As consumers are bracing themselves for a sharp rise in water bills, modernisation of London's Victorian water networks has become inevitable.
Sewage is treated and turned into electricity
In London, more than half of the water mains are more than 100 years old, and around a third are over 150.
The ageing pipes do not seem to affect the quality of water, but the problem is that they leak.
Thames Water has recently proposed a £4bn investment programme to upgrade the city's oldest water mains and sewers.
In the past, explains Thames Water's Chris Shipway, the focus has been on water treatment.
"That has led to very high water quality and very high quality of treated water being returned into rivers after the sewage treatment process," he added.
"Now the emphasis needs to change and we need to concentrate on the infrastructure: the pipes that carry the water to the homes and the sewage away from it."
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
The company says that replacing water mains in London is its top priority.
The aim is to reduce leakage, a problem concentrated in north London, where corrosive clay soils, combined with heavy traffic, have damaged the pipes.
"Reducing leakage reduces the amount of water that's wasted so we can deliver a more efficient supply," he said.
The 'Great Stink'
London's water and sewerage systems were first developed in Victorian times.
Until the second half of the 19th century, Londoners were still drinking water from the same portions of the Thames that the open sewers were discharging into.
After several outbreaks of cholera, it was in the wake of the Great Stink of 1858, when the stench from the Thames caused the parliament to go into recess, that authorities finally took action.
The construction of the central London sewer system was overseen by Sir Joseph Bazalgette, and started in 1859.
In 1859, he used 318 million bricks to create a series of underground tunnels to transfer London's sewage to the east of the city.
The sewage is now processed and turned into electricity at treatment works in Beckton and Crossness.
Dealing with the city's sewage has become increasingly complicated over the 150 years.
Maintenance works are often prompted by the need to comply with EU regulations.
"Many of the Brussels directives are to do with standards of discharge from sewage treatment works, for instance," said a spokesman for Water UK, which represents water and wastewater service suppliers.
"The work that the industry is doing to help meet European directives is enormous and has been very successful."
Climate change has also had an impact on the aging system.
Flash floods are increasingly frequent, and reducing the number of homes at risk from sewage floods is a priority.
London has a combined sewer system that deals both with water flushed from homes and incoming rainfall from drains.
In addition to that, more and more green spaces have been concreted over.
There is less grass to soak up rainfall, and concrete means water runs off much more quickly and straight into the sewers.
A plan submitted by Thames Water to water industry regulator Ofwat a few months ago proposed spending £ 520m to reduce the number of homes at risk.
"We need to build underground storage tanks to increase the capacity of the sewer network so it can cope with heavy storms," Chris Shipman said.
"We also need to make changes to the size of pipes and to pumping stations," he added.
However, the ageing mains do not have an impact on the quality of water.
Extremely high quality standards have been confirmed for the third year running, the Drinking Water Inspectorate said.