What Is an Individual Wastewater Disposal System?

What You Need to Know About an Individual Wastewater Disposal System

Septic system construction
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An individual wastewater disposal system (IWDS) takes the wastewater from a single home or business, separates out solids, and purifies it, typically using the soil in a drain field on the property. It's also known as a septic system and can be an environmentally friendly way to get rid of wastewater if the system is well designed and properly maintained.

Learn more about the different types of IWDS and the factors that go into deciding which IWDS would be the best for your circumstances.

What Is an IWDS?

Homes and businesses that are not connected to a centralized municipal sewage system for the treatment of wastewater must have some type of IWDS. This setup is more common in the country or the outlying suburbs of a city; homes and businesses in cities are almost always incorporated into the sewage system, and people who live there typically pay a monthly fee for sewage treatment as part of their bill for water use.

An IWDS handles wastewater from toilets, sinks, and clothes washers. It generally consists of an underground septic tank, a system of pipes, and a drainage field. The system is designed to prevent contaminated water from entering groundwater or running off aboveground.

Alternate names: septic system, onsite wastewater treatment system

How Does an IWDS Work?

There are several alternatives to a traditional septic system if a particular property isn't well suited for the standard setup. But an IWDS generally works in the following way:

A pipe directs all sources of wastewater from your home into a septic tank, which is built to last a long time and may be constructed of concrete, fiberglass, or polyethylene. Inside the tank, solids settle to the bottom and start to decompose. Oil and grease float at the surface.

In order to maintain the septic system and extend its life, the septic tank must be pumped regularly—every three to five years—to remove the floating scum and the sludge at the bottom.

Wastewater exits the tank through a screened pipe that helps prevent nonliquids from leaving the tank. It enters the drain field through a series of perforated pipes that allow water to seep through gravel and then into the ground. (A geotextile may be used to prevent soil from collecting within the gravel and reducing its effectiveness.) As the water trickles down through the soil, it is purified by microbes. By the time any of the water reaches groundwater level several feet below the surface, it should be free of contaminants.

How to Choose an IWDS

The process of selecting a type of IWDS starts once the ground has been broken. Many options may no longer be suitable due to the type of soil, topography, and other conditions that can affect the location of the wastewater system.

The type of IWDS that's best for your property depends upon five major factors:

  • Soil permeability
  • Seasonal water table depths
  • Size and shape of the building
  • Location of the building
  • Uses of the property

Soil Permeability

Permeability is the rate at which water will move through soil. Soils that have low clay content usually will have high permeability rates and so require a smaller wastewater absorption area. Soils with high clay content, typically reddish in color, are not the best types of soil for IWDSs. But soils that are too highly permeable are also not ideal because the water may move through the soil too quickly to allow for efficient purification.

In most states, the owner of a proposed building site that doesn't have access to a sewer system must conduct a percolation, or perc, test to determine the permeability of the soil. If the site fails the perc test, the owner may not construct a building with a typical IWDS there.

Vertical Separation to Seasonal Water Tables

States differ on the number of vertical feet of soil that must be between the lowest depth of the drain field and the highest depth of groundwater, but many require 4 feet. Because the groundwater table can rise and fall with the seasons, depending on the amount of rainfall that has been received, the highest level of the groundwater must be the depth used to determine the vertical separation. If the water table is too high in a certain location and the required number of feet of vertical separation can't be achieved, the wastewater treatment system can't be built there.

A period of heavy rain that saturates the ground may make it difficult for the drainage field to do its job. It can result in water backing up into the septic tank or untreated wastewater building up near the IWDS.

Property Size and Shape

Some parcels of property are so small or narrow that certain types of wastewater systems cannot be physically installed. Municipal code regulations regarding the minimum installation distance from property lines, buildings, water wells, and other features of the property may also affect the type and layout of the IWDS.

Sometimes the combination of parcel shape with low permeability soils will reduce the IWDS options available. Size is the factor that most often renders a property unsuitable for an individual wastewater system. With enough property to work with, an effective wastewater system can be designed at almost every site.

Home Placement on the Property

Sometimes the area on the property selected for the building is also the best site for the wastewater system. In this case, a more expensive system or even an off-site system may be required.

Building Use

A few types of wastewater systems, such as aerobic treatment units, need to be used daily to function properly. These systems are not recommended for buildings that are used only occasionally, such as weekend or vacation homes.

Alternatives to a Traditional IWDS

There are seven common alternatives to the traditional IWDS with gravel and perforated pipes. The one you might choose depends on factors unique to your property. Some of the options require the construction of a pumping station and other components that require regular maintenance.

These alternatives are typically more expensive to build and operate than a traditional IWDS, but they may allow for construction on a site that wouldn't otherwise be suitable for a septic system.

Mound System

Effluent is pumped into an aboveground mound consisting of a gravel bed or gravel trenches on top of a thick layer of sand. The wastewater is purified before it moves into the natural soil below the mound. This system is typically used in a location with high groundwater or a tight clay soil.

A high layer of bedrock may also complicate the construction of an IWDS and prevent the use of a traditional system.

Drip Distribution System

In a drip distribution system, a pump tank distributes effluent through a supply line and then into a network of drip tubes that are no more than a foot below the surface. The wastewater is treated by the soil below, so there's no need for a potentially unsightly mound.

Sand Filter System

This type of system is similar to the mound system in that it uses sand as the purifying medium, but in this case, the sand is inside a large box that's typically made of concrete, fiberglass, or polyethylene. The wastewater goes from the septic tank to a pump tank, which regulates the rate at which the effluent is sent through the sand filter. From there, it may flow or be pumped into a drain field.

Chamber System

In this system, wastewater flows into a series of large plastic chambers that resemble bottomless pipes. Because of their size, the chambers allow for a bigger absorption area than a traditional IWDS and are useful in areas with a high water table.

Aerobic Treatment Unit

Systems that use an aerobic treatment unit (ATU) are like municipal sewage treatment plants for an individual building. Oxygen is injected into the ATU, where bacteria break down wastes. Additional treatment may be necessary after effluent goes through the ATU to ensure pathogens are destroyed.

Evapotranspiration System

This type of system is different from the others in that the effluent evaporates from an open-topped, sand-filled tank whose drain field is lined with a waterproof material. This system can be considered only in an arid climate where evaporation consistently exceeds rainfall.

Constructed Wetland System

This system involves constructing a manmade wetland that has the same waste treatment properties as natural wetlands. Wastewater flows into an area filled with suitable plants, and the plants and associated microbes remove nutrients and pathogens. The wetland is typically built above a waterproof liner, and wastewater may flow from the wetland into a drain field for additional treatment underground.

Key Takeaways

An individual wastewater disposal system purifies the wastewater from a single property.


Another name for an IWDS is a septic system.


The type of IWDS that may be constructed on a given property depends on several factors, including the permeability of the soil and the size and shape of the property.


If a typical IWDS can not be built on a certain site, there are alternative systems that may be constructed instead.