Understanding the Functioning of Sewer and Septic Systems

Home Improvement

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

After reading the article on How Toilets Work, many readers are curious about what happens next. This article will explore the inner workings of sewer systems to understand how they manage the billions of gallons of wastewater produced worldwide every day!

Why is a Sewer System Necessary?

Fun Fact:

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

Each time something is flushed down the toilet or washed down the sink’s drain, it produces sewage (also known as wastewater in polite society). One may wonder why this wastewater cannot be dumped onto the ground outside the house or into a nearby stream. There are three primary reasons why wastewater cannot be released into the environment:

1. It smells: Releasing wastewater into the environment results in an unpleasant odour.
2. It contains harmful bacteria: Human waste contains coliform bacteria (e.g. E. coli) and other bacteria that can cause disease. When water gets infected by these bacteria, it becomes a health risk.
3. It contains suspended solids and chemicals that harm the environment: Wastewater contains nitrogen and phosphates that act as fertilizers, promoting the growth of algae. Excessive algae growth can block sunlight and foul the water. Wastewater also contains organic material that bacteria in the environment will begin to decompose. As they do, these bacteria consume oxygen in the water, leading to a lack of oxygen that kills fish. Suspended solids in wastewater make the water appear murky and affect the ability of many fish to breathe and see. Increased algae, reduced oxygen, and murkiness destroy the ability of a stream or lake to support wildlife, and all the fish, frogs, and other life forms quickly die.

No one wants to live in a place that smells bad, is full of deadly bacteria, and cannot support aquatic life. That is 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 the houses are too far apart to install a sewer system, people install their private sewage treatment plants known as septic tanks.

A septic tank is a large concrete or steel tank buried in the yard that might hold 1,000 gallons (4,000 liters) of water. The wastewater enters the tank at one end and exits at the other. The tank looks similar to this in cross-section:

The article explains how a septic tank system works. The tank has three layers: the scum layer, sludge layer, and a clear water layer. The wastewater enters the tank from the house and is broken down by bacteria. The gases produced by the bacteria are vented through a pipe that goes up to the roof. The water flows out of the tank and into a drain field, where it is absorbed and filtered by the ground. The size of the drain field depends on how well the ground absorbs water. A septic system is a passive system that operates on gravity. In urban areas, a sewer system collects wastewater and takes it to a treatment facility.

The article also explains why manhole covers are round. They are round to prevent accidents because a square or rectangular cover could fall down the manhole.

In an ideal scenario, a sewer system would rely solely on gravity to function, similar to a septic system. Pipes from each building or house would connect to a large sewer main that runs down the middle of the street. The sewer main is usually 3 to 5 feet in diameter and occasionally a vertical pipe runs up from the main to the surface, covered by a manhole cover, to allow for maintenance purposes. These sewer mains gradually flow into larger pipes until they reach the wastewater treatment plant. To facilitate gravity, the wastewater treatment plant is usually located in a low-lying area and the sewer mains will follow creeks or streams that naturally flow downhill. However, in cases where the lay of the land does not cooperate, a grinder-pump or lift station is used to move the wastewater uphill.

Once the water reaches the wastewater treatment plant, it goes through one, two or three stages of treatment depending on the sophistication of the plant. The first stage, called primary treatment, allows solids to settle out of the water and scum to rise, similar to a septic tank. The solids are collected for disposal, either in a landfill or an incinerator. Primary treatment is simple and involves a screen followed by pools or ponds to enable the water to sit so that the solids can settle out. Primary treatment removes about half of the solids, organic materials and bacteria from the water. If the plant only does primary treatment, then chlorination is used to kill the remaining bacteria and the water is discharged.

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

The wastewater treatment process involves several stages to remove contaminants from the water. The first stage is called primary treatment, which uses physical processes to remove large solid waste and debris from the water. The second stage, known as secondary treatment, uses biological processes to remove dissolved and suspended organic materials. The third stage, called tertiary treatment, involves chemical and physical processes to remove contaminants such as phosphorous and nitrogen from the water. Chlorine is added to the water to kill any remaining bacteria, and the water is then discharged. The effectiveness of the wastewater treatment plant is measured by several indicators such as pH, BOD, dissolved oxygen, suspended solids, total phosphorous and nitrogen, chlorine, and coliform bacteria count. These indicators need to be monitored closely due to the large quantity of wastewater produced. The wastewater treatment process is crucial for protecting the environment and public health.

FAQ

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

A sewer system is a centralized network of pipes and treatment plants that carries wastewater from homes and businesses to a treatment facility. A septic system is an individual underground system that treats and disposes of wastewater from a single household or building.

2. How does a sewer system work?

A sewer system uses gravity or pumps to transport wastewater from homes and businesses to a treatment plant, where it is treated and discharged into the environment. The system includes pipes, manholes, lift stations, and treatment facilities.

3. What happens to the wastewater in a treatment plant?

At a treatment plant, wastewater is treated through a series of physical, chemical, and biological processes to remove pollutants and contaminants. The treated water is then discharged into a river or ocean, or used for irrigation or industrial processes.

4. How do septic systems work?

A septic system consists of a septic tank and a drainfield. The septic tank collects and treats wastewater from a household or building. Solids settle to the bottom of the tank and are decomposed by bacteria, while liquid effluent flows out of the tank and into the drainfield, where it is filtered and treated by the soil.

5. How often does a septic system need to be pumped?

A septic system should be pumped every 3-5 years, depending on the size of the tank and the household usage. Regular pumping helps prevent solids from accumulating in the tank and clogging the drainfield.

6. What are some signs that a septic system is failing?

Signs of a failing septic system include sewage backups, slow drains, foul odors, and wet spots or lush vegetation in the drainfield area. If you notice any of these signs, you should have your system inspected and repaired by a professional.

7. Can you use a garbage disposal with a septic system?

While it is not recommended, you can use a garbage disposal with a septic system if you take care to limit the amount of food waste and grease that goes down the drain. Excessive food waste and grease can clog the system and cause problems.

8. Can you build on top of a septic system?

Building on top of a septic system is not recommended, as it can damage the system and cause it to fail. If you need to build on the property, you should consult with a professional to determine the location of the system and ensure that it is not disturbed.

9. How can you maintain a septic system?

To maintain a septic system, you should have it inspected and pumped regularly, conserve water by fixing leaks and using water-efficient appliances, avoid flushing non-degradable items like wipes and feminine hygiene products, and avoid using harsh chemicals that can kill the beneficial bacteria in the tank.