Short definition
An aerobic treatment unit (ATU) is a mechanically aerated septic alternative used where soil conditions make a standard gravity (anaerobic) septic system infeasible. Oxygen-loving bacteria digest waste faster and more completely, so the effluent leaving the tank is much cleaner — clean enough to use marginal drainfields that wouldn’t work for gravity septic. ATUs need power, regular service, and a maintenance contract.
What it is
Standard septic relies on anaerobic (oxygen-intolerant) bacteria slowly digesting waste in a sealed tank. An ATU uses a mechanical aerator or blower to pump oxygen into the tank, which dramatically speeds up biological breakdown and produces effluent with much lower BOD (biochemical oxygen demand) and TSS (total suspended solids).
The trade-offs:
- More effective — works on poor-percolation soils, near shorelines, and on small lots where a conventional drainfield won’t fit.
- Always-on power — aerator runs continuously; multi-day power outages cause problems.
- Annual inspection required under WA rule (WAC 246-272A), versus every 3 years for gravity septic.
- Maintenance contract typically required — most WA counties require a service agreement with a certified provider.
Why it matters to a homeowner
If you’re buying a property with an ATU, the math is different from a gravity-septic property:
- Installation cost: $10,000–$25,000 typical (vs. $4,000–$8,000 for standard gravity septic).
- Annual maintenance contract: $200–$500/yr.
- Electricity: roughly $15–$30/month to run the aerator.
- Annual inspections in addition to standard pumping cycles.
- Eventual unit replacement at 15–25 years, similar to a major appliance.
The same applies if your existing drainfield has failed and the soil won’t pass a percolation test for replacement gravity septic — an ATU may be the only legal alternative. WA Department of Health regulates ATUs through the State Board of Health and county health jurisdictions.
When you’ll encounter this term
- New construction in a WA county with strict percolation requirements (Olympic Peninsula, Mason County, San Juan Islands).
- Drainfield failure on a site where soil won’t support a replacement standard system.
- Pre-purchase inspection on a property with an existing ATU — factor in the maintenance contract and inspection cadence.
- Shoreline-protection-zone construction where effluent quality requirements are higher.
Common variants / not the same as
- ATU vs. anaerobic septic. ATU has powered aeration; anaerobic is gravity-only.
- ATU vs. sand filter / mound. Those are downstream-treatment improvements (after the tank). ATU is in-tank aeration.
- ATU vs. POWTS. POWTS (private on-site wastewater treatment system) is the umbrella WA term; ATU is one type.
Common failure modes
- Aerator/blower motor failure → reverts to anaerobic, effluent quality drops, alarms trigger.
- Power outage → brief outages OK, multi-day outages problematic.
- Filter clogging at the discharge — annual service item.
- Owner deferring maintenance — drainfield damage results, warranty often voided.
Washington note
Most active ATU jurisdictions in WA: Mason, Kitsap, Jefferson, Clallam, San Juan counties — areas with marginal soils and strict shoreline protection. King and Pierce counties have ATUs in unincorporated rural fringes. Per WAC 246-272A, ATUs are classified as alternative on-site sewage systems and require annual inspection unless the local health officer specifies more frequent inspections. Approved unit lists are maintained by each county health department; common WA-approved brands include Orenco AdvanTex, Norweco Singulair, and Hoot.