Understanding the Functioning of Sewer and Septic Systems

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Home Improvement

Photo courtesy photos.innersource.com
The water treatment plant in Libertyville, Illinois.

After reading the article on How Toilets Work, readers often wonder about the process that takes place after flushing the toilet. This article will delve into the inner workings of sewer systems, explaining how they handle the billions of gallons of wastewater produced worldwide, every day!

Why Do We Require Sewer Systems?

Did You Know?

NASA has designed technology being used to make sewage treatment eco-friendlier. To learn more about cool NASA innovations, check out this interactive animation from Discovery Channel.

Every time we flush the toilet or wash something down the sink’s drain, we create wastewater, also known as sewage. One might wonder why not dump this wastewater outside the house or into the nearby stream? Three key aspects of wastewater make it unsuitable for release into the environment:

  1. It is malodorous. Directly releasing wastewater in the environment can lead to a foul smell.
  2. It contains harmful bacteria. Human waste naturally contains coliform bacteria, such as E. coli, and other bacteria that can cause diseases. Infected water can become a health hazard.
  3. It contains suspended solids and chemicals that can impact the environment. For instance:
    Wastewater comprises nitrogen and phosphates that act as fertilizers, encouraging algae growth. Excessive algae can block sunlight and foul the water.
    Wastewater contains organic material that bacteria in the environment will start decomposing. Bacteria consume oxygen in the water, leading to a decrease in oxygen levels, which kills fish.
    The suspended solids in wastewater can make the water look murky, impacting the ability of fish to breathe and see.

    The increased algae, reduced oxygen, and murkiness can harm the ability of a stream or lake to support wildlife, and all fish, frogs, and other life forms die quickly.

No one would want to reside in a location that smells bad, contains deadly bacteria, and cannot support aquatic life. That’s why communities build wastewater treatment plants and impose regulations against the release of raw sewage into the environment.

Private Treatment: The Septic Tank

In rural areas where homes are spaced far apart, making the installation of a sewer system too expensive, people construct their private sewage treatment plants known as septic tanks.

A septic tank is a large concrete or steel tank that is buried in the yard. The tank can hold up to 1,000 gallons (4,000 liters) of water. Wastewater flows into the tank at one end and leaves the tank at the other end. The cross-section of the tank looks like this:

The septic tank system is a simple way of treating wastewater in areas where sewer systems are not available. The tank is made up of three layers, with the scum layer floating on top, sludge layer sinking to the bottom, and clear water layer in the middle. The water from the house goes into the tank and then flows into a drain field made of perforated pipes buried in trenches filled with gravel. The water is slowly absorbed and filtered by the ground in the drain field. The size of the drain field depends on how well the ground absorbs water. A septic system uses gravity and is a passive system. In urban areas, a sewer system collects wastewater and takes it to a wastewater treatment facility. Manhole covers are round to prevent accidents.

Ideally, a sewer system is powered by gravity, similar to a septic system. Pipes from buildings flow into a sewer main, which is usually located in the middle of the street and can be 1 to 1.5 meters in diameter. Manholes are placed at the surface to provide access for maintenance. The sewer main connects to larger pipes that lead to the wastewater treatment plant located in a low-lying area. If the landscape does not allow gravity to do all the work, a grinder-pump or lift station is used to move the wastewater uphill. Once at the treatment plant, the wastewater goes through one, two or three stages of treatment, depending on the plant’s sophistication. Primary treatment allows solids to settle out of the water, while secondary treatment removes organic materials and nutrients with the help of bacteria.

The article discusses the process of wastewater treatment and the different stages involved. The first stage involves removing large solids and debris through screening and settling. The second stage, called primary treatment, removes smaller solids and organic matter through sedimentation and aeration. The third stage, known as tertiary treatment, varies depending on the community and composition of the wastewater, but typically involves the use of chemicals to remove phosphorous and nitrogen, as well as filter beds and other types of treatment. Chlorine is added to the water to kill any remaining bacteria before it is discharged.

The effectiveness of wastewater treatment plants is measured on several scales, including pH, BOD, dissolved oxygen, suspended solids, total phosphorous and nitrogen, chlorine, and coliform bacteria count. These indicators need to be closely monitored because communities produce a large amount of wastewater, with discharge levels ranging from 10 million to 100 million gallons per day.

The article also provides links to related articles on HowStuffWorks, as well as other helpful resources.


1. What is the difference between a sewer system and a septic system?

A sewer system is a network of underground pipes that transport wastewater from homes and businesses to a treatment plant for processing. A septic system, on the other hand, is a self-contained system that treats and disposes of wastewater on-site, usually in a homeowner’s yard.

2. How does a sewer system work?

Wastewater from homes and businesses flows through pipes into a sewer system. The sewer system then transports the wastewater to a treatment plant for processing. At the treatment plant, the wastewater is treated to remove contaminants, such as bacteria and viruses, before it is released into the environment.

3. How does a septic system work?

A septic system consists of a septic tank and a drainfield. Wastewater from the home flows into the septic tank, where solid waste settles to the bottom and is broken down by bacteria. The liquid wastewater then flows into the drainfield, where it is filtered through layers of soil before being absorbed into the ground.

4. What are the benefits of a septic system?

One benefit of a septic system is that it can be more cost-effective than connecting to a sewer system. Septic systems also allow homeowners to have more control over their wastewater treatment and disposal. Additionally, septic systems can be used in rural areas where sewer systems are not available.

5. What are the potential problems with a septic system?

If a septic system is not properly maintained, it can lead to problems such as backups, clogs, and odors. A failing septic system can also contaminate groundwater and surface water, which can pose a health risk to humans and animals.

6. How can I maintain my septic system?

To maintain a septic system, it is important to have it inspected and pumped regularly by a professional. Homeowners should also be careful about what they put down their drains, as certain materials can clog or damage the system. Additionally, homeowners should avoid planting trees or shrubs near the septic system, as roots can damage the pipes and tank.

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