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Pollution of Waterbodies

Treating textile wastewater can make a difference

A.K.M.A. Quader
THE textile processing industries produce wastewater which is one of the main sources of pollution of the rivers, canals and wetlands in and around Dhaka as well as many other places in the country. Effluents from these industries contain suspended solids. large amount of dissolved solids, un-reacted dyestuff and other chemicals that are used in different stages of dyeing, fixing, washing and other processing.
The textile wastewater treatment technologies aim at achieving color removal and reduction of Total Dissolved Solids (TDS), Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD) and pH correction. Treatment processes include physical, chemical and biological methods; and they have merits and limitations. To meet the environmental requirements, researchers have been making efforts to use a combination of two or more methods for producing an environmentally acceptable effluent from the treatment facility. Combination of chlorination and chemical coagulation is reported to meet the environmental requirements for textile wastewater treatment.
Chlorination of wastewater
was practiced for reducing BOD as long ago as in 1859 in England and this was effectively employed for treating industrial waste water in Germany and USA. Because of high cost of chlorine, the industry then looked for alternative chemicals and treatment system which has led to chemico-physico treatment process requiring more equipment and increased investment and operating costs.
Chlorine is also a raw material for large number of more valuable chlorinated organic and inorganic products. However, where chlorine is cheap or available as an otherwise unusable product, its use in the treatment of wastewater can be a viable and practical option.
Sources of chlorine can be either chlorine gas in cylinder or in-situ electrochemical chlorine. The aqueous chlorine as a chemical oxidant produces one atom of oxidizing power per mole of chlorine. Thus 71 kg of Chlorine will deliver 16 kg of Oxygen for the desired oxidation process. In spite of the complexities of chlorine oxidation process, chlonne offers the advantage for its ability to destroy colours, odours, organics, inorganics, cyanide, phenols, detergents etc. Moreover, chlorination renders treated water disinfected.
The Department of Environment (DoE), as per the Environment Conservation Rules-1997 (ECR-1997), has set the following standards for the effluents to be discharged from composite textile plant and large processing unit.
DoE has not yet established methods/procedures for measuring these parameters. The standard does not specify any value/standard for dissolved oxygen (DO), COD, free chlorine and colour. For aesthetic reason, the treated water shall be colourless and clear and odourless. If the limit for BOD is 150 ppm, the effluent is unlikely to have significant DO at the point of discharge.
The organics present in the wastewater can react with dissolved chlorine forming chloro-organo compounds.
The reaction depends on the type of organics (alkanes, alkenes etc), pH and availability of chlorine. Efforts are made to minimise the formation of these compounds. Chlorination of drinking water or treated wastewater for disinfection has the same limit and still recognised as the most effective means for disinfection.
Why this process?
With the passage of the Environment Conservation Act in 1995 and the notifica-tion of Environment Conservation Rules in 1997, the Department of Environment, Bangladesh has since then pursuing the textile sector in particular to build and operate wastewater treat ment plants.
In view of the investment cost of an ETP (Effluent Treatment Plant) plus its operating expenses for chemicals and aids which are imported items, the industry has been responding slowly. The wastewater treatment method based on chlorine gas if adopted by this sector, the situation can change in favour of ETP operation as the process is simple and can be operated at a relatively low cost because of low cost of chlorine in the country.
Functions of chlorination such as: elimination colour and odour, reduction of BOD and COD, oxidation of metal ions, destruction of cyanides
and phenols, removal of scum, grease and detergents, and control of foams and flies plus disinfection serve all the objectives of the treatment of textile wastewater composition of which is not as wild as sewage. The handling of chlorine gas may appear risky, but its wide use in water treatment in hundreds of locations by the process industries and WASA assures that it can be handled safely by following related procedures.
Treating textile wastewater with chlorine gas
For the past six months from November 2008, the textile wastewaters have been treated with chlorine gas in the existing ETPs at a number of textile composite industries around Dhaka. The ETPs have been slightly modified to introduce chlorinated water from the chlorine dispenser. The ETPs have been producing effluents that meet the requirements of ECR-97(Schedule 12-B) according to the tests and analysis performed at the reputed labora-tories including DoE. Operating experiences reveal that the cost of chlorine consumed is very low compared to the chemicals and aids used previously.
Win-win situation
The priority and emphasis today are to make the industry treat its wastewater. The objective is compliance by the industry to achieve the standards for effluents by operating an ETP. DoE's performance is to be judged by how many industries operate ETP for treating their wastewater and to what extent the set standards are achieved and thus water bodies preserved.
This effective but cheap approach based on chlorine gas provides an opportunity for both the industry and DoE to perform in a win-win situation. With true and honest commitment to serve the
nation and its people by ensuring a clean environment, both the parties should strive hard to operate ETPs for complying with ECR-97 requirements. The textile wastewater treatment with chlorine gas can make all the difference by doing something positive rather than being smart by doing nothing purposefully.
DoE, Bangladesh created in 1995 now working with two gazette notifications. Whatever, we expect that DoE will come forward to support all endeavors to push for effective and efficient use of ETPs in textile industries and thus protect and preserve our environment, especially water bodies.
Dr. AK.MA. Quader is Professor of Chemical Engineering. BUET, Dhaka