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If you have an onsite sewage (OSS) system, you are required to operate and maintain it properly. For those who have a pump to pump the wastewater (effluent) into the drainfield or filtering system, you will be required to keep written records. Why? Because a well operated and maintained system treats the wastewater completely, and therefore protects the public's health, ground water, lakes, streams and drinking water wells.
Septic tank lid on property

How do I know where my OSS is?

There may be septic tank access covers exposed in your yard (see photo.) If no covers are visible, please contact Eastgate Environmental Health at 206-477-8050 and ask for a copy of the "as built" drawings for your property or search and download a property's as-built drawings.

As built drawings are the drawings submitted to Public Health in order to get a building permit. These drawings show where the system has been placed.

What are the components of my OSS and what do they do?

The on-site sewer system collects, treats and disposes of household wastewater. In order to do that, there are two steps: waste separation and percolation through soil.

  1. Septic tank. All of the household waste, from your toilet, bath, laundry, sinks, etc. drains into the septic tank. In the tank, it separates. The light particles rise to the top creating a scum layer, heavier particles sink to the bottom creating a sludge layer, and leaving a clearer liquid in the middle. When the sludge and scum layers get too thick, the tank needs to be pumped. The liquid remaining between the sludge and scum layers is partially treated wastewater called "effluent." There are 2 chambers in the septic tank which makes the effluent go through the separation process twice.

  2. The soil treatment effluent then moves to a soil treatment system, such as a drainfield under the yard, or a mound. In a conventional OSS, the gravity moves the effluent from the septic tank into and through the drainfield soil. For more information, see "How a septic system works".

    Most new OSS now use an electric pump chamber rather than gravity to pump the effluent into the soil treatment system. When your system includes a pump chamber, it is a 3rd component that needs monitoring and maintenance.

How does the septic tank work?

When the wastewater from sinks, toilets, tubs, garbage disposals, and laundry leaves the house it goes into the septic tank. This tank has 2 consecutive chambers in which a simple separation takes place.

The lighter waste floats to the top, and forms a "scum layer." The heavier waste sinks to the bottom and forms the "sludge layer." The clear, or "clarified" liquid in the middle is what will go out to the drainfield after the second separation chamber.

While in the septic tank, anaerobic bacteria eats and digests suspended food and waste further clarifying the wastewater that goes to the drainfield. The clearer the wastewater is, the easier it is for the drainfield to complete the purification process.

See additional details and images.

How does the drainfield work?

The drainfield is either spread out under your yard or built into your yard as a mound or other soil treatment system. No matter what kind of system you have, the function is the same.

The drain field completes the treatment. The effluent (partially treated wastewater) is piped into the drainfield, where it must trickle through sandy or loamy type soil for 2 or 3 feet. During this trickle down, aerobic bacteria and minerals in the soil break down the remaining organic material and kill the remaining germs. The soil also chemically locks up chemicals like phospates.

If treatment is not completed during this part of the process, waste can contaminate the ground water, streams, lakes and create a Public Health hazard.

What happens during the rainy season - is my drainfield compromised?

It can be if the drainfield is saturated, and the "effluent" (wastewater) doesn't have 2 to 3 feet to trickle down through. This can cause a Public Health hazard.

However, current regulations prevent building drainfields where soils might become saturated. Therefore, your drainfield should work well year round unless:

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

What kind of laundry soap should be used, powder or liquid?

Current research indicates that it doesn't matter which you use, powder or liquid, but what DOES MATTER are 2 things: 1) how much detergent you use per load, and 2) how often you wash clothes or dishes.

An important way to maintain your system is to do loads of wash over several days rather than all in one day. Use the dishwasher sparingly, too.

Remember: the system works best when wastewater has time to separate, and the bacteria have time to break down the organic matter. Too much water going into the system too often rushes water through the system too fast.