An aerobic treatment unit, or ATU, uses a blower to inject oxygen into the wastewater, encouraging aerobic bacteria to break it down far more thoroughly than the anaerobic settling in a standard septic tank does. The result is a noticeably cleaner effluent, which is why ATUs are often chosen on lots with poor soil, a small footprint, or proximity to sensitive water, where a smaller or less demanding drainfield is needed.
Where a conventional tank simply lets solids settle and relies entirely on the drainfield’s soil for treatment, an ATU actively treats the wastewater itself before it ever reaches the field, using mechanical aeration and, in many designs, an additional settling or clarification stage afterward.
That higher level of treatment comes at a real ongoing cost: an ATU needs continuous electricity to run the blower, and most state codes require a service contract with regular professional inspection and maintenance, unlike a conventional tank which needs only periodic pumping. Skipping that maintenance defeats the purpose, since a neglected ATU can perform worse than a well-maintained conventional tank.
Because effluent quality is higher, some jurisdictions allow a reduced drainfield size, reduced setbacks, or use on soils that would not otherwise qualify, which is the practical reason a property owner ends up with an ATU rather than a choice made for its own sake.
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It actively injects oxygen to support aerobic bacteria breaking down wastewater, producing a cleaner effluent than the passive settling a standard anaerobic tank provides.
Yes, it needs continuous electricity and, under most state codes, a service contract with regular professional inspection, unlike a conventional tank that only needs periodic pumping.
Poor soil, a small lot, or proximity to sensitive water can make a conventional system unsuitable; an ATU's cleaner effluent can allow a smaller drainfield or use on soil that wouldn't otherwise qualify.
Guide last reviewed 2026-08-31.