Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative

Abstract

The control of water pollution has become of increasing importance in recent years. The release of dyes into the environment constitutes only a small proportion of water pollution, but dyes are visible in small quantities due to their brilliance. Tightening government legislation is forcing textile industries to treat their waste effluent to an increasingly high standard. Currently, removal of dyes from effluents is by physio-chemical means. Such methods are often very costly and although the dyes are removed, accumulation of concentrated sludge creates a disposal problem. There is a need to find alternative treatments that are effective in removing dyes from large volumes of effluents and are low in cost, such as biological or combination systems. This article reviews the current available technologies and suggests an effective, cheaper alternative for dye removal and decolourisation applicable on large scale.

Introduction

Textile industries consume large volumes of water and chemicals for wet processing of textiles. The chemical reagents used are very diverse in chemical composition, ranging from inorganic compounds to polymers and organic products Mishra and Tripathy, 1993, Banat et al., 1996, Juang et al., 1996. The presence of very low concentrations of dyes in effluent is highly visible and undesirable (Nigam et al., 2000). There are more than 100,000 commercially available dyes with over 7×10⁵ ton of dye-stuff produced annually Meyer, 1981, Zollinger, 1987. Due to their chemical structure, dyes are resistant to fading on exposure to light, water and many chemicals Poots and McKay, 1976a, McKay, 1979. Many dyes are difficult to decolourise due to their complex structure and synthetic origin. There are many structural varieties, such as, acidic, basic, disperse, azo, diazo, anthroquinone based and metal complex dyes. Decolouration of textile dye effluent does not occur when treated aerobically by municipal sewerage systems (Willmott et al., 1998).

Through the formation, in 1974 of the Ecological and Toxicological Association of the Dyestuffs Manufacturing Industry (ETAD), aims were established to minimise environmental damage, protect users and consumers and to co-operate fully with government and public concerns over the toxicological impact of their products (Anliker, 1979). Over 90% of some 4000 dyes tested in an ETAD survey had LD₅₀ values greater than $\text{2×10}{}^3\text{mg}\text{/}\text{kg}$. The highest rates of toxicity were found amongst basic and diazo direct dyes (Shore, 1996).

In Great Britain, such matters are regulated by the Environment Agency (EA) for England and Wales, and the Scottish Environment Protection Agency (SEPTA), (Willmott et al., 1998). Government legislation is becoming more and more stringent, especially in the more developed countries, regarding the removal of dyes from industrial effluents. Environmental policy in UK, since September 1997, has stated that zero synthetic chemicals should be released into the marine environment. Enforcement of this law will continue to ensure that textile industries treat their dye-containing effluent to the required standard. European Community (EC) regulations are also becoming more stringent (O’Neill et al., 1999).

There are many structural varieties of dyes that fall into either the cationic, nonionic or anionic type. Anionic dyes are the direct, acid and reactive dyes (Mishra and Tripathy, 1993). Brightly coloured, water-soluble reactive and acid dyes are the most problematic, as they tend to pass through conventional treatment systems unaffected (Willmott et al., 1998). Municipal aerobic treatment systems, dependent on biological activity, were found to be inefficient in the removal of these dyes (Moran et al., 1997).

Nonionic dyes refer to disperse dyes because they do not ionise in an aqueous medium. Concern arises, as many dyes are made from known carcinogens such as benzidine and other aromatic compounds (Baughman and Perenich, 1988). Weber and Wolfe (1987) demonstrated that azo- and nitro-compounds are reduced in sediments, and similarly Chung et al. (1978) illustrated their reduction in the intestinal environment, resulting in the formation of toxic amines. Anthroquinone-based dyes are most resistant to degradation due to their fused aromatic ring structure. The ability of some disperse dyes to bioaccumulate has also been demonstrated (Baughman and Perenich, 1988).

This review illustrates the critical study of the most widely used methods of dye removal from dye-containing industrial effluents. These methods have been discussed under three categories: chemical, physical and biological. Currently the main methods of textile dye treatment are by physical and chemical means (Table 1) with research concentrating on cheaper effective alternatives.