A £100MILLION, seven-year upgrade is planned for Glasgow's sewers.

Scottish Water plans work to improve the quality of water returned to the Clyde and its tributaries.

And it will also provide badly needed extra capacity for future developments across the city.

The company says it is too early to discuss what the project will entail.

But it is understood one option could involve a three- mile long shallow tunnel under roads and rail lines.

Another option being considered is a deeper tunnel which would require a pumping station to be built.

Depending on the final choice, the 4.5m diameter tunnel will be constructed at depths of between 10m to 30m. It is likely to run across an area of old mine workings raising potential technical problems.

According to a trade journal, the project will take around seven years to complete from the time the contract is awarded.

A Scottish Water spokesman said: "We are investigating the provision of a waste water transfer and storage system – a sewer or network of sewers which holds and conveys waste water – as part of planned investment to improve our waste water infrastructure in Greater Glasgow."

Waste water from toilets, wash hand basins and kitchen sinks travels through underground pipes to a treatment plant before it is cleaned and put back into the Clyde.

The spokesman added: "The improvements will help protect and enhance the natural environment of the River Clyde and its tributaries, will help tackle flooding and the effects of climate change and will provide Greater Glasgow with a sustainable waste water system fit for the 21st century.

"We are working to determine the feasibility of a number of options as part of our planned investment.

"One of these options could be a proposed sewer tunnel which would be constructed to alleviate the pressure on some of the problem areas in Glasgow's drainage network and provide additional storm water storage.

"It is too early to say whether this option will be chosen but none of our planned work will interfere with the Glasgow 2014 Commonwealth Games.

"Details of all our plans will be released once our investigations have been concluded."

vivienne.nicoll@ eveningtimes.co.uk