G. Gurumurthy

The direct contact evaporation technique increases the heating efficiency or the fuel value up to 85 per cent.





Coimbatore , Sept. 27

WITH handling of textile effluents still remaining a hot topic in the region, an environmental engineering consultancy group from Tirupur has come out with a technology solution that seeks to treat the spent dye (dye bath), the core pollutant in wet textile processing, using hot gas.

The consultancy group Neelakantaa Pollution Preventers (NPP) of Tirupur, headed by an NRI chemical engineer Mr S.R. Ramaswamy, says its technology targets the spent dye bath, through "the direct contact evaporation process" that eliminates collection of sludge from the effluent treatment cycle itself.

The spent dye bath accounts for 70 per cent of the pollutants in textile effluents.

(This is in contrast to the existing practice of removing the colour from the effluents using nano filtration and separating salt/dyestuffs and pure water from dye-bath using the reverse osmosis system. The higher wear and tear of the membrane encountered by the users of the reverse osmosis for treating the dye-bath is considered a deterrent in managing the hosiery effluent.)

The direct contact evaporation technique, according to Mr Ramaswamy, envisages use of hot gas generated at 800-1000 degree Celsius against the mass of spent dye bath or the left-over concentrated dye solution that comes out as reject after the completion of the dyeing process.

The spent dye solution comprising the residual dye-stuffs, salt (sodium chloride or the common salt) and soda rejects, when left untreated becomes sludge and thereby poses disposal problems.

A good portion of this cold spend dye bath when it comes into direct contact with the hot gas gets evaporated leaving the rest of the wastes such as the salt, soda and dye chemical to crystalise into a coloured salt slurry.

This slurry is then centrifuged and sent to a roaster to burn off the coloured chemicals stuck with the salt residues. This de-dyed crude salt is further treated with hydrochloric acid to neutralise its soda content and converted into pure salt using a crystalliser.

The main advantage of this technique, according to Mr Ramaswamy, is that the gas required for burning the spent dye bath into steam could be generated by using the same fuel the dyeing units would use for the heating the boiler. The direct contact evaporation technique increases the heating efficiency or the fuel value up to 85 per cent. . This way, it paves way for total elimination of the textile sludge in effluents and at the same time ensuring the `0' discharge by allowing 100 per cent recovery of water and salt.

As for the cost for setting up direct contact evaporation treatment plant, the environmental consultancy group says that the initial investment cost would work out to Rs 300 per litre of dye bath and a plant to handle 10,000 litre capacity of dye bath would be Rs 30-40 lakh. However, Mr Ramaswamy is of the view that investment cost would come down with higher capacity. The cost for a plant to treat one lakh litre of dye-bath per day is estimated to come down to Rs 100 per litre or Rs 1 crore for the project.

The NPP has commissioned a pilot plant to treat 10,000 litre dye-bath per day at a private dyeing house in Sultanpet near Tirupur, which according to the NPP members performed well.

Ms Vasundhra Devi, Director, of NPP, says that the cost of investment made for the plant could be recovered in six years.