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About septic systems

On-site sewage systems, also known as OSS or septic systems, collect, treat and dispose of household wastewater through two steps: waste separation and percolation through soil. In the first step, wastewater moves through the septic tank where lighter particles (scum) and heavier particles (sludge) separate, leaving behind a clear layer of wastewater (effluent). In the second step, effluent moves into a filtering system (the drainfield) where wastewater is further treated by filtration through sandy or loamy soil where aerobic bacteria and minerals in the soil remove any remaining germs. Treating wastewater through OSS prevents the contamination of groundwater and freshwater sources and protects the public from health hazards.


OSS owner requirements

As an OSS owner, you need to have your septic system regularly inspected and maintained (which may include repairs and replacements as your septic system ages). Septic systems can have a direct impact on groundwater and freshwater sources if they are not properly maintained. You can also make sure to operate your system properly, for example by throwing all food waste into the compost or trash can, instead of tossing it down your drains.


Contact a certified maintainer to inspect your septic system


How to locate your OSS

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 acccess cover

Learn more about septic maintenance:

How septic systems work

  • Septic tanks
  • Pump tanks
  • Drainfield

Types of septic systems

  • Gravity drainfields
  • Pressure distribution drainfields
  • Sand filter
  • Mound systems
  • Technical review of these systems and others

Additional resources

Order free brochures at 206-477-8050 or toll-free, 800-325-6165 ext. 6-4932 for the following:

  • For homeowners
    Including condos, duplexes and mobile homes.

  • For real estate professionals
    Information for your clients who are selling or buying homes or businesses with an on-site septic system.

  • For non-residential buildings
    Including food establishments, businesses, schools, churches, etc.

See also: Be SepticSmart, materials from the Environmental Protection Agency (EPA)


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