How Sewer and Septic Systems Function

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A water treatment plant in Libertyville, Illinois.

After reading the article How Toilets Work, many readers might wonder what happens next after flushing the toilet. In this article, we will examine one of the internal workings of sewer systems to comprehend how they manage billions of gallons of wastewater produced globally every day!

Why Sewer System is Essential?

Fun Fact:

NASA technology is being used to make sewage treatment more friendly to the environment. Learn more about cool NASA innovations in this interactive animation from Discovery Channel.

Each time you flush the toilet or wash anything down the sink’s drain, you produce sewage (also known as wastewater in polite society). One question that many people might ask is, “Why can’t we dump this wastewater onto the ground outside the house or into a nearby stream?” The following are the three main reasons why releasing wastewater into the environment is not recommended:

  1. It stinks. If you release wastewater directly into the environment, things get very smelly very fast.
  2. It contains harmful bacteria. Human waste naturally contains coliform bacteria (e.g. E. coli) and other bacteria that can cause disease. Once water becomes infected with these bacteria, it becomes a health hazard.
  3. It contains suspended solids and chemicals that affect the environment. For example:
    Wastewater contains nitrogen and phosphates that, being fertilizers, encourage the growth of algae. Excessive algae growth can block sunlight and foul the water.
    Wastewater contains organic material that bacteria in the environment will start decomposing. When they do, these bacteria consume oxygen in the water. The resulting lack of oxygen kills fish.
    The suspended solids in wastewater make the water look murky and can affect the ability of many fish to breathe and see.

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

Nobody wants to live in an area that stinks, has deadly bacteria, and cannot support aquatic life. That’s why communities build wastewater treatment plants and enforce laws against the release of raw sewage into the environment.

Private Treatment: The Septic Tank

In rural areas where houses are located far apart, and installing a sewer system would be too expensive, people install their sewage treatment plants. These are known as septic tanks.

A septic tank is a large concrete or steel tank 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. The tank looks something like this in cross-section:

The article discusses how septic tanks work and the process of treating wastewater. The septic tank has three layers – the scum layer on top, the sludge layer at the bottom, and a clear water layer in the middle containing bacteria and chemicals like nitrogen and phosphorous. Wastewater flows into the septic tank from the sewer pipes in the house, and as new water enters, it displaces the water already there, which flows out of the septic tank and 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. A septic system is powered by gravity and is a passive system. In urban areas, a sewer system is constructed to collect wastewater and take it to a wastewater treatment facility. Finally, the article answers the question of why manhole covers are round, which is because it avoids accidents.

The most ideal sewer system is one that relies solely on gravity, just like a septic system. Pipes from each building or house are connected to a sewer main that runs down the center of the street, which is usually around 1-1.5 meters in diameter. The sewer main occasionally has a vertical pipe that runs from the main to the surface and is covered by a manhole cover, allowing access to the main for maintenance purposes. The sewer mains are connected to increasingly larger pipes until they reach the wastewater treatment plant. To assist gravity in doing its job, the wastewater treatment plant is situated in a low-lying area, and the sewer mains follow creek and stream beds, which naturally flow downhill. However, when the lay of the land proves to be difficult for gravity to work, a grinder-pump or lift station is used to move the wastewater uphill.

Once the wastewater reaches the wastewater treatment plant, it undergoes one to three stages of treatment, depending on the plant’s sophistication. The first stage, called primary treatment, is similar to a septic tank. It lets the solids settle out of the water while the scum rises. The solids are then collected for disposal. Primary treatment involves a screen followed by a set of pools or ponds that allow the water to sit and the solids to settle out. Primary treatment can remove half of the solids, organic materials, and bacteria from the water. If the plant only does primary treatment, the water is chlorinated to kill the remaining bacteria and then discharged.

The second stage, known as secondary treatment, removes organic materials and nutrients. Bacteria aid in this process, and the water flows to large, aerated tanks where bacteria consume everything they can. The wastewater then flows to settling tanks where the bacteria settle out. Secondary treatment can remove up to 90% of all solids and organic materials from the wastewater.

The wastewater treatment process involves several stages, including primary treatment, secondary clarifiers, and tertiary treatment. The third stage varies depending on the community and wastewater composition and may involve chemical treatment, filter beds, and chlorine disinfection. The efficiency of treatment plants is measured by various factors such as pH, BOD, dissolved oxygen, suspended solids, total phosphorous and nitrogen, chlorine, and coliform bacteria count. These measures are crucial due to the significant amount of wastewater produced by communities, ranging from 10 to 100 million gallons per day. To learn more about related topics, visit HowStuffWorks articles and other useful links provided.


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

A sewer system is a network of pipes that carries wastewater from homes and businesses to a treatment facility, where it is cleaned and discharged into a body of water. A septic system, on the other hand, is a self-contained system that treats and disposes of wastewater on the property where it is generated. Septic systems are typically used in rural areas where there is no access to a municipal sewer system.

2. How does a septic system work?

A septic system consists of a large underground tank that is buried on the property. Wastewater from the house flows into the tank, where it is held long enough for solids to settle to the bottom and oil and grease to rise to the top. The liquid in the middle layer is then discharged into a drain field, where it is filtered through layers of soil and eventually absorbed into the ground.

3. What are some common problems with septic systems?

Some common problems with septic systems include clogs or blockages in the pipes, a full or overflowing tank, and damage to the drain field. These issues can be caused by a variety of factors, such as flushing non-biodegradable materials down the toilet, using too much water, or neglecting regular maintenance and pumping of the tank.

4. How often should a septic system be pumped?

The frequency of septic tank pumping depends on several factors, such as the size of the tank, the number of people in the household, and the volume of wastewater generated. As a general rule, septic tanks should be pumped every 3-5 years to prevent buildup of solids and maintain proper functioning of the system.

5. Can I add anything to my septic system to improve its performance?

Yes, there are several things you can do to help your septic system function properly. These include conserving water by fixing leaks and using low-flow fixtures, avoiding flushing non-biodegradable materials down the toilet, and adding bacteria supplements to the tank to aid in the breakdown of solids. It is important to consult with a professional before adding any supplements to ensure they are safe and effective for your specific system.

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