The Thames Tideway Tunnel is a deep-level sewer along the tidal section of the River Thames in London, running from Acton in the west to Abbey Mills in the east, where it joins the Lee Tunnel which connects to Beckton Sewage Treatment Works. The tunnel is designed to capture almost all the raw sewage and rainwater from combined sewers which would otherwise overflow into the river during heavy rain. The sewage can be stored in the tunnel until it can be treated at Beckton.
Bazalgette Tunnel Limited (BTL), backed by investors Allianz, Amber Infrastructure, Dalmore Capital and DIF, is the licensed infrastructure provider for the tunnel's finance, building, maintenance and operation. On 3 November 2015, the license award was made by Ofwat, ensuring the start of the project. Since then, BTL also trades as Tideway.
Construction of the Tideway Tunnel began in 2016 and the project was due to be completed by 2024, but the COVID-19 pandemic delayed this to early 2025. The first sewage flowed into the tunnel in September 2024. It became fully operational in February 2025, and was officially opened in May 2025.
The estimated capital costexcluding financing, operations and maintenancewas £3.8bn with an additional £1.1bn for preparatory works. Due to the COVID-19 pandemic, additional costs of £233m were incurred. The 2021-22 annual report gave an updated cost of £4.3bn. The final cost was £5bn.
The main tunnel has an internal diameter of and runs at a depth of between at the western end, and in the east. The tunnel drains 34 of the most polluting combined sewer overflows and is expected to lead to the overflows operating for 3.7% of the time on a maximum of four days per year at the time of commissioning.
Background
Built between 1859 and 1875, Sir Joseph Bazalgette's original London sewerage system was designed to capture both rainwater and the sewage produced by four million people. As a failsafe, to prevent sewage backing up and flooding people's homes, Bazalgette's system had the ability to overflow into the Thames via 57 combined sewer overflows (CSOs) along the banks of the river.
In 2001, London's population density was 18,457 people per square kilometre, compared to just 6,825 per square kilometre in Bazalgette's day. As the population grew, so did development, and this altered the natural drainage of the sewerage catchment so that more rainfall flowed into London's sewers, rather than being naturally absorbed into permeable ground. A permeable area twice the size of Hyde Park was lost to hard surfacing every year up to 2001.Daily Telegraph 8 June 2011
In the 2011 census returns Greater London hosted just over 8.1 million residents and Thames Water in a leaflet of that year explained how its works and how sewers needed expansion. The older inner boroughs have combined sewerage systems which cannot cope with the extra hard standings. As London continued to grow the new developments and suburbs were equipped with separated foul water and rainwater drainage pipe work. Extending such separate systems would stop sewage flooding of homes and businesses, as well as into the rivers and the estuary, would reduce carbon emissions, and be cheaper in the long term, than the multitude of pumped water tanks otherwise needed to stop homes flooding plus the Tideway tunnel. Pipes would be laid in a number of streets in each borough each year over say twenty years, limiting traffic diversions. By 2011 overflows into the river took place more than once a week and as little as 2 mm of rainfall could trigger a discharge. The mean total of this was , that is 39 million tonnes, of storm sewage entering the river.Thames Water Needs report (Summer 2010) accessed: 16 January 2014 These discharges, a mixture of raw sewage and rainwater, needed to be cut to comply with the EU's Waste Water Treatment Directive (UWWTD). If cut to such levels, it would restore the ecology of the river to that of 1865–70 or better, that is, before such growth took place.
Options assessment Instigated in 2001, the Thames Tideway Strategic Study,Thames Tideway Strategic Study – Executive Summary (2005). Accessed 17 January 2014 conducted by a group comprising Thames Water, the Environment Agency, DEFRA and the Greater London Authority, was intended to assess the impact of the CSO discharges into the Thames and to identify objectives and propose potential solutions, while keeping costs and benefits in mind.
Outcomes After four years, the Thames Tideway Strategic Study report was published in 2005, and outlined the following objectives: To protect the ecology of the Tideway; To reduce aesthetic pollution due to sewage-derived litter; and Protect the health of recreational water users
Potential strategies Four potential strategies were discussed: Adoption of source control and sustainable urban drainage; Separation of foul and surface drainage and local storage; Screening, storage or treatment at the discharge point to river; and In-river treatment
After evaluation of alternatives it was decided that only one strategy, the screening, storage or treatment at the point of discharge, would meet the estuary objectives fully. The remainder were found to be either impractical or insufficient to provide a solution, although the evidence from the outer London Boroughs is that separated storm drainage can sustainably prevent all pollution. In a 2006 report commissioned for Ofwat, Jacobs Babtie strongly recommended using the computer model of the existing system to do detailed studies of the effects of laying separate storm water pipes, but their advice and that from Ofwat in 2007 concerning the poor value of the tunnel was not followed. A number of parties questioned the integrity of the TTSS study, in particular the dismissal of separation and future SUDs/blue-green infrastructure as a solution. Some groups that opposed the tunnel stated that it is an unsustainable 19th-century solution to a 21st-century problem. They argued that rainwater from roads should be sent directly to canals, rivers or the estuary, and that landowners could install soakaways for their roof run off, making them eligible for discounted water treatment bills. This would reduce pressure on the foul system and take away the need for the tunnel. In November 2010 the draft National Policy Statement for Waste Water (NPSWW) was issued, and eventually published largely unaltered, despite Parliamentary objections that it promoted the Tideway Tunnel as the only solution.
Screening, storage and treatment The three-part solution to implement screening, storage and treatment was collectively known as the London Tideway Improvements.
Stratford to Beckton: Lee Tunnel This earliest phase was the Lee Tunnel: a deep, broad storage and conveyance tunnel dug and lined from Jenkins Lane, Beckton to Abbey Mills, Stratford, sloping down the reverse way. This tunnel, running down to deep from Abbey Mills to Beckton is forecast to capture , or 16 million tonnes annually from the greatest-polluting CSO point in London. Thames Water awarded the £635 million construction contract to the MVB joint venture of Morgan Sindall, VINCI Construction Grands Projets and Bachy Soletanche, in January 2010. Construction began in 2010 and on 28 January 2016 the Mayor of London, Boris Johnson, confirmed the tunnel open.
Sewage treatment works modernisation The second part was the £675 million project to modernise and extend London's five major sewage treatment works to treat more sewage, thus greatly reducing the need for storm discharges to the river: Mogden Sewage Treatment Works – a £140 million upgrade to extend treatment capacity by 50% Crossness Sewage Treatment Works – a £220 million upgrade to extend treatment capacity by 44% Beckton Sewage Treatment Works – a £190 million upgrade to extend treatment capacity by 60% Riverside Sewage Treatment Works – an £85 million upgrade to improve water quality and produce renewable energy on site Long Reach Sewage Treatment Works – a £40 million upgrade in Dartford
These intend to improve the standard to which the sewage is treated, thus improving the Tideway's water quality.
Acton to Stratford The final part is the Thames Tideway Tunnel, which intercepts outflows from London's most polluting CSOs, stores them and directs (conveys) them to sewage treatment works for processing.
Thames Tideway Tunnel planning and consultation
Initial design and phase 1 consultation Following the Thames Tideway Strategic Study, Thames Water consulted with relevant authorities to get feedback from stakeholders who would potentially be affected by the construction of the Thames Tideway Tunnel. Thames Water sought feedback on the proposed tunnel routes and potential locations of construction sites.
Three tunnel routes were considered: River Thames Routesuch an alignment, broadly, would follow the river from west London to Beckton STW and would cut across the Greenwich Peninsula, reducing the length of the tunnel where there are no CSOs to be intercepted. Rotherhithe Routea similar alignment, but this would have cut additionally across the Rotherhithe Peninsula, reducing the length of the main tunnel by approximately but requiring longer connection tunnels from some CSOs. Abbey Mills Routethis route would connect the Thames Tunnel to the head of the Stratford to East Ham part at Abbey Mills Pumping Station. This would follow the same route but deviate north-east to Abbey Mills. The main tunnel length would be about less than the River Thames Route and save about £900 million.
Route and sites selection A long list of 373 potential sites was created using a desktop survey of the land on either side of the 34 most polluting CSOs (as identified in the Thames Tideway Strategic Report). These sites were then further evaluated against more detailed planning, engineering, environmental, property and community considerations resulting in a shortlist of sites.
The three tunnel routes, as well as the shortlist of sites, were then put out for consultation between September 2010 and January 2011. In total 2,389 feedback forms (both online and hard copy), 480 pieces of correspondence and five petitions were received.
In response to the comments received, changes and improvements to some of the sites, including the potential use of alternative sites and alternative technical solutions, were considered. Based on this a round of interim engagement took place from March to August 2011. Residents around 11 specific sites were sent letters explaining that these sites were being considered as alternative sites, and invited to attend drop-in sessions to pose questions and gain a better understanding of the project. In total ten two-day sessions and one community liaison meeting were held. These were attended by over 800 people. In all 168 comment cards and 147 pieces of site specific correspondence were received and considered.
Based on this first round of consultation and interim engagement it was recommended that, for the project to be as cost-effective as possible and cause the least disruption, while still meeting the requirements of the UWWTD, the preferred scheme for the Thames Tideway Tunnel would need to involve:
the Abbey Mills route a main tunnel long with an internal diameter of ; direct interception of 21 CSOs; indirect interception of a further 12, and a local solution for the remaining CSO; selection of five out of 52 possible access shafts to be built including three to be combined with CSO interception; and selection of seventeen out of 71 CSO alternate positionings from the final shortlist.
The new preferred route and sites were then sent out for a second round of public consultation and feedback.
Phase 2 and targeted consultation The second phase of consultation was carried out between November 2011 and February 2012 when local authorities, land owners, local businesses and communities were consulted on: