Types of Septic Systems | US EPA

The design and size of a septic system can vary widely, from within your neighborhood to across the country, due to a combination of factors. These factors include household size, soil type, site slope, lot size, proximity to sensitive water bodies, weather conditions, or even local regulations. Below are ten of the most common types of septic systems used, followed by illustrations and descriptions of each system. The list is not all-inclusive; there are many other types of septic systems.

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See below for illustrations and descriptions of various types of conventional and alternative septic systems. More detailed information on specific technologies can be found in Fact Sheets.

Conventional Systems:

• Septic Tank
• Conventional System
• Chamber System

Alternative Systems:

• Drip Distribution System
• Aerobic Treatment Unit
• Mound Systems
• Recirculating Sand Filter System
• Evapotranspiration System
• Constructed Wetland System
• Cluster / Community System
• Community-Scale Options

Septic Tank

A septic tank is a buried, watertight tank designated and constructed to receive and partially treat raw domestic sanitary wastewater. Heavy solids settle to the bottom of the tank while greases and lighter solids float to the top. The solids stay in the tank while the wastewater is discharged to the drainfield for further treatment and dispersal.

Conventional System

A conventional decentralized wastewater treatment system consists of a septic tank and a trench or bed subsurface wastewater infiltration system, known as a drainfield. A conventional septic system is typically installed at a single-family home or small business.

The gravel/stone drainfield is a design that has existed for decades. The effluent is piped from the septic tank to a shallow underground trench of stone or gravel. A geofabric or similar material is then placed on top of the trench so sand, dirt, and other contaminants do not enter the clean stone.

Effluent filters through the stone and is then further treated by microbes once it reaches the soil below the gravel/stone trench.

Gravel/stone systems are relatively large in overall footprint and may not be suitable for all residential sites or conditions.

Chamber System

Gravelless drainfields have been widely used for over 30 years in many states and have become a conventional technology replacing gravel systems. They take many forms, including open-bottom chambers, fabric-wrapped pipe, and synthetic materials such as expanded polystyrene media. The gravelless systems can be manufactured with recycled materials and offer a significant savings in carbon footprint.

An example of a gravelless system is the chamber system. The chamber system serves as an alternative design to the gravel/stone system. The primary advantage of the chamber system is increased ease of delivery and construction. They are also well suited to areas with high groundwater tables, where the volume of influent to the septic system is variable (e.g., at a vacation home or seasonal inn), in an area where gravel is scarce, or in areas where other technologies such as plastic chambers are readily available.

This type of system consists of a series of connected chambers. The area around and above the chambers is filled with soil. Pipes carry wastewater from the septic tank to the chambers. Inside the chambers, the wastewater comes into contact with the soil. Microbes on or near the soil treat the effluent.

Drip Distribution System

The drip distribution system is a type of effluent dispersal that can be used in many types of drainfields. The main advantage of the drip distribution system is that no large mound of soil is needed as the drip laterals are inserted into the top 6 to 12 inches of soil. The disadvantage of the drip distribution system is that it requires a large dose tank after the septic tank to accommodate the timed dose delivery of wastewater to the drip absorption area. Additional components, such as electrical power, are necessary for this system, requiring an added expense and increased maintenance.

Aerobic Treatment Unit

Aerobic Treatment Units (ATUs) use many of the same processes as a municipal sewage plant, but on a smaller scale. An aerobic system injects oxygen into the treatment tank. The additional oxygen increases natural bacterial activity within the system that then provides additional treatment for nutrients in the effluent. Some aerobic systems may also have a pretreatment tank and a final treatment tank including disinfection to further reduce pathogen levels.

The benefits of this system are that it can be used in homes with smaller lots, inadequate soil conditions, in areas where the water table is too high, or for homes close to a surface water body sensitive to contamination by nutrients contained in wastewater effluent. Regular life-time maintenance should be expected for ATUs.

Mound Systems

Mound systems are an option in areas of shallow soil depth, high groundwater, or shallow bedrock. The constructed sand mound contains a drainfield trench. Effluent from the septic tank flows to a pump chamber where it is pumped to the mound in prescribed doses. Treatment of the effluent occurs as it discharges to the trench and filters through the sand, and then disperses into the native soil.

While mound systems can be a good solution for certain soil conditions, they require a substantial amount of space and periodic maintenance. 

Recirculating Sand Filter System

Sand filter systems can be constructed above or below ground. Effluent flows from the septic tank to a pump chamber. It is then pumped to the sand filter. The sand filter is often PVC-lined or a concrete box filled with a sand material. Effluent is pumped under low pressure through the pipes at the top of the filter. The effluent leaves the pipes and is treated as it filters through the sand. The treated wastewater is then discharged to the drainfield.

Sand filters provide a high level of treatment for nutrients and are good for sites with high water tables or that are close to water bodies, but they are more expensive than a conventional septic system.

Evapotranspiration System

Evapotranspiration systems have unique drainfields. The base of the evapotranspiration system drainfield is lined with a watertight material. After the effluent enters the drainfield, it evaporates into the air. Unlike other septic system designs, the effluent never filters to the soil and never reaches groundwater.

Evapotranspiration systems are only useful in specific environmental conditions. The climate must be arid and have adequate heat and sunlight. These systems work well in shallow soil; however, they are at risk of failure if it rains or snows too much.

Constructed Wetland System

A constructed wetland mimics the treatment processes that occur in natural wetlands. Wastewater flows from the septic tank and enters the wetland cell. The wastewater then passes through the media and is treated by microbes, plants, and other media that remove pathogens and nutrients. The wetland cell typically consists of an impermeable liner, and gravel and sand fill, along with the appropriate wetland plants, which must be able to survive in a perpetually saturated environment.

A wetland system can work via either gravity flow or pressure distribution. As wastewater flows through the wetland, it may exit the wetland and flow into a drainfield for further wastewater treatment into the soil.

Cluster / Community System

A cluster (or community) decentralized wastewater treatment system is under some form of common ownership and collects wastewater from two or more dwellings or buildings. It conveys the wastewater to a treatment and dispersal system located on a suitable site near the dwellings or buildings. It is common to find cluster systems in places like rural subdivisions.

Community-Scale Options

Within a region, county, city, town, or neighborhood, the systems used to treat wastewater can vary significantly and may include both centralized and decentralized treatment options. The most appropriate system is selected based on factors such as cost, land availability, population density, and environmental conditions. In this illustrated example of a town, several types of decentralized systems are shown: a conventional septic system with a drainfield serving a single home, an onsite cluster system serving a commercial shopping center, and an offsite cluster system serving multiple houses and an apartment building.

The table below the graphic describes each of the four examples of decentralized and centralized wastewater treatment.

Community-Scale Options

Type of System Description 1. Decentralized- Individual Onsite System A conventional decentralized individual onsite system (septic system) consists of a septic tank and a subsurface wastewater infiltration system, known as a drainfield. A septic system is typically installed at a single-family home or small business. Also shown are a cesspool (outdated and ineffective method of wastewater disposal with no treatment) and a “straight pipe” that discharges raw wastewater directly into a nearby stream (prohibited due to the severe health and environmental risks it poses). Learn more about how your septic system works. 2. Decentralized-Commercial Onsite Cluster System A decentralized commercial onsite cluster system is usually owned by a single property owner and collects wastewater from one or more dwellings or buildings, such as a restaurant or small business. It conveys the wastewater to a treatment and dispersal system located on the property and is typically managed by a decentralized wastewater professional. 3. Decentralized-Offsite Cluster System A decentralized offsite cluster (or community) decentralized system is under some form of common ownership and collects wastewater from two or more dwellings or buildings. It conveys the wastewater to a treatment and dispersal system located on a suitable site near the dwellings or buildings and is typically managed by a decentralized wastewater professional. 4. Centralized-Sewer System Centralized systems are publicly or privately owned sewer systems. They treat wastewater in a single, centralized location. Sewers collect municipal wastewater from homes, businesses, and industries and deliver it to a treatment plant for processing. After wastewater is treated, it is reused or discharged to surface water or ground water. Learn more about small wastewater systems.

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

The article How Toilets Work leads many readers to ask the next logical question: "So what happens after I flush the toilet?"

In this article, we will look at one of inner workings of sewer systems so that you can understand how they handle the billions of gallons of wastewater that the world produces every day!

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Why Do We Need a Sewer System?

Did You Know?

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 you wash something down the sink's drain, you create sewage (also known in polite society as wastewater). One question that many people might ask is, "Why not simply dump this wastewater onto the ground outside the house, or into a nearby stream?" There are three main things about wastewater that make it something you don't want to release into the environment:

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 (for example, 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.

No one wants to live in an place that stinks, is full of 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.­

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Private Treatment: The Septic Tank

In rural areas where houses are spaced so far apart that a sewer system would be too expensive to install, people install their own, private sewage treatment plants. These are called septic tanks.

A septic tank is simply a big concrete or steel tank that is buried in the yard. The tank might hold 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:

In this picture, you can see three layers. Anything that floats rises to the top and forms a layer known as the scum layer. Anything heavier than water sinks to form the sludge layer. In the middle is a fairly clear water layer. This body of water contains bacteria and chemicals like nitrogen and phosphorous that act as fertilizers, but it is largely free of solids.

Wastewater comes into the septic tank from the sewer pipes in the house, as shown here:

A septic tank naturally produces gases (caused by bacteria breaking down the organic material in the wastewater), and these gases don't smell good. Sinks therefore have loops of pipe called P-traps that hold water in the lower loop and block the gases from flowing back into the house. The gases flow up a vent pipe instead -- if you look at the roof of any house, you will see one or more vent pipes poking through.

As new water enters the tank, it displaces the water that's already there. This water flows out of the septic tank and into a drain field. A drain field is made of perforated pipes buried in trenches filled with gravel. The following diagram shows an overhead view of a house, septic tank, distribution box and drain field:

A typical drain field pipe is 4 inches (10 centimeters) in diameter and is buried in a trench that is 4 to 6 feet (about 1.5 m) deep and 2 feet (0.6 m) wide. The gravel fills the bottom 2 to 3 feet of the trench and dirt covers the gravel, like this:

The water is slowly absorbed and filtered by the ground in the drain field. The size of the drain field is determined by how well the ground absorbs water. In places where the ground is hard clay that absorbs water very slowly, the drain field has to be much bigger.

A septic system is normally powered by nothing but gravity. Water flows down from the house to the tank, and down from the tank to the drain field. It is a completely passive system.

You may have heard the expression, "The grass is always greener over the septic tank." Actually, it's the drain field, and the grass really is greener -- it takes advantage of the moisture and nutrients in the drain field.

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Urban Wastewater Systems

In urban and suburban areas where people are packed closer together and where there is a lot more wastewater to treat, the community will construct a sewer system that collects wastewater and takes it to a wastewater treatment facility.

Photo courtesy photos.innersource.com
Water treatment plant in Libertyville, IL Why are manhole covers round?

Because it avoids accidents. Since manhole covers are round, it is impossible for a cover to fall down the manhole. If they were square or rectangular, they could.

In the ideal case, a sewer system is completely gravity-powered, like a septic system. Pipes from each house or building flow to a sewer main that runs, for example, down the middle of the street. The sewer main might be 3 to 5 feet (1 to 1.5 m) in diameter. Periodically, a vertical pipe will run up from the main to the surface, where it is covered by a manhole cover. Manholes allow access to the main for maintenance purposes.

The sewer mains flow into progressively larger pipes until they reach the wastewater treatment plant. In order to help gravity do its job, the wastewater treatment plant is usually located in a low-lying area, and sewer mains will often follow creekbeds and streambeds (which flow naturally downhill) to the plant.

Normally, the lay of the land will not completely cooperate, and gravity cannot do all the work. In these cases, the sewer system will include a grinder-pump or a lift station to move the wastewater up over a hill.

Photo courtesy photos.innersource.com
Screw pumps

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). Here's what each stage does:

• The first stage, known as primary treatment, does the same thing a septic tank does. It allows the solids to settle out of the water and the scum to rise. The system then collects the solids for disposal (either in a landfill or an incinerator).

  Primary treatment is very simple -- it involves a screen followed by a set of pools or ponds that let the water sit so that the solids can settle out.

  Photo courtesy photos.innersource.com
  Primary screen Photo courtesy photos.innersource.com
  Primary clarifiers

  Primary treatment might remove half of the solids, organic materials and bacteria from the water. If the plant does no more than primary treatment, then the water is chlorinated to kill the remaining bacteria and discharged.

• The second stage, known as secondary treatment, removes organic materials and nutrients. This is done with the help of bacteria -- the water flows to large, aerated tanks where bacteria consume everything they can. Photo courtesy photos.innersource.com
  Aeration tank

  The wastewater then flows to settling tanks where the bacteria settle out. Secondary treatment might remove 90 percent of all solids and organic materials from the wastewater.

  Photos courtesy photos.innersource.com
  Secondary clarifier
• The third stage, known as tertiary treatment, varies depending on the community and the composition of the wastewater. Typically, the third stage will use chemicals to remove phosphorous and nitrogen from the water, but may also include filter beds and other types of treatment. Chlorine added to the water kills any remaining bacteria, and the water is discharged. Photo courtesy photos.innersource.com
  Final clarifier Photo courtesy photos.innersource.com
  Chlorination tank

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