GUIDE // SYSTEM TYPES
Aerobic vs anaerobic septic systems.
Which one is right for your lot?
If your soil will not perc, or you are building on a tight lot near water, an aerobic treatment unit may be your only option. If your soil is good, a conventional anaerobic system is cheaper to install, cheaper to run, and easier to live with. Here is the honest side-by-side from a working septic pumper in West Tennessee.
SIDE BY SIDE
The honest comparison
Both systems do the same job: separate solids from liquids and return treated effluent to the soil. The difference is how they treat the waste and what your site can support.
How it treats waste
Anaerobic (conventional)
Bacteria break down solids without oxygen inside a sealed tank. Effluent flows out to a gravity drainfield.
Aerobic (ATU)
An air pump forces oxygen into a treatment chamber. Aerobic bacteria break waste down far more completely before it leaves the unit.
Install cost
Anaerobic (conventional)
Roughly $3,000 to $7,000 for a typical residential install in West Tennessee.
Aerobic (ATU)
Roughly $10,000 to $20,000 installed. More components, more labor, and a control panel.
Power
Anaerobic (conventional)
None. Fully passive.
Aerobic (ATU)
Runs on 120V. The air pump and control panel add about $15 to $30 per month to the power bill.
Maintenance
Anaerobic (conventional)
Pump the tank every 3 to 5 years. That is essentially it.
Aerobic (ATU)
Required service contract in most states. Inspections every 4 to 6 months, plus pumping. Air pump diaphragms and filters wear out.
Site requirements
Anaerobic (conventional)
Needs good soil that will absorb effluent. Poor soil, high water table, or a small lot can rule it out.
Aerobic (ATU)
Works on lots that will not perc for a conventional system. Cleaner effluent means a smaller or shallower drainfield.
Lifespan
Anaerobic (conventional)
Tank 30+ years. Drainfield 20 to 30 with care.
Aerobic (ATU)
Treatment unit 15 to 20 years. Air pump 3 to 5 years before rebuild or replacement.
WHEN AEROBIC WINS
Pick aerobic if
- Your soil will not perc for a conventional gravity system
- You have a high water table or shallow bedrock
- The lot is too small for a full-size drainfield
- You are near a lake, creek, or well and need cleaner effluent
- The county health department requires an ATU for your site
WHEN ANAEROBIC WINS
Pick anaerobic if
- Your soil percs and you have room for a proper drainfield
- You want the lowest install cost and lowest ongoing cost
- You do not want a required service contract or monthly power draw
- You are replacing an existing conventional system on the same footprint
FAQ
Common questions
What is the main difference between aerobic and anaerobic septic systems?
Anaerobic systems treat waste with bacteria that live without oxygen inside a sealed tank, then send effluent to a gravity drainfield. Aerobic systems (also called ATUs, aerobic treatment units) pump air into the tank so oxygen-loving bacteria break waste down more completely. The aerobic effluent is much cleaner, which is why they can be used on sites where a conventional system will not work.
Is an aerobic septic system worth the extra cost?
Only if your site needs it. If your soil percs and you have room for a normal drainfield, a conventional anaerobic system is cheaper to install, cheaper to run, and simpler to maintain. If your soil is heavy clay, the water table is high, or the lot is small, an aerobic unit may be the only way to get an approved system. That is when the extra cost is worth it.
How often does an aerobic system need maintenance?
Most states, including Tennessee, require a service contract on aerobic units. That usually means an inspection every 4 to 6 months to check the air pump, filters, chlorinator if fitted, and effluent quality. The tank itself still needs to be pumped, though typically less often than a conventional system.
Do aerobic septic systems smell?
A properly running aerobic unit should not smell. Aerobic bacteria produce carbon dioxide and water instead of the methane and hydrogen sulfide anaerobic bacteria produce. If an aerobic system smells, the air pump is usually failing or the unit is overloaded.
How long does an aerobic septic system last?
The treatment unit itself lasts 15 to 20 years in most cases. The air pump is the wear part and typically needs a rebuild or replacement every 3 to 5 years. Skipping service kills the aerobic bacteria and shortens the life of the entire system.
Can I convert my anaerobic system to aerobic?
Sometimes. If the existing tank is sound, an aerator can be retrofitted to convert it to an aerobic unit. It is not always cheaper than a fresh install and it has to be approved by the county health department. We are happy to look at your setup and tell you honestly what makes sense.
READY?
Not sure which system fits your lot?
We pump and service both conventional and aerobic systems across Jackson and West Tennessee. If you are building, replacing, or just trying to figure out what you already have, give us a call and we will walk your site with you.
Also useful: Signs your septic system is failing and How often to pump a septic tank.