An advanced treatment unit, usually called an aerobic treatment unit or ATU, is a septic system that pumps air into the tank. That air feeds oxygen-loving bacteria, which break the waste down faster and more completely than a conventional tank does. The effluent comes out cleaner, which is why ATUs go in on lots where a regular septic system wouldn’t work.
The trade-off is that an ATU is a small machine rather than a passive tank. It needs electricity every day, it has parts that wear out, and in many jurisdictions it comes with a maintenance contract you’re required to keep for as long as you own it.
How it works, briefly
Most household ATUs have three stages. A pretreatment chamber, sometimes called a trash trap, lets heavy solids settle out, much like a regular septic tank. An aeration chamber comes next, where a compressor or blower pushes air through the wastewater. A settling chamber at the end lets the remaining solids drop out before the treated water leaves the unit. Some systems add a disinfection step, often chlorine tablets, before the water goes to a drain field or spray field.
A conventional septic tank does its work without oxygen and without power. The comparison of aerobic and anaerobic systems covers the difference in detail.
When an ATU is required
An ATU is one of several alternatives to a conventional septic system, and it’s usually the answer when a conventional system can’t be permitted. The US EPA lists the situations where aerobic units make sense:
- Smaller lots without room for a full-size drain field
- Soil that can’t treat wastewater properly on its own
- A water table too high for a conventional drain field
- Homes close to lakes, rivers, bays, or other water that’s sensitive to nutrients
The EPA’s technical fact sheet on aerobic treatment adds another common one, replacing a failed septic system on a lot where a new conventional field won’t fit or won’t pass.
Whether one is required on your property is usually a county decision, so ask your local health department. Two regions show how differently the rules can play out.
Florida. Much of the state has a high water table, one of the conditions the EPA names, and ATUs are common as a result.
Coastal New York and New England. On Long Island, the concern is nitrogen reaching bays and drinking water. Suffolk County requires innovative and alternative systems for certain construction projects submitted on or after July 1, 2021, and those systems have to reduce total nitrogen in the effluent to 19 mg/l or less. A basic ATU isn’t automatically built to hit that target. The EPA fact sheet notes that a standard aerobic unit may release more nitrates to groundwater than a septic system. In nitrogen-sensitive areas, ask whether a unit is approved for nitrogen reduction, not only whether it’s aerobic.
The maintenance contract most buyers don’t expect
This is where most of the surprise cost lives.
A conventional tank needs pumping every few years and an occasional inspection. An ATU needs regular service from someone qualified to work on it. The EPA fact sheet describes maintenance as more labor-intensive than for a septic system, requiring semi-skilled personnel, and usually handled through routine contract service.
Many jurisdictions turn that recommendation into a legal requirement. In Lee County, Florida, the health department requires the homeowner to keep a valid maintenance contract with an approved maintenance entity for the life of the system. The first contract runs two years, and every renewal runs at least one year. The maintenance entity has to inspect or service the unit at least twice a year and send its reports to the county. The homeowner also has to keep a current operating permit, renewed every two years, and the county inspects the system at least once a year.
Some states let homeowners take over parts of the maintenance after the first contract period, with training. Others don’t allow it at all. Check with your local health department before assuming anything.
| Conventional septic | ATU | |
|---|---|---|
| Needs electricity | Only if it has a pump | Yes, every day |
| Moving parts | Few or none | Compressor or blower, controls, often pumps |
| Routine service | Pumping and inspection every few years | Several service visits a year |
| Service contract | Rarely required | Required in many jurisdictions |
| Operating permit | Varies by county | Required in some jurisdictions |
| Alarm | Only on pump systems | Usually |
| Cost to operate | Lower | More expensive to operate, per the EPA |
The EPA fact sheet lists “more expensive to operate than a septic system” as the first disadvantage, and its list of drawbacks also includes requiring electricity and having mechanical parts that can break down. Prices vary too much by region and system type to quote a useful national figure here.
When the power goes out or the compressor fails
Without power, the air stops. The bacteria that need oxygen stop doing their job, and treatment quality drops. If your system has a pump that moves water out to the field, that stops too.
Most ATUs have an alarm. The EPA fact sheet notes that most aerobic units come with an alarm and control system to detect mechanical breakdown. When the septic alarm goes off, it usually means a high water level or a component that has stopped working. Check whether a breaker has tripped, cut back on water use, and call your maintenance provider.
During a power outage, go easy on water. Every load of laundry or long shower adds to a tank that isn’t treating or moving water. If the alarm stays on after power comes back, call for service.
Compressor and blower failures top the list of mechanical trouble. The EPA lists blower or aerator failure, pump clogging, electrical motor failure, and failing controls as the major mechanical issues with these units. Catching these early is part of what a maintenance contract pays for.
Don’t open the unit to look inside. Septic gases are dangerous, and the lids on these systems are there for trained service people.
Living with one day to day
An ATU is more sensitive to what goes down the drain than a conventional tank. The EPA notes that large amounts of certain cleaning agents, grease, and floating debris can upset the treatment process and cause equipment problems. What you shouldn’t flush applies here with even less room for error.
Sudden heavy loads are another weak point. The fact sheet lists “subject to upsets under sudden heavy loads” as a disadvantage. Spread out laundry, and think twice before draining a hot tub into the system.
If the system uses chlorine tablets for disinfection, use only tablets made for wastewater treatment. In Texas, the state environmental agency warns specifically against substituting swimming pool tablets.
Buying a house that already has one
An ATU on a listing isn’t a reason to walk away, but it calls for more questions than usual.
- Is there a current maintenance contract? Ask for a copy, when it expires, and whether it transfers to a new owner.
- Is the operating permit current, if your county requires one?
- Can you see the service records? A unit that’s been serviced on schedule is a very different purchase from one that’s been ignored.
- When was it last pumped?
- How old is the compressor or blower?
Where a contract is required for the life of the system, as in Lee County, the obligation doesn’t end when the house sells. It becomes yours at closing. Buying a house with a septic system walks through the inspection process, and for an ATU it’s worth hiring an inspector who works on aerobic systems specifically.
Common questions
How much does ATU maintenance cost each year?
It depends on your county’s requirements, how many service visits the contract includes, and your provider. It’s a recurring cost that a conventional system mostly doesn’t have, so get quotes from two or three licensed maintenance providers before you buy or install.
Who can service an aerobic treatment unit?
In many jurisdictions, only a licensed or approved maintenance provider. Florida counties like Lee keep a list of approved maintenance entities. Some states let homeowners do part of the work after the first contract period, with training. Your local health department can tell you the rules where you live.
What happens if I move into a house with an ATU?
You inherit the system and, where they’re required, the maintenance contract and operating permit. Get copies of the contract and the service records before closing, and confirm the contract transfers or plan to sign a new one right away.
Can I switch an ATU back to a conventional septic system?
Usually not. The ATU was probably installed because a conventional system couldn’t be permitted on that lot. If the conditions haven’t changed, the requirement hasn’t either.
Sources
- US EPA, Types of Septic Systems
- US EPA, Decentralized Systems Technology Fact Sheet: Aerobic Treatment
- Florida Department of Health in Lee County, Aerobic Treatment Units
- Suffolk County Department of Health Services, Sanitary Code Article 19
- Texas Commission on Environmental Quality, Basics for Septic Systems