

Disperse dyes

Disperse dyes are poorly soluble in water and are applied in the form of fine powder dispersed in the dye bath. During high-temperature dyeing, they can diffuse into synthetic fibres and fix themselves there permanently.

Dyeing process in textile industries

The textile and leather sector ranks third in the Moroccan industry, after the chemical/parachemical and food industries. It is divided into three sub-sectors:

Leather and footwear
Clothing and linings
Textile

The textile subsector occupies a privileged position within the textile and leather sector, comprising 520 units divided between large, medium and small enterprises. This subsector employs nearly 200,000 people in Morocco, making it the largest industrial employer in the country, representing 40% of national industrial employment. The positioning of textile companies in the value chain is illustrated in figure 1.

Figure 1: Positioning of the textile industry.

Yarn Process

Textile fibers are transformed into yarn through grouping and twisting operations that allow them to be bound together. The main steps in creating a yarn are summarized and illustrated in the figure below:

Figure 2: Main steps in the wire process.

Fabric development

Fabrics are created by two main methods: weaving, which involves interlacing one set of yarns with another, and knitting, which uses crochet needles to interlock one or more sets of yarns through a series of loops. The figure below illustrates the processes of fabric formation, particularly for flat fabrics such as sheets and clothing.

Figure 3: Finishing treatment.

This operation aims to improve the appearance, durability and functionality of fabrics by transforming unfinished products (woven, knitted) into finished consumer goods. This intensive water finishing process involves several steps, as illustrated in figure 4. These steps include treatments in chemical baths (preparation, dyeing, printing and finishing) and often require additional steps of washing, rinsing and drying.

Figure 4: Finishing process.

The most commonly used textile technology consists of several steps: desizing, scouring, bleaching, mercerizing and dyeing [14]:

Sizing is the first preparation step where sizing agents such as starch, polyvinyl alcohol (PVA) and carboxymethylcellulose are added to strengthen the fibres and reduce the risk of breakage.
Desizing is a step that involves removing sizing substances before weaving.
Scouring is used to remove impurities from the fibres by using an alkaline solution (usually sodium hydroxide) to break down the natural oils, fats, waxes and surfactants, and to emulsify and suspend these impurities in the scouring bath.
Bleaching is used to remove unwanted colours from the fibres using chemicals such as sodium hypochlorite and hydrogen peroxide.
Mercerizing is a continuous chemical process to increase the dye absorption capacity, luster and appearance of fibers. During this step, a concentrated alkaline solution is applied, followed by an acid wash before the dyeing step.
Dyeing is the process of adding color to fibers, which requires large volumes of water, not only for the dye bath, but also for rinsing. Depending on the dyeing process, various chemicals such as metals, salts, surfactants, biological agents, sulfide and formaldehyde may be added to improve the adsorption of dyes on the fibers.

The textile industry generally uses a large amount of chemicals, including:

Detergents and caustics, used to remove dirt, grit, oils, and waxes. Bleaching is also used to improve the whiteness and brightness of fabrics.
Sizing agents, added to improve the weaving process.
Oils, used to facilitate spinning and knitting.
Latex and glues, which serve as binders in various processes.
Dyes, fixing agents, and various inorganic materials, which are used to give fabrics the bright and vibrant colors demanded by the market.
A wide variety of specialty chemicals, such as softeners, stain release agents, and wetting agents, used to improve the final properties of textiles.

Many of these chemicals remain in the final product, while the remainder is removed from the fabrics and released into textile waste.

Printing is the process of decorating textiles by applying pigments, dyes, or other materials in a patterned manner. The four main methods of textile printing are block printing, roller printing, screen printing, and heat transfer printing. In each method, the application of color, usually as a thick paste, is followed by fixing (usually by spraying or heating) and then removing excess color by washing. Printing styles are classified as direct, discharge, or resist.

Washing jeans

Washing jeans is an operation intended to partially remove the indigo dye used to color cotton jeans. This operation is carried out at a temperature of about 60 ° C, using pumice stone or enzymes, with the addition of auxiliary products. The water used for washing jeans must be slightly softened and heated. There are two types of washing for jeans:

Chemical washing

There are three main methods for washing jeans:

Bleaching: This process uses an oxidizing agent such as sodium hypochlorite or potassium permanganate (KMnO4) added during washing, with or without the use of pumice stone. The discoloration obtained varies depending on the concentration of the bleaching agent, the temperature, the quality of the bleaching solution and the duration of the treatment. Intense whitening is generally obtained with a short treatment time.
Enzymatic washing: This type of washing is more environmentally friendly. It consists of applying organic enzymes that degrade the cellulose of the fabric. Once the desired color is achieved, the enzyme is neutralized by changing the alkalinity or temperature of the bath. The final treatment includes a looping and a softening cycle.
Acid washing: This process involves spraying clothes with pumice stone previously soaked in a solution of sodium hypochlorite or potassium permanganate, which produces localized whitening and a marked contrast between the bright blue and the uneven white. This wash enhances the color contrast of the fabric, sometimes using an optical brightening agent. The advantage of this process is that it saves water, as no additional water is required.

Mechanical washing

Here are two other methods of washing jeans:

Stone washing: In this process, newly dyed jeans are placed in large washing machines where they are hit with pumice stones or volcanic rocks. This process gives the jeans a soft and attractive appearance. The composition, hardness, shape, size and porosity of the stones can vary, making them versatile. However, this process is expensive and requires a high investment. The pumice stone adds a worn or faded effect by acting like sandpaper, removing some of the dye particles on the surface of the jeans.
Micro-sanding: This finishing process involves sanding the fabrics with sandpaper or an abrasive chemical to make the surface soft and hairless. This can be done before or after dyeing. The process uses a series of cylindrical rollers arranged horizontally, covered with sandpaper or an abrasive chemical, to create a soft surface. The jeans are pulled over the surface of the sanding rollers to achieve a raised finish. This process results in some reduction in color.

Figure 5: Washing jeans.

Impact of textile processes on the environment

Water use

Water is used for washing, cleaning and steam production activities. To provide a quantitative perspective on water use, here you will find data from some Moroccan textile companies. Performance indicators related to water use have also been calculated.

The data presented in table 2 provide an example of the amount of water consumed by these companies. However, this information represents only a sample and provides only a partial overview of the sector on this aspect.

Table 2: Example of water use in Moroccan textile companies.

Wastewater

Wastewater discharges generated by the textile industry include cleaning water, process water, cooling water and water from steam production. The amount of water used varies considerably depending on the specific processes implemented and the production capacity. For example, a company producing 135 tons of finished products generates on average 105,800 m³ of wastewater per year.

To provide additional information on wastewater effluents, a dataset for a Moroccan textile company, audited under the BAT4MED project, is presented below. The data in table 3 provide an example of the amount of wastewater produced by this company. However, this information represents only a sample and gives only a partial overview of the sector.

These data relate to an annual production of 3,200,000 pieces.

Table 3: Example of wastewater production in a Moroccan textile company.

The high volumes of wastewater generated by the textile production process are mainly due to the following operations:

Desizing: This step is one of the major sources of wastewater pollution. It involves the discharge of large quantities of sizing agents used in the weaving processes.
Pretreatment: This process produces wastewater containing natural and synthetic polymers as well as various other potentially toxic substances. Although bleaching is not a major source of pollution, scouring removes impurities present in the fibres, leaving only water as a by-product of the peroxide reaction.
Dyeing: This operation is responsible for the majority of the effluent. The wastewater comes mainly from the dye baths and wash waters, which contain by-products, residual dyes and auxiliary chemicals. Other pollutants may include solvents used for cleaning.

The wastewater from these operations is characterized by:

High temperature;
Traces of chemicals;
Coloration due to dyes used during dyeing and printing operations;
A high salt concentration with a correspondingly high pH.

The effluents generated by textile processes are detailed in table 4.

Table 4: Textile processes (BAT4 MED)

Effects of textile dyes on the environment

Organic dyes used in textile colouring must have high photocatalytic stability. This stability makes conventional aerobic treatments ineffective in degrading them, leading to their persistence in natural water resources in the absence of tertiary treatment. Environmental concerns regarding residual dye content and residual colour in textile effluents are therefore important, particularly for textile operators who discharge wastewater directly into watercourses, as well as for commercial operators, in terms of compliance with the requirements for residual dyes in treated effluents [15].

Concentrations of dyes above 1 mg/L in watercourses, resulting from direct discharges of textile effluents, whether treated or not, can lead to public complaints. Such high concentrations disrupt the reoxygenation capacity of receiving waters and block sunlight, which inhibits biological activity. This seriously affects aquatic life, particularly by disrupting the photosynthesis process of aquatic plants and algae [2]. The direct discharge of textile wastewater into natural waters, such as rivers, thus pollutes these environments and harms flora and fauna.

Depending on the duration and concentration of exposure, dyes can have acute and/or chronic effects on aquatic organisms. The depletion of dissolved oxygen in water, one of the most serious impacts of textile discharges, is of particular concern, as this oxygen is essential for the survival of aquatic organisms. This phenomenon also hinders the natural water purification process. In addition, dyes used in textile industries present a potential hazard, as they can transform into toxic and/or carcinogenic by-products, especially under anaerobic conditions [16].

Several dyes have been shown to be toxic to fish and mammals. They also inhibit the growth of microorganisms and negatively impact aquatic flora and fauna. In addition, many dyes and their breakdown products have been shown to be toxic to aquatic life, including aquatic plants, microorganisms, fish, and mammals [17].

Eutrophication

Reactive dyes bind to cellulose, wool and polyamide fibres. In this process, the dyes can also bind to NH2 groups, and their biological assimilation can lead to eutrophication. Indeed, nitrates released by the action of microorganisms on these dyes, when present in excessive quantities, become toxic to aquatic life, particularly fish, and compromise the quality of drinking water. Their absorption by aquatic plants promotes their excessive proliferation, which leads to oxygen depletion by inhibiting photosynthesis in the deeper layers of rivers and stagnant waters.

Under-oxygenation

When a significant amount of organic matter, including synthetic dyes, is introduced into the aquatic environment through occasional discharges, the natural regulatory processes become unable to compensate for the oxygen consumption by bacteria. One study estimates that the degradation of 7 to 8 mg of organic matter by microorganisms is enough to consume all the oxygen contained in a liter of water [18].

Color-turbidity-odor

The accumulation of organic dyes in waterways leads to various problems, including the appearance of bad tastes, bacterial proliferation, foul odors and abnormal water colorations. One study estimated that a coloration can be perceptible to the human eye from a concentration of 5 x 10⁻⁶ g/L [19]. Beyond the unsightly appearance, these coloring agents can also interfere with the transmission of light in water, thus blocking the photosynthesis process of aquatic plants.

Persistence

Synthetic organic dyes are compounds that cannot be eliminated by natural biological degradation [20]. Their persistence in the environment is strongly linked to their chemical structure:

Unsaturated compounds are less persistent than saturated compounds.
Alkanes are less persistent than aromatic compounds.
The persistence of aromatic compounds increases with the number of substituents.
Halogen substituents increase the persistence of dyes more than alkyl groups.

Bioaccumulation

A substance accumulates in an organism that does not have specific mechanisms to prevent its resorption or to eliminate it once absorbed. Thus, several dyes and their transformation products have been shown to be toxic to aquatic life, including fish, which are capable of accumulating various organic substances [17]. Species at the top of the food chain, including humans, are therefore exposed to these toxic substances, with concentrations that can reach up to a thousand times the initial levels present in the water.

Cancer

Textile dyes, even in their natural state, can be carcinogenic and are associated with health risks, including intestinal and brain cancer, as well as fetal abnormalities [21]. These dyes can bind to the NH2 group, and their biological assimilation tends to accelerate genotoxicity and microtoxicity [22, 23]. The highest health risk associated with these substances comes from their adsorption and degradation products through the gastrointestinal tract, as well as from the formation of hemoglobin adducts, disrupting blood formation. The median lethal dose (LD 50) values reported for aromatic azo dyes range from 100 to 2,000 mg/kg body weight [24].

Several dyes are capable of damaging DNA, which can lead to the formation of malignant tumors. Electron-donating substituents in the ortho and para positions can also increase the carcinogenic potential of these dyes. Among the azo dyes most known for their toxicity are direct black 38, a precursor of benzidine, and azodisalicylate, a precursor of 4-phenylenediamine. Degradation products that induce cancer in humans and animals include 2-nitroaniline, 4-chloroaniline, 4,4-dimethylenedianiline, 4-phenylenediamine, nitrosamines, dimethylamines, etc. [25].

Inhalation of waste from cotton, linen or hemp can cause byssinosis, a respiratory disease. Byssinosis is today one of the most significant health problems in the textile industry, particularly during drying processes, where it can exceed legal limits and cause hearing problems. The use of dyes and pigments is also linked to various irreversible effects on human health, which can directly affect workers at the application site and, later, consumers during the life cycle of the products.

Another crucial piece of information concerns the absorption of dyes by the human body. Dyed clothing, when in prolonged contact with the skin, can be absorbed, especially when body temperature is high and pores are dilated. Once absorbed, dyes and/or heavy metals tend to accumulate in the kidneys, bones, heart and brain. Health effects can be serious when the level of accumulation exceeds the permissible limit, with particularly serious consequences in children, as this accumulation can affect their growth and potentially their lives.

Wastewater treatment of textile industries at the end of the chain

• Many finishing activities in the industrial sector require water and generate wastewater, particularly during the degumming of printed fabrics. For this reason, wastewater treatment techniques have been developed, allowing textile companies to recover high-quality water after treatment, reusable in the process (for example, for the first rinsing steps), thus reducing the water footprint of textile production. Among these techniques:

Mixed wastewater treatment with recycling of approximately 60% water

The recycling of the effluent is achieved by partial on-site treatment of mixed textile water, following the following steps:

Equalization and neutralization: The process starts with an equalization of approximately 20 hours, followed by neutralization.
Activated sludge treatment: A special system composed of loop reactors is used to completely remove biodegradable components. Organic compounds and oxygen, acting as a buffer, are temporarily adsorbed by lignite (cotton residual cellulose).
Adsorption of dyes and non-biodegradable compounds: Lignite coke powder is used to adsorb dyes and other compounds that are difficult or non-biodegradable.
Flocculation/Precipitation and flotation: These steps remove sludge, which is an essential operation in the process.
Fixed bed gravel filter filtration: This filtration removes organic components and suspended solids, with fixed beds including activated carbon.
Remaining flow discharge: The remaining water flow is discharged into the river.
Storage and conditioning of treated water: The treated wastewater is stored in a tank and conditioned with ozone to prevent any biological activity.

This treatment allows to reduce the volume of wastewater and the non-recycled water has a very low content of residual organic components. In addition, this technique allows to save about 50% of neutral salts. However, its main disadvantage remains its high cost, although this can be amortized over time.

Recycling of textile industry wastewater by membrane treatment of separate flows

Membrane filtration via the treatment of separate flows allows water to be recovered and reused in the production process. This membrane technique is particularly suitable for companies processing fabrics, mainly cotton, where wastewater mainly comes from rinsing operations. About 90% of the water used can be recycled by most of these processes.

The main advantage of this technique lies in the water saving and the reduction of wastewater discharges. However, in Morocco, its high cost in terms of investment and maintenance is a major drawback.

Use of photo-oxidation for the third water purification treatment

This technique can be used in addition to the biological degradation methods commonly used in conventional treatment plants. It eliminates biodegradable and non-biodegradable organic compounds present in textile effluents. Developed on a semi-industrial scale, this treatment takes place in a homogeneous medium with an acidic pH, and consists of the addition of Fe²+/H₂O₂/UV, either in the presence of light or in the dark. Ultraviolet radiation is used to improve the performance of the reaction, which is very energetic and causes the covalent bonds of organic molecules to break, making the material biodegradable.

The advantage of this photo-oxidation technique is that it can reduce the organic pollutant load by up to 50%. However, it is essential to obtain a basic pH (pH=8) to precipitate the iron in solid form, otherwise the output samples will have high coloration, iron content, and turbidity, resulting in suspension of solids in the treated water.

Purification of industrial and mixed wastewater by combined membrane filtration and sonochemical technologies

Ultrafiltration (UF) combined with ultrasonication (US) via membranes allows to remove dyes and reuse auxiliary chemicals for dyeing, or to concentrate dyes and auxiliary products to produce purified water. Ultrafiltration is effective in removing particles and macromolecules, while the ultrasound effect is generated by cavitation bubbles. During cavitation, the bubbles implode, locally creating intense heat and high pressure.

The advantage of this technique lies in the combination of ultrafiltration and ultrasonic treatment technologies, which seems to be a promising approach for wastewater treatment. The particularities of each of these technologies – physical separation and sonochemical oxidation – allow to reduce the pollutant load of the mixed wastewater studied.

Proper treatment of industrial wastewater by implementing a combination of appropriate purification techniques

This treatment takes place in three stages:

First stage: This purification stage is intended to remove solid particles and sedimentary matter present in the wastewater.
Second stage: This purification phase focuses on the removal of organic substances and nutrients.
Third stage: The objective of this last stage is to purify the wastewater by removing compounds that are difficult to separate from the water.

Here are some combined wastewater treatment techniques:

Biological treatment
Dosage of activated carbon in biological treatment
Membrane bioreactor
Chemical precipitation (coagulation-flocculation)
Microfiltration, ultrafiltration
Nanofiltration, reverse osmosis
Sand filtration
Oxidation
Ozonization
Evaporation
Incineration

The implementation of a combination of these wastewater treatment techniques makes it possible to reduce the amount of impurities that affect the environment (soil, water). The choice of the type, configuration and size of the wastewater treatment plant, as well as the associated costs, are determined according to the discharge situation, the quantity of wastewater to be treated and the volume to be treated.

Removal of disperse dyes from textile effluents using biological sludge

Biological sludge from domestic wastewater treatment plants has an adsorption capacity for disperse dyes and organic matter. This technique has been successfully tested on dyes such as disperse red 60 and disperse blue 60, showing an increased adsorption capacity with acclimatized biological sludge. The main advantage is the reduction of COD, BOD, TKA, and dye levels in the wastewater.

Anaerobic degradation of textile dye bath effluents using Halomonas sp.

This technique uses halophilic and halotolerant bacteria of the genus Halomonas to reduce the COD and colour of effluents containing reactive dyes. It allows the decomposition of refractory organic compounds, providing further treatment or complete mineralisation of the compounds.

Removal of dyes from synthetic and natural textile effluents by one- and two-phase anaerobic systems

This process decolorizes azo dyes, such as Congo Red, from synthetic and natural textile effluents. The two-phase treatment has superior stability, with a major role played by the acid-generating reactor in dye reduction. The removal of COD and dyes is the main advantage.

Biosorption of reactive dye from textile wastewater by non-viable biomass of Aspergillus niger and Spirogyra sp.

This technique uses the biomass of the fungus Aspergillus niger and the algae Spirogyra sp. as biosorbents to remove reactive dyes from textile effluents. It improves water quality by reducing potentially toxic, carcinogenic and mutagenic substances.

Chemical Coagulation/Flocculation for Dye Removal in Textile Effluents

Chemical coagulation and flocculation involve the addition of chemicals to change the state of dissolved solids and facilitate their removal by sedimentation. This technique is particularly effective as a pre-treatment, improving the quality of wastewater.

Electrochemical Oxidation for the Treatment of Industrial Textile Effluents

Electrochemical oxidation is a promising method for treating textile effluents, reducing COD, total solids (TS), total dissolved solids (TDS) and total organic carbon (TOC). It achieves maximum color removal efficiency (96%) in 60 minutes.

Removal of Reactive Dyes from Wastewater by Adsorption on Coconut Activated Carbon

The adsorption of dyes on coconut activated carbon (CAC) is optimal at acidic pH values of 1 to 3, and the adsorption capacity increases with carbon dosage. This process reduces hazardous chemicals in textile wastewater treatment plants, with powdered activated carbon (PAC) being preferred due to its high adsorption capacity.

Biosorption of Anionic Textile Dyes by Fungal and Yeast Biomass

Non-viable fungal and yeast biomass has been used for the biosorption of various textile dyes. This technique is attractive because of the availability of these microorganisms as industrial by-products, offering cost savings and reduction in the toxicity of textile effluents.

References

[1] Suteu, D.; Zaharia, C.; Malutan, T. Biosorbents Based on Lignin Used in Biosorption Processes from Wastewater Treatment (chapter 7). In: Lignin: Properties and Applications in Biotechnology and Bioenergy, Ryan J. Paterson (Ed.), Nova Science Publishers, 27 (2011a). pp., ISBN 978-1-61122-907-3, New York, U.S.A.

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