What Is an Aerobic Treatment Unit (ATU)?

An aerobic treatment unit (ATU) is a septic treatment tank that pumps air into the wastewater. The EPA explains that the extra oxygen boosts natural bacterial activity, which gives the effluent more treatment before it goes to the drain field.

Explanation

Florida's septic rule defines an ATU as a unit "which introduces air into sewage" to treat it within a tank (Rule 62-6.002(3)). The EPA says ATUs work like a small sewage plant and can be used on smaller lots, with poor soil, where the water table is too high, or near water that is sensitive to nutrients. It adds that owners should expect regular maintenance for the life of the unit.

Florida sets extra rules for ATUs:

• Approved models only: units designed for up to 1,500 gallons a day must be certified to an NSF/ANSI standard: Standard 40 Class I, Standard 245 (nitrogen reduction) or Standard 350 (water reuse) (Rule 62-6.012(1)).
• Alarm: a visual and audio warning device must be installed where it will alert the people in the home if the unit fails (Rule 62-6.012(2)(c)).
• Operating permit: valid for 2 years and renewed every 2 years (s. 381.0065(4), F.S.). The state's base fee is $100; county health departments add their own fees.
• Service contract: the owner must keep a current maintenance agreement with a maintenance entity permitted by DEP, which must inspect the unit at least twice a year. The owner of an owner-occupied single-family home can instead be permitted as the maintenance entity for their own unit after training certified by the manufacturer's representative (s. 381.0065(4)(v), F.S.).
• State inspection: the law calls for buildings that use an ATU to be inspected by the department at least once a year.

Not every ATU counts as a nitrogen-reducing system. For Florida's nitrogen rules, DEP accepts Florida-approved NSF 245-certified units, not every aerobic model.

Published ranges from Fixr and HomeGuide put a new aerobic system at about $10,000–$40,000 or more. InspectApedia puts a yearly maintenance contract at about $175–$600.

When to seek help

• The ATU alarm goes off. Call your maintenance company.
• Your operating permit is about to expire, or you do not know its status. Ask your county health department, or DEP in Marion County.
• You are buying a home with an ATU. Ask for the operating permit and the service contract.
• Your maintenance contract has lapsed.

Learn more in conventional vs. aerobic septic systems in Florida and Florida septic regulations. Use the real-estate septic inspection checklist when buying, or compare aerobic system service companies.

Related terms

• Nitrogen-reducing (enhanced) septic system
• Drain field (leach field)
• Mound system
• Sludge and scum layers