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.
Presence of colour and its causative compounds has always been undesirable in water used for either industrial or domestic needs. Colour is a visible pollutant. Common man may not object to the discharge of colourless effluents laden with toxic and hazardous pollutants. On the other hand the discharge of coloured effluents, though less toxic, are often objected by the public on the assumption that colour is an indicator of pollution. It is therefore, not surprising to note that colour in wastewater has now been considered a pollutant that needs to be treated before discharge. Different colouring agents like dyes, inorganic pigments, tannins, lignins etc usually impart colour. Amongst complex industrial wastewater with various types of colouring agents, dye wastes are predominant. More than 8100 chemically different types of dyes are currently manufactured and the biggest consumers of these dyes are the textile industries, paper and pulp industries, dye and dye intermediates industries, pharmaceutical industries, tannery and Kraft bleaching industries which are probably the most potential contributors as far as colour pollution is concerned. Colour removal is one of the daunting tasks faced by the textile finishing, dye manufacturing, pulp and paper, Kraft bleaching and tannery industries among others. These industries consume large quantities of water and are therefore a source of considerable colour pollution. Colour is contributed by phenolic compounds such as tannins, lignins (2–3%) and organic colourants (3–4%) and with a maximum contribution from dye and dye intermediates, i.e. reactive/ azo/ acid/ vat dye etc. Among the above, dyes are difficult to be decolourized due to their complex structure, synthetic origin and recalcitrant nature, which makes it obligatory to remove them from industrial effluents before being disposed into hydrological systems. These dyes include several structural forms of dyes such as acidic, reactive, basic, disperse, azo, diazo, anthraquinone based and metal-complex dyes. The only thing in common is their ability to absorb light in the visible region. Effluents discharged from textile, dyestuff and other industries to natural water bodies and wastewater treatment systems are currently causing significant health concerns. In particular colour removal, is of late a major scientific interest, as indicated by the multitude of related research reports. There are more than 8000 chemical products associated with the dyeing process listed in the colour index.
In this article the problems of colour in industrial wastewaters is reviewed.
The discharge of coloured effluents, though less toxic is presented by the public on the assumption that colour is an indication of pollution. The colour of water, polluted with organic colourants, reduces when the cleavage of the –C=C– bonds, the –N=N– bonds and heterocyclic and aromatic rings occurs. The absorption of light by the associated molecules shifts from the visible to the ultraviolet or infrared region of the electromagnetic spectrum. There are about 12 classes of chromogenic groups, the most common being the azo type, which makes upto 60–70% of all textile and tannery dyestuff produced, followed by the anthraquinone type. A dye house effluent typically contains 0.6–0.8 g dye/l. Hence colour and dye removal, in particular, has recently become an area of major scientific interest. Wastewater treatment using physical, chemical and biological or combinations of these methods are well established for colour removal.
Among the physio-chemical processes, adsorption technology is considered to be one the most effective and proven technology having potential application in both water and waste water treatment. Adsorption is a rapid phenomenon of passive sequestration and separation of adsorbate from aqueous/gaseous phase on to solid phase. Adsorption techniques have gained favour recently due to their efficiency in the removal of pollutants to stable compounds for conventional methods. Adsorption is an economically feasible process that produces a high quality product. Decolouration is a result of two mechanisms adsorption and ion exchange and is influenced by many physio-chemical factors such as dye/sorbent interaction, sorbent surface area, particle size, temperature, pH, and contact time. Adsorbents, which contain amino nitrogen, such as chitin, tend to have a significantly larger adsorption capacity in acid dyes. There have been many examples of low cost adsorbents made from waste materials, for removal of dye and coloured organic matter from aqueous media that are of low cost. There are various materials by which adsorption can be carried out, a few of them are discussed below.
Activated carbon is the most commonly used method of dye removal by adsorption. Powdered activated carbon has a reasonably good colour removing capacity when introduced in a separate filtration step. High removal rates are seen for cationic mordant and acid dyes. The removal is moderate for sulphur, dispersed, direct and reactive dyes. The adsorptive capacities of dyes onto non-biological waste materials, such as activated carbon, will also depend on the surface charge of the adsorbent in contact with water. For carbon, the surface charge will be neutral thus physical adsorption will predominate. This results in activated carbon having a high adsorption capacity for both acid and basic dyes. Oxygenated coconut shell activated carbon-fly ash-china clay are found to be very effective for the removal of basic dyes brilliant blue (BB 69) and brilliant red (BR 22) from industrial effluents. Maximum removal of dye (98%) was achieved. Performance is dependent on the type of carbon used and the characteristics of the wastewater to be treated. Carbon has to be reactivated as disposal of the concentrates is of concern. Reactivation normally results in 10–15% loss of the sorbent and hence there is a need for regeneration to make it cost effective.
Ion exchange has not been widely used for the treatment of dye-containing effluents or colour removal, mainly due to the option that ion exchangers cannot accommodate a wide range of dyes. Wastewaters containing colour is passed over the ion exchange resin until the available exchange sites are saturated. Employing this method, both cationic and anionic dyes can be removed from effluents successfully. Advantages of this method include no loss of adsorbent on regeneration, reclamation of solvent after use and the removal of soluble dyes. One major disadvantage is the high operation cost. Organic solvents are expensive and the ion exchange method is not very effective for disperse dyes. Standard ion exchange systems have not been widely used for treatment of dye-containing effluents. The reason for this is probably the general opinion that ion exchanges cannot accommodate a wide range of dyes and further perform poorly in the presence of other additives in wastewaters. One of the effective anion exchangers gaining applicability in the recent years is quarternized cellulose. Hydrolyzed reactive dye binds to it through columbic association of the dye sulphonate group with resin quaternary amines, or with additional interactions i.e. hydrogen bonding, Van der Waals forces. The effectiveness of dye removal from wastewater depends on the type and number of such interactions. However, as chloride concentration increases the binding rate between dye and resin diminishes, while the addition of NaOH completely prevents binding. The latter findings mean that NaOH can be used as a regeneration agent of resin saturated with dye.
Chemical de-colourization methods:
Oxidation is the most commonly used chemical decolouration processes due to its simple handling. As described in the early years of research, modern dyes are resistant to mild oxidation conditions such as those, which exist in biological treatment systems. Suitable colour removal, therefore, must be accomplished by more powerful oxidizing agents such as chlorines, ozone, Fenton’s reagents, UV/peroxide, UV/ozone, or other oxidizing techniques or combinations. Chlorine has been proved to be a good dye-oxidizing agent and applied at low capital and operating costs. However the potential of chorine to form undesirable compounds with nitrogen containing components of the wastewater has limited the acceptability of this method. Hydrogen peroxide (H2O2) is one of the widely used agent that needs to be activated by some means, for example, ultraviolet light. Many methods of chemical decolouration vary depending on the method in which H2O2 is activated. Chemical oxidation removes the dye from the dye-containing effluent by oxidation, resulting in aromatic ring cleavage of the dye molecules.
Fenton’s reagent (hydrogen peroxide, activated with Fe(II) salts) is very suitable for the oxidation of toxicants present in wastewaters, which inhibit biological treatment. Chemical separation uses the action of sorption or bonding to remove dissolved dyes from wastewaters and shown to be effective in decolourizing both soluble and insoluble dyes. Advantages of this process include COD, colour and toxicity reduction. Since the mechanism involves flocculation, impurities are transferred from the wastewater to the sludge, which is still ecologically questionable. It has conventionally been incinerated to produce power, but such disposal seems to be far from environment friendly. The performance is dependent on the final floc formation and its setting quality, although cationic dyes do not coagulate at all. Acid, direct, vat, mordant and reactive dyes usually coagulate, but the resulting floc is of poor quality and does not settle well yielding mediocre results. Fenton’s reagent performs best with high dye concentrations and low initial pH. However the initial concentration of dye bath after pH adjustment was markedly inhibited by the presence of sodium chloride, which is found in wastewater from reactive dyeing. A comparative study of the oxidation of disperse dyes by an electrochemical process, ozone, hypochlorite and Fenton’s reagent showed that the Fenton’s later was the best among the oxidation processes studied in terms of COD reduction and colour removal. This technology was found to be effective in decolourizing a wide range of dyes. Fenton’s reagent proceeds by oxidizing ferrous to ferric iron with simultaneous splitting of H2O2 into hydroxide ion and hydroxyl radical. The latter oxidizes the dye while the former precipitates with ferric iron together with organics. Ferric seems as effective as ferrous iron, and only a few mg/l is required if high temperature is used. Complete decolourization was obtained after the Fenton’s reagent stage. The final COD reduction after activated sludge treatment was only 80 mg/l.
Ozonation: The use of ozone was first pioneered in the early 1970’s and is a very good oxidizing agent due to its high instability (Eo=2.07 V) compared to chorine (Eo=1.36 V) and H2O2 (Eo=1.78 V). It can selectively oxidize unsaturated bonds (e.g. –C=C– or –N=N–) and aromatic structures. Oxidation by ozone will lead to the degradation of chlorinated hydrocarbons, phenols, pesticides and aromatic hydrocarbons. Ozone can react both in direct and indirect pathways. Direct pathways involve the ozone molecule itself as the electron acceptor. Hydroxide ions catalyze the auto decomposition of ozone to hydroxyl radicals in aqueous solutions. Hydroxyl radicals are very strong, non-selective oxidants (Eo = 3.06 V) that react with organic and inorganic chemicals with rate constants upto 109 times higher than ozone. Therefore at low pH ozone may often efficiently target unsaturated chromophoric bonds in a dye molecule via direct reactions. At higher pH, indirect reactions of ozone may lead to a less efficient process by the indiscriminant oxidation of all parts of dye molecule in addition to other scavengers in solution. Ozonation is one of 254 the most effective means of decolourizing dye-laden wastewater and has been shown to achieve high colour and effluent COD removal with improved biodegradability. The dosage applied to the dye-containing effluent is dependent on the total colour and residual COD to be removed. With no residual or sludge formation and no toxic metabolites, ozonation leaves the effluent with no colour and low COD suitable for discharge into aqueous systems. One major advantage is that ozone can be applied in its gaseous state and therefore does not increase the volume of wastewater and sludge. The disadvantage of ozonation is its short half-life (typically being 20 min) demanding continuous application making it a cost intensive process. Treatment of dyeing process wastewater with ozone followed by chemical coagulation showed, 62% removal of colour after ozonation and the constituent compounds were efficiently removed using Ca(OH)2 as coagulant. Decolouration efficiency of >99% was achieved in multistage treatment of high strength dye wastewaters. The ozonation process leads to complete decolourization with a very short retention time, however effective mineralization of the dye was not observed. Ozone is more effective in reducing the colour in comparison to COD and TOC. It was reported that at an ozone dose (∆O3) of 230 mg/l, maximum reduction efficiencies of colour, COD and TOC were 86, 22 and 15% respectively. Ozonation was also very effective in increasing the biochemical oxygen demand of the pulp mill effluent.
Sodium hypochlorite (NaOCl) Chemical oxidation of coloured wastewater is also possible with Chlorine compounds. This method attacks the amino group of the dye molecule by the Cl+ which initiates and accelerates azo-bond cleavage. This method is not suitable for disperse dyes. Decolouration rates increase with increasing chlorine concentration and decreasing pH of the medium. Dyes containing amino or substituted amino group on the naphthalene ring i.e., dyes derived from aminonapthol- and naptylamino-sulphonic acids are most susceptible for chlorine decolouration. However, one aspect which has come to force in the recent years is that for environmental reasons, the future use of chemicals containing chlorine and the release of aromatic amines, which are carcinogenic, or otherwise toxic molecules should also be restricted. However, it should be noted that although about 40% of the pigments used worldwide contain chlorine this corresponds to only less than 0.02% of the total chlorine production.
Electro chemical oxidation Electrochemical treatment of coloured wastewater is considered as one of the advanced processes, and a potentially powerful method of pollution control, offering high removal efficiencies. Electrochemical processes generally have lower temperature requirement when compared to other non-electrochemical treatments and hence do not require any additional chemicals. The required equipment and operation is generally simple. The controls are easy and the electrochemical reactors are compact, and prevent the production of unwanted by-products. It is believed that the main oxidizing agent in electrochemical process is hypochlorite ion or hypochlorous acid produced from the naturally occurring chloride ions. Hydroxyl radical or other reactive species also participate in the electrochemical oxidation of organics. The process would be relatively non-specific, which is, applicable to a variety of contaminants. This electrochemical oxidation can be achieved either directly or indirectly at the anode, or indirectly using appropriate anodically generated inexpensive reagents. The breakdown metabolites are generally not hazardous leaving it safe for treated wastewater to be released back into waterways. Electrochemical method shows efficient and economical removal of dyes and high efficiency for degradation of recalcitrant pollutants. Electrochemical method which was developed in mid 1990’s has been successfully applied to treat various wastewater i.e. land fill leachate, refractory organic pollutants including lignin, EDTA, wastewater containing polyaromatic organic pollutants, tannery effluents and textile dye waste water. The electrochemical method of oxidation for colour removal is more efficient for the treatment of textile dye wastewater for the dyeing stage than for total dyeing and finishing stages. During dyeing, colour reduction is about 100% with only 6 min of electrolysis. It was observed that 90–93% colour removal was achieved by prolong electrolysis (neutral medium pH 7, 4.5 Adm-2 current density, 220 min electrolysis time) and was concluded that sodium chloride enhances the destruction of the dye. electrochemical method to be effective in removal of colour and reduction of COD of azo dye effluents of two leading textile industries in a flow reactor. Maximum colour removal (95.2%) was achieved at flow rate of 5 ml/min and current density of 29.9 mA/cm2.
Coagulation and precipitation Hydrolyzing metal salts of iron and aluminium are widely used as primary coagulants to promote the formation of aggregates in wastewater and reduce the concentration of colourants and other dissolved organic compounds. Short detention time and low capital cost makes chemical coagulation a widely used technique. The high cost of chemicals for precipitation as well as for pH adjustments, problems associated with dewatering and disposing of generated sludge, and high concentration of residual cation levels which remains in the supernatant are some of the limitations of this method. Treatment with chemicals like aluminium sulphate, ferrous and ferric sulfate, ferric chloride, calcium chloride, copper sulphate etc. either alone or in combination for removal of colour from individual dye wastes as well as composite mill waste are investigated. These studies indicate the feasibility of chemical coagulation/precipitation for colour removal and suggest that colour removal is accomplished by aggregation/precipitation and adsorption of colouring substances onto the polynuclear coagulant species and on to hydrated flocks. The removal mechanism is governed by the fact that the major part of the pore surface area exists in pores from 10 to 1000 µm and the co-polymers have the advantage of large pore diameter upto the order of 400 lm enhancing the process of adsorption on flocs. Ullman reported around 80% colour removal from pulp mill effluents after employing a flocculation-sedimentation unit and alum coagulation. The American Water Works Association (AWWA) suggests coagulation as best treatment for removal of colour from wastewater.
The ability of biological treatment process for decolourization of industrial effluents is ambiguous, different and divergent. Observations indicate that dyes themselves are not biologically degradable since microorganisms do not utilize the colour constituents as a source of food. Most currently used laboratory methods for biodegradation involve aerobic microorganisms, which utilize molecular oxygen as reducing equivalent acceptor during the respiration process. Yet, environmental conditions with lack of molecular oxygen are not uncommon. In these anoxic and hypoxic environments, microorganisms survive by using sulphates, nitrates and carbon dioxide etc as electron acceptors. With respect to future improvements, research data is increasingly being gathered which indicates that certain dyes are susceptible to anoxic/anaerobic decolourization. An anaerobic step followed by an aerobic step may represent a significant advancement in biological treatment and decolourization in the future. An advantage of biological treatment over certain physico chemical treatment methods is that over 70% of the organic material present that is measured by the COD test may be converted to bio solids.
Aerobic trickling filter plant in some treatment systems were removing between 34 and 44% of the dye colour and that percentage removal of colour for different industrial effluents are different by different biological treatment methods. The heavy metals contained in the dye molecule tend to inactivate the microorganism. Biological colour removal from a cotton textile effluent containing an azo reactive dye and reported colour removal of 90%.
Anaerobic biological treatment helps in decolourization of the dyes, by rendering them amenable to further aerobic treatment and degradation. Anaerobic bioremediation allows azo and other water-soluble dyes to be decolourized. This decolouration involves an oxidation-reduction reaction with hydrogen rather than free molecular oxygen aerobic system. Typically, anaerobic breakdown yields methane and hydrogen sulphide. Primary degradation and decolourization of dyes with azo-based chromophore can be achieved by the reduction of the azo bond (–N=N–). In many cases the decolouration of reactive azo dye under anaerobic conditions is a co-metabolic reaction. These reductive cleavage reactions usually occur with low specific activities but are extremely unspecific with regard to the organism involved and the dyes converted. In this unspecific anaerobic process, low molecular weight redox mediators (e.g. flavins or quinones), which are enzymatically reduced by the cells, are very oftenly involved. These reduced mediator compounds reduce the azo group in a purely chemical reaction.
R1 – N = N – R2 + 4e– + 4H+ → R1-NH2 + R2NH2
R1+R2 are aromatic substituents in dye molecule.
Azo dyes also acts as an oxidizing agent for the reduced flavin nucleotides of the microbial electron chain. It is reduced and decolorized concurrently with re-oxidation of the reduced flavin nucleotides, however co substrates are required for decolouration to proceed at a variable rate. The amines produced by the reduction of the azo dyes are colourless but they are very resistant to further degradation under anaerobic conditions. . Applicability of a thermophilic UASB anaerobic system as a unit operation for the decolouration of synthetic textile dye wastewater clearly indicates that it has significant advantage over an equivalent mesophilic system and can effectively decolourize such a wastewater with a higher efficiency. Among the different reactors studied anaerobic filter and UASB reactor gave good colour removal efficiencies. A major advantage of this anaerobic system apart from the decolouration of soluble dyes is the production of biogas. Biogas can be reused to provide heat and power and in turn reduce energy costs.
Advanced oxidation processes (AOP) have led their way in the treatment of aqueous waste and is rapidly becoming the technology of choice for many applications. AOP’s have been described as oxidation processes that are based on the generation of hydroxyl (*OH) radical intermediates. The direct photolysis of H2O2 by the photon of wavelength shorter than 370 nm produces two (*OH) radicals that can oxidize most organic pollutants. AOPs based upon hydrogen peroxide ozone and ultraviolet radiation have been among the most commonly investigated. AOPs are capable of achieving desired on site colour destruction. If carried out to its ultimate stage AOP can completely oxidize organic pollutants to CO2 water and salts. Partial oxidation can result in increased biodegradability of pollutants so that residual organic compounds can be removed through conventional biological treatment. There are a growing number of successful commercial applications of these AOPs and their combinations. The *OH quantum yield has been determined to be 0.017 at 360 nm for Fe3+–H2O2 photolysis. Combination of several processes may be another alternative for the improvement of treatment efficiency. It is reported that a combination of the oxidants H2O2 and O3 in the presence of UV irradiation is more effective to destroy hazardous and refractory pollutants than UV/H2O2 or UV/O3 photoxidation process. But at the same time cost for UV/H2O2/O3 treatment is higher compared to UV/H2O2 or UV/O3 process. reported the removal of colour from textile waters by *OH radical oxidation. The colour removal was markedly related with the amount of *OH formed. The optimum pH for both colour removal and *OH formation occurs at pH 3–5. Upto 96% of colour was removed within 30 min under the studied conditions. All oxidation processes were capable of completely decolourizing the wastewater within 30 min. Decolourization proceeded fastest by the O3/Mn(II) process, whereas the O3/H2O2 combination was more efficient in the removal of dissolved organic carbon (DOC) and UV254 nm.