Introduction

The last couple of years has seen a massive expansion in the chemicals industry; however, with this progress came the concomitant pollution of the environment with a significant range of xenobiotics (dye). Dye compounds form one such category of novel environmental contaminants and are used through a large number of industrial processes, most notably the textile, leather, plastics, food, pharmaceuticals and for manufacturing paints industries. Azo dyes represent the largest and most important group of dyes (Dafale et al. 2008a) mainly due to the simple synthesis. During manufacturing and usage of azo dyes an estimated amount of 10-15% is released into the environment. The release of these compounds into the environment is undesirable, not only because of their color but also because many azo dyes and their breakdown products are toxic, mutagenic, carcinogenic (Dos Santos et al. 2005) and negative influence on the living organisms.

Azo dyes can be reduced by azo reeducates in liver cells, skin surface micro flora, kidney cells and intestinal bacteria and thus aromatic amines (aryl amines) are released. Walker (1970) pointed out that the metabolism of azo dyes in mammal (dog) occurred when the result showed the product of azo dye (sulphanilic acid) was found in the urine of dogs after they consumed Orange I. Over the years epidemio-logical studies have shown the evidence that long-term occupational exposure to certain aromatic amines such as benzidine, 4-aminobiphenyl and 2-naphthylamine, that are used in dye industries increases the risk of developing cancer. To avoid the bad effect of azo group the concentration (mg/kg) of each harmful aromatic amine in dye products should not exceed 150 mg/kg. Although some azo group consisting compound have some bad effect but it is very important group for colour formation. Most of the colour component are consist of azo group. It's proper utilization can avoid it's bad effect. For this reason as a environmentalist we should know details about it.

Different physical, chemical and biological techniques can be applied to remove dyes from wastewater (Cooper 1993; Southern 1995; Vandevivere et al. 1998; Hao et al. 2000; Robinson et al. 2001). Each technique has its technical and economical limitations. Most physico-chemical dye removal methods have drawbacks because they are expensive, have limited versatility, are greatly interfered by other wastewater constituents, and/or generate waste products that must be handled. Alternatively, biological treatment may present a relatively inexpensive way to remove dyes from wastewater.

Biodegradation is defined as biologically mediated breakdown of chemical compounds. When biodegradation is complete the process is called mineralization i.e. the total breakdown of organic molecules into water, carbon dioxide and/or any other inorganic end products (Bennett and Faison 1997). Biodegradation of the dye by certain groups of fungi during dye removal has been extensively demonstrated. It has been demonstrated that under certain environmental conditions various microorganisms are able to transform azo dyes to noncolored products or even completely mineralize them (Stolz 2001; Xu et al. 2007). These synthetic compounds are resistant to aerobic degradation by bacteria. The strong electron-withdrawing character of the azo group stabilizes these aromatic pollutants against conversions by oxyge-nases (Kulla et al. 1983; Pagga and Brown 1986; Haug et al. 1991). In studies with more than 100 azo dyes tested in aerobic activated sludge systems, only a few were actually biodegraded (Shaul et al. 1991). Due to their recalcitrance in aerobic environments, the azo dyes eventually end up in anaerobic sediments, shallow aquifers, and groundwaters (Baughman and Perenich 1988) or in the gastrointestinal tracts of higher animals (Chung et al. 1992).

Under anaerobic conditions, azo dyes are readily cleaved via a four-electron reduction at the azo linkage generating aromatic amines (Brown and Laboureur 1983; Brown and Hamburger 1987). Biodegradation of dyes has been the subject of a large number of research papers and several review articles have been published (Walker 1970; Levine 1991; Chung and Cerniglia 1992; Bumpus 1995; Banat et al. 1996; Delee et al. 1998; McMullan et al. 2001; Stolz 2001; Pearce et al. 2003; Forgacs et al. 2004). Especially anaerobic azo dye reduction has been thoroughly investigated and most researchers agree that it is a non-specific and presumably extracellular process, in which reducing equivalents from either biological or chemical source are transferred to the dye.

The bacterial metabolism of azo dyes is initiated in most cases by a reductive cleavage of the azo bond, which results in the formation of colorless aromatic amines. Anaerobic reduction of the azo dyes is relatively easy to achieve, but the products have been found to be biorecalcitrant under anaerobic conditions (Stolz 2001). This observation coupled with the fact that many aromatic amines are completely degraded under aerobic conditions has led to the proposal that anaerobic-aerobic systems might be effective in achieving the complete biodegradation of azo dyes (Supaka et al. 2004).

Azo dyes are generally persistent under aerobic conditions (Isik and Sponza 2004c). However, under anaerobic conditions, they undergo facile reductive fission, yielding colorless aromatic amine compounds (Stolz 2001) , that in turn generally require aerobic conditions for their biodegradation (Sponza and Isik 2002a; Ekici et al. 2001). Azo dyes require an anaerobic and an aerobic phase for their complete biodegradation. Combined anaerobic aerobic treatment is therefore the most logical strategy for the complete removal of azo dyes from wastewater.

A lot of research papers have reported the results of combined anaerobic-aerobic bioreactor treatment of azo dye containing wastewaters. This review article summarizes the results of those research studies and discusses the feasibility of combined anaerobic-aerobic treatment for the complete removal of azo dyes from wastewater.

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