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MODULE-5 Wastewater Management
Large On-Site Sewage Systems
Large On-Site Sewage Systems (LOSS) is a traditional on-site sewage system that is used by two or more adjacent properties. The use of shared systems can be cost effective in comparison to individual on-site systems when alternative treatment is used in nitrogen sensitive areas. Instead of multiple individual drainfields, a larger combined drainfield is used. Shared systems are ideal in higher density areas where the location of individual drainfield areas is impractical. This encourages the design of cluster or open space developments. With flows from 3,500 to 14,500 gallons per day (gpd), DOH regulates these as LOSS.
Washington's Large On-site Sewage System (LOSS) regulation, Chapter 246-272B WAC, requires minimum lot areas of 12,500 square feet within subdivisions except when a planned unit development (PUD), which includes clustered units, is proposed. A PUD provides flexibility to design a site that promotes an Open Space Residential Design, especially on sites with shallow depths to groundwater or with areas of poor soils on a portion of the site that make it difficult to prove each individual lot can support its own system.
The minimum lot size for a property in a cluster development using a shared system does not have to be controlled by the septic system design as the system can be located on its own separate lot. Then the density is controlled by the local zoning and subdivision codes.
Based on the site and soil considerations, there is also a set of criteria outlined for the location of a LOSS. Permits are issued for these systems and must be renewed annually. LOSS must be owned by a public entity and must have an approved management and monitoring plan. The recently enacted Chapter 70.118B RCW authorizes DOH to regulate and adopt rules for LOSS. It also authorizes local jurisdictions to issue civil fines to small systems that fail to comply.