Storm Drain vs. Sewer System

If you’ve ever noticed a sewer gas smell in bathroom, it’s definitely a good idea to get that checked out by a professional. Noticing this kind of smell may also get you thinking about how modern day sewers actually work.

It’s important to note there are different types of sewer systems that serve different purposes. With modern advances in technology, our industrialized society is fortunate to benefit from procedures and machinery that not only purify water, but also promote better health for us and for our natural environment.

So what’s the difference between a storm sewerand a sanitary sewer? It may seem like every sewer system is the same, but that actually couldn’t be farther from the truth.

This in depthguide will educate you about the differences between the various types of sewers so you know exactly what happens when you run your water or flush your toilet.

What is a Storm Sewer?

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A storm sewer is designed to carry rainfall and is typically located within curbs, as seen in the photo above. Storm sewers can also carry water that results from snow melting. You’ll find storm sewers within basement floor drains in older buildings as well, along with alleys and driveways. There is often a logo, most often a fish, or some kind of writing that warns that anything poured down the storm sewer ends up in a local water source.

The main purpose of a storm sewer is to carry away excess rain, hence the name “storm” sewer. Once the rainfall flows through the opening of the storm sewer, it travels through underground pipes and drains to the ocean or nearby creeks, canals or rivers, as mentioned.

It’s important to note that this water is untreated, meaning whatever goes into the storm drain is exactly what enters the water source on the other end.

What is a Sanitary Sewer?

A sanitary sewer does what it sounds like it does – it carries sewage – not rainwater. Any sewage that flows through the underground pipes goes directly to a wastewater treatment facility where it’s professionally treated before it’s discharged. This wastewater treatment certainly eliminates that smell coming from toilet through a specialized process.

Sewage is treated at this wastewater facility to limit the amount of pollution that goes into the water source. Before the sewage is actually released into a particular body of water following its treatment, it has to pass the regulations mandated by the National Pollutant Elimination Systems Permit.

The sewage that flows through a sanitary sewer can originate from the bathroom, laundry room or kitchen sink. These types of sewers take sewage from residential homes and commercial buildings.

Historical note – during the 19th and 20th centuries, sewers that were originally left open were closed and covered with cast iron to improve health conditions of nearby residents.

In the 21st century, it’s extremely important that the pipes that make up a sanitary sewer are tightly and properly sealed to prevent any sewage from leaking out prior to going through the purification treatment.

The image below illustrates the difference between storm sewers and sanitary sewers.


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Here are a few reminders about how you can positively impact the environment when dealing with both types of sewer systems:

  • If you care about our waterways, never dump any chemicals down a storm drain!
  • Within your home, don’t pour any harsh chemicals down the drain; instead, take these chemicals to a local hazardous waste collection site so they’re disposed of properly.
  • Grass clippings and other types of litter can actually cause sewer systems to flood, so never intentionally pour those down any storm drains.

Most cities have sanitary systems and storm sewers separated. When you look at the manhole covers you see in the street, you’ll be able to tell which leads to a wastewater plant and which leads directly to a body of water.

As you can see below, this particular sewer system is a sanitary sewer, as denoted by the writing.


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Regardless of which type of sewer you’re talking about, all sewers do deteriorate over time. If there are cracks in any pipes, it’s no longer necessary to completely excavate and remove the entire pipe; nowadays, a special cement mixture coats the pipe under high pressure and successfully seals all the cracks.

Hopefully this guide makes you feel more educated about each type of sewer system. Thanks to advances in modern technology, we have sewage systems that function properly.

And if you ever notice a sewer gas smell in bathroom, give your local plumber a call to get it checked out by a professional!