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About septic systems

On-site sewage systems, also known as OSS or septic systems, collect, treat and dispose of household sewage through two steps: waste separation and percolation through soil. In the first step, sewage from all plumbing fixtures drains into the septic tank. Lighter particles (scum) and heavier particles (sludge) separate, leaving behind a clear layer of sewage (effluent). In the second step, effluent moves into the drainfield, which is a filtering system where sewage is further treated by filtration through soil where aerobic bacteria and minerals in the soil helps to remove any remaining germs and some chemical contaminants.

Treating sewage through septic systems prevents the contamination of groundwater sources, lakes, streams, and beaches, and protects the public from health hazards. Septic systems recharge groundwater aquifers, increase stream flows, and reduce the energy needs to pump sewage to a centralized treatment plant. Septic systems provide many benefits – especially when they are properly designed, installed, and maintained.


OSS owner requirements

As a septic system owner, you need to have your septic system regularly inspected and maintained (which may include repairs and replacements as your septic system ages). If septic systems are not properly maintained, we no longer receive their benefits. Instead, they can have harmful impacts on groundwater and pollute lakes, streams, and beaches. They can even cause sewage to surface in your yard or back up into your house. To prevent this, a septic system owner also needs to operate their system properly, for example, by throwing all food waste into the compost or trash can, instead of tossing it down the drain.

How to locate your septic system

To locate your septic system, look for a septic tank access cover in your yard (see photo). Then download a copy of the OSS record drawing for your property, which can show you the location of the underground parts of your septic system.

Septic tank access covers

The State code gives these requirements for how often a septic system must be inspected.

  • Conventional gravity systems need to be inspected every 3 years.
  • All other types of septic systems need to be inspected once a year (unless the manufacturer specifies they should be inspected more frequently).
  • All septic systems that serve a food service establishment must be inspected once every year.

This project has been funded wholly or in part by the United States Environmental Protection Agency under Assistance Agreement PC-01J18001 to the Washington State Department of Health. The contents of this document do not necessarily reflect the views and policies of the Environmental Protection Agency, nor does mention of trade names or commercial products constitute endorsement or recommendation for use.


Link/share our site at www.kingcounty.gov/oss/care