United Utilities is about to start a multi-million pound deep clean of a maze of underground water mains running east to west across Lancashire.

The zigzagging pipework lies underneath Longridge, Preston, Kirkham and Blackpool, fed from the huge 27" Hodder Aqueduct, which underwent its own £70 million refurbishment last year. Between them, the jungle of pipes, supply drinking water to more than 30,000 household taps daily.

Warrington based United Utilities, which serves 7 million customers with clean water daily across the North West, said specialist cleaning teams would work at night starting in Longridge in March, moving through Preston's Fullwood district, into Kirkham finishing seven months later in Blackpool by September 2010.

Ray Melia, Project Engineer for the mammoth cleaning job said:

"Last year we spent around £70 million cleaning the large 27" Hodder Aqueduct, which is over 90 years old itself, and whilst the Hodder and the feeder pipes have been cleaned before, we need to carry out ongoing maintenance to remove deposits that naturally build-up inside them.

"We use a technique called Robust Flushing. At various controlled opening points on the water network, we systematically flush short lengths of water main, and with the aid of pressure already inside the water main, we can remove all the deposits”.

The utility firm said that harmless sediment had settled at the bottom of the pipework over the years, and flushing would remove sediment at various flushing points that have been identified in parts of Longridge, Preston, Kirkham and Blackpool.

The Hodder large diameter trunk mains system consists of two gigantic 27" and 36" cast iron pipes running side-by-side, which run from Hodder water treatment works and Stocks reservoir near the Forest of Bowland to the Fylde coast. Some sections of the pipes are big enough for an adult to stand up in.

Between them, the huge underground pipes supply up to 219 million litres of water a day to 850,000 people in Lancashire. The first 27" diameter pipe was laid in the 1920's and the 36" diameter pipe in the 1950's. Since the pipes were put in place deposits of iron and manganese which occur naturally in raw water from upland reservoirs have built up on the inside of the pipes. Although not harmful to health, the deposits can result in discolouration of drinking water.