Why textile dye is so hard to remove from waste water
During the dyeing process, the dye is dissolved into the process water and it is still there when the process water later is released as effluent. The effluent contains high content of dyestuff, surfactants and other additives that are generally made up of organic compounds with a complex structure.
The dissolved dye compounds of the effluent are resistant to light, acids, bases and oxygen, as these are the desired properties of the dyed clothes. It’s therefore difficult to treat textile wastewater with conventional methods as these organic compounds have poor bio-degradability.
The main environmental concern of textile wastewater lies in the dissolved organic dye compounds as some of them are aromatics and considered carcinogenic.
How to remove dye from textile industry wastewater with ozone
With high concentration ozone it is possible to break down the dissolved organic dye compounds and thereby reduce the amount of colour and enhancing the biodegradability of the wastewater. The most efficient way to remove dissolved dye is using a combination of biology, filtration and ozone.
Ozone has been used successfully for removal of colour from textile wastewater streams in plants around the world as well as in other industrial wastewater processes. In wastewater treatment, ozone is often used in conjunction with biological treatment systems such as activated sludge. Organic dyes are mostly refractory due to their large molecular size and they can not be easily removed by adsorption on activated sludge. In some cases ozone has been used before the
biological process, but mainly after biological treatment. If the wastewater is hardly biodegradable or toxic, activated sludge pre-treatment is an option. In textile wastewater processes, a 20-30% improvement in the action of the biological system has been observed.
Ozone is effective in removing the colour from all dyes used in textile processing. The amount of ozone can vary depending on a number of factors: how much colour was removed in the biological process, the type of dye used, where ozone is applied in the process, etc. Knowing the proper amount of ozone required to meet the colour removal objective for the receiving water body is critical to the economics of the ozone system. In general it is not easy to predict the amount of ozone required, so in virtually all cases where specific previous experience is not available, pilot testing is employed.
About 1 mg Ozone per mg dye is required to achieve 95% colour removal, although this ratio varies by dye type. The ratio increases to about 1.5 for 100% removal. Reaction times were on the order of 10 minutes. In the textile industry a typical dosage might be 15 mg/l post biological treatment, but the levels could easily reach 25 mg/l. It is important to note that the ozone dose only needs to make the dye compound uncoloured and not necessarily completely mineralize the material.
- A large textile manufacturer SP Apparels, Tamil Nadu, India, required a very high capacity machine, 7.5 kg (7,500) to treat the effluents from their production process. This was also done successfully.