The treatment of wastewater is one of the most important subjects in pollution control. In the textile industry, the main pollution source of wastewater comes from the dyeing and finishing industry. Major pollutants include high suspended solids (SS), chemical oxygen demand (COD), biochemical oxygen demand (BOD), heat, colour, acidity and basicity and other soluble substances. All pollutants, except colour, can be reduced by general chemical and physical methods. Nowadays, the colour of wastewater cannot effectively be dealt with by a single method. Therefore, the major problem of dye wastewater is the colour that is produced by the residual dyes during the dyeing process. Despite the large amount of water in waste streams, residual dyes in drainage water can accumulate. That is why the concentration of dyes and the colour of streams will increase gradually and the transparency of streams will also be reduced.
There are several methods used to de-colourize dye wastewater, but they cannot be effectively used individually. Means the coagulation process effectively de-colourizes insoluble dyes, such as disperse dyes, but does not work well for soluble dyes. There is also a large amount of sludge created during the coagulation process. Also, the oxidation process, effectively de-colourizes almost all dyes except disperse dyes, but does not remove COD. The effectiveness of de-colourization by the oxidation process will also be reduced by impurities in the wastewater. The activated carbon absorbent is not for insoluble dyes and biological system to de-colourization is not effective because most dyes are toxic to organism.
Here I discuss about Fenton’s Reagent, because it not only oxidise dyes but also acts as coagulant and also increased dissolved oxygen in wastewater. Hydrogen peroxide is common reagent in dyeing process, so this method is expected to be more efficient in treating dye effluent.
Destroy of Dye molecule:
In presence of strong acid, hydrogen peroxide and ferrous ions are more stable. When hydrogen peroxide is present in aqueous system containing organic substance in presence of ferrous ion in strong acid a redox reaction takes place.
Dissociation of hydrogen peroxide and acidified ferrous ion is as below.
H2O2 + Fe2+ —-> Fe3+ + HO● + OH–
The hydroxyl radical generated in this reaction would attack organic substance of unsaturated dye molecule. So that chromophore of dye molecule would be destroyed.
HO● + RH —-> H2O + R●
Ferric ion generated in above reaction forms complex with hydroxide ion and complex have tendency to polymerize in acidic medium. Thus the decolorized dye molecule in wastewater can be coagulated and precipitated. This process removes COD and also reduces amount of sludge.
Effect of pH on de-colourization:
When pH of system is lower than 3.5, the efficiency of colour removal is higher and COD reduction is also 88-90%. But if system pH goes above 4, then de-colorization is not notable.
Amount of hydrogen peroxide:
Study shows that, 580-590 ppm of peroxide required for reactive dyes, 870-880 ppm of peroxide for acid dyes and 290-300 ppm for remaining types of dyes.
Amount of ferrous:
Dosage of ferrous also affect de-colorization. Higher the dosage better the de-colorization.
Effect of temperature:
In higher temperature, de-colourization is better. i.e. redox reaction and de-colourization can be accelerates by raising temperature of effluent.