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On-site Sewage Systems (OSS)

Small on-site sewage systems, also known as septic systems, treat wastewater from private residences and restaurants and are used extensively statewide. Defined in Chapter 246-272A WAC, small on-site sewage systems (OSS) are those sewage systems that have flows of less than 3,500 gallons per day. There are about 950,000 OSS in Washington. OSS that are properly designed, installed, and maintained can effectively treat residential wastewater for a long time.  However, system failures can and do occur.  When this happens both public health and the environment are threatened. An OSS failure can be caused by bad system design, improper maintenance, or simply because the system has reached the end of its life expectancy.  System owners are often not aware when their OSS has stopped functioning properly and failing OSS are not detected. There are many documented cases where failing OSS have polluted surrounding areas. This site explains the importance of properly functioning OSS, how some areas are environmentally more sensitive to pollution from OSS, and the different roles state agencies, local health jurisdictions, and homeowners play in managing small on-site sewage systems. It also provides help and information on OSS management for both homeowners and local health jurisdictions.

Resource / Study Guide for Licensed Onsite Sewage System Installers

Washington State Onsite Sewage Code:

Washington State Approved Septic Tanks link:

Washington State Approved treatment and distribution products (drain field):

Washington State Recommended Standards and Guidance Documents:

EPA On-site Wastewater Treatment Systems Manual, EPA 625/R-00/008: