Buying a Home on Septic in WA: Gravity vs Pressure vs ATT Systems
A failed on-site septic system can cost $20,000 to $45,000+ for an engineered replacement. Here is what every Washington buyer needs to know about system types, county pumping rules, and drainfield inspection.
Zachary Bradley
Licensed Home Inspector · WA Lic #26014533
Key Inspector Takeaways
- ✓Washington county health departments require an Operation & Maintenance (O&M) inspection and pumping report upon property sale.
- ✓Conventional gravity systems require deep, permeable soils; rocky or clay soils require pressure distribution or aerobic treatment units (ATUs).
- ✓Driving vehicles or placing heavy structures over drainfield lines crushes pipes and causes catastrophic backup.
- ✓Camera scopes between the house and septic inlet baffle verify absence of tree root blockages.
The Anatomy of an On-Site Sewage System (OSS)
Every septic system has two core components: the septic tank (which separates solids, scum, and effluent liquids) and the drainfield / soil dispersal component (which purifies effluent through natural soil microbiology).
If solids overflow into the drainfield because of lack of pumping, the soil pores clog with biomat, causing sewage to surface in the yard.
Comparing Septic Technologies in SW Washington
1. Conventional Gravity: Simplest system with lowest maintenance, relying purely on downhill slope.
2. Pressure Distribution: Uses an electric submersible pump to dose effluent evenly across drainfield laterals on shallow soils.
3. Aerobic Treatment Units (ATUs) & Glendon Mounds: Engineered systems for high water tables and river valleys like the Chehalis basin.
Frequently Asked Questions
Related Southwest WA Inspection Services
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