The bacteria absorb iron in order to survive and grow. In so doing they reduce the iron concentration in water a hundredfold, from 2 micrograms per litre down to 0.02 micrograms. The EC standard is 0.2 micrograms.
The bacteria have been in service at the Roydon treatment works in Essex since February. It uses water with a high iron level from an underground supply and serves 140,000 people in Harlow and the north-eastern edge of London. Its quality compliance manager, Ian Watson, said yesterday that in their first six months, the microbes had performed reliably. The bacteria grow in sand filters through which the water trickles. When their population reaches a high level the flow of water is temporarily reversed, washing out most of them. These are then removed; meanwhile the few remaining microbes stuck on grains of sand in the filter begin to multiply again.
Iron can also be removed by using chlorine or bubbling large quantities of air through the water. But when Three Valleys came to refurbish its ageing Roydon works, bacteria were chosen because it estimated they were the cheapest option.
Some chlorine is still added during treatment to kill bacteria which, unlike the iron-absorbing types, can infect humans.
Three Valleys is owned by the French company Generale des Eaux, along with several other smaller UK water companies. Bacterial removal of iron is already used in France but is new in Britain.
A water company is to give its three million consumers a say in the level of their bills. Welsh Water yesterday became the first of the big 10 water concerns to produce an annual report for its customers after criticism of the latest round of increased charges.
It will spend pounds 1m - 0.5 per cent of its annual operating costs - to find out the customers' views on its investment programme.