Why Sometimes You Can Smell the Sewer Even Though You Mostly Can't

5 reasons you might get a sudden whiff of something unpleasant.

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Sommer Mathis

If you're lucky enough to live in a modern metropolis in the developed world, you probably don't spend much time thinking about the sewer. Chief among the myriad joys of being alive today as opposed to say, 200 years ago, is surely that the paths we traverse no longer smell, almost all the time, like stale urine and excrement.

Except, of course, when they do. Underground sanitary sewerage systems are wonderful inventions, but they're by no means infallible. Anyone who's ever set foot in New York City at the height of an August heatwave can attest to that fact. Spend enough time walking around any American city, and you'll smell it, too: A random whiff of stinky sewer.

A few weeks back in fact here in Washington, D.C., on a temperate, sunny day, it happened to me in three different parts of the city: a residential street in Foggy Bottom; a sidewalk in central downtown, near Metro Center; and a major intersection two blocks from my home in the Shaw neighborhood. In each of these spots at different times during the course of a single day, I could detect that distinct, sickly sweet smell of the sewer.


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There hadn't been a storm for days and it wasn't particularly hot, so I wondered if there might have been something going on in the sewers citywide, an odorous epidemic of some kind. Normally, I don't smell or even notice the sewer in any of these spots. So I called Barry Lucas, DC Water's program manager for the sewer system, to find out what might have been happening. Lucas reassured me there wasn't anything specific going on that day. Instead, he thinks I was probably just unlucky. Turns out there are a lot of reasons why you might suddenly be accosted by sewer gas for no apparent reason, none of them particularly pleasant. Lucas walked me through them all:

"HOW LONG IT'S BEEN IN THE SEWER"
Many big city wastewater systems accept flow from neighboring jurisdictions. At least some of what's running through the sewers in the District of Columbia, for example, has traveled from Loudoun County, Virginia, some 45 miles away. Lucas explains that the longer any "material" has been in the sewer, the more likely it is to be stinky — or in his words, if it's made a long journey, it'll have "higher concentrations of sulfides" and get some "nice anaerobic processes" going. Yum!

"IT'S ALMOST LIKE SHAKING UP A SODA"
D.C.'s sewer system, like most of its era, operates primarily thanks to gravity. This means things keep flowing in the direction of the sewer's ultimate destination, the wastewater treatment plant, without the need for a lot of pumping. But it also means that variations in slopes and grading within the system can lead to differences in what Lucas terms "turbulence." "If the flow is very turbulent" in any particular spot, Lucas says, "you would tend to get the production of more sewer gases."

"DEPENDS ON THE LOCATION"
In addition to variations in "turbulence," no two sewer lines are necessarily going to be the same distance from the surface. The minimum sewer line depth in D.C. is 4 feet, while Lucas says the average is more like 6 to 8 feet. Others, especially in newer parts of the city, could be buried as far as 50 to 100 feet underground, so you'd obviously be much less likely to smell those. Stop in a random spot that happens to be only 6 feet above the flow, however, and you could very well end up with a nose full of methyl mercaptan.

The same holds true for how far sewer odors have to travel before they reach your nose. If, say, a sewer manhole has lots of vents in it, Lucas says, or if you happen to be standing right next to a storm drain connected to a combined sewer system (more on that in a minute), you might be more likely to notice a foul smell.

"THERE MAY JUST HAVE BEEN A RUSH"
Little as we'd like to be reminded, sanitary sewerage systems are flowing underneath our feet at all times, but the amount of material present in the flow changes constantly. In other words, as Lucas puts it, "everyone may have flushed at the same time." Seriously? "Yes. If you stayed in that same spot all day you may notice it periodically, depending on how the wind is blowing." Now there's a fun science experiment to try at home: Can you tell when several of your neighbors evacuate their bowels at the same time?

"TEMPERATURE, WHETHER IT'S RAINED"
This wasn't the case for me on the day in question, but weather does indeed play a part, Lucas says, especially for older sewer systems like Washington, D.C.'s that still contain elements of a combined system (a combined sewer system "conveys both sanitary sewage and storm water in one piping system," as DC Water's website explains). So if there's been a storm recently, both the concentrations of sanitary material and the "turbulence" of the combined system can be affected. Likewise, "the warmer it tends to be, the more odors are going to be generated."

Combined systems can also just be smellier by nature. In D.C., the remaining combined sewers are concentrated around the oldest parts of the city, near downtown (ah ha!) and Georgetown, where storm drains are collecting not just rain but grease from restaurants, antifreeze and motor oil from the surface of roads, and plenty of garbage. When this noxious ooze reaches the stuff you just flushed down the toilet, all sorts of chemical reactions can occur.

So does DC Water spend any time trying to minimize sewer odors? Absolutely, Lucas says. In the hot summer months, in fact, his crew will regularly dose the system with neutralizing chemicals such as magnesium hydroxide or ferrous sulfate. They've also been concentrating on areas with "a history of odors," like a few downtown parks and the C&O Canal, and constructing a number of odor control facilities that act "almost like hood vents."

"The human nose is very sensitive to a lot of the sewer smells," Lucas says. Yep, even the man in charge of the sewers can admit that.

All photos by Sommer Mathis

About the Author

  • Sommer Mathis is editor of CityLab. Previously she spent five years editing and reporting on the D.C. metro area at DCist.com and TBD.com.