Discolouration of water

When water leaves our supply works it is clear, bright and free of unwanted particles. This page explains why sometimes water may be discoloured when it reaches your taps and what you can do to clear it.

We test it rigorously to make sure it stays this way, however, the water can become discoloured before it reaches your tap.

The two most common problems are when the water becomes brown or when it becomes cloudy and white. 

Both happen after the water has left the treatment works, either in the mains distribution system or in domestic plumbing.

Milky, chalky or bubbly discolouration is caused by tiny bubbles of air. It is quite harmless and the water will clear if you leave it to stand.

Air can enter water in the distribution system, particularly when mains have been drained to carry out repairs, but more often it comes in through a faulty fitting in your property, for example through part of the tap.

The air dissolves in the water under pressure. The pressure is released when you turn on the tap. The air then comes out of solution forming millions of tiny bubbles which give the water a cloudy, white colour.

It is similar to what happens when a fresh bottle of lemonade is opened. These bubbles then rise slowly to the surface and the water clears from the bottom upwards.

Where the water is hard white particles can occur after the water has been heated.

These are harmless flakes of limescale, calcium carbonate, which have formed in the pipework or in the kettle.

This discolouration is almost always caused by rust (iron oxide) which collects on the inside of iron water mains and service pipes which run from the property to the water main.

Rust can be dislodged by a disturbance to the pipe, such as a burst main or rapid changes in the direction or speed of water flow.

The rust temporarily gives the water a brown colour. We work very hard to stop this discolouration with a programme of mains flushing, cleaning, relining and replacement.

Stains may occur after dishwashing where the detergent has not been rinsed off properly or they may be caused by the salt used to soften the water for rinsing.

A rinse aid recommended by dishwasher manufacturers may stop the problem.

An oily film may form on the surface of boiled water if you have galvanised iron pipes and fittings. Very small, flat crystals of a zinc compound are formed and then float on the surface.

These form a shimmering layer which has nothing to do with oil. 

However, they do show that either you are filling the kettle without first flushing the pipe or that the galvanised pipes in the property are in need of attention.