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What is a dye contaminated effluent?Trade effluent is any effluent (liquid waste) that is discharged from any premises where a trade or industry is carried out. A dye contaminated effluent, then, is one which contains residual dye, here a dye is defined as a soluble substance suitable for staining or colouring, which has an affinity to the substrate that it is being applied to. What is the situation of the textile effluents treatment?The treatment of textile effluents is of interest due to their toxic and esthetical impacts on receiving waters. While much research has been performed to develop effective treatment technologies for wastewaters containing azo dyes, no single solution has been satisfactory for remediating the broad diversity of textile wastes. 
The nature of waste from the textile industry depends on the type of factory, the processes being operated and the fibres used. In general, however, textile wastewater is highly coloured. Around 10-15% of all the dyes used in the industry are released into the environment during manufacture or usage. Human and ecological health concerns have prompted the government to require textile effluent discharges to have increasingly lower colour and nitrogen levels. Despite being aware of the problem, many textile manufactures have failed to adequately remove azo dye compounds from their wastewaters. The amount of waste produced from textile dyeing can be minimised by a number of measures, including: • Low liquor dyeing. • Improved dye fixation. • Discontinuation of overflow rinsing, where possible. • Segregation of hot/ cold effluent and heat recovery. • Replacement of certain toxic dyestuffs. • Banning of chlorine based carriers/ levellers. • Reuse of dye liquors on repeat shades. • Replacement of wasteful two stages dyeing of blends with single stage dyeing. • Automatic dispensing and control systems. Are textile effluents toxic?The presence of potentially toxic compounds in wastewaters from textile dyeing industries (dyehouses) has led to environmental research to identify methods that can effectively treat these wastewaters. Colour in wastewater is highly visible and affects esthetics, water transparency, and gas solubility in water bodies. Besides the problem of colour, there is a general concern that some azo dyes either are toxic or can be modified biologically to toxic or carcinogenic compounds. Azo dyes are the most toxic of the dye types. Many studies have been conducted showing the toxic potential of azo dyes. The problem associated with azo dyes is created by the dye metabolites. After releasing dyes into the aquatic environment, they may be converted into potentially carcinogenic and/or mutagenic amines. 
Substituted benzene and naphthalene rings are common constituents of azo dyes, and have been identified as potentially carcinogenic agents (IARC, 1982). While most azo dyes themselves are non-toxic a significantly larger portion of their metabolites are (Ganesh, 1992). Most dyes that have been shown to be carcinogenic are no longer used; however, a complete investigation of all dyestuffs is impossible (Brown and DeVito, 1993). An investigation of several hundred commercial textile samples revealed that nearly 10 percent were mutagenic in the Ames test (McCarthy, 1997). Another study conducted on 45 combined effluents from textile finishing plants showed that 27 percent of the wastewater samples were mutagenic in the Ames test (McCarthy, 1997 as cited in Jager, 1996). The potential for toxic effects to the humans, resulting from the exposure to dyes and dye metabolites, is not a new concern. As early as 1895 increased rates in bladder cancer were observed in workers involved in dye manufacturing (Rehn, 1895). Other concerns are the impurities within commercial dye products and the additives used during the dyeing process. Many textile effluents contain heavy metals that are complexed in the azo dyes. High concentrations of salt are often used to force fibre-reactive dyes out of solution and onto substrates (Zollinger, 1991). These compounds can cause high electrolyte and conductivity concentrations in the dye wastewater, leading to acute and chronic toxicity problems. Understanding the dye structures and how they are degraded is crucial to understanding how toxic by-products are created. Brown and DeVito (1993) have compiled a three-part list of the biological mechanisms thought to be responsible for carcinogenic activation of azo dye compounds. This list is based on an extensive review of the literature regarding azo dye toxicity, and places each mechanism in order of their frequency of citation. Brown and DeVito (1993) postulate that: - Azo dyes may be toxic only after reduction and cleavage of the azo linkage, producing aromatic amines. - Azo dyes with structures containing free aromatic amine groups that can be metabolically oxidized without azo reduction may cause toxicity. - Azo dye toxic activation may occur following direct oxidation of the azo linkage producing highly reactive electrophilic diazonium salts. Research in the area of dye wastewater treatment includes identifying the toxic constituents of dyes and associated processing waters as well as assessing potential toxicity associated with the use of ozone treatment to reduce colour (Carrière et al. 1993; Keqiang et al. 1994; Fouche 1995; Law 1995). Wastewater colour is one of the major problems facing industries involved in dyeing processes. Wastewater from dyehouses often carries high concentrations of excess dye that fails to adhere to substrate fibres. What are the treatment solutions for the textile effluents?The implementation of strict legislation which rules the discharge of coloured water combined with an increase awareness of that negative environmental impact of these dyestuffs has resulted in an increasing number of studies on the biodegradation of dyes in recent years. 
These studies include ozonation, oxidative processes, photochemical, etc, that are chemical degradation techniques. Adsorption, activated carbon, membrane filtration are physical treatments of coloured Wastewaters. The aerobic and anaerobic biological treatments are now widely used, being an effective option for effluent decolourization. Industrial treatment of wastewaters in modern treatment facilities consists of various stages: Mechanical treatment , where large particulate impurities are pulverized, and sand, petroleum and various fat-containing products, sewage sludge, and wet sediment are removed. Biological treatment , including absorption of finely disperse organic and inorganic pollutants from wastewaters by the surface of a microorganism and then decomposition of the absorbed substances inside the cells of the microorganisms during the biological processes that take place there (oxidation, reduction); ammonium compounds are oxidazed by bacteria into nitrates and nitrites and oxygen is then split off and secondarily used for oxidation of organic substances. Treatment of wastewaters with microorganisms has its own advantages and disadvantages and is a function of many factors – temperature, solar radiation, oxygen supply, biocenosis composition, presence of nutrients and toxins, etc. Chemical degradation techniques: • Oxidative processes. The main oxidising agent is hydrogen peroxide (H 2 O 2 ). It sometimes requires activation by UV light, for example. • Fentons reagent (H 2 O 2 -Fe(II) salts). This is suitable for the treatment of effluents which are resistant to biological treatment or poisonous to live biomass. Performance of this method is dependent on good floc formation and settling quality. A disadvantage is the generation of sludge and its disposal. • Ozonation – a good oxidising agent. The dosage applied to a given effluent is dependent on the total colour and residual COD to be removed. Advantage – used in gaseous state, therefore does not affect the volume of wastewater or sludge. Disadvantage – costly; ozone has a short half-life. • Photochemical – dye molecules are degraded to CO 2 and H 2 O by UV treatment in the presence of H 2 O 2 . Degradation is caused by the production of high concentrations of hydroxyl radicals. Disadvantage – depending on the initial materials, additional by products may be produced (e.g. halides, metals, inorganic acids, etc). Advantage – no sludge is produced and foul odours are greatly reduced. • Sodium hypochloride (NaOCl) – initiates and accelerates azo-bond cleavage. The use of chloride for dye removal is becoming less frequent due to negative effects when released into waterways. • Electrochemical destruction – advantages – little or no use of chemicals; no sludge production; breakdown metabolites are generally not hazardous, therefore treated wastewaters can be released back into waterways. Physical treatments • Adsorption – efficient in the removal of pollutants too stable for conventional methods. Decolourisation is a result of two mechanisms: adsorption and ion exchange, and is influenced by many physico-chemical factors, e.g. sorbent surface area, particle size, pH, contact time. • Activated carbon – the most commonly used method of dye removal by adsorption. It is very effective for adsorbing cationic, mordant and acid dyes, and to a slightly lesser extent, dispersed, direct, vat, pigment and reactive dyes. Performance is dependent on the type of carbon used and the characteristics of the wastewater. Disadvantage: activated carbon is expensive; it has to be reactivated, which can result in 10-15% loss of sorbent. • Membrane filtration – can clarify, concentrate and separate dye continuously from effluent. Advantages compared to other methods are resistance to temperature, to an adverse chemical environment, and to microbial attack. Disadvantages – disposal of the residue, high capital cost and the need for membrane replacement. What are the major bottlenecks for dyed effluent treatment?Due to the highly variable nature of biological treatment systems and especially textile effluents, there are a number of factors that may affect the biodegradation rate of dyes. Non-dye related parameters such as temperature; pH, dissolved oxygen or nitrate concentrations, type and source of reduction equivalents, bacteria consortium, and cell permeability can all affect the biodegradation of dyes and textile effluents. Dye related parameters such as class and type of dye (i.e. reactive-monoazo), reduction metabolites, dye concentration, dye side-groups, and organic dye additives could also affect the biodegradability of azo dye wastewaters. Temperatures, which are too high or too low, can result in the exclusion of a particular group of microorganisms. The wastewater pH can affect the proper functioning of both anaerobic and aerobic organisms. An exponential increase in the decolourization rate is observed by decreasing the pH, but this relationship depends on the dye being tested. Nitrate and especially oxygen may play an important role in determining the rate of dye reduction. The presence of oxygen generally inhibits the degradation of azo dye chromogens. Obligate aerobes might actually decolorize azo compounds under temporary anoxic conditions. However, high nitrite or nitrate concentrations in the mixed liquor of activated sludge plants could significantly inhibit dye removal. Without the necessary reduction equivalents to optimize bacterial respiration and growth, dye reduction may be inhibited. Often, bacterial cultures are unable to proliferate when an azo dye is the sole carbon and nitrogen source. Therefore, additional, readily biodegradable sources may be necessary. In the absence of oxygen an azo compound will act as the sole oxidant, and its reduction rate will be governed by the rate of formation of the electron donor The presence of oxygen may out compete the azo dye as the preferred oxidizer of reduced electron carriers in the respiration chain, and thus limit the reduction of azo linkages. The type of bacteria or consortium used for dye biodegradation will undoubtedly affect the reduction rate. In general however, aerobic microbes do not have the ability to substantially decolorize azo dyes, but can oxidize the dye metabolites. The converse applies to anaerobic microbes. A final non-dye related factor is the cell permeability and the cell wall adsorption of azo dyes. Effects of dye absorption by the cell wall: (1) Dye adsorption follows Freundlich adsorption isotherms at low dye loads per weight of biomass, but exhibits a high variability; (2) Depending on the dye, subsequent reduction may take place or the dye may remain in the cell wall; (3) Adsorption does not inhibit the reduction rate of microbes that exhibit the ability to reduce azo dyes. 
Very little of the dye added to a biological reactor will be leached from the biomass when placed in a landfill. This might suggest that the dye is effectively reduced after adsorption to the cell wall or that very little dye is actually adsorbed. Cell permeability might play an important role in dye biodegradation. Dyes that are not reduced by whole cells are effectively degraded by cell extracts from both facultative anaerobes and obligate aerobes. This suggests that many cells might be capable of dye biodegradation, but are limited by the permeability of their cell walls. The azo dye structure can play a significant role in the dye biodegradation rate. Depending on the number and placement of the azo linkages, some dyes will biodegrade more rapidly than others. In general, the more azo linkages that must be broken will cause the reduction rate to be slower. While there are not a large number of studies that specifically address this factor. Poly-azo dyes are less likely to degrade than mono- or diazo dye types. Fibre-reactive azo dyes often contain solubilizing side groups, as well as a nucleophilic reactive group. Depending on the nature of these groups, biodegradation might be inhibited. 
Sorption of dye to sludge depends on the type, number, and position of the substituents in the dye molecule. Hydrophilic sulpho groups reduce the dye removal through sorption, and conversely, sorption is increased by the presence of hydroxyl, nitro, and azo groups in the dye molecule. The production of toxic by-products or the presence of toxic dye additives may also inhibit biodegradation. High salt concentrations are not uncommon in textile effluents and may result in adverse conditions for biodegradation. Dispersing and solubilizing agents may also create inhibitory conditions for dye reduction. The production of inhibitory dye metabolites, cause a decrease in biodegradation. Anaerobic degradation of simulated textile effluent generates metabolites that are toxic to some aerobic organisms. The distribution of dye intermediates plays an important role in determining the aerobic biodegradability of some dyes. Low dye removal may be attributed to the presence of intermediates that are less susceptible to microbial degradation or that act as inhibitory agents. Inhibition of various microorganisms to dye metabolites is frequently cited throughout the literature and is assumed to be a key factor when treating wastewaters containing azo dyes. A final and important factor to evaluate is the initial dye concentration of the wastewater. Elevated dye concentrations may cause a drop in percent dye removal. Furthermore, the inhibition may be directly related to the effects of increased dye metabolite formation due to higher dye concentrations. Tolerable influent concentrations are likely specific to individual or related groups of dyes. |
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