Types of Septic Systems

Gravity Pressure Distribution Mound Sand Filter Other Types Outdated

There are a number of different types of septic (on-site sewage) systems. Different systems exist because a property's soil and water conditions can vary. It's important to know what type of system you have so you can properly maintain it.

Find out what type of system you have – If you live in Clallam, Clark, Island, Jefferson, King, Kitsap, Pierce, Skagit, Skamania, or Thurston counties you can get information about your septic system by using the onlineRME Property Search. For other counties, or if you can't find information online about your system, contact your local health department.

Gravity System

A gravity system consists of three parts: septic tank, drainfield, and soil beneath the drainfield. A gravity system requires at least 3 feet of native suitable soil beneath the drainfield.

How a Gravity System Works

As wastewater flows from the house into the septic tank through the inlet baffle, heavy solids settle to form a sludge layer on the bottom of the tank. Lighter materials, including oil and grease, float to the top forming a scum layer. The wastewater liquid in the middle flows through the outlet baffle into the next component of the system.

Regarding the outlet baffle:

  • If your system doesn't already have one, consider installing an outlet baffle filter. These filters fit into the outlet baffle of your septic tank and add an extra barrier that prevents suspended solids from moving into the drainfield. Don't forget to clean your filter every 6 to 12 months.
  • If you have a concrete baffle, you may want to hire someone to retrofit a plastic baffle in its place. Concrete baffles tend to degrade in septic tanks due to corrosive gases. A filter can be easily installed on a plastic outlet baffle.

Gravity systems typically use a distribution box (d-box) to equally distribute the wastewater into each lateral pipe in the drainfield. Once the wastewater reaches the lateral pipes, it flows out of small holes into a gravelled trench eventually reaching the surrounding soil. Oxygen loving bacteria and other microbes treat the wastewater by removing pathogens. This final stage of treatment is critical in protecting groundwater and surface water.

Pressure Distribution System

Pressurized systems include a pump chamber that collects treated wastewater from the septic tank. A pressure distribution system is used when the soil and site conditions require controlled dosing, as when there's just 2-3 feet of native suitable soil beneath the drainfield. The pump chamber contains a pump, pump control floats, and a high-water alarm float. The pump action can be controlled either by the use of control floats or by timer controls.

How a Pressure Distribution System Works

Wastewater goes from the septic tank and flows into the pump chamber. The pump itself is located on the floor of the pump chamber. There are floats inside the chamber used to control the pump. The On/Off float turns the pump on as the wastewater rises to a preset level. The pump spreads the wastewater equally throughout the drainfield lines. When the wastewater level inside the pump chamber drops, the pump turns off and gives the drainfield soil time to absorb the wastewater. If the pump fails or too much wastewater enters the chamber, the highest float will rise and sound an alarm on a control panel near the house. Silence the alarm and contact a licensed professional.

Note: Not all pump tanks use a float system. Some systems use a timer control panel rather than On/Off floats.

Mound System

The mound is a drainfield bed that is raised above the natural soil surface with a specific sand fill material, all covered by suitable cover soil. They can be used when there's only 1 or 2 feet of native suitable soil. Within the sand fill is a gravel-filled or gravelless chamber bed with a network of small diameter pipes.

How a Mound System Works

The pump sends wastewater to the mound drainfield bed in controlled doses for even distribution to the pipes. The wastewater leaves the pipes under low pressure through the small holes in the pipes and trickles downward through the gravel and into the special sand fill. The wastewater is treated as it moves through the sand and into the natural soil.

Sand Filter System

A typical sand filter is a PVC-lined or concrete box filled with a specific sand fill material. A network of small diameter pipes is placed in a gravel-filled or gravelless chamber bed on top of the sand. They can be used when there's only 1.5 feet of native suitable soil under the receiving drainfield.

How a Sand Filter System Works

The pump tank sends wastewater to the sand filter box in controlled doses for even distribution to the pipes. The wastewater leaves the pipes, trickles downward through the gravel, and is treated as it filters through the sand. A gravelled underdrain collects and moves the treated wastewater to either a second pump chamber for discharge to a pressure distribution drainfield or to a gravity flow drainfield. The second pump chamber may be located in the sand filter box.

Other Types

There are several other types of systems for use on properties that don't have enough native suitable soil depth to provide adequate treatment alone. Aerobic Treatment Units (ATU) and BioFilter systems are a couple system types in this category. Contact your local health department for more information on these types of systems.

Aerobic Treament Unit (ATU)

Aerobic treatment units can be used as a pretreatment device in places having a minimum of 1 to 1.5 feet of native suitable soil under the receiving drainfield. A blower, or aerator, injects air into the ATU enhancing the aerobic microbial action. This type of unit will often require some form of disinfection such as chlorine or UV treatment before the wastewater enters the drain field. An ATU is required by the manufacturer and often the local health jurisdiction to be inspected at least annually or sometimes more often by a manufacturer-certified representative and/or local health jurisdiction representative.


BioFilters are designed and installed on properties where there's as little as 1 foot of native suitable soil available. The product is stand-alone and requires no additional drainfield-like component. The manufacture requires BioFilter-approved professionals to design, install, and perform maintenance on these systems.

Outdated Systems

Septic Tanks

In the past, many different containers have been used as septic tanks and they may no longer be safe or effective. Prior to the 1970s, septic tanks could have been made from cinderblocks, wood, 55-gallon drums, or concrete (manufactured or homemade). From about 1965 to 1975, single compartment concrete or steel tanks were common. Metal tanks will corrode and have been known to collapse. If you have an older system installed prior to the 1970s, find out what type of septic tank you have by contacting your local health department. Older systems may need to be upgraded so they are safe and effective, or they may need to be abandoned and replaced with a new septic system.


Older septic systems were sometimes built with no septic tank, for either part or all of the wastewater. Untreated wastewater going directly into a bottomless tank is a cesspool. This type of system may meet the definition of a failure and would need to be abandoned and replaced with a new septic system.

Seepage Pit

Wastewater enters a septic tank receiving first-level treatment and then flows into a bottomless chamber, the seepage pit, which is usually several feet in depth. Necessary oxygen is not usually present at these depths to provide final treatment, allowing untreated wastewater to discharge into the ground. This may meet the definition of a failure and would need to be abandoned and replaced with a new septic system.

Contact your local health department to identify outdated systems and get advice on septic system upgrades or replacements.

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