An aerobic septic system, often called an aerobic treatment unit (ATU), treats household wastewater with added oxygen before it leaves the property. A conventional septic system relies on a tank and the soil. An aerobic system adds an air pump and extra treatment steps, which usually means better treatment in difficult sites and a higher ownership burden for the homeowner.
If your home has an aerobic system, the practical questions are what it needs from you, what service costs, and what an alarm means. This guide covers those, and explains how an aerobic system differs from a conventional one.
What is an aerobic septic system?
An aerobic treatment unit works on the same basic idea as a municipal sewage plant, scaled down for one home. Bacteria that need oxygen break down waste faster than the bacteria in a conventional septic tank, and a mechanical aerator supplies the oxygen. University extension guidance describes the typical layout as a pretreatment tank, an aeration chamber, a settling chamber, and a disinfection step.
The treated water then goes to a final dispersal area. Depending on local rules and the site, that can be a spray field, a drip system, an absorption area, a sand filter, or an evapotranspiration bed.
How an aerobic system works, step by step
- Pretreatment tank. Wastewater leaves the house and enters a tank where solids settle, much like a conventional septic tank.
- Aeration chamber. An air pump or blower bubbles air through the wastewater. Oxygen-loving bacteria digest much of the organic waste.
- Settling chamber. Remaining solids settle out so clearer water moves on.
- Disinfection. Many systems disinfect the water with chlorine tablets, UV light, or another method before dispersal.
- Dispersal. The water goes to a spray field, drip lines, an absorption area, or another approved dispersal method.
Most systems also include an electrical control panel, a float or level switch, and an alarm. Because the system depends on electricity and moving parts, a power outage or pump failure affects it more than it would a gravity-fed conventional system.
Aerobic vs. conventional septic systems
| Feature | Conventional septic system | Aerobic septic system |
|---|---|---|
| Treatment | Tank plus soil treatment | Tank, aeration, settling, usually disinfection |
| Power needed | Usually none for a gravity system | Yes, for the air pump and controls |
| Moving parts | Few or none | Air pump, controls, alarm, sometimes a spray pump |
| Homeowner upkeep | Inspection and pumping | Regular service, often under a contract, plus inspection and pumping |
| Typical use | Sites with suitable soil and space | Sites where soil, space, or water conditions limit a conventional system |
| Cost to own | Lower | Higher upfront and ongoing |
For the basics of the conventional design, see how septic systems work. The EPA also describes the main system types on its types of septic systems page.
When is an aerobic system used?
Local regulators decide which systems are allowed on a given lot. Aerobic systems are commonly considered when soil drains poorly, the water table is high, the lot is small, or a conventional drain field would not meet local rules. Some jurisdictions also require a higher level of treatment near water or in sensitive areas.
Your county or state health department is the authority on what your property needs. The permit and as-built plan for your home usually show which system is installed. If you do not have them, see how to find septic system records.
Aerobic septic system cost
Published cost figures for aerobic systems vary a lot, and there is no single authoritative national price. Many online cost guides repeat each other without citing data. Treat the figures below as rough planning ranges, not quotes, and get written local estimates.
- Installation. Installed cost depends on the unit, the site, permits, and the dispersal method, and published ranges are wide. Online guides commonly put a typical installed system somewhere in the high four figures to the low five figures, with complex sites costing more. For comparison with other system types, see septic system cost and septic tank installation cost.
- Annual service contract. InspectAPedia, citing Montana data, lists a basic plan at roughly $200 to $300 per year and a more extensive plan at about $500 to $600 per year, with repairs and parts extra. Prices differ by provider and region, so compare quotes for the same scope of work.
- Electricity. The air pump runs on a schedule set by the unit. Published estimates of the yearly electric cost range from under $100 to a few hundred dollars depending on the unit, local rates, and run time, so ask your service provider about your model.
- Pumping. The settled solids still need removal from time to time. See how often to pump a septic tank for the general guidance, and ask your service provider how often your unit needs it.
Before you buy a home with an aerobic system, ask for the service history and the current service contract. A system with missing maintenance records is a larger risk than the same system with a clean record.
Maintenance: what you and your service provider do
Aerobic systems need more attention than conventional ones. Extension guidance generally treats the air pump as a key part of the system, and notes that control panels, alarms, filters, and disinfection equipment need periodic inspection.
Typical professional service visits can include:
- Checking the air pump or blower, controls, and alarm
- Checking the level and condition of the sludge layer
- Replenishing disinfection supplies such as chlorine tablets, where used
- Inspecting spray or drip lines and heads
- Taking readings such as chlorine or pH, where required by the permit
How often visits happen depends on your permit and contract. Many plans and some state rules call for about three professional inspections a year, roughly every four months, and InspectAPedia describes service plans built around that kind of schedule. Some states, including Texas, require the owner to hold a maintenance contract with a qualified provider for at least the first two years after a new aerobic system is installed. After that, rules differ: some jurisdictions let trained owners do the maintenance and others keep requiring a contract. Check your state and county rules.
Tasks homeowners are often asked to do:
- Listen for unusual noise from the air pump
- Watch for warning lights and alarms
- Check the discharge area for pooling or odor
- Clean or inspect the air pump inlet filter on the schedule in the manufacturer’s manual; intervals vary by unit
- Protect spray heads from lawn equipment and damage
- Keep records of every service visit
Check your own unit’s manual, your permit, and your service contract for the schedule that applies to you. Texas A&M’s guide to living with an aerobic treatment unit and spray field explains the owner’s responsibilities under Texas rules, and Oklahoma State University publishes a fact sheet on aerobic treatment units.
Water use and what goes down the drain
Aerobic systems can be upset by the same things that harm conventional systems: heavy water use in a short time, grease, wipes, harsh chemicals, and large amounts of bleach or antibacterial products. Spread laundry across the week and fix leaks promptly. For more, see what not to put in a septic tank and septic system water usage.
Common aerobic system problems and alarms
The most common signal is the alarm. It usually points to a high water level, which can come from a failed pump, a stuck float, or a clogged outlet line. An alarm is not automatically an emergency, but treat it seriously if drains are slow, gurgling, or backing up.
- Alarm sounding or light on. Cut back on water use right away (skip laundry and long showers), follow the steps in your manual, and call your service provider. Light colors and alarm codes vary by manufacturer, so use your unit’s manual to tell an aeration problem from a high-water alarm. Our guide to a septic alarm going off explains safe first steps.
- Air pump not running. Without air, the bacteria that treat the water slow down and the system can become overloaded. Call your service provider. This includes a power outage: ask your provider how long your unit can safely sit without power and what to do during a long outage.
- Odor. A strong sewage smell near the tank or spray area needs attention. See septic smell for how to narrow down the cause.
- Clogged or damaged spray heads. Heads can be damaged by mowers and tools, and may need cleaning or replacement.
- Ponding in the dispersal area. This can signal overload or a blockage in the discharge lines.
Keep the contact information for your service provider next to the control panel.
Mistakes that shorten the life of an aerobic system
Most expensive aerobic repairs trace back to a few avoidable habits:
- Switching off the air pump to save electricity. The treatment bacteria need a steady oxygen supply. A pump that stays off for long periods can reduce treatment and may overload the dispersal area.
- Ignoring an alarm or warning light because the drains still work. Many failures are cheaper to fix when caught at the first alarm.
- Skipping service visits or letting a required service contract lapse. Some permits treat a lapsed contract as a violation.
- Flushing wipes, grease, or large amounts of bleach and antibacterial cleaners, which can harm treatment and clog filters and spray heads.
- Running many loads of laundry in one day. A surge of water can push solids through the system before they settle.
- Driving or parking over the tank, control area, or dispersal area, or planting deep-rooted trees nearby.
If you are unsure whether a habit is safe for your unit, check the manual or ask your service provider before you change it.
Safety around treated wastewater
Treated effluent from an aerobic system is cleaner than raw wastewater, but it is not drinking water and is not the same as clean irrigation water. Rules for spray and drip dispersal, including where the water may go and how close people may be, come from your permit and your local health department, so follow them and ask the department if you are unsure about contact with the dispersal area.
Never open or enter a septic tank or a treatment chamber. These spaces can contain hazardous gases and low oxygen, and only a trained professional should open them.
Questions to ask before buying a home or installing a system
- What type of aerobic unit is installed, and what is the model and age?
- Is a service contract required by the permit, and who holds it?
- What do the last two years of service records show?
- Where does the treated water go, and has the dispersal area been inspected?
- Have there been alarms, pump replacements, or repairs?
- What is the annual cost of service, electricity, and pumping?
- Is the unit still under the manufacturer’s or installer’s warranty?
An aerobic unit is one of several designs. For how it compares with conventional, mound and sand filter systems, see types of septic systems.
Frequently asked questions
What is an aerobic septic system?
It is a septic system that adds oxygen to wastewater with an air pump so that oxygen-loving bacteria can treat it faster, usually followed by settling and disinfection before the water is dispersed.
Is an aerobic septic system better than a conventional one?
Aerobic systems often provide a higher level of treatment and can work on sites where a conventional system cannot. They also cost more to install and maintain and depend on electricity and mechanical parts.
How often does an aerobic septic system need service?
It depends on your permit, the manufacturer, and your service contract. Many plans call for about three inspections a year, and some states require a maintenance contract for at least the first two years after installation.
How much does an aerobic septic system cost?
Published installed-cost ranges are wide and inconsistent, and annual service is commonly reported in the hundreds of dollars. Get local written quotes because site conditions and permit rules drive the price.
Can I maintain an aerobic system myself?
Owners can usually do basic checks, such as listening to the pump and watching the alarm, but many jurisdictions require service by a licensed provider, at least for the first years. Check your local rules before skipping a service contract. If you are buying a home with an aerobic system, see also our septic inspection when buying a house guide and our overview of septic system lifespan.
What does it mean when my aerobic system alarm goes off?
It usually signals a high water level, a pump problem, or a clogged outlet. See our septic alarm guide and contact your service provider.
Sources and further reading
- U.S. EPA: Types of Septic Systems
- Texas A&M AgriLife: Living with an Aerobic Treatment Unit and Spray Field
- Oklahoma State University Extension: Aerobic Treatment Units fact sheet
- InspectAPedia: Aerobic septic system maintenance costs
- InspectAPedia: Aerobic septic alarm diagnosis
This article provides general U.S. homeowner information. Aerobic system rules, required service, and costs vary by state, county, and manufacturer. Always follow your permit, your unit’s manual, and the advice of a licensed local septic professional.