Fat solidifies in the sewers

The severity and regularity of flash floods is expected to increase as more of London's green spaces are concreted over and the population rises.

Thames Water said although the basic system was sound, new investment was needed to maintain and modernise it.

Work to prevent sewage flooding during heavy rainfall is planned by the firm.

In 1859 Sir Joseph Bazalgette used 318 million bricks to create an underground grid to transfer London's sewage to the east of the city.

PROBLEMS ON THE SEWERS About 100 tonnes of fat, oil and grease are dumped in London's sewers each year Half the 100,000 blockages Thames Water clears each year are caused in this way, which costs about �7m to remove "Flushers" once cleared a 150ft solidified slug of hardened fat with pick axes from sewers in the Leicester Square area - it took them eight weeks

It is now processed and turned into electricity at treatment works in Beckton and Crossness.

London's unusual combined sewer system deals both with water flushed from homes and incoming rainfall from drains.

Thames Water says climate change is expected to increase the regularity and severity of flash floods, placing greater demands on the system.

This will be compounded by the vanishing areas of green spaces able to soak up rainfall and a 800,000 population increase predicted by 2016.

Up to �500m is already being invested by Thames Water to tackle the risk of properties being flooded with sewage when rainfall is heavy.

The firm also spends �7m every year removing solidified cooking fat from sewers which is a particular problem in Soho, central London, where a 150ft slug of fat took eight weeks to remove recently.

On Wednesday, Environment Minister Elliot Morley visited the 1,300-mile network of brick tunnels and cathedral like chambers that run under the city.