Understanding the Functioning of Sewer and Septic Systems

Home Improvement

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

After reading the article ‘How Toilets Work’, many readers are curious to know what happens to the wastewater after it is flushed. This article will delve into the inner workings of sewer systems and how they manage millions of gallons of wastewater generated around the world every day!

Why is a Sewer System Essential?

Did You Know?

NASA technology is being utilized to make sewage treatment eco-friendly. Find out more about innovative NASA creations in this interactive animation from Discovery Channel.

Whenever you flush the toilet or pour something down the sink drain, it produces sewage, also referred to as wastewater. One may wonder why this wastewater cannot be released outside the house or into a nearby stream. Here are the three main reasons why:

1. It produces a foul odor. If wastewater is released into the environment, it can start producing an unpleasant smell immediately.
2. It contains harmful bacteria. Human waste naturally contains bacteria, such as coliform bacteria (e.g., E. coli), that can lead to illnesses. Once this water is contaminated with such bacteria, it becomes hazardous to health.
3. It contains chemicals and suspended solids that have an impact on the environment. For instance:
   Wastewater consists of nitrogen and phosphates that promote algae growth. Overgrowth of algae can block sunlight and pollute the water.
   Wastewater contains organic matter that bacteria in the environment begin to decompose. When they do, they consume oxygen from the water. The resulting oxygen depletion kills fish.
   The suspended solids in wastewater make the water appear murky and can hinder the breathing and vision of several fish species.
   
   The excessive algae, reduced oxygen, and murkiness make the stream or lake incapable of sustaining aquatic life, leading to the death of fish, frogs, and other organisms.

No one wants to live in an area that smells bad, is full of harmful bacteria, and does not support aquatic life. Therefore, communities establish wastewater treatment plants and enforce laws against releasing raw sewage into the environment.

Private Treatment: The Septic Tank

In rural areas where houses are far apart and installing a sewer system is expensive, people set up their own sewage treatment plants, called septic tanks.

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

The article explains the process of how a septic tank system works. The tank has three layers which are the scum layer, sludge layer, and clear water layer. The scum layer is formed when anything that floats rises to the top, while the sludge layer is formed when anything heavier than water sinks to the bottom. The clear water layer contains bacteria and chemicals such as nitrogen and phosphorous that act as fertilizers. The wastewater comes from the sewer pipes in the house and enters the septic tank. As new water enters the tank, it displaces the water that is already there, and this water flows into a drain field. The drain field consists of perforated pipes buried in trenches filled with gravel, and the water is slowly absorbed and filtered by the ground. The size of the drain field depends on how well the ground absorbs water. The septic system is powered by gravity, which means it is a passive system. In urban and suburban areas, where there is a lot more wastewater to treat, they use a sewer system that collects wastewater and takes it to a wastewater treatment facility. Finally, the article ends with the explanation of why manhole covers are round to avoid accidents.

Ideally, a sewer system would rely solely on gravity, similar to a septic system. Each building would have pipes that connect to a main sewer line, which runs down the street, typically 3 to 5 feet in diameter. The main sewer line is equipped with vertical pipes that lead to the surface and are covered by manhole covers for maintenance purposes. The sewer lines gradually increase in size until they reach the wastewater treatment plant. The plant is usually situated in a low-lying area, and the sewer lines follow creekbeds and streambeds to take advantage of the natural downhill flow. However, sometimes the lay of the land does not allow for this natural flow, and a grinder pump or lift station may be used to transport wastewater uphill.

Once the wastewater reaches the treatment plant, it undergoes one, two, or three stages of treatment, depending on the complexity of the plant. The first stage, primary treatment, is similar to that of a septic tank. Solids settle out of the water and scum rises to the top. The solids are then collected for disposal. Primary treatment involves a screen and a series of pools or ponds that allow the water to sit, allowing the solids to settle out. This stage removes around half of the solids, organic materials, and bacteria from the water. If the plant only performs primary treatment, the water is chlorinated to eliminate any remaining bacteria before being discharged.

The second stage, secondary treatment, removes organic materials and nutrients using bacteria. The water flows into large, aerated tanks where bacteria consume all they can. The wastewater then flows to settling tanks where the bacteria settle out. This stage can remove up to 90% of the solids and organic materials from the wastewater.

The article discusses the different stages of wastewater treatment and the indicators used to measure the effectiveness of the treatment plant. The first stage, known as primary treatment, involves removing large solids and grit from the water. The second stage, secondary treatment, uses biological processes to remove organic matter and suspended solids from the water. The third stage, tertiary treatment, varies depending on the community and may involve the use of chemicals and filter beds to remove phosphorus and nitrogen from the water. The article also discusses the importance of monitoring indicators such as pH, BOD, dissolved oxygen, suspended solids, total phosphorus and nitrogen, chlorine, and coliform bacteria count. These indicators are important because wastewater treatment plants can produce large quantities of wastewater, and discharge levels can range from 10 million to 100 million gallons per day. The article concludes with links to related articles and resources.

FAQ

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

A sewer system is a network of pipes and pumps that transport wastewater from homes and businesses to treatment plants for processing. A septic system, on the other hand, is a self-contained wastewater treatment system that is installed on a property and treats and disposes of wastewater on site.

2. How does a sewer system work?

A sewer system uses gravity and pumps to transport wastewater from homes and businesses to treatment plants. Wastewater enters the system through pipes in homes and businesses and flows downhill to larger pipes in the street. These pipes then flow to even larger pipes, eventually leading to the treatment plant. The treatment plant then processes and cleans the wastewater before releasing it back into the environment.

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

Once the wastewater arrives at the treatment plant, it is first screened to remove any large objects or debris. Then, it undergoes a series of processes including sedimentation, aeration, and disinfection to remove contaminants and pathogens. The treated wastewater is then discharged into a nearby body of water or used for irrigation.

4. How does a septic system work?

A septic system is comprised of a septic tank and a drain field. Wastewater from homes and businesses flows into the septic tank, where it is held and separated into layers. The heavier solids settle to the bottom of the tank and are broken down by bacteria, while the lighter materials and liquids float to the top and exit the tank through an outlet pipe. The remaining liquid then flows into the drain field where it is filtered through layers of soil and cleaned before returning to the groundwater.

5. What are some common problems with sewer and septic systems?

Common problems with sewer systems include clogs, backups, and leaks in pipes. These issues can lead to overflows and sewage spills, causing health hazards and environmental damage. For septic systems, common problems include a buildup of solids in the tank, clogs in the drain field, and leaks in the tank or pipes. These issues can cause a backup of wastewater and potential contamination of groundwater.

6. How can I maintain my sewer or septic system?

Maintaining your sewer or septic system involves regular inspections, cleaning, and pumping. For sewer systems, it is important to avoid flushing non-biodegradable items and to dispose of hazardous materials properly. For septic systems, it is important to avoid overloading the system with excessive water usage and to have the tank pumped every 3-5 years.

7. Can I install a septic system on any property?

No, not all properties are suitable for a septic system. Factors such as soil type, topography, and proximity to water sources must be considered when determining if a property is suitable for a septic system. It is important to consult with a professional before installing a septic system.

8. How can I tell if my sewer or septic system is failing?

Signs of a failing sewer system include slow drains, foul odors, and sewage backups. Signs of a failing septic system include wet areas or standing water in the drain field, foul odors, and slow draining fixtures. If you notice any of these signs, it is important to have your system inspected by a professional.