Tertiary Treatment Processes
It is worthwhile to mention that textile waste contains significant quantities of non-biodegradable chemical polymers. Since the conventional treatment methods are inadequate, there is a need for the efficient tertiary treatment process.
Oxidation techniques:
A variety of oxidizing agents can be used to decolorize wastes. Sodium hypochlorite decolourizes dye bath efficiently. Though it is a low-cost technique, it forms absorbable toxic organic halides (AOX) [4]. Ozone on decomposition generates oxygen and free radicals and the later combines with coloring agents of effluent resulting in the destruction of colors [5].
Arslan et al. investigated the treatment of synthetic dye house effluent by ozonization, and hydrogen peroxide in combination with Ultraviolet light [6]. The main disadvantage of these techniques is it requires an effective sludge producing pretreatment.
Electrolytic precipitation & Foam fractionation:
Electrolytic precipitation of concentrated dye wastes by the reduction in the cathode space of an electrolytic bath been reported although extremely long contact times were required. Foam fractionation is an experimental method based on the phenomena that surface-active solutes collect at gas-liquid interfaces. However, chemical costs make this treatment method too expensive [7].
Membrane technologies:
Reverse osmosis and electrodialysis are important examples of membrane process.
The TDS from wastewater can be removed by reverse osmosis [8]. Reverse osmosis is suitable for removing ions and larger species from dye bath effluents with high efficiency (up to > 90%), clogging of the membrane by dyes after long usage and high capital cost is the main drawbacks of this process.
Dyeing process requires the use of electrolytes along with the dyes. Neutral electrolyte like NaCl is required to have high exhaustion of the dye. For instance, in cotton dyeing, NaCl concentration in the dyeing bath is in the range of 25-30 g/l for deep tone and about 15 g/l for a light tone but can be as high as 50 g/l in exceptional cases. The exhaustion stage in reactive dyeing on cotton also requires a sufficient quantity of salt.
Reverse osmosis membrane process is suitable for removing high salt concentrations so that the treated effluent can be re-used again in the processing. The presence of electrolytes in the washing water causes an increase in the hydrolyzed dye affinity (for reactive dyeing on cotton) making it difficult to extract.
In electrodialysis, the dissolved salts (ionic in nature) can also be removed by impressing an electrical potential across the water, resulting in the migration of cations and anions to respective electrodes via anionic and cationic permeable membranes. To avoid membrane fouling it is essential that turbidity, suspended solids, colloids, and trace organics be removed prior to electrodialysis.
Electrochemical processes:
They have lower temperature requirement than those of other equivalents non-electrochemical treatment and there is no need for additional chemical. It also can prevent the production of unwanted side products. But, if suspended or colloidal solids were a high concentration in the wastewater, they impede the electrochemical reaction. Therefore, those materials need to be sufficiently removed before electrochemical oxidation [9].
Ion exchange method:
This is used for the removal of undesirable anions and cations from wastewater. It involves the passage of wastewater through the beds of ion exchange resins where some undesirable cations or anions of wastewater get exchanged for sodium or hydrogen ions of the resin [10]. Most ion exchange resins now in use are synthetic polymeric materials containing ion groups such as sulphonyl, quarternary ammonium group, etc.
Photo catalytic degradation:
An advanced method to decolorize a wide range of dyes depending upon their molecular structure [11]. In this process, photoactive catalyst illuminates with UV light, generates highly reactive radical, which can decompose organic compounds [12].
Adsorption:
It is the exchange of material at the interface between two immiscible phases in contact with one another. Adsorption appears to have considerable potential for the removal of color from industrial effluents [13].
Owen (1978) after surveying 13 textile industries has reported that adsorption using granular activated carbon has emerged as a practical and economical process for the removal of color from textile effluents [14].
Thermal evaporation:
The use of sodium persulfate has better oxidizing potential than NaOCl in the thermal evaporator. The process is eco-friendly since there is no sludge formation and no emission of the toxic chlorine fumes during evaporation. Oxidative decolorization of reactive dye by persulphate due to the formation of free radicals has been reported in the literature [15].
Effluent Treatment Practices:
The textile industry encompasses a range of unit operations, which use a wide variety of natural and synthetic fibers to produce fabrics. Textile units generally follow various treatment schemes. Two of such are shown in scheme 1 and 2 as typical cases.