The Thames Tideway Tunnel is a deep-level sewer along the tidal section of the River Thames in London, running 25 kilometres (16 miles) 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 cost – excluding financing, operations and maintenance – was £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 7.2 m (24 ft) and runs at a depth of between 30 m (98 ft) at the western end, and 70 m (230 ft) 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. 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 39,000,000 m3 (3.9×1010 L), that is 39 million tonnes, of storm sewage entering the river. 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, 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.
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