Thames Tideway Tunnel
As the UK’s largest water supplier, it’s our responsibility to protect our major water source, the River Thames. By improving our sewage networks, we can promote a healthier river environment.
One of our largest improvement projects is the Thames Tideway Tunnel, or super sewer.
Delivered by Tideway, this 25km long tunnel has been bored under the River Thames. At 7.2m wide, it's the width of three London buses. This extra capacity will help the sewer cope with the growing demands of our population and environment.
What the Thames Tideway Tunnel will do
The Thames Tideway Tunnel will collect sewage so it can be treated before entering the river. First, it will pipe sewage to Abbey Mills. Then it will join the Lee Tunnel, to deliver all the collected untreated sewage to our Beckton works. Beckton will treat the sewage and return clean water to the environment. This will make sure the river is clean, sustaining a rich, diverse ecology for London. We can use the sewage waste to generate more renewable energy, too.
Over the last 30 years, we've seen a dramatic clean-up of the Thames. The raw sewage that overspills each year is its last major source of pollution. The new tunnel will improve the river's appearance and provide habitats for aquatic wildlife. It will also reduce the amount of sewage-related litter in the Thames. In turn, this will reduce the amount ingested by wildlife.
The tunnel will capture all the ‘first flush’ from the sewers after heavy rain. This contains sediment built-up during periods of dry weather and causes the most damage. Instead of over 50 sewage spills a year, there will only be around four. These will mostly contain surface water runoff after heavy storms.
Why we need it
Designed by Sir Joseph Bazalgette, London's original sewerage system was built between 1859 and 1875. As a combined sewer, it captured rainwater runoff as well as waste from a population of four million.
To prevent sewage backing up and causing flooding, Bazalgette designed combined sewer overflows (CSOs). At times of heavy rain, the sewer's contents could overflow via CSOs along the banks of the river.
Since then, London's population has doubled and is still growing. Climate change is causing extreme weather, including periods of prolonged, heavy rainfall. These demands cause the CSOs to release sewage into the river more frequently than planned. To keep up with these changes, we need to upgrade the system.
The implementation plan for these upgrades was created by the Thames Tideway Strategic Study in the early 2000s.
The plan
The Thames Tideway Tunnel project is a three-part scheme. The first two stages were completed by Thames Water and are already delivering the desired environmental benefits.
First phase
First, we upgraded five Thames Water sites; Beckton, Crossness, Mogden, Riverside and Long Reach. This resulted in a 40% reduction of storm discharges to the River Thames.
Second phase
In 2016, we built the Lee Tunnel. It intercepts the Tidal Thames' largest combined sewer overflow (CSO) at Abbey Mills.
The Lee Tunnel is a 7.2m diameter, 7km long tunnel. It runs from Abbey Mills pumping station in Stratford, to Beckton sewage treatment works in Newham. This resulted in another 15% reduction.
Third phase
The third and final part of the London Tideway Improvement scheme is the Thames Tideway Tunnel. Construction started in 2016 by a separate company called Bazalgette Tunnel Ltd (also known as Tideway). It's due to enter commissioning in 2024. It will begin to intercept CSOs along the River Thames from Acton in West London to Abbey Mills in East London.
When complete, this third and final stage will reduce CSO overflows along the Thames by 40%. This 40% reduction will add to the 55% already achieved by the first two phases delivered by Thames Water. In total, the volume discharged will be reduced by 95%, depending on the weather.
Learn more about the benefits of the Thames Tideway Tunnel.
Paying for the Thames Tideway Tunnel
The Thames Tideway Tunnel will cost £4.1bn to complete. We've spent an extra £1.1bn on preparatory works. The cost of the project is being paid for by our 15 million wastewater customers, through their bills.
We'll continue working as efficiently as we can, to keep customers’ wastewater bills as low as possible.
Take a look at our other investment projects to see what else we’re doing to improve our network and waterways.
Preparing for the Tideway Tunnel
We carried out studies in the Channelsea River and surrounding watercourses. This helps us to understand the existing ecology, including water quality and fish populations. It also establishes a baseline for water quality, before connecting the Tideway Tunnel.
The results found:
- Water quality is improving
- 14 species of fish in the Channelsea River
This indicates the river is healthy and supports a wide variety of fish and other wildlife. It can be considered a good quality urban river system.