Ruled out

Sanganer, a small industrial town near Jaipur, is famous for hand block printed fabrics. But for its residents, dyeing units are a serious problem. Says Purshottam Singh: "Sometimes the supplied water is red and at other times bluish. How can we drink this?".

Sanganer's problem is typical of areas where industrial units operate freely. Colour in water is a pointer to contaminants ranging from dissolved organic material, high levels of disinfectants and excess of its by-products and inorganic contaminants like metals. The average hazard limit varies from five to 30 colour units, but colour can be objectionable even at 15 units.

What you see, what you get?
The limits for colour in potable water are based on visual or "aesthetic" parameters. Given a choice between visibly coloured water and a colourless source that may be more unsafe, people tend to opt for the latter. Colour is also determined by a visual comparison with a standard solution (see box: Gauging colour in water).

New drinking water standards prescribed by the Bureau of Indian Standards (is 10500) set colour standards at five colour units as the desirable limit and 25 colour units as permissible limit in the absence of alternate source.

The us Environmental Protection Agency (usepa) lays down 15 colour units as the secondary standard for drinking water in the us . These standards are non-enforceable guidelines regulating contaminants that may cause skin or tooth discoloration or aesthetic effects such as taste, odour, or colour in drinking water.

usepa also recommends secondary standards for water systems but does not require compliance. States, however, may adopt them as enforceable. On the other hand, the Drinking Water Inspectorate of England and Wales has prescribed the colour standard for water supply at 20 colour units. Unlike India and the us, these standards are not only prescribed but are enforceable in the uk .

No fix on industrial discharge
Colour standards for industrial discharge vary. Countries like Mauritius have set up standards for colour discharged from dye manufacturing, textile dyeing and paint units.

In India, however, the Central Pollution Control Board (cpcb) does not stipulate any standard for colour in the wastewater discharged from industries, except relating to dyes and dye intermediates. It just recommends that, "All efforts should be made to remove colour and unpleasant odour as far as practicable." For dyes and dye intermediates, the standard for colour in the effluent that's discharged has been set at 400 Hazen units.

But colour standards are not completely indicative of the pollution in water and effluents that can be caused due to colour. Total suspended solids, total dissolved solids, foaming agents and metals like aluminium, copper, iron and manganese are indicative of colour in water and should also be considered when tracing pollution caused by colour in water bodies.

Similarly, certain dyes in wastewater include toxic metals. The presence of metals in dyes can be caused by the use of mercury or other metals as catalysts during dye making. Mercury, besides being hazardous to health, is a metal that can also cause pollution problems during the manufacturing process. Some dyes also include metals as an integral part of the dye molecule.