Combined sewer

Based on Wikipedia: Combined sewer

When It Rains, It Pours—Into the River

Picture a heavy rainstorm hitting a major city. Water cascades off rooftops, flows down streets, and disappears into drains. In roughly 860 American communities—home to about 40 million people—that rainwater is mixing with something far less pleasant: raw human sewage. And when there's too much of it, the whole foul mixture gets dumped, untreated, directly into rivers, lakes, and coastal waters.

This isn't a design flaw. It's the design.

Welcome to the world of combined sewers, one of urban infrastructure's most enduring—and problematic—legacies from the Victorian era.

One Pipe to Carry It All

A combined sewer is exactly what it sounds like: a single pipe system that carries both sewage from toilets and sinks, and stormwater runoff from streets and rooftops. When the sun is shining, these systems work reasonably well. Sewage flows to treatment plants, gets cleaned up, and everyone's happy.

But when it rains? The equation changes dramatically.

Stormwater isn't just water. As rain travels over roofs and pavement, it picks up a remarkable cocktail of contaminants: oil and grease from cars, heavy metals, pesticides, animal waste from pets and wildlife, soil particles, and whatever else happens to be lying around. All of this joins the sewage already flowing through the pipes.

The problem is volume. Combined sewer systems were typically built to handle between three and 160 times the normal dry-weather sewage flow. That sounds like a lot until you experience a serious thunderstorm or spring snowmelt. When flows exceed capacity, something has to give.

The Overflow Problem

Engineers aren't stupid. They knew that sometimes the system would be overwhelmed. So they built in what Americans call "storm-water regulators" and the British more bluntly call "combined sewer overflows," or CSOs. These are essentially relief valves—carefully designed structures that divert excess flow away from treatment plants that would otherwise be damaged by the deluge.

Where does that diverted flow go? Into the nearest body of water. Untreated.

Some of these overflow points rarely activate. Others discharge every single time it rains.

The consequences are grim. Beach closures. Shellfish harvesting bans. Drinking water contamination. In the worst cases, the system backs up before reaching the overflow points, and raw sewage bubbles up through toilets into people's homes. Imagine coming home to find your basement flooded with a toxic mixture of your neighbors' waste and street runoff. Now imagine the cleanup bill.

The First Foul Flush

There's a particularly nasty wrinkle to this problem. During dry weather, a film of biological material accumulates on the inner surfaces of combined sewers—a biofilm of bacteria and organic matter clinging to the pipes. When a storm hits, the initial surge of water scours this accumulated gunk loose and sweeps it along.

This "first foul flush" means that the very beginning of an overflow event is often the most polluted. Late summer storms, following weeks of dry weather, tend to be the worst of all.

How Did We End Up With This System?

The history of combined sewers is really the history of cities learning, slowly and painfully, what happens when millions of people live in close proximity.

Before the 19th century, urban sanitation was astonishingly primitive by modern standards. Chamber pots were emptied into streets. Animals were slaughtered in open-air "shambles"—butcher stalls where blood and offal ran freely. Draft horses deposited manure everywhere. The streets of major cities were, quite literally, covered in excrement.

Early sewers weren't designed for human waste at all. They were simply channels to carry rainwater away from streets and into rivers. Open gutters running down the middle of roads. Urban streams conscripted into drainage duty. The idea of treating sewage before discharge didn't exist yet.

Then came the great urban infrastructure projects of the late 19th and early 20th centuries. Cities across Europe and North America covered their open sewers with pipes, arched tunnels, and concrete channels. Streets were paved with impermeable surfaces like asphalt—a technology developed by Scottish engineer John Loudon McAdam, whose name gives us "macadam" and "tarmac."

When engineers built these enclosed systems, they faced a choice: build one set of pipes for sewage and another for rainwater, or combine everything into a single system.

One pipe was cheaper. And since most cities didn't have sewage treatment plants yet anyway, there seemed to be no advantage to keeping the streams separate. Everything was going into the river untreated regardless.

Besides, street runoff at the time was often heavily contaminated with animal waste. Keeping it separate from human sewage might have been a distinction without a difference.

The World Changed, But the Pipes Didn't

The 20th century transformed urban environments in ways the Victorian engineers couldn't have anticipated. Horses gave way to automobiles. Municipal slaughterhouses replaced street-side butchering. Piped water replaced rain barrels. Streets and sidewalks were increasingly covered with impermeable surfaces that sent rainwater rushing into drains rather than soaking into the ground.

Meanwhile, cities finally started building sewage treatment plants. But these plants were designed for dry-weather flows. Nobody could afford to build treatment capacity for peak storm events—the plants would sit idle most of the time and still couldn't handle the truly massive deluges.

So the overflow structures stayed. The untreated discharges continued. And as cities grew and climate patterns shifted toward more intense rainfall events, the problem got worse.

Separate But Unequal

Modern sewer systems take a different approach. They build two completely separate pipe networks: sanitary sewers for toilets and drains, and storm sewers for rainwater. The sanitary sewage goes to treatment plants. The stormwater, being relatively clean, can often be discharged directly to waterways or allowed to infiltrate into the ground.

This separation eliminates CSO events entirely. No matter how hard it rains, the treatment plant only receives sanitary sewage at predictable volumes.

So why don't cities with combined systems simply separate them?

Cost. Staggering, almost incomprehensible cost.

Consider Washington, D.C. In 2011, the city separated the sewers in just four small neighborhoods. The price tag: 11 million dollars. And that was the easy part—areas where separation was physically and economically feasible. For many combined systems, the existing infrastructure is so deeply embedded in the urban fabric that building parallel pipes would require tearing up virtually every street, disrupting every building, and spending billions.

In 2005, southeast Michigan estimated it would need 2.4 billion dollars to address its CSO problem. By that point, having spent nearly a billion dollars, they had reduced untreated discharges by 85 percent—from 30 billion gallons per year to around 10 billion. An impressive achievement, but even after all that money, billions of gallons of sewage-contaminated water still flowed into lakes and rivers annually.

The Tunnel Solution

If you can't prevent overflows from happening, perhaps you can prevent them from reaching waterways. That's the logic behind CSO storage tunnels—massive underground caverns that capture overflow during storms and hold it until the treatment plant has capacity to process it.

Washington, D.C., is betting big on this approach. Starting in 2013, the city began constructing the "Clean Rivers Project": four deep storage tunnels totaling 18 miles in length, running alongside the Anacostia River. When complete in 2030, the system will reduce overflows to that river by 98 percent.

These tunnels are engineering marvels. They must be large enough to hold enormous volumes of mixed sewage and stormwater, deep enough to collect flow from existing sewer lines by gravity, and equipped with pumping systems to empty them back to treatment plants after storms pass.

But they come with their own challenges. Stored sewage doesn't wait patiently. Leave it too long and it goes septic—the biological processes that occur in stagnant waste generate hydrogen sulfide gas (the smell of rotten eggs) and can make the eventual treatment much more difficult. Careful management of storage timing is essential.

Going Green

There's another approach that doesn't involve massive tunnels or parallel pipe systems: reduce the amount of stormwater entering the sewers in the first place.

This is the philosophy behind "green infrastructure"—a collection of techniques that mimic natural water cycles in urban environments. Green roofs absorb rainfall before it can run off. Permeable pavement lets water soak into the ground. Rain gardens and bioswales capture and filter runoff. Trees and other vegetation intercept precipitation and release it slowly through evaporation.

None of these measures can single-handedly solve a major city's CSO problem. But together, distributed across thousands of properties, they can meaningfully reduce the peak flows that trigger overflows. Philadelphia has made green infrastructure a centerpiece of its CSO control strategy, recognizing that the city simply cannot afford the tunnel-only approach.

There's an appealing elegance to this solution. Rather than building ever-larger systems to move water away from cities as fast as possible, green infrastructure works with natural processes to slow water down and let it infiltrate where it falls. The approach also provides co-benefits: reduced urban heat island effects, improved air quality, enhanced property values, and better quality of life.

The Monitoring Revolution

You can't manage what you can't measure. For decades, combined sewer systems operated largely in the dark—utilities knew overflows happened during storms, but had limited real-time information about where, when, and how much.

That's changing rapidly. Sensors have gotten cheaper. Communications technology has improved. Software can now process enormous streams of data to identify patterns and predict problems.

Modern CSO management increasingly relies on "real-time decision support systems"—networks of sensors throughout the sewer system that feed data to control rooms where operators can adjust flows, open and close gates, and direct water to available storage capacity. Instead of each overflow point operating independently according to fixed rules, the system can be managed as a whole to minimize total discharges.

These monitoring networks also help identify bottlenecks. Sometimes a single problem point—a collapsed pipe, an undersized junction, a malfunctioning pump—causes a disproportionate share of overflow events. Finding and fixing these chokepoints can dramatically improve system performance at relatively modest cost.

Regulation and Reality

In 1994, the United States Environmental Protection Agency (EPA) issued a policy requiring municipalities to address CSO pollution. Congress made this mandatory by amending the Clean Water Act in 2000. Cities were required to implement "nine minimum controls" and develop long-term plans to reduce overflows.

The policy tried to balance environmental protection with economic reality. It recognized that CSO control is site-specific—what works in one city may not work in another. It acknowledged that communities have limited budgets. It allowed flexibility in setting water quality goals based on local conditions.

But it also had teeth. Cities that failed to make progress faced enforcement actions, consent decrees, and penalties. Pittsburgh, Seattle, Philadelphia, and New York are all operating under federal consent decrees that legally compel them to solve their CSO problems on specified schedules. The EPA can impose both upfront penalties and ongoing "stipulated penalties" for missing milestones.

The United Kingdom has taken similar action. The Environment Agency identified "unsatisfactory intermittent discharges" and issued directives requiring action. Canada adopted nationwide standards in 2009, including requirements to remove floating material from CSO discharges and prevent overflows during dry weather.

A Different Kind of Sewage Scandal

Combined sewer overflows get attention because they're dramatic—massive discharges of sewage into waterways during storms. But there's a quieter, more insidious problem lurking in many cities: properties whose sewage connections were never properly hooked up to begin with.

In older cities, some buildings have their waste pipes connected not to the sewer system, but to ancient networks of local streams and rivers that were long ago buried and forgotten. These "misconnections" discharge untreated sewage constantly—not just during storms, but every single day, rain or shine.

The owners often have no idea. Their toilets flush. Their sinks drain. Everything seems to work. They don't know that their waste is flowing directly into a buried stream that eventually emerges into a river.

Estimates suggest there could be tens of thousands of such misconnected properties in London alone. Unlike CSO events, which are at least designed into the system and occur at known locations, misconnections are essentially hidden pollution sources scattered randomly across the urban landscape.

The Long View

Combined sewers represent a kind of infrastructure debt—decisions made over a century ago that we're still paying for today. The Victorian engineers who built these systems weren't wrong given what they knew. They were solving the urgent public health crises of their time: cholera, typhoid, the horrific mortality rates of industrial cities choked with waste.

They succeeded. Combined sewers, even with their overflow problems, were vastly better than open gutters and contaminated wells. Life expectancy in cities improved dramatically.

But the solutions of one era become the problems of the next. We inherited systems designed for a world of horse-drawn transportation, permeable streets, and no sewage treatment. We live in a world of climate change, intense rainfall events, and environmental standards that would have seemed impossibly ambitious to our great-grandparents.

The fixes are expensive, slow, and unglamorous. Nobody wins elections by promising better sewer tunnels. But the work continues, city by city, year by year. Southeast Michigan's 85 percent reduction in untreated discharges represents real rivers getting cleaner, real beaches reopening, real shellfish becoming safe to eat.

It's not a problem that will be solved quickly or cheaply. But it is being solved—one overflow at a time.