1. Standard Gravity System

How it Works: Wastewater flows by gravity from the house into a septic tank (where solids settle) and then directly into a series of perforated pipes set in gravel trenches (the drainfield).
Site Conditions: Requires deep, well-draining soil and a low water table.
Cost & Maintenance: Most affordable to install and maintain; completely passive (no pumps or electricity).

2. Pressure Distribution System

How it Works: Uses an electric pump and dose tank to push effluent evenly throughout the entire drainfield under pressure at controlled intervals.
Site Conditions: Used when soils are shallow or the property has a slope.
Cost & Maintenance: Moderate cost; requires routine monitoring of the pump, float switches, and electrical panel.

3. Capping Fill System

How it Works: A shallow drainfield trench is dug partially into natural soil, and clean topsoil (a "cap") is built up over the top to create sufficient depth above groundwater or bedrock.
Site Conditions: Common in areas with high seasonal water tables or shallow bedrock.
Cost & Maintenance: Moderate cost; earthwork is required, and the capped mound must be protected from vehicles and heavy livestock.

4. Sand Filter System

How it Works: Effluent is pumped from the septic tank into a lined bed of specially graded sand. Microbes in the sand treat the wastewater before it is pumped to a smaller drainfield.
Site Conditions: Used on challenging lots with very poor soil drainage, high water tables, or tight lot boundaries.
Cost & Maintenance: Higher installation cost; requires periodic flushing of manifold lines and pump maintenance.

5. Alternative Treatment Technology (ATT) Systems

How it Works: Uses powered mechanical equipment (aerobic treatment units) to break down waste inside a primary chamber, producing highly treated effluent before sending it to a reduced-size drainfield.
Site Conditions: Used in environmentally sensitive areas, waterfront properties, or lots with minimal buildable area.
Cost & Maintenance: Highest initial cost; Oregon DEQ requires homeowners to maintain an active service contract with a certified maintenance provider for regular monitoring and reporting.

Bedroom Count vs. Permit Capacity: In Oregon, septic systems are sized by daily wastewater flow based on the number of bedrooms—not bathrooms. A home permitted for a 3-bedroom septic system cannot legally be marketed or sold as a 4-bedroom home, even if an extra room has a closet.

The Reserve / Replacement Area: Oregon DEQ rules require properties to maintain an undisturbed "reserve area" set aside on the lot. If the primary drainfield fails, this designated space must be available for a replacement system. Buyers must ensure outbuildings, driveways, or patios have not encroached on the reserve area.

Property Alterations: Adding a shop, pool, or garage requires checking setbacks from the existing tank, drainfield, and reserve area (typically 10 to 20 feet minimum clearance).

Key Inspection Steps:

Locating & Exposing Lids: Access lids over the tank compartments and pump chamber are dug up for access.

Structural Tank Inspection: The inspector checks concrete, plastic, or fiberglass tanks for cracks, root intrusion, or deteriorating baffles.

Pumping & Solids Level Check: The tank is pumped out to measure sludge and scum accumulation, and to inspect the bottom of the tank.

Pump & Control Panel Check: For mechanical systems (Pressure Distribution, Sand Filter, ATT), the inspector tests high-water alarms, float switches, and pump amperage.

Drainfield Evaluation: The inspector evaluates the drainfield for signs of failure, such as standing effluent, spongy grass, or sewage odor. Hydraulic load tests may be performed to test absorption.