The public demand for color-free waste discharge to receiving waters and tougher color standards have made decolorization of a variety of industrial wastes a top priority. Unfortunately, with the complicated color-causing compounds, the decolorization of these wastes is a difficult and challenging task. This article first describes the background information of dye molecules and dye waste characteristics. The methods for color measurements and standards are then discussed. Different techniques including almost all the known physical, chemical and biological techniques are described for decolorization. Each process alone may not be able to meet the requirements. A combination of these processes, for example, chemical-biological, biological-chemical, chemical-physical, chemical-chemical, etc. is often used. The formation of intermediates during the decolorization process is emphasized. These byproducts may be more toxic than the parent compounds. Thus, the extent of the mineralization in waste decolorizatio...

Decolorization of Wastewater

Kinetics and thermodynamics of Methylene Blue adsorption on Neem (Azadirachta indica) leaf powder

Water pollution control is presently one of the major thrust areas of scientific research. While coloured organic compounds generally impart only a minor fraction of the organic load to wastewaters, their colour renders them aesthetically unacceptable. Stringent regulating measures are coaxing industries to treat their waste effluents to increasingly high standards. Colour removal, in particular, has recently become an area of major scientific interest as indicated by the multitude of related research reports. During the past two decades, several decolourization techniques have been reported, few of which have been accepted by some industries. There is a need to find alternative treatments that are effective in removing dyes and colourants from large volume of effluents, which are cost-effective, like the biological or integrated systems. This article reviews some of the widely used and most promising industrial wastewater decolourization methods. Data on decolourizing efficiencies of different causative agents, obtained by means of different physical, chemical and biological methods are discussed. Further a critical review is made on the various treatment methodologies and emerging technologies with a note on their advantages and disadvantages.

Decolourization of industrial effluents - available methods and emerging technologies - a review

The residual dyes from different sources (e.g., textile industries, paper and pulp industries, dye and dye intermediates industries, pharmaceutical industries, tannery, and Kraft bleaching industries, etc.) are considered a wide variety of organic pollutants introduced into the natural water resources or wastewater treatment systems. One of the main sources with severe pollution problems worldwide is the textile industry and its dye-containing wastewaters (i.e. 10,000 different textile dyes with an estimated annual production of 7.105 metric tonnes are commercially available worldwide; 30% of these dyes are used in excess of 1,000 tonnes per annum, and 90% of the textile products are used at the level of 100 tonnes per annum or less) (Baban et al., 2010; Robinson et al., 2001; Soloman et al., 2009). 10-25% of textile dyes are lost during the dyeing process, and 2-20% are directly discharged as aqueous effluents in different environmental components. In particular, the discharge of dye-containing effluents into the water environment is undesirable, not only because of their colour, but also because many of dyes released and their breakdown products are toxic, carcinogenic or mutagenic to life forms mainly because of carcinogens, such as benzidine, naphthalene and other aromatic compounds (Suteu et al., 2009; Zaharia et al., 2009). Without adequate treatment these dyes can remain in the environment for a long period of time. For instance, the half-life of hydrolysed Reactive Blue 19 is about 46 years at pH 7 and 25°C (Hao et al., 2000). In addition to the aforementioned problems, the textile industry consumes large amounts of potable and industrial water (Tables 1, 2 and Fig. 1) as processing water (90-94%) and a relatively low percentage as cooling water (6-10%) (in comparison with the chemical industry where only 20% is used as process water and the rest for cooling). The recycling of treated wastewater has been recommended due to the high levels of contamination in dyeing and finishing processes (i.e. dyes and their breakdown products, pigments, dye intermediates, auxiliary chemicals and heavy metals, etc.) (Tables 3, 4 and 5) (adapted from Bertea A. and Bertea A.P., 2008; Bisschops and Spanjers, 2003; Correia et al., 1994; Orhon et al., 2001).

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Textile Organic Dyes – Characteristics, Polluting Effects and Separation/Elimination Procedures from Industrial Effluents – A Critical Overview

Virtually all the known physicochemical and biological techniques have been explored for treatment of extremely recalcitrant dye wastewater; none, however, has emerged as a panacea. A single universally applicable end-of-pipe solution appears to be unrealistic, and combination of appropriate techniques is deemed imperative to devise technically and economically feasible options. An in-depth evaluation of wide range of potential hybrid technologies delineated in literature along with plausible analyses of available cost information has been furnished. In addition to underscoring the indispensability of hybrid technologies, this article also endorses the inclusion of energy and water reuse plan within the treatment scheme, and accordingly proposes a conceptual hybrid dye wastewater treatment system.

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M.P Roche

Papers

Treatment of a wastewater resulting from dyes manufacturing with ozone and chemical coagulation