Wikipedia Deep Dive

London sewer system

13 min read

The Smell That Changed Everything

In the summer of 1858, the British Parliament faced an enemy it could not debate, legislate, or ignore. The River Thames had become so foul with human waste that lawmakers hung sheets soaked in chloride of lime over the windows of the Houses of Parliament, desperate to block out what became known as the Great Stink. It didn't work. The smell was so unbearable that parliamentary sessions were cut short, and members fled the building.

This was the crisis that finally forced London to build one of the most ambitious engineering projects of the Victorian era — a sewage system so well designed that much of it still operates today, more than 150 years later.

A River of Human Waste

To understand why the Thames smelled so apocalyptic, you need to understand what early nineteenth-century London was using it for. The river was, quite literally, an open sewer. Every chamber pot, every cesspit, every drain in the city eventually emptied into its waters. The same waters that people drew drinking water from. The same waters where children played along the banks.

The consequences were devastating.

Cholera — a disease caused by the bacterium Vibrio cholerae, which spreads through contaminated water — tore through London repeatedly. Tens of thousands died. A physician named John Snow figured out as early as 1849 that the disease spread through contaminated water, not through "miasma" or bad air as most believed. He famously removed the handle of a water pump on Broad Street during the 1854 outbreak, stopping a local epidemic in its tracks.

But even Snow's brilliant detective work couldn't overcome the prevailing wisdom. Most people, including most scientists and politicians, remained convinced that disease traveled through foul-smelling air. The irony is almost too perfect: it took unbearable smell, not scientific evidence about water contamination, to finally motivate action.

The Man Who Saved London

Enter Joseph Bazalgette. If you've never heard his name, you should have — he's one of the most consequential engineers in history, responsible for saving more lives than almost any military general or political leader you could name.

Bazalgette was the Chief Engineer of the Metropolitan Board of Works, which is a rather dull title for a man who would reshape the geography of London itself. According to the BBC, "Bazalgette drove himself to the limits in realising his subterranean dream." That dream was nothing less than a complete reimagining of how a city handles its waste.

The concept was elegantly simple. Instead of letting sewage flow into the Thames in central London where people lived and worked, he would intercept it. Six massive tunnels — called interceptor sewers — would run roughly parallel to the river, catching the waste before it could reach the water. These tunnels would then carry everything downstream, far away from the populated areas, where it could be safely discharged into the Thames Estuary.

Simple in concept. Staggering in execution.

An Underground Cathedral of Brick

The numbers involved in Bazalgette's project still astonish. The six main interceptor sewers stretched nearly 100 miles in total length. These were fed by 450 miles of main sewers, which in turn connected to some 13,000 miles of smaller local sewers. The construction consumed 318 million bricks — a number so large it's difficult to comprehend. Picture a brick. Now picture 318 million of them.

Workers excavated 2.7 million cubic meters of earth and poured 670,000 cubic meters of concrete. The project took six years, from 1859 to 1865, though elements continued for years afterward.

Bazalgette made one decision that would prove particularly prescient. He used Portland cement — a relatively new material at the time — to strengthen the tunnels. This was considered innovative, even risky. But the cement-reinforced tunnels proved remarkably durable. When engineers inspected them 150 years later, they were still in excellent condition.

He also built in extra capacity. When calculating the diameter of the pipes, Bazalgette allegedly doubled his initial estimates, saying something to the effect of "we're only going to get one chance at this." That foresight gave London decades of additional capacity as its population exploded.

Where Gravity Fails

There was a problem with the elegant gravity-based design. London is relatively flat, and sewage needs to flow downhill. In some areas — Chelsea, Deptford, Abbey Mills — the natural elevation simply wasn't enough to keep things moving.

The solution was pumping stations, and Bazalgette turned these functional necessities into architectural statements. The Abbey Mills Pumping Station, in particular, became known as the "cathedral of sewage" for its ornate Victorian Gothic design. It seems almost absurd to lavish such architectural attention on a building whose primary purpose is pumping human waste. But the Victorians understood something we've largely forgotten: infrastructure is a statement about civic pride. If you're going to build something that will last a century and serve millions of people, why not make it beautiful?

The pumping stations lifted the sewage high enough to continue its gravity-fed journey eastward. North of the Thames, most sewage eventually flows to the Northern Outfall Sewer, which delivers it to Beckton Sewage Treatment Works — one of the largest treatment facilities in Europe. South of the river, the Southern Outfall Sewer carries waste to Crossness, another facility whose original Victorian pumping station has been preserved as a museum of industrial archaeology.

More Than Just Sewers

Bazalgette's project reshaped London in ways that went far beyond waste management. To build the low-level sewer on the north bank of the Thames, he had to reclaim land from the river itself. This became the Victoria Embankment — that broad walkway along the Thames that tourists stroll today.

The Embankment project killed multiple birds with one stone. It contained the sewer, yes, but it also created new roads to relieve London's impossible traffic congestion. It provided space for public gardens where previously there had been only fetid mudflats. And running beneath it all, alongside the sewer, went the tunnels for the Circle line of the London Underground.

When the Victoria Embankment officially opened on July 13, 1870, it was celebrated as a triumph of engineering and urban planning. Which it was. But it was also a triumph of creative thinking — recognizing that a sewage project could simultaneously solve problems of transportation, public space, and aesthetics.

The Science of Clean

Moving sewage away from central London was only half the battle. The other half was figuring out what to do with it once it arrived at the outfall points.

Initially, the answer was simple: dump it into the Thames Estuary during outgoing tides, letting the ocean carry it away. This was better than polluting central London, but it was hardly a permanent solution. As London grew and environmental standards evolved, something more sophisticated was needed.

A chemist named William Dibdin, working for the Metropolitan Board of Works in the late nineteenth century, conceived of using biological processes to treat sewage. The basic idea — which still forms the foundation of sewage treatment today — is to use bacteria to break down organic waste. You're essentially harnessing the same decomposition that happens in nature, just speeding it up and containing it in controlled facilities.

Throughout the twentieth century, London's sewage treatment capabilities improved dramatically. The goal shifted from merely removing waste from sight to actually cleaning it before discharge. Today, the water released from treatment plants into the Thames is clean enough that the river supports fish populations again — including salmon, which had been absent for over a century.

When Victorian Engineering Meets Modern Demand

Bazalgette's system was a marvel. But it was designed for a London of roughly three million people. Greater London today has a population approaching nine million. The sewers, despite their generous original sizing, have reached their limits.

The original system was engineered to handle about 6.5 millimeters of rainfall per hour. That sounds like a lot — it's a fairly steady rain — but London's weather can be more violent than that. When storms dump heavy rain quickly, the system simply cannot keep up.

Here's where it gets unpleasant. London's sewers are what engineers call "combined sewers," meaning rainwater and sewage travel through the same pipes. This made sense in the Victorian era, but it creates a serious problem during heavy storms. When rainfall exceeds the system's capacity, the mixture of rainwater and raw sewage has to go somewhere. That somewhere is often the Thames.

These events are called "combined sewer overflows," and they happen with disturbing frequency. Raw sewage enters the Thames perhaps fifty times a year. Each major overflow can dump millions of tons of untreated waste into the river. The environmental and health consequences are significant.

A Tunnel Under the River

After years of study and debate, London settled on a massive solution: the Thames Tideway Tunnel, approved in 2014 and completed in February 2025 at a cost of £5 billion.

The Tideway Tunnel is essentially a giant storage tank running underneath the riverbed. At 7.2 meters in diameter and 25 kilometers long, it stretches from Acton in west London to the Lee Tunnel in the east, which connects directly to the Beckton treatment works. When storms overwhelm the Victorian sewers, the overflow is captured in this tunnel rather than being dumped into the Thames. The stored sewage can then be pumped to treatment facilities once the storm passes and capacity becomes available.

It's a brute-force solution — essentially adding a massive underground reservoir to a system that lacks sufficient capacity. But sometimes brute force is what's needed. The tunnel is expected to reduce sewage discharges into the Thames by around 95 percent.

The Other Scandal

The Tideway Tunnel addresses the problem of storm overflows, but there's another sewage issue lurking in London's streets that no amount of tunnel building will solve.

Scattered across the city are thousands of properties whose drains are connected not to the sewer system but to London's ancient network of small rivers and streams. Many of these waterways have been buried and forgotten, flowing through underground culverts, but they're still there — and they still eventually empty into the Thames.

When a building is "misconnected" in this way, its sewage flows directly into these hidden rivers. No treatment. No interception. The homeowner often has no idea their waste isn't going where it should. The problem is particularly common in older properties that predate the modern sewer connections, but it also happens when newer plumbing work is done incorrectly.

Experts estimate there could be tens of thousands of misconnected properties across London. Fixing them is a slow, property-by-property process that requires identifying the problem, tracing the pipes, and reconnecting to the proper sewers. It's not glamorous work, and it doesn't lend itself to grand engineering solutions. But every misconnected property means raw sewage entering London's waterways every single day.

The Weight of History

There's something profound about the fact that modern London still depends on infrastructure designed in the 1850s. When you flush a toilet in Chelsea or Shoreditch or Southwark, your waste begins a journey through tunnels that Joseph Bazalgette drew with pen and ink more than 160 years ago.

His system has outlasted the British Empire it served. It has survived two World Wars, including bombing raids that struck London repeatedly. It has adapted, imperfectly but persistently, to serve a population three times larger than it was designed for.

The Victorians thought about infrastructure differently than we often do today. They built for centuries, not decades. They assumed their grandchildren's grandchildren would inherit what they created, and they built accordingly. The Portland cement Bazalgette specified, the generous pipe diameters, the architectural beauty of functional buildings — all of it reflects a long-term thinking that can feel almost foreign in our age of planned obsolescence.

In Fiction and Memory

London's sewers have captured the imagination of writers and filmmakers for as long as they've existed. Neil Gaiman's novel Neverwhere imagines a parallel London existing in the tunnels beneath the city. Terry Pratchett's Dodger features a protagonist who makes his living in the pre-Bazalgette sewers alongside figures like the great engineer himself. Anne Perry's Dark Assassin uses the construction of the system as its central plot element.

Even Disney got in on it: Professor Ratigan, the villain of The Great Mouse Detective, uses the sewer system as his hideout. For a certain type of story — tales of hidden worlds, of what lurks beneath the surface, of secrets the city keeps — London's sewers are irresistible.

This imaginative fascination makes sense. There really is a hidden city beneath London, a network of tunnels and chambers stretching for thousands of miles. Workers called "flushers" still descend into Bazalgette's tunnels to maintain them. They find strange things down there: crystallized fat deposits the size of buses, lost artifacts, the occasional historical curiosity. In 2017, a "fatberg" weighing 130 tons — congealed cooking fat, wet wipes, and other debris — was discovered blocking a sewer in Whitechapel. A section of it now resides in the Museum of London.

The Lesson of the Great Stink

Perhaps the most enduring lesson of London's sewers is how they came to exist at all. For decades, proposals to modernize the city's waste management were ignored because they were expensive. People died of cholera by the tens of thousands, and still the plans gathered dust. Scientific evidence about waterborne disease was dismissed.

What finally forced action was the unbearable reality of the Great Stink. When Parliament itself became uninhabitable, when the wealthy and powerful could no longer escape the consequences of their inaction, the money suddenly appeared. The will suddenly materialized.

It's a pattern that repeats throughout history, in infrastructure, in public health, in environmental protection. We know what needs to be done. We have known for years, sometimes decades. But we wait until the crisis is undeniable, until it affects those with the power to act, until delay is no longer possible.

London got its sewers. They have served the city remarkably well, a testament to Victorian ambition and engineering skill. But they came later than they should have, and the delay cost countless lives.

Today, as London adds new tunnels and struggles to trace misconnected drains, the city is still grappling with the infrastructure of waste. The problems are different in scale and technical detail, but the fundamental challenge remains: how do you manage the unglamorous necessity of moving human waste away from human lives? How do you convince people to invest in what they cannot see, in pipes running beneath streets they walk without thinking about what flows below?

Joseph Bazalgette answered those questions for his era. We're still working on answers for ours.