2-Septic Design.pdf

This is an On-Site Sewage System (OSS) permit documentation package for Mason County, Washington, issued for a 3-bedroom residential repair/replacement system at 3891 W Skokomish Valley Road, Shelton, WA 98584.

The permit (SWG2026-00113) was issued on June 16, 2026, by Jeff Wilmoth with current fees of $845.00 and an expiration date of June 10, 2029. The system design includes a 1,200-gallon trash tank, 1,200-gallon pump tank (single compartment units), a BNR 500 treatment device, a pressure distribution system with six 50-foot laterals at 1.25-inch diameter with Schedule 40 PVC, and a trench/bed drainfield configuration. The system operates at 270 gallons per day with a design capacity of 360 gallons per day, supported by 900 square feet of primary, designed, and reserve area. The septic tank capacity is 1,200 gallons (trash/BNR 500 configuration). A Liberty Pumps 280 Series 1/2 HP submersible effluent pump, with specifications detailed in the pump curve provided, handles transport to the drainfield. The design includes pressure laterals with 60 orifices spaced 3/16 inches apart, manifold placement on Schedule 40 PVC, and transport piping of 2-inch diameter. The system requires a control panel with counter, elapsed meter, and event counter, utilizing either transducers or floats. Installation must be performed by a Mason County Certified Installer, with the designer being Cindy Waite (360-701-0205). The permit conditions require Mason County installation approval prior to system component backfill, septic designer/engineer installation approval, proper drainfield depth maintenance, and submission of an Asbuilt Form, Record Drawing, and Installation fee for final approval. The design was approved June 15, 2026, and is valid through June 10, 2027, contingent upon the permit not expiring and site conditions remaining unchanged.