Aerobic septic systems
An aerobic treatment unit (ATU) is a small version of a city sewage plant.
How an aerobic treatment unit works
It blows air into the wastewater so oxygen-loving bacteria can break down waste faster and more completely than in a plain septic tank.
Texas A&M AgriLife Extension says these units remove 85 to 98 percent of the organic matter and solids, producing cleaner effluent than a conventional septic tank.
The usual treatment train has five parts. A trash tank catches solids that settle or float. An aeration chamber gets air from an air pump (aerator) pushed through diffusers. A clarifier lets spent bacteria settle out. A chlorinator disinfects the water. A pump tank then doses the treated water to spray heads or drip lines.
Disinfection is needed when the water is sprayed on a lawn. Oklahoma State University Extension notes that drip irrigation dispersal does not require disinfection.
Oklahoma's rule requires a chlorine residual of at least 0.2 mg/l in the pump tank when chlorine is used with spray irrigation.
| Part | What it does |
|---|---|
| Trash (pretreatment) tank | Holds back solids that sink and grease that floats |
| Aeration chamber and air pump | Air is pumped through diffusers so aerobic bacteria can digest the waste |
| Clarifier | Calm chamber where bacteria and leftover particles settle out |
| Chlorinator | Tablet chlorinator disinfects the water before it is sprayed |
| Pump tank | Stores treated water and doses it out; carries the high-water alarm float |
| Spray heads or drip lines | Spread the treated water over the lawn or into the top of the soil |
Sources: US EPA, Types of Septic Systems (last updated May 4, 2026); Texas A&M AgriLife Extension, L-5302 Aerobic Treatment Unit (Lesikar, 9-08); Oklahoma State University Extension, PSS-2275 Aerobic Treatment System (Abit and Jones, print code 0714); Texas A&M AgriLife Extension, L-5344 Tablet Chlorination (Weaver and Lesikar, 9-08); Oklahoma DEQ, OAC 252:641 Individual and Small Public On-site Sewage Treatment Systems (amendments effective 9-15-24)
When an aerobic system is used
The EPA says ATUs suit smaller lots, poor soils, high water tables and homes near water bodies sensitive to nutrients.
Oklahoma State University Extension says more advanced systems are required where soil is too clayey, too sandy or too shallow for a conventional system, and that ATUs had gained popularity in Oklahoma.
In Oklahoma an ATU can only be used for daily flows from 100 to 1,500 gallons. A spray field after an ATU is allowed even where perc results rule out a conventional field, if it is sized with the Group 5 soil criteria.
In Arkansas an ATU may replace a septic tank only at the sole discretion of the Department of Health.
Sources: US EPA, Types of Septic Systems (last updated May 4, 2026); Oklahoma DEQ, OAC 252:641 Individual and Small Public On-site Sewage Treatment Systems (amendments effective 9-15-24); Arkansas State Board of Health, Rules Pertaining to Onsite Wastewater Systems (effective Sep 5, 2024)
Maintenance: chlorine tablets
The chlorinator only works with tablets in it. Texas A&M AgriLife recommends checking it weekly and adding tablets as needed, putting in two to five at a time so they do not jam.
Use only calcium hypochlorite tablets made for wastewater. Never use swimming pool tablets, which are often trichloroisocyanuric acid, work too slowly, and can form an explosive compound if mixed with calcium hypochlorite.
Handle tablets with gloves, open the container in a ventilated area, and do not store them in the house, garage or near metal tools.
If you have a service contract, it will probably say that you, the homeowner, add the tablets.
Sources: Texas A&M AgriLife Extension, L-5344 Tablet Chlorination (Weaver and Lesikar, 9-08); Texas A&M AgriLife Extension, B-6234 Living with an Aerobic Treatment Unit and Spray Field (01/10)
Maintenance: alarms, air pump, sludge and service visits
Listen for the air pump (compressor) running, check chlorine often, watch for rotten egg odors, and check the spray heads. Oklahoma State University Extension says faulty spray heads are a common problem.
Keep power on to the unit all the time. The bacteria need a constant air supply. Clean the air pump's intake filter or screen.
Have the trash tank checked for sludge. Oklahoma State University Extension suggests checking at least every six months and pumping when sludge fills 40 percent of the tank's liquid capacity. Texas A&M AgriLife suggests a schedule similar to a septic tank, every 2 to 3 years.
If an alarm sounds, call your maintenance provider, do not try to repair it yourself, and cut back on water use until it is fixed.
Oklahoma law requires the installer to maintain a new aerobic system for two years at no extra cost, including chlorine and sludge checks every six months. After that the owner is responsible for maintaining it or hiring someone.
In Arkansas, an ATU distributor must have factory-trained service staff able to respond within 48 hours, licensed as Certified Monitoring Personnel.
The EPA says systems with pumps, float switches or mechanical parts should generally be inspected once a year. In Texas, Texas A&M says inspections are usually every 4 months (a Texas requirement, shown for comparison).
| Task | How often | Source |
|---|---|---|
| Check chlorine tablets, add as needed | Weekly | Texas A&M L-5344 |
| Listen for air pump, check odor, chlorine and spray heads | Often (regular simple checks) | OSU PSS-2275 |
| Measure trash tank sludge | At least every 6 months | OSU PSS-2275 |
| Pump trash tank | When sludge reaches 40% of liquid capacity, or about every 2 to 3 years | OSU PSS-2275; Texas A&M L-5302 |
| Professional inspection | Generally once a year (EPA); Oklahoma installer checks every 6 months in the first 2 years | EPA; OAC 252:641-10-3 |
| After a vacation | Cut water use for at least two days on return | OSU PSS-2275 |
Sources: Oklahoma State University Extension, PSS-2275 Aerobic Treatment System (Abit and Jones, print code 0714); Texas A&M AgriLife Extension, L-5302 Aerobic Treatment Unit (Lesikar, 9-08); Oklahoma DEQ, OAC 252:641 Individual and Small Public On-site Sewage Treatment Systems (amendments effective 9-15-24); Arkansas State Board of Health, Rules Pertaining to Onsite Wastewater Systems (effective Sep 5, 2024); US EPA, How to Care for Your Septic System (last updated Aug 17, 2026); Texas A&M AgriLife Extension, B-6234 Living with an Aerobic Treatment Unit and Spray Field (01/10)
Pros and cons versus a conventional system
Pros: cleaner effluent (85 to 98 percent removal), works on small lots, poor soils and high water tables, and needs less vertical soil separation in Oklahoma than a conventional field. HomeAdvisor notes an aerobic system needs a smaller leach field.
Cons: the EPA says to expect regular lifetime maintenance. It needs power around the clock, chlorine handling, and service visits. The bacteria suffer when a home sits empty or is used only on weekends, so ATUs struggle at vacation homes. It also costs more to install.
People and pets should stay off the spray field, since the water is treated wastewater, not drinking water.
Sources: US EPA, Types of Septic Systems (last updated May 4, 2026); Oklahoma State University Extension, PSS-2275 Aerobic Treatment System (Abit and Jones, print code 0714); Texas A&M AgriLife Extension, L-5302 Aerobic Treatment Unit (Lesikar, 9-08); Texas A&M AgriLife Extension, B-6234 Living with an Aerobic Treatment Unit and Spray Field (01/10); HomeAdvisor, Septic System Installation Cost (updated Jun 20, 2026)
What an aerobic system costs [EST]
[EST] HomeAdvisor (updated Jun 20, 2026): an aerobic septic system costs $10,000 to $20,000 to install, versus $3,000 to $8,000 for a conventional anaerobic system.
[EST] This Old House (modified 03/12/2026) lists the same ranges, sourced from Angi estimates.
[EST] HomeAdvisor (updated Jun 17, 2026): adding bacteria to an aerobic system costs $50 to $500, most often $100 to $200.
No cost guide from the preferred list could be read for the yearly price of an aerobic maintenance contract, so that figure is left out.
Sources: HomeAdvisor, Septic System Installation Cost (updated Jun 20, 2026); HomeAdvisor, How Much Does Septic Tank Pumping Cost? (updated Jun 17, 2026)
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Find a licensed septic proCommon questions
How does an aerobic septic system work?
Wastewater settles in a trash tank, then an air pump bubbles air into an aeration chamber so bacteria digest the waste. A clarifier settles out solids, a chlorinator disinfects the water, and a pump tank sends it to spray heads or drip lines.
What chlorine tablets go in an aerobic system?
Only calcium hypochlorite tablets approved for wastewater. Texas A&M AgriLife warns never to use swimming pool tablets, which work too slowly and can form an explosive compound when mixed with wastewater tablets.
How often does an aerobic system need service?
Check tablets weekly and the trash tank sludge at least every six months. The EPA says systems with pumps and mechanical parts should generally be inspected once a year. In Oklahoma the installer must maintain a new system for its first two years.
How much does an aerobic septic system cost?
[EST] HomeAdvisor (Jun 20, 2026) puts installation at $10,000 to $20,000, compared with $3,000 to $8,000 for a conventional system.