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Septic tanks

Septic tanks above ground

Double compartment septic tank anatomy

All conventional septic systems have a septic tank, which is usually a large buried rectangular or cylindrical container made of concrete, fiberglass or polyethylene. Wastewater from all plumbing fixtures drains into the septic tank where heavy solids settle to the bottom—and bacterial action produces digested sludge and gases—and lighter solids—such as grease, oils and fats—rise to the top to form a scum layer.

Systems constructed before 1975 in King County usually have single compartment tanks. Those built since 1975 are usually two compartment tanks.

Solids that are not decomposed remain in the septic tank. It is important to remove solids from the tanks periodically by pumping them out. If they are not removed, they will continue to accumulate until they overflow into the drainfield. This eventually leads to drainfield plugging and drainfield failure. The first signs may be slow draining fixtures, however the system may fail by discharging sewage effluent to the ground or back-up into the house.

Double compartment septic tank anatomy

Double compartment septic tank anatomy

Single compartment septic tank anatomy

Single compartment septic tank anatomy

Wastewater leaving the septic tank is a liquid called effluent. While it has received some treatment in the septic tank, it is still unpleasant smelling and contains disease-causing organisms, organic wastes, and other pollutants. This effluent needs to go through the next stage of wastewater treatment (the drainfield) or it can lead to significant environmental and public health problems.


In the drainfield, liquid from the septic tank flows through pipes in the yard for final treatment by organisms in the soil.

The effluent trickles through sandy or loamy soil for 2 or 3 feet, where aerobic bacteria and minerals in the soil break down the remaining organic material and kill the remaining germs. The soil also locks up chemicals such as phosphates.

Current regulations prevent building drainfields where soils might become saturated. Therefore, your drainfield should work well year-round (even during the rainy season) unless:

  • It is very old
  • The drainfield area has changed (caused by extensive landscaping changes, increased building, etc.), or
  • You use more water than the system is designed for.

If the drainfield is saturated and/or effluent doesn't have 2 to 3 feet to trickle down through, treatment cannot be completed. This can lead to contamination of groundwater and fresh water sources.

Your yard should also have a "reserve area" where your replacement drainfield will be located if the drainfield fails.

For landscaping and maintenance tips, please see Landscaping your drainfield.

Pump tanks

Some systems have a pump tank in addition to a septic tank. This pump tank contains a sewage effluent pump, control floats, and a high-water alarm. The control floats are set so that a specific volume of effluent is sent to the drainfield. This specific amount is referred to as a dose. When effluent in the pump tank reaches the "on" float, the pump is activated and pumps effluent out until it reaches the "off" float (See diagram below.)

See brochure on "Understanding and Caring for Your Septic Tank System" by the Washington State Dept. of Health.

Pump tank anatomy

Pump tank anatomy

If the "on" float or pump fail to work, an alarm will activate once effluent reaches the alarm float. Once this alarm is activated there is some emergency storage space available in the pump tank before the system overflows or backs up into the house. Homeowners should be aware of the location of the alarm device (see your as-built information for details). The alarm device is commonly located under the kitchen sink or in the garage and can be deactivated; however, this will not resolve the problem and, in such an event, homeowners should immediately begin emergency water conservation measures and call an On-site System Maintainer or contact the Eastgate Health District Office at 206-477-8050.