Effluent Parameters & Ways to treatment (Detailed Guide)
Optimizing procedures for the treatment of dye-containing wastewater is an enormous task, made extremely complex by the hundreds of dyestuffs commercially available in the market. Dyes among various classes e.g., azo, anthraquinone (reactive/vat/disperse) exhibit different chemical characteristics & hence respond differently to treatments aimed at their removal. The complexity of textile dye wastewater prohibits a universal effluent treatment system. Detailed wastewater characterization is an integral part of determining the wastewater methods to be applied. The quantification values of impurities and hazardous ingredients are governed & part of individual geographic area’s legislation with frequent modifications and so not depicted.
Dyehouse effluents are generally discharged into local authority sewer lines/rivers or any other line (e.g., centrally coordinated effluent system), but in all these modes of discharges, there are certain permissible limits of impurities and toxicity. Dyehouse effluent undergoes great variations in content and ph. It is therefore necessary to discharge into a primary storage tank to obtain comparative uniformity by randomization. It also helps in the sedimentation of some solids. Excess acidity or alkalinity is corrected by a dosing mechanism, which is controlled by a pH meter. Excess of caustic alkalinity can be removed by using the carbon dioxide in the flue gas from the boiler. By this means alkalinity can be reduced to ph 9 at virtually no cost.
Effluent Treatment Methods
One of the most common effluent treatment methods is that of precipitation. This involves precipitation of insoluble salts, coagulation of colloidal material, and flocculation. The degree of contamination or impurities are examined by some important parameters like,
- COD value: -it is the measure of O2 consumed by organic matter in the water in presence of a chemical oxidizing agent. The point is important because reduction of oxygen in water directly affects aquatic life. It is typical of dye house effluent that COD is higher than BOD. This is based on the amount of potassium dichromate used in oxidizing the organic matter in the wastewater.
- Suspended solids: -Floating insoluble chemicals mainly consists of insoluble oils/solvents, short fibers, insoluble dyes and compounds that have precipitated in the effluent because of change in temperature and ph. The sample of water is investigated after settling. Finally filtered through a gooch crucible and the residue is dried and weighed.
- The ph of the effluent should be preferably between 5 and 9.it can be most conveniently ascertained with universal indicators.
- BOD: – It represents the amount of organic matter oxidisable by the microorganism in the presence of oxygen. Basically, BOD is determined by estimating the dissolved oxygen in the water both before and after incubation at 20 degrees C. for five days.
- Color: -This is visible pollution. The presence of color resists light transmission into deeper levels and prohibits photosynthesis. In general, a dyeing & printing house discharge 9-10% of the dyestuffs it consumed into effluent. Dyes are not easily biodegradable due to its molecular design to with stand various fatness properties. Most dyes are not highly toxic, but the color presence is undesirable in wastewater but can be removed by oxidation or adsorption on sedimentation.
- Toxic chemicals: – For the textile industry the major offenders here are presence of heavy metals such as chromium and copper, organochlorine compounds from moth proofing agents and sulphides from dyeing sulphur colours. The presence of heavy metals directly reduced the ability of biodegradability.
- Synthetic detergents: -Alkyl benzene sulphonates detergents with the branched alkyl chain such as derived from propene tetramer causes heavy foaming because of their low rate of biodegradability. Elimination of branched chain substituents in detergents can make great improvement in foaming problem.
The highest standards of effluent treatment require combinations of different types of treatment, despite the expense; in today’s time, it is more relevant due to regulatory controls to be maintained day in day out.
The reactive dye effluent is associated with the most serious ecological problems arising from a high salt load. So, the fundamental approach to the objective of minimizing contamination quantity in effluents by salts/unfixed dye is,
- Lowering the liquor ratio.
- Avoiding high temperature dyeing. The low temp. Dyeing involves higher reactivity and hence the hydrolyzed dye may reduce in quantity.
- Optimizing the dye recipe.
- Developing novel dyes with low salt consumption and high fixation.
The most initial stage involves the treatment of oxidizing and reducing agents targeted at removal of color and ready for the next stage. Chemical coagulation is also used to treat organic dyes and pigments. The textile plant often uses coagulation and flocculation compounds such as lime, alum, ferric salts, or polyelectrolytes followed by sedimentation. The disadvantage of using coagulation and flocculation is the generation of sludge in big amounts that require disposal. So coagulant is not the final answer for color removal too.
Wastewater Treatment
Wastewater treatment by bisulphite catalyzed sodium borohydride reductive technology is commercialized for the removal of metal cations and use of cationic polymer along with also removing color. Although the actual treatment procedure depends on the specific spices in the wastewater. The general guideline is as under.
- Ph adjustment 5-6
- Bisulphite addition typically0.2 to 0.5 gpl Na2S2
- Borohydride solution addition to an orp-500 to -600 Mv (or until the desired colour reduction is achieved)
- addition of cationic agent –polyamine or aluminum base coagulant
- Carbon adsorption, peroxide oxidation or biological post treatment.
The life expectancy of commercial dyes
(Courtesy Environment Chemistry of Dyes & Pigments by A. Reife & H. S. Freeman-John Wiley & Sons Inc.)
Dye class life expectancy-years
Acid 15
Basic 14.5
Disperse 13.4
Direct 19.5
Reactive 15.4
Activated carbon adsorption is a very helpful method in the reduction of color effluent with various classes of dyes, but it is not proven efficient and is also not economical in practice. To make it efficient and economical it is used in tandem with polymer flocculation, chemical reduction, and biodegradable treatment. Particularly azo dyestuffs are soluble due to sulphonic groups and induced polarity; they are not readily absorbed on tot the no-polar carbon surface. The dyes must be degraded, and their degraded products are easily absorbed.
The technologies that involve reverse osmosis and membrane filtration, adsorption on active carbon, or generation of coagulants by electrochemical techniques are costly propositions.
