United Utilities' has reached a new milestone in the construction of its new £100 million Davyhulme sludge treatment plant.

Two giant gas storage holders in the form of giant plastic bubbles have now been put in place at the site.

The PVC storage bags will be used to capture the biogas produced by the new sludge treatment process. At 20 metres high and 23 metres across the giant spheres are the biggest of their kind in Europe. Each bubble will be able to store 9,000 cubic metres of gas.

The £100 million new Sludge Balanced Asset Programme (SBAP) plant will treat around 91,000 tonnes of wastewater sludge from Davyhulme and sites in Lancashire.

Existing sludge facilities at Davyhulme can meet around two-thirds of the plant's energy requirements but the new improved process is expected to generate enough biogas every year to make the site completely self-sufficient in power with excess to be fed into the national grid. United Utilities expects to be able to reduce its carbon footprint by 32,000 tonnes carbon dioxide (tCO2e) per year compared with existing processes.

Steve Mogford, United Utilities' chief executive, said:

"This is an exciting milestone for the project because it represents the new low-carbon future for Davyhulme. These "space-age" bubbles are where the biogas from the process will be stored before being used to generate electricity and heating.

"United Utilities is showing real leadership by embracing the latest technology for this scheme. It's an excellent example of the very real benefit our record £3.6bn investment is bringing to people and the environment across the North West ."

United Utilities said that when the SBAP plant is finished in 2013 it will put Davyhulme on the map as the "green energy" capital of the North West.

Black and Veatch are contractors on the project.