Designing Your On-Site Sewage System (OSS)

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Whatcom County requires that OSS are designed by a licensed designer and installed by a licensed installer. Following is some background information that you should be discussing with your selected designer:

OSS Design Considerations

The first step to proper use of septic systems occurs before installation. Location of your tank and drainfield can be a cause of OSS failure. Placing a drainfield in an area that does not have adequate space and proper soil types can lead to problems before you even move in. If a hook-up to the community sewage system is not an option where you live, discuss these steps with a licensed designer for proper placement and installation.

  • Design your system for present and future needs.  Are you planning to expand the house or your family size in the future?  If your septic system demands will change, plan ahead.
  • Check for proper soil content to allow for effective drainage (soils can be too compact or too porous, effluent needs time to travel through the soil to allow for filtration).  This is required by Whatcom County Code 24.05.110.
  • Generally, according to the Environmental Protection Agency (EPA), the lower the soil permeability, the larger the drainfield required for adequate filtration and treatment.
  • Ensure adequate distance between the soil layer and wet-season upper level of the water table.
  • If your property is on the shoreline, place the septic system as far away from shoreline as possible.
    • Whatcom County Code for minimum freshwater setbacks in the Lake Whatcom watershed:
      • Septic tank: 50 feet
      • Drainfield edge: 100 feet
  • Redirect all surface water flow away from your drainfield.  Stormwater runoff from gutters, driveways, as well as lawn watering can overflow the drainfield and picks up other bacterial contaminants in their travels.
  • Record the exact location of the tank and drainfield for future inspections.
  • Plant grass or shallow-rooted, low maintenance, low water use plants above the drain field.  This assists the drain field with oxygen exchange and water evaporation.  Plants with deep root systems such as trees and berries can clog up the system and should be avoided.  You also want plants that are easily removed when the system requires repair or maintenance.  Plants that should be avoided include: water-loving plants (watering will saturate the soils around the drainfield), poplar, willows, lilacs, berries and root vegetables.
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