The characteristics of the effluents from different sections of the mill are given, the characteristics of the different process effluents; vary from mill to mill, as they are directly related to water usage.
The effluents have higher values of pH, total solids, TDS, BOD, COD, alkalinity, etc., than the standard values prescribed by the BIS and the Pollution Control Board.
Pollutional Load Contributed by the Mills:
It clearly indicates that the organic load depends on the volume of the effluent discharged due to the dilution factor.
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Harmful Effects of Textile Effluent:
The untreated effluents, if discharged into streams, cause rapid depletion of the dissolved oxygen of the receiving streams. The condition is aggravated due to the settlement of the suspended substances and subsequent decomposition of the deposited sludges in an anaerobic condition. The alkalinity and toxic substances like sulphides and chromium affect life and also interfere with the biological treatment processes. Some of the dyes have a toxic effect on aquatic life.
The colour, often, renders the water unfit for use for some industrial purposes in the downstream side. Streams containing as little as 25 ppm sodium hydroxide have been reported to be deadly for fish.
Pollutional Control in Textile Industry:
The treatment plant set up, for any effluent control, is considered to be expenditure without return. Considering this fact, it is necessary to introduce techniques whose cost of treatment is minimal. This can be best achieved by reduction of pollution at source.
For this purpose, good house-keeping, closer process-control and process chemical-substitution are the best solutions. Simultaneously, these steps minimize the cost of treatment. It is also essential to educate the working staff so that they develop a sense of concern for pollution control and realize that it will be in their interest, too.
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By closer control of processes, the pollution load can be reduced by 30-40 percent. For this purpose, the following measures should be implemented: Reduction in chemical usage :
1. In the finishing operation, the starch is used in large quantities. The unutilized eventually joins the effluent stream. Its amount should be optimized though it is estimated that it will always remain somewhat in excess of what is required. Obviously, this excess should be diverted to a different route. In summer, it should be sun-dried and in winters, it should be evaporated and the solid material, thus, obtained should be disposed of as solid waste.
2. In bleaching operations, chlorine and peroxides are used. They should be mixed with dyeing effluent in order to decolorize it.
3. In kiering mercerizing processes, caustic soda is used in large quantities. It should be recycled by readjusting its concentration.
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Recovery and Re-use of Chemicals: Many chemicals can be recovered and re-used, e.g., caustic soda, starch, etc.
Process Modification: Certain changes in processes can reduce the amounts and concentrations of waste materials, e.g., change in dyeing method (use of standing bath).
Flow or Volume Reduction: The waste flow may be reduced by the re-use of process water, counter flow wastes, segregation of weak waste for disposal without treatment, changes in processes and other similar means.
Process Chemical Substitution:
The pollution load in the textile industry can be reduced effectively by process chemical substitution. The following substitutions are suggested:
1. Substitution of low BOD synthetic detergents for soap.
2. Substitution of low BOD sizes, e.g. carboxy methyl cellulose, hydroxy methyl cellulose, polyvinyl alcohol, etc., for high BOD starches.
3. Replacement of acetic acid in dyeing with an inorganic salt, e.g., ammonium sulphate or chloride.
4. Use of low BOD dispersing, emulsifying, leveling, etc., agents in place of high BOD agents.
5. Reduction in the use of caustic soda in kiering.
Ganapati and Masselli have reported that the total pollution load of the textile mill can be reduced to a large extent by process chemical substitution.
Souther has also made some suggestion for the abatement of pollution in the textile industry, e.g. process chemical substitution, closer process control and good housekeeping.
Masselli etaf have emphasized certain preliminary practices to reduce the quantity and strength of the textile effluent. They also observed that closer control of cotton kiering, sizing and the amount of chemicals used in various other processes may reduce the pollution load up to 30 percent. They have also found process chemical substitution to be an effective means for pollution abatement.
Treatment Methods:
Suitable combinations of chemical and biological methods briefly described here are considered essential for proper treatment.
Chemical Treatment:
Neutralization and chemical coagulation are the main chemical methods used for the treatment of textile effluent. Alum, ferrous sulphate, ferric sulphate and ferric chloride are generally used as the coagulant for the reduction of BOD and colour from the effluent.
Sulphuric acid or lime is used for pH control. Each coagulant, when used with a specific waste, has its own optimum isoelectric point (pH for maximum coagulation). Some coagulant aids are also used in conjunction with the coagulants.
Biological Treatment:
The trickling filter, activated sludge and dispersed growth aeration are considered suitable for this purpose. Textile effluent has also been treated successfully by the stabilization pond method, in combination with sewage.
Trickling filtration is generally desirable due to its flexibility, lower operating cost, and capability of handling shock loads of effluents. Activated sludge treatment leads to greater BOD reduction, but requires large units to provide the long detention periods usually needed and also requires highly qualified supervision.
Dispersed growth aeration generally gives a somewhat lower BOD reduction than activated sludge treatment but does away with the sludge problem. It also takes a minimum of operation and maintenance.
Dietrich found that mixtures of electrolytes containing Al3+, Fe3+, Se, Si4+ and Ca” are more effective flocculating agents than those containing only Fe3+ cations. When the mixture is applied to textile plant sewage at 50 mg/l, it successfully reduces the total pollution load. Nemerow enumerated chemical coagulation, trickling filtration, the activated sludge process and dispersed growth aeration for the treatment of textile effluent.
In conclusion, a combination of physical-chemical followed by biological treatment for the treatment of textile effluent is considered the most suitable.