One of the current overflow sites in London. Picture: Tideway Tunnel.

The 25-kilometre Tideway Tunnel will follow the Thames and is designed to catch overflow from the system that currently pumps sewage into the river on a weekly basis that adds up to “tens of millions of tonnes” each year.

Under the new plan, 22 huge shafts up to 70 metres deep will funnel sewage down to the tunnel below where it will be carried to a treatment plant

Deputy delivery manager of the central section of the site, Malcolm Orford, said in some cases the vast “plumbing job” will mean dropping 50 tonnes of sewage per second from 14 storeys high.

“That’s a truly phenomenal amount of energy,” he said. “That is not dissimilar to the same kind of flow in the whole River Thames on a summer’s day ….it is enormous.”

The vast volume means engineers will angle the sewage flow to ensure it spins in a vortex, similar to the way water drains down a plug, so it doesn’t “smash everything”.

‘Rag’ is the stuff fatberg’s are made of. Picture: Tideway TunnelSource:Supplied

The new tunnel will intercept existing sewage pipes before it reaches the river. Picture: Tideway TunnelSource:Supplied

New tunnel. Picture: Tideway TunnelSource:Supplied

Old tunnel feat. a giant fatberg.Source:Supplied

The engineer said they have also explored the idea of an energy recovery system but found the “lumpy” material and warm underground environment meant it wasn’t practical.

“Because it’s dry and warm and occasionally gets a really good soaking of nutrient rich sewage it just grows algae like you wouldn’t believe and so it all gums up. So sadly we can’t install energy recovery,” he said.

It's a fascinating insight into the level of engineering expertise that has gone into creating the country’s largest industrial water project that is set to eliminate the overflow of sewage into the Thames.

Currently, the city of eight million people relies on a Victorian era network of sewers designed by Sir Joseph Bazelgatte in 1868 after the “great stink” in the city.

Mr Orford said the current system, which uses the Thames as an overflow outlet, is still in “amazing condition”. However rapid population growth and urban development means the river is flooded with sewage once or twice a week rather than once or twice a year. Tidal flow means it can remain trapped washing back and forth for months.

The vast tunnel, which will fit two double deckers inside it, has been planned for 17 years and is due for completion in 2024. It’s hoped it will serve the city for the next 100 years.

It won’t solve the fatberg problem, that has to sometimes be manually dug out by “flushers”.Source:news.com.au

The tunnel bore used to drill through rock travel about 100 metres per week. Picture: Tideway TunnelSource:Supplied

Where the 25km tunnel will run. Picture: Tideway TunnelSource:Supplied

At present, vast tunnel bores have been lowered into the ground from three major sites where they drill the tunnel in sections at a rate of around 100 metres per week.

It’s hugely complex work dodging the city's labyrinth of underground rail lines, utility cables and archaeological sites, not to mention an unexploded bomb from World War 2 found recently in the river.

As for the notorious “fatbergs” lying underneath London that block pipes and horrify residents, Thames Water spokesman Miles Evans said the super sewer won’t do anything to ease the “perfect combination” of sanitary products, wet wipes, oil and grease that cause blockages in the first place.

“The whole network of sewers stays the same. Thames Water still have to go and clear those blockages. We have teams working around the clock keeping London flowing,” he said.

“If it’s particularly bad we have to send people in to manually shovel it out. They use high pressure water jets to flush it through.”

Visit the Tideway Tunnel website for more information.