Cathedrals on the Marshes
The story of London’s sewer system, from Bazalgette to the Thames Tideway Tunnel
Written by Nick Maini for the Greater London Project
Sewage management is one of civilisation’s greatest achievements. A defining chapter in the history of sanitation unfolded in Victorian Britain, particularly in London, where a pioneering sewage system was built. That system transformed the city’s sanitary conditions and has remained an essential part of London’s infrastructure ever since.
By preventing disease outbreaks and protecting clean water supplies, effective sewage systems have allowed dense populations to develop along with modern cities. A vast network of infrastructure that operates largely unseen beneath streets and within pumping stations or treatment facilities has transformed cities from the disease-ridden environments of the past into the clean, liveable spaces we enjoy today.
Cesspools, night soil men & cholera
For most of its history, London relied on cesspools for sewage management, which were underground pits that collected and stored sewage from buildings. Cesspools were emptied by “night soil men” (also known as “gong farmers”) who sold the contents as fertiliser for agricultural use outside the city. This system remained the primary disposal method until London’s rapid urban expansion in the 19th century undermined its efficacy. The city’s population doubled from 1 million to 2 million between 1801 and 1851, then nearly tripled to 5.6 million by 1891. Cesspools frequently overflowed as housing density increased and many houses began to be connected to street sewers that were originally designed only for rainwater. These sewers emptied directly into the Thames, turning London’s main water source into an open sewer. The introduction of water closets (flush toilets) worsened the situation by dramatically increasing household wastewater volume, creating serious public health risks.
In 1854, Dr. John Snow and Rev. Henry Whitehead tracked down the source of a cholera outbreak in Soho to a contaminated water pump on Broad Street, disproving the prevailing “miasma theory” (that disease spread through foul air) and establishing the connection between contaminated water and disease transmission.
Sir Joseph Bazalgette
Following several more cholera outbreaks and the "Great Stink of 1858", a summer during which the stench from the polluted Thames became so unbearable that Parliament considered relocating to Henley, the government commissioned the Metropolitan Board of Works, led by Sir Joseph Bazalgette, to deliver a new sewage system.
Bazalgette’s sewage system transformed the city’s sanitary conditions and delivered critical infrastructure that has protected public health for over 150 years. Bazalgette’s system consisted of over 1,100 miles of street sewers, 82 miles of main interceptor sewers, 318 million bricks, and four major pumping stations to divert sewage eastward away from central London. This vast infrastructure project not only improved public health dramatically but also reshaped London through the creation of the Victoria, Albert and Chelsea Embankments, for which 52 acres of land were reclaimed from the river.
The London plans were heavily informed by the pioneering work of James Newlands, who had designed a sewage system in Liverpool during the late 1840s which had dramatically improved public health outcomes.
Cathedrals for Sewage
Bazalgette's pumping stations at Abbey Mills and Crossness are perhaps the most remarkable features of the entire London sewage system. Designed by the architect Charles Driver to include magnificent Gothic Revival features, Romanesque brickwork, campanile/minaret style chimneys, and ornate decorative ironwork, their grandeur is a testament to the magnitude of Victorian civic ambition. These pumping stations, far from being purely utilitarian facilities for sewage management, were conceived as architectural masterpieces that came to be known colloquially as the “Cathedrals on the Marshes”.
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Bazalgette’s combined sewage system
The system Bazalgette designed is a combined sewage system, meaning that clean rainwater and sewage wastewater (from toilets, bathrooms and kitchens) are conveyed in the same pipes to a sewage treatment works.
During heavy rainfall, the capacity of the sewer system could be exceeded, and sewage treatment works could be overwhelmed by the increased volume, meaning sewage would flood homes, roads and open spaces. But in Bazalgette’s system, this risk is mitigated by combined sewer overflows (CSOs). CSOs are just like overflow valves on bathtubs. During periods of heavy rainfall, they allow excess volume in the sewage system to flow directly into the Thames.
Though far-sighted, the system’s Victorian creators did not anticipate the volume of wastewater that would eventually be entering the London sewage system. The original Bazalgette design was for a rate of untreated sewage discharges into the River Thames of 12 per year. By the 2010s, however, the system was discharging about once per week. In a typical year 39 million cubic metres of sewage overflows into the river, but it can be well over 120 million cubic metres in higher rainfall years, with serious consequences for water quality, ecological health, and public health.
Bazalgette had calculated that his Victorian infrastructure would need to provide for a population of at least 4 million. By the time of Bazalgette's death in 1891, the population had already exceeded 4 million. By the 2010s, Bazalgette’s system was serving over 8 million Londoners (a number that has continued to grow in the years since).
The Thames Tideway Tunnel
Given the volume of untreated water polluting the River Thames, a remedial plan was drawn up in 2000, the Thames Tideway Strategic Study (TTSS).
This study recommended the construction of a single, large tunnel which could store the excess backlog of sewage wastewater and rainwater generated during periods of heavy rainfall (which would otherwise overflow into the river) until it could be treated at the sewage works in Beckton.
The Thames Tideway Tunnel project, as it became known, involved the UK’s largest ever planning application up to that point in time, with 125,000 pages submitted in total. The hard copy was stored on a 16m-long shelf. The two copies that had to be hand-delivered to the Planning Inspectorate, each weighing one tonne, were transported in separate vans.
Thames Water, the monopoly water utility business for the Greater London area, was deemed operationally unsuited to manage the Tunnel's risks due to the project’s scale and complexity, so Bazalgette Tunnel Limited (Tideway) was created as a special-purpose company through a public-private partnership to finance and deliver the project.
Firstly, a competitive tender was run for investors, and delivered a record low private sector financing rate for this kind of monopoly infrastructure, primarily due to the inclusion of a government backstop that protected investors against certain low-probability but high-impact risk events. Additionally, the main tunnel was split into three target price contracts (for West, Central, and East) under a “Win Only One Rule”. This diversified the risk of project failure by ensuring that each company could secure a maximum of one section. And the target price model of risk/reward sharing incentivised contractors to share financial responsibility, with parties absorbing 50% of any cost overruns and benefiting from 50% of any underspends.
Besides this, the project offers a case study in nuanced stakeholder management amid impassioned resistance against the development.
Phil Stride, the executive who oversaw the Tunnel’s delivery, chronicled the drawn-out and often contentious engagement process in his book, revealing how the project’s leadership navigated challenging community dynamics.
“Many local residents questioned the actual need to connect the associated CSOs to the Thames Tideway Tunnel. ‘Stop the Shaft’ were the principal catalyst behind local opposition at Barn Elms… There were a number of very challenging and emotionally charged public meetings… The most notable points of the meeting were that Stephen Greenhalgh had the whole audience chanting in opposition of the project.
One of the most surreal experiences in the whole engagement process was the meeting at St Mary’s Church Hall in Putney, with Graham Stevens of Bluegreen UK and a very tolerant Justine Greening, to discuss blue-green alternatives to the project and how nanotechnology would soon make our solution redundant. We concluded that said nanotechnology was still being developed, and is yet unproven.”
Source: Phil Stride (2019) The Thames Tideway Tunnel: Preventing Another Great Stink
As with almost all UK large-scale public infrastructure projects in recent decades, the estimated cost for the project inflated over time. The 2021-22 annual report gave an updated cost of £4.3bn, while the final cost was reported to be around £5bn.
In summary
The Thames Tideway Tunnel was first proposed in 2000. Today, 25 years later, it is finally fully operational (as of February 2025) and projected to reduce untreated wastewater flowing into the Thames by 95%.
Just as Bazalgette’s original system has served London for over 150 years, the Thames Tideway Tunnel promises to protect the Thames and serve Londoners for generations to come.
For London, the Tunnel has solved a critical infrastructure and environmental challenge. In doing so, it offers an example of how a modern city can deliver transformative projects at scale amid increasing complexity and competing stakeholder interests.
There are still plenty of challenges. Sewage treatment processes at facilities such as Beckton, now the largest sewage treatment works in Europe, are imperfect. Likewise, a permanent wastewater monitoring network for public health surveillance has yet to be established, following the collapse of the temporary solution delivered during the COVID-19 pandemic.
The story of London’s sewage system is far from over.
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I love the Victorian ambition in the design of Abbey Mills Pumping Station, and I enjoyed seeing the architectural inspirations you picked out.