Septic Glossary: Terms You’ll See on This Site

Area of Special Concern: A TPCHD-designated zone with extra septic siting and design scrutiny due to sensitive water bodies nearby. Pierce County's only current example is the Key Peninsula, covering shellfish districts at Rocky Bay, Burley Lagoon, and Filucy Bay.

ATU (Aerobic Treatment Unit): An engineered septic system that actively adds oxygen to speed up wastewater treatment, often required on sites where a conventional gravity system won't work.

Baffle: A barrier inside a septic tank that controls the flow of wastewater between chambers and helps keep solids from moving into the drainfield.

Conditional RSS: A Report of System Status issued despite an unresolved defect, allowing a real estate sale to proceed while the buyer completes repairs within 90 days of closing.

Drainfield (Leach Field): The area where treated wastewater from the septic tank is released into the soil for final filtration. Also called a leach field.

Gravity System: The most common, least expensive type of conventional septic system, relying on gravity rather than pumps to move wastewater from the tank to the drainfield.

Mound System: An engineered septic system built above natural grade, often required on sites with a high water table or shallow soil.

O&M (Operation & Maintenance) Inspection: A required periodic evaluation of a septic system's condition, performed by a certified specialist. How often it's required depends on the system's risk tier.

OSS (On-Site Sewage System): The formal regulatory term for what most homeowners just call a septic system — any wastewater treatment system serving a single property rather than connecting to a municipal sewer.

Percolation Test (Perc Test): A test measuring how quickly water drains through soil at a site, used to determine what kind of septic system a property can support.

Permit-Exempt Well: A private well that doesn't require a full water right permit, subject to gallons-per-day withdrawal limits that vary by watershed (WRIA) in Washington.

Pressure Distribution System: A septic system that uses a pump to evenly distribute wastewater across the drainfield, rather than relying on gravity alone — often used where the drainfield needs more uniform dosing.

Record Drawing: The official as-built diagram of a septic system's components and layout, created after final installation inspection. Needed to properly locate and maintain the system later.

Report of System Status (RSS): A required evaluation from TPCHD confirming a septic system's condition before a Pierce County property can be sold. Good for one year and cannot be renewed.

Risk Tier: Pierce County classifies every septic system as High, Moderate, or Low risk, which determines how often an O&M inspection is required — annually, every 3 years, or only when triggered by a sale or repair.

Sand Filter: An engineered septic system that filters wastewater through a sand bed before it reaches the drainfield, often used on sites with poor natural soil filtration.

Setback: The minimum required distance between a septic system component (tank, drainfield) and a well, property line, water body, or structure.

Sludge / Scum Layer: The solid waste that settles (sludge) or floats (scum) inside a septic tank. When these layers get too thick, they need to be pumped out before they reach the drainfield.

Sole Source Aquifer: An EPA designation for an aquifer that serves as the primary drinking water source for an area, triggering extra scrutiny for septic siting and design. Much of central Pierce County falls under this designation.

TPCHD (Tacoma-Pierce County Health Department): The single local health jurisdiction responsible for septic permitting, inspection, and enforcement across all of Pierce County, both incorporated cities and unincorporated areas.

WAC 246-272A: The Washington state regulation that sets statewide design, construction, and maintenance standards for septic systems. Individual counties like Pierce administer and enforce these standards locally, which is why requirements can differ county to county.