How does an aerobic septic system work?

A conventional tank works without added oxygen. An aerobic treatment unit, or ATU, pumps air in on purpose, and the EPA compares it to a small municipal sewage plant.

What are the stages of an aerobic system?

Texas A&M describes four stages:

1. Pretreatment. A trash tank holds back plastics and other solids that microbes can't break down.
2. Aeration. An air pump pushes air through diffusers, and microbes use that oxygen to break down waste.
3. Settling. In a clarifier, microbes and other solids settle out of the treated water.
4. Final treatment and dispersal. The water is disinfected, stored in a pump tank and released, often through a spray field.

Not every unit follows that exact pattern. The EPA notes that pretreatment is optional in some, and that treated water can go to a subsurface leach field or, in some cases, the surface.

How clean is the water?

Cleaner than what leaves a septic tank, but still not drinking water. Texas A&M says the unit removes 85 to 98 percent of organic matter and solids, while an older EPA fact sheet reports 70 to 90 percent for suspended growth units.

How is it different from a regular septic system?

What can go wrong?

Most problems come back to the microbes. Texas A&M lists these weak points:

• Without enough air, the microbes can't survive.
• Grease, food waste, paper and excess water can overload the unit.
• Toxic cleaners can kill the microbes.
• Long stretches of low use, such as vacations, weaken them.
• Alarms for high water or low air pressure need prompt service.

For the upkeep side, read our guide to maintaining an aerobic system.

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