London relies on a 150-year-old sewer system that was built for a population of 4 million – less than half its current size of almost nine million people. The soaring pressure on the system has seen millions of tonnes of raw sewage discharged into the River Thames each year. Normally, sewage flows through London’s existing sewage system to the main treatment works around the capital, but, when it rains, the system fills to capacity and simply overflows. Even a light drizzle of rain causes untreated sewage to spill into its waters.
(Map image data copyright 2023 Google)
With the city’s population expected to increase to 11 million by 2050, to describe as timely the new ‘super sewer’ now being built under the Thames to intercept most of those spills and clean up the river would be something of an understatement. Some 25km long, 7.2 metres in diameter and boasting a storage capacity of 1.25 million cubic metres, the Thames Tideway Tunnel will run from Acton Storm Tanks in west London to the Abbey Mills pumping station in east London, joining up with the already-built Lee Tunnel. The new tunnel will capture the most polluting overflows and divert them into the new tunnel between 33 and 65 metres below ground.
Due for completion in 2025 – and being delivered by Tideway, the company set up to design, build, finance, commission and maintain the tunnel – it is expected to protect the river for at least the next 120 years, intercepting, storing and ultimately transferring sewage waste away from the River Thames.
A major component of the project involves the Beckton Works, part of the original 1864 waste treatment system for London. The largest sewage plant in Europe, Beckton covers over 250 acres and treats the waste of more than 3.5 million people. Engineers are working on a programme that would see the works expanded and improved, so 60% more sewage can be treated.
UPDATE
So, where is the project at right now? Tideway reports that all six tunnel boring machines have finished their drives, marking the completion of the tunnelling phase. Complex and challenging, this called for the establishment of a suitable path that would avoid all of the other tunnels running under London, such as road and Underground.
Meanwhile, Tideway has confirmed that the tunnel’s secondary lining is underway on the final, eastern section of the London super sewer, with all of the secondary lining elsewhere complete. This will continue on the concluding 5.5km stretch of the tunnel between Bermondsey and Abbey Mills pumping station into next year. Secondary lining is required to provide added strength to the tunnel to ensure the super sewer “can do its job for generations to come, while providing a smooth surface over which the sewage flows can travel when the tunnel is up and running”, says Tideway. Primary lining was completed for the entire sewer in April 2022.
Meanwhile, in mid-November 2022, engineers at the Blackfriars Bridge foreshore site began removing the eastern cofferdam (35 sheet piles in total), following completion of the east section of the new river wall. The cofferdam is split into two sections – east and west – to provide a dry working area. The piles, 15m long, were driven up to five metres into the riverbed. With the sheet piles gone, the river wall constructed to contain the sewer interception structures will be visible, opening up a new area of public space that will remain once Tideway has completed its operations. The western section of the cofferdam will remain in place into 2023.
Other work has also been progressing at pace across the 24 construction sites. Giant bronze ventilation columns, installed for the first time on the central section of the super sewer project, are now visible as 5m-high columns at Victoria Embankment. Nineteen of these ventilation columns are required across the Tideway project to regulate the gases within the super sewer. For those above ground, many unaware of what has been going on some distance down beneath their feet, these are clear signs that major change is coming to their city’s sewage system.
The ventilation columns will serve as an exhaust system, removing air displaced by flows into London’s new super sewer. This air will be filtered through air treatment chambers. Each column has been inscribed with newly commissioned poetry commemorating the local area’s heritage, often with reference to London’s ‘lost rivers’.
In the midst of such evocative memories, Sir Joseph William Bazalgette, the man responsible for London’s existing sewer network a century and a half ago, should also be celebrated. He was chief engineer of London’s Metropolitan Board of Works and, in response to the ’Great Stink of 1858’, responsible for the creation of what was to be the first sewerage system for central London, instrumental in improving public health and tackling epidemics such as cholera.
Three generations of the Bazalgette family recently visited Chambers Wharf where they descended into the main ‘super sewer’ tunnel to see the system designed to prevent millions of tonnes of sewage from polluting the River Thames. Among them was British television executive and producer Sir Peter Bazalgette, who said his great-great-grandfather would have been “immensely proud and utterly flabbergasted” at the scale and ambition of the project.
“He couldn’t have boring machines of the sort they have today or the quality of the hydraulics they are putting the secondary lining in. None of that would have been imaginable to him. But some things have stayed the same, such as the need for pumps. That was the same principle Sir Joseph used,” said Bazalgette.
As far as the cost of the scheme is concerned, Tideway’s overall estimate has increased to £4.3bn from its original budget of £3.52bn – with warnings in last year’s annual report that further increases were possible.
BOX: Raw end of the deal
The urgent need to deliver a new sewer for Londoners is all too clear, with the dumping of raw sewage into UK rivers often provoking outrage at the damage caused to the environment and health – none more so than the 2 billion-plus litre discharge into the Thames over a period of two days in October 2020. The source was Mogden wastewater treatment works in Isleworth, west London, which released the equivalent of 400 Olympic swimming pools of untreated waste into the river. This is in comparison to half a billion litres of waste from the site spilled into the river in the whole of 2016.
Overflows are permitted and legal, if sewage networks are likely to be overwhelmed in extreme circumstances, such as storms. Sarah Bentley, chief executive of Thames Water, claimed Mogden had been “inundated” with extra waste, due to heavy rain, when the massive raw sewage discharge occurred. In a Water Quality and Rivers Report released at the time, she commented: “They were the wettest days on record and we struggled to treat both the permitted amount, but also just struggled to treat the sewage. On that day, there was enough rainfall to fill Loch Ness.”