The 3 Types of Sewer Systems – How Each Work

Most people don’t think about their sewer systems until something goes wrong. And that something involves the use of substantial amount of money to fix or replace.


Sewer systems are very important. They provide a way of removing human waste and wastewater from homes and businesses without any adverse effect on the environment or our health.

There are 3 types of sewer systems. These are sanitary sewers, storm sewers and combined sewers. Sanitary sewers carry wastewater from the house to the sewage treatment plant while storm sewers drain rain water and melting snow into rivers and streams. Combined sewers are a combination of sanitary and storm sewers.

In short,

  • Sanitary sewer systems – Carries waste from toilets, sinks, showers, washings machines etc. to the city’s sewer line and out into a treatment plant. Also known as foul sewers.
  • Storms sewer systems– Carries surface water (from rain and melting snow) to water bodies like streams and rivers. Also known as surface water sewers.
  • Combined sewer systems – Carries both house waste and surface water to sewer treatment plants and water bodies.

The main difference between separate and combined sewer systems is that separate sewer systems carry either waste from the house or surface water, while combined sewer systems carry both.  Combined sewers are connected to a sewer treatment facility and also drain to a water body when they overflow.

Separate systems of sewerage are preferred in modern plumbing than combined sewer systems. This is because there is no room for overflow, which results in raw/untreated sewage flowing into water bodies.

To gain an in-depth knowledge of how the 3 sewer systems work, I will walk you through each one of them.

1. Sanitary Sewer Systems

Credit: Sonoma City

A sanitary sewer system refers to the sewer line which runs underground from your house and is connected to the public sewer line at the street. The private side of is also known as the main line or sewer lateral.

Waste is removed from your house via gravity. The sewer line is strictly installed with a certain pre-calculated slope which helps the waste to drain out without clogging the system which would result in backups.

Private sanitary sewer lines have a diameter of 4 inches but some could have a diameter of 6 inches. Their depths in the ground vary depending on the slope used and how far the house is from the street.

Your sanitary sewer line has 2 sections. These are the upper and the lower laterals.

The upper lateral is the section of the sewer line between your house and your property line. To gain access to the main line for sewer line inspection and cleaning, a sewer cleanout is usually installed on the upper lateral.

A sewer cleanout could be inside the house (in the basement) or outside the house but very close to the wall of the house. It will be sticking out a few inches from the ground with a square nut at the top of the cap.

The lower lateral is used to connect the upper lateral to the city’s sewer line. Although the upper lateral is strictly the responsibility of the homeowner, take your time to call your city to check if they are responsible for the lower lateral or if you are. It is different for different areas.

Once the waste from your house enters the public sewer line in the street, it is transported to the sewage treatment facility. There, it is treated before being circulated again for use.

As you can imagine, public sewer systems are installed across areas of varying altitudes. As such, the city cannot depend on gravity to transport the sewage to the treatment plants.

The sewage goes through a series of pumping stations along the path of the public sewer lines until it reaches the final destination. Any clogs or sewer backups in these lines is strictly the responsibility of the city.

As I mentioned clogs or sewer backs in your private sewer laterals are your responsibility. You should therefore avoid flushing wet wipes, rags, diapers, sanitary towels and anything else that is not human waste or toilet paper.

Avoid also pouring grease down the kitchen drain. Instead, pour it in sealable containers and trash it with your other home waste.

The public side of the sanitary sewer lines is accessed through manholes for maintenance. Private sewer laterals as I mentioned are accessed via sewer cleanouts and if the house has no cleanout a plumber will need to pull out a toilet or access the line from the plumbing vent.

I would recommend having your sewer line inspected and cleaned frequently especially if you have an old line or have lots of trees in your property. It is costly but better than having to deal with sewer backups.

2. Storm Sewer Systems

As we have seen, sanitary sewer systems remove waste and waste from inside the house, specifically from all fixtures with a drain. What then should you use to remove water from around your property to prevent flooding?

Storm sewers are designed to carry surface water also known as surface run off as a result of rainfall, melting snow and irrigation run off from around the house.

Houses and businesses are have storm drains installed which basically looks like manholes through which the surface water drains and flows out through underground pipes to the nearest water body.

If you have a French drain in your property, it is very possible that the French drains carries surface water from a place where it is likely to flood and drain it out into a storm drain.

It is important to mention that storm sewers do not drain into any sewage treatment facility. The runoff is deposited straight into the nearest river or stream.

It is therefore important to make sure that you do not pour/dump any toxic waste into a storm drain. The toxic waste will negatively impact water resources, marine life and basically the entire environment.

Just like sanitary sewer, storms sewer can also clog but they don’t clog as frequently as sanitary sewers. The responsibility is on the homeowner if the clog happens inside their property while out in the street the city will be responsible.

You can prevent storm sewer clogs by removing dirt/debris from around your storm drain and picking up tree leaves from your property frequently.

3. Combined Sewer Systems

Credit: New York City

Combined sewer systems are designed to carry domestic waste, surface runoff and sometimes even industrial waste in the same pipe to a sewer treatment plant.

On the surface, combined sewer systems may look practical and even economical to run but that is however not the case. I will tell you why.

During dry weather, combined sewer systems work seamlessly and all the domestic waste, storm runoff (if any) and industrial waste are transported and treated in the sewage treatment facility. That however does not happen during wet weather.

Storm runoff combined with the other types of waste will overwhelm the sewer systems and that is they these systems are designed with an overflow to the nearest stream or river. These overflows are known as combined sewer overflows (CSOs).

The bad thing is that the CSOs not only contain storm runoff but also untreated sewage from homes and industries. These toxic wastes are deposited directly into the water bodies and will definitely negatively impact them.

Another thing to know about combined sewer systems is that they can result in a large volume of wastewater reaching the sewage treatment plant. The plant will therefore be overwhelmed and not treat the wastewater properly.

According to EPA, CSOs are an environmental concern for about 772 cities in the United States which have the combined sewer systems.

Wrap Up

And basically those are the 3 main types of sewer systems. I hope you enjoyed reading this post.

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