In developing countries like Bangladesh, Srilanka, China, Philippines and Mauritius, the textile sector plays a crucial role in these countries Gross Domestic product . Textile industry generates high amount of wastewater in this region especially due to the fact that there is not much scope of wastewater treatment. Various chemical and large amount of water is used during the whole process, approximately 200L of water is used to produce 1kg of textile . The World Bank estimates that textile dyeing and treatment contribute up to 17-20 percent of total industrial water pollution. As a result, the liquid waste produced by the industry is quite large and contains inorganic and organic compounds , some of which are carcinogenic and are threats to environment and human health.
The wastewater coming out of textile industry usually goes through various primary, secondary and tertiary treatment mechanism like flocculation, chemical coagulation, simple sedimentation, aerated lagoons, aerobic activated sludge, trickling filters, reverse osmosis, electro dialysis and others . Suspended solids, most of oil and grease and gritty materials are removed in primary treatment. Secondary treatment involves reduction of BOD, phenol, color control etc. Tertiary treatment is where the remaining wastes are removed. Although these treatment processes remove most of the wastes from water, but they are not always effective in removing all kinds of dyes and trace metals like Cr, As, Cu and Zn .
Carbon Nanotubes (CNTs) are comparatively new technology and their use have become widespread in catalysis, adsorption, and separation. Due to their large specific surface area, small pore size, hollow and layered structures CNTs have proven to possess great potential as adsorbents for removing organic and inorganic contaminants from textile wastewater.
Classification of Textiles and dyes
Cotton, woolen, and synthetic—are the three main categories of textiles. Also they are referred to as cellulous, protein and synthetic fibers depending upon the used raw materials.
Estimation shows around 107-109 kg of dyes are produced every year worldwide . Fibers, alongside fabrics to be produced determine the type of dye and chemical needs to be used. Chromophores, a group of atoms are the reason why dyes make our world colorful. The intensity is caused by electron withdrawing or donating substituents, called auxochromes . Van der Waals forces, hydrogen bonds and hydrophobic interactions are what causes dyes to get stuck with fibers. When dye and fiber have the opposite charge and create covalent bond, they have been found to be the strongest dye-fiber attachment .
Dyes also contain heavy metal like lead (Pb), chromium (Cr), cadmium (Cd) and copper (Cu). The chart below shows common type of dyes used for three different types of fiber :
Table 1: Types of fiber and dyes in textile
|Type of fiber||Common dyes|
|Cellulose (Cotton, Rayon, linen, ramie, hemp and lyocell)||· Reactive (remazol, procion MX and cibacron F)
· Direct dye (congo red, direct yellow 50 and direct brown 116)
· naphthol dyes (fast yellow GC, fast scarlet R and fast blue B)
· indigo dyes (indigo white, tyrian purple and indigo carmine)
|Protein (wool, angora, mohair, cashmere and silk)||· acid dyes (azo dyes, triarylmethane and anthraquinone dyes)
· lanaset dyes (Blue 5G and Bordeaux B)
|Synthetic (polyster, nylon, spandex, acetate, acrylic, ingeo and polypropylene)||· dispersed dyes (yellow 218 and navy 35)
· basic dyes (basic orange 37 and basic red 1)
· direct dyes (congo red, direct yellow 50 and direct brown 116)
Textile wastewater composition and nature
Substantial variations in the COD and BOD, pH, color and salinity characterizes Textile wastewater due to the composition of the dye effluent changing with the textile product . Currently, the dyes are with color-display groups and polar groups within the realm of aromatic and heterocyclic compounds. Their structure is creates difficulty to degrade the printing and dyeing wastewater as they are complex and stable .
Usually textile processing include desizing, scouring, bleaching, mercerizing and dyeing processes. First preparation step is sizing, where agents are added to provide strength to the fibers and minimalize breaking. Desizing removes sizing materials, then weaving is done. Impurities from the fibers to breakdown natural oils, fats, waxes and surfactants, by scouring bath. Next step is bleaching where unwanted colors are removed from the fibers. Mercerizing increases dye-ability and fiber appearance. Last but not the least, Dyeing is done to add color to fibers. The whole process requires large volumes of water. Depending on the dyeing process, many chemicals are added to improve dye adsorption onto the fibers.
Table 2: Different steps of textile production and wastewater nature
|Process||Chemical Composition||Wastewater Nature|
|Sizing||Starch, waxes, Carboxymethyl Cellulose (CMC), Polyvinyl Alcohol
(PVA), wetting agents.
|High in BOD, COD|
|Desizing||Starch, CMC, PVA, fats, waxes, pectin||High in BOD, COD, SS, dissolved solids (DS)|
|Bleaching||Sodium Hypochlorite, Cl2, NaOH, H2O2, acids, Surfactants, NaSiO3, Sodium Phosphate, short cotton fiber||High alkalinity, high SS|
|Mercerizing||Sodium Hydroxide, cotton wax||High pH, low BOD, high DS|
|Dyeing||Dyestuffs Urea, reducing agents, oxidizing agents, Acetic acid, detergents, wetting agents.||strongly colored, high BOD, DS, low SS, heavy metals|
|Printing||Pastes, urea, starches, gums, oils, binders, acids, Thickeners, cross-linkers, reducing agents, alkali||Highly colored, high BOD Oily, appearance, SS, slightly alkaline|
Table 3: Characteristics of Textile Effluent and Standards
|Parameter||Textile effluent ||EPA |
|Total BOD (mg/L)||80-60000||15|
|Oil and grease (ppm)||10-30||10|