The waste water generated by textile industries are considered most polluting among all industrial sectors because of volume discharged and effluent composition. During the last few years, new and/or tighter regulations coupled with increased enforcement concerning wastewater discharges have been established in many countries.
The main sources of wastewater generated by the textile wet-processing industry originate from the washing (or scouring) and bleaching of natural fibers and from the dyeing and finishing steps. During the operations on fibers, these processes generate wastewaters of great chemical complexity and diversity. The chemical composition of textile mill effluents is also changing rapidly. Most significant is the current popularity of cotton fabrics and bright colours leading to greater usage of reactive and azo dyes and most important cause of shifting wastewater composition are the new and tighter restrictions on discharged effluents.
The nature of textile wastewaters has been reviewed in terms of process chemicals used and in terms of the classical parameters BOD, COD, TSS, TS and contents of N, P, and heavy metals.
Dye molecules consist of a chromagen. There are about 12 classes of chromogenic groups, the most common being the azo type which makes up to 60-70% of all textile dyestuffs produced, followed by the anthraquinone type. A second classification of dyes is based on their mode of application to textiles and distinguishes acid, reactive, metal-complex, disperse, vat, mordant, direct, basic and sulphur dyes. Research on textile effluent de-colourization has often focused on reactive dyes. This is due to three main reasons. First, reactive dyes represent an increasing market share, currently about 20-30% of the total market for dyes, because they are used to dye cotton which makes up about half of the world’s fiber consumption. Second, a large fraction, typically around 30%, of the applied reactive dyes is wasted because of dye hydrolysis in the alkaline dye bath. Third, conventional wastewater treatment plants.
Since dyes are intentionally designed to resist degradation, it is no surprise that little dye degradation occurs in activated sludge systems. Among azo dyes tested, only a very few were degraded aerobically. The degree of stability of azo dyes under aerobic conditions is proportional to the structural complexity of the molecule. Under anoxic conditions, however, azo dyes are readily cleaved to aromatic amines since the azo bond can serve as electron acceptor in the electron transport chain provided a carbon source is available and nitrates absent. The by-products of the azo bond cleavage, aromatic amines, are not further metabolized under anaerobic conditions but are readily bio-degraded in an aerobic environment.
Due to new innovations in process chemicals used, the biodegradability of textile wastewaters has been increasing steadily during recent years. The persistent molecules present in textile effluents belong to very diverse chemical classes, each used in relatively small amounts. Aside from dye molecules, these include dyeing auxiliaries such as polyacrylates, phosphonates, sequestering agents (e.g. EDTA), de-flocculation agents (lignin or naphthalene sulphonates), antistatic agents for synthetic fibers, carriers in disperse dyeing of polyester, fixing agents in direct dyeing of cotton, preservatives (substituted phenol), and a large number of finishing auxiliaries used for fire, moth, and waterproofing.
Textile effluents tend to inhibit only slightly the heterotrophic activity within activated sludges. Unlike heterotrophs, chemoautotrophic nitrifying bacteria are often substantially inhibited in activated sludges fed with textile effluents.
Numerous large-scale studies have been conducted to assess the toxicity of textile effluents toward aquatic life. Typical LC50 (96 h) values amount to 5-6% (v/v) with bleaching, dyeing, or mixed textile effluents, using a freshwater fish as test organism.
Most textile wet processes, such as sizing, spinning, weaving, desizing, dyeing, scouring and washing consume large amounts of surfactants. Combined municipal and textile effluents contain peak concentrations of 11 and 67 mg/l anionic and non-ionic surfactants respectively. A large fraction of the non-ionic surfactants used in textile processes are alkyl phenol ethoxylates as the textile industry is the largest consumer of these surfactants. Alkyl phenols are, however, much more toxic than the ethoxylated forms, with maximum accepted concentration in the low ppb range.
AOX and heavy metals:
Traditionally, sodium hypochlorite was usually preferred to for the bleaching of cotton and linen H2O2 because of superior whiteness, lower cost and possible fiber damage by radicals arising from H2O2 decomposition. Hypochlorite bleaching effluents contain up to 100 mg/l AOX including considerable amounts of carcinogenic chloroform. Hypochlorite is being superseded by alkaline peroxide bleaching. Certain reactive dyes are AOX. Given their carcinogenic nature, the discharge of AOX is restricted.
Heavy metals concentrations in dyebath effluents, typically in the 1-10 mg/l.