Flushing toilets enable most Americans to make their own waste disappear as if by magic, but most would be hard-pressed to answer this simple question: When you flush, where does it go?

Septic tank owners, about 20 percent of Americans, are most likely to be able to give an accurate answer, because they’re responsible for the maintenance of their own sewage-disposal systems. A flush from one of their toilets sends wastewater to a tank buried on their property, where the waste products separate into solid and liquid layers and partially decompose. The liquid layer flows out of the tank and into a drainfield that disperses it into the soil, where naturally occurring microbes remove harmful bacteria, viruses, and nutrients. The solid layer stays behind in the form of sludge that must be pumped out periodically as part of routine maintenance. If the tank is properly designed and maintained, those bacteria, viruses, and nutrients stay out of groundwater and surface water that people may use for drinking water, and they never reach surface water bodies where people swim or boat.

The vast majority of the 80 percent of Americans who don’t use septic tanks are served by municipal water-treatment plants. Waste from their homes is whisked immediately off the premises, never to be seen, smelled, or considered again. Pipes carry waste from these homes to wastewater-treatment plants that, in some ways, work like a septic tank on a very large scale.

Just as in a septic tank, the solid and liquid wastes are separated first in a process known as primary treatment. Next, as in a septic tank’s drainfield, bacteria break down contaminants in a process called secondary treatment. After that, treatment with chlorine kills the remaining bacteria. Then, in some communities, special treatment technologies remove contaminants that are of special concern, such as phosphorus or nitrogen. When the process is complete, the treated waste meets regulatory standards and is released to a nearby water body—that is, if all goes well. If all doesn’t go well—perhaps the treatment plant suffers an outage or there’s more waste than the plant was designed to treat—untreated waste can be released to surface water.

As treatment plants age across the United States and as the country’s population grows, these releases are becoming more problematic, contributing to the serious surface-water problems that crop up frequently in the news. Harmful algal blooms like the one that cost Toledo, Ohio, its drinking water last summer, fish kills like the one recently reported off Long Island, and the much-discussed dead zone in the Gulf of Mexico are all fed by phosphorus, nitrogen, and other contaminants found in the untreated sewage that, according to EPA estimates, flows out of America’s treatment plants during the 23,000 to 75,000 sanitary-sewer overflows that happen per year.

The causes of these water-quality issues are complex, because the same pollutants can be washed into surface water from agricultural land, industrial sites, and fertilized lawns dotted with pet waste, but the 3 to 10 billion gallons of untreated waste released from our sewage-treatment plants per year cannot help but have an impact.

Specifically, they affect the water you swim in and the water you drink.

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A number of studies, including this one from 2010, have found that emergency room visits for gastrointestinal distress increase after a heavy rain. These illnesses are believed to spike after a storm because rainwater washes pathogens into lakes and rivers used for recreation and drinking water. A 2015 study published in Environmental Health Perspectives goes a step further than earlier research by pointing to a common type of municipal sewage-treatment system, combined-sewer systems, as an important factor in these illnesses.

The EPA has called overflows from combined sewer systems “the largest category of our Nation’s wastewater infrastructure that still need to be addressed,” affecting Americans in 32 states, including the District of Columbia. The agency has been working with municipal water systems to address the problem for decades and much progress has been made, but to understand why it’s taking so long, you have to consider history. You also have to consider the massive costs that come with making changes to public works that have served millions of people for more than a century.

Combined sewers collect human waste, industrial waste, and stormwater runoff into a single pipe for treatment and disposal. (In other municipalities, these waste streams are handled separately.) In dry weather, a combined sewer ordinarily carries a relatively low volume of waste, delivering it to publicly owned treatment works, or POTWs for short, that are designed to handle that flow. In plain terms, when a combined sewer system is functioning properly, you can generally trust that when you flush, the contents of the toilet bowl end up where they’re supposed to go.


Things change when it rains in communities served by combined sewers. Because a combined system must handle surges of stormwater, rainfall markedly increases the volume of waste that its equipment must handle, making this type of sewage system particularly likely to overflow into surface water. As these diagrams show, they were designed to do this as a fail-safe for system failures that were intended to be rare but aren’t any longer. If you’re accustomed to a faint smell of sewage in the streets after a rainstorm, these diagrams will show you why.

Unfortunately, the receiving waters for these rain-induced spills are sometimes the same water bodies that are used for drinking water, and sometimes people swim there, too. And sometimes the overflow is so significant that the stormwater-and-sewage mixture backs up into the streets where people walk.

Is it any wonder that rainy weather often triggers a spike in stomach bugs and beach closures?

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Given what’s at stake, why are upgrades to aging systems taking so long? Consider this map of the 772 American communities with combined-sewer systems.

The majority of combined sewer systems are located in the Northeast and Great Lakes regions. (EPA)

Most combined systems are concentrated in the older cities of the Northeast and the Great Lakes region, but they also exist in other older cities as far-flung as Atlanta, Memphis, and San Francisco. In other words, the systems that pose risks today happen to be the ones—state-of-the-art when they were built, but not today—that are in some of the biggest cities in America, which have a combined population of approximately 40 million people.

If you’re feeling relieved to see that your hometown isn’t marked on the map, remember that fecal-coliform bacteria don’t always stay close to home. Waste spilled into the Ohio River affects everyone down the Ohio and the Mississippi, and it contributes to the ongoing woes of the Gulf of Mexico. Even if you don’t live in the Northeast, along the Ohio, in the Great Lakes region, along the Mississippi, or on the Gulf Coast, bear in mind that 40 percent of the commercial seafood caught in the continental U.S. comes from the Gulf of Mexico. In other words, when Cincinnati’s sewer system overflows into the Ohio, it intrudes into the food chain of a lot of people.

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The EPA calls combined sewers “remnants of the country's early infrastructure.” The first sewers weren’t designed to handle the constant and huge stream of wastes from our toilets, because they were invented when we didn’t have any toilets. Sewers were originally built to solve the problems of cities that were flooded with their own refuse—garbage, animal manure, and human waste left in the open rather than in a privy or latrine—during every rainstorm. To prevent that flooding, the fouled stormwater was shunted out of town and into the nearest handy receptacle, which was often a lake, river, stream, or ocean.

When flush toilets became common in the mid-1800s, they were piped into these existing sewers, introducing much more human waste, as well as a large volume of water that had never been there before. In some ways, this was a design feature, not a bug, because the burst of stormwater flushed out pipes that might have otherwise gotten clogged. This flush of rainwater also diluted the waste before it hit a nearby river.

In time, though, dilution wasn’t enough to keep waterways safe and attractive, and sewage treatment plants were invented to clean up the waste stream before releasing it to water bodies. Newer cities, which were starting from scratch, generally handled stormwater separately from human and industrial wastes from the start, but cities whose sewer systems had always been combined continued to treat both waste streams together.

As the older cities grew larger, their combined-treatment systems struggled to keep up, and growing populations weren’t the only factor. Time itself exacerbated their woes. In Hoboken, for example, some sewer lines date back to the Civil War. Common sense says that pipes that have been buried for a century and a half tend to leak. Over time, they also get clogged with debris or even congealed cooking oil, resulting in narrowed pipes that overflow even more easily.

When narrowed pipes are already overloaded, the added influx of stormwater when it rains becomes just too much water. Now, some cities experience overflows with less than a quarter-inch of rain, with resulting risks to human health. It is common for cities with combined-sewer systems to advise citizens to stay out of the water for days after any rainfall. And now the Environmental Health Perspectives study suggests that after a very heavy rain, those overflows may be affecting their communities’ drinking water, too.

What is being done? Combined sewers have been an EPA priority for many years and, after decades of significant effort, the numbers are starting to move in the right direction, but this is not a problem that can be turned around quickly or cheaply. New York City’s combined sewers are still the single largest source of pathogens to the New York Harbor system, according to the New York Department of Environmental Protection. A single 2014 storm triggered a release into Lake Erie from Detroit, Michigan, of more than 44 million gallons of raw sewage from sanitary sewers and almost 3 billion gallons from combined sewers, and such releases from Detroit and the other cities with sewer outfalls on Lake Erie contribute to the fact that it blooms with algae every summer. Last summer, one of those algal blooms cost Toledo its drinking water for two days, and this year’s harmful algal blooms were projected to be even worse than last year’s.

As with any engineering project, the benefits of reducing overflows to zero—an effort estimated by the EPA in 2004 to cost $88.8 billion—must be weighed against its cost.

“We mustn’t forget the hugely successful effort in the 1970s and 1980s to provide secondary treatment at virtually every sewage-treatment plant in the country,” said Wayne Huber, a professor emeritus of Civil and Construction Engineering at Oregon State University. As an example, he describes what happened in Portland, Oregon, where a system of tunnels now contains 90 percent of the city’s stormwater surges. “Portland spent about $500 million on its deep tunnels and pumping system,” Huber said. “This has reduced the number of releases into the Willamette River from maybe 50 to 100 per year to five to ten per year.”

Huber also highlights Philadelphia’s “green technology” strategy to reduce overflows to the Delaware and Schuylkill rivers. Since avoiding massive construction is often synonymous with avoiding massive expenditures, Philadelphia’s use of approaches like rain gardens and green roofs to divert stormwater from the waste stream going to its treatment plants could serve as a model for other municipalities struggling with the same problems.

Huber cautions against relying on a single approach, saying that “green technology seeks to avoid large investments in infrastructure by keeping stormwater out of the combined sewer in the first place, but in heavily urbanized areas that is seldom an option, hence the massive storage projects that we see in cities like Chicago.”

On the individual level, people concerned about wastewater can give some thought to the fertilizer, pesticides, trash, and animal waste that wash off of lawns and into sewer systems, lakes, rivers, and oceans. As citizens, they can also advocate at local, state, and federal levels for improvements. People can reduce stormwater flow by planting their own rain gardens and green roofs—and by being judicious about the way they water our lawns and wash their cars. Sometimes, doing the right thing is as simple as being careful about what goes into storm drains and toilets.

After hearing about the plume of sewage, littered with used condoms and tampons, that emanated from Philadelphia’s sewer outfalls prior to the city’s upgrade, it’s hard to look at flushing the toilet the same way. If Americans want to be able to drink tap water or swim at beaches after it rains, they have to keep trying to improve wastewater infrastructure, even if the size of the problem boggles the mind.