The Vyrnwy Aqueduct, a huge pipeline which send millions of gallons of water a day across Shropshire, is being given a £60m clean up.
United Utilities said the project would take 10 years to complete.
The Victorian-built aqueduct, which passes close to Oswestry, helps send water from Lake Vyrnwy in Powys, Wales, to Liverpool.
Work is to begin in the autumn, with an exhibition being held about it in Oswestry, on 11 August.
The aqueduct serves about 900,000 customers in north-west England.
The aqueduct can carry 210 million litres (46 million gallons) of water a day.
A spokesman said the clean up needed to be done because of water quality.
"The aqueduct between Oswestry and Prescot in Liverpool is actually made up of three pipes, each up to three-and-a-half feet in diameter," he said.
"The two oldest date back to the 1880s and are made of cast iron."
These can fur up on the inside, he said, and harmless deposits of iron and manganese can get into the water.
The deposits cause discolouration which, he said, customers do not like.
The first phase of the scheme will clean the section of steel pipe between Oswestry and Malpas and will take about a year.