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Large On-site Sewage Systems - What are they?

ax100 illustrationLarge On-site Sewage Systems (LOSS) convey, store, treat, and provide subsurface soil treatment and disposal of domestic sewage.  Their design flow is between 3,500 to 100,000 gallons per day. 

LOSS offer an alternative to centralized municipal sewage treatment plants and can serve from 10 to about 370 individual residences, or equivalent flows from schools and churches, campgrounds and recreation vehicle parks, resorts or state park sites, or smaller cities or towns. 

Department of Health LOSS Program staff review and approve LOSS project applications state-wide. The LOSS rule is Chapter 246-272B WAC, developed under authority of Chapter 70.118B RCW. The revised rule became effective July 1, 2011. 

All existing LOSS are required to obtain and renew annual operating permits from the Department of Health.  Some LOSS were previously regulated and permitted by either the Washington State Department of Ecology or local health jurisdictions.  The regulatory authority for these LOSS transferred to Health when the revised rule became effective.

Traditionally, on-site treatment occurs through a septic tank and drainfield, located on or near the property where the sewage is produced. Other higher level treatment methods, in combination with a drainfield, are becoming more common, particularly when there is a concern about nutrients such as nitrates in the groundwater or when a drainfield is less able to provide adequate treatment. 

Certain types of wastewater and sewage are not allowed to be treated by or flow into a LOSS, including industrial wastewater and stormwater.  The Department of Ecology reviews, permits, and regulates systems that treat industrial wastewater and stormwater.