Aerobic septic systems: how they work, what they cost, and the upkeep
Updated
The short answer
An aerobic septic system uses an air pump to bubble oxygen through the sewage so aerobic bacteria break it down faster and more completely than in a conventional tank. The cleaner effluent can then be sprayed or dripped over the yard or sent to a smaller field. A 3-bedroom aerobic system typically costs $8,800 – $21,400 installed, plus roughly $300–$650 a year for a service contract and electricity.
How urgent · Plan for it
If a site evaluation says you need one, plan for the running costs as well as the install. If you already have one, the service contract and a working air pump are what keep it legal and working.
How an aerobic treatment unit works
An aerobic treatment unit (ATU) is a septic tank with extra stages. Sewage first settles in a trash tank, then moves into an aeration chamber where an air pump keeps it oxygenated. Oxygen-loving bacteria break down waste much faster than the anaerobic bacteria in a conventional tank. A clarifier lets the remaining solids settle back, and in most spray systems the water passes a chlorinator before a pump sends it to spray heads or drip lines.
When an aerobic system is used
Aerobic systems are chosen where the soil can't treat conventional effluent: small lots, clay, shallow soil, high water tables, and sites near lakes or wells. They're also common where the county requires a higher level of treatment. In Texas and parts of the South they're the standard system on many new builds.
What an aerobic septic system costs
| Item | Typical cost |
|---|---|
| Installed system, 3 bedrooms | $8,800 – $21,400 |
| Aerobic treatment unit alone | $4,000 – $8,000 |
| Service contract | $200 – $500 a year |
| Electricity for the air pump | $100 – $200 a year |
| Chlorine tablets (spray systems) | $50 – $150 a year |
| Replacement air pump | $300 – $900 |
| Pumping (every 2–5 years) | $300 – $750 |
Planning ranges; the installed figure is from the site's cost engine for typical soil and average labour. Not quotes.
Aerobic septic system maintenance
Aerobic systems need attention that conventional ones don't. Many states require an ongoing service contract or a certified operator — Texas, for example, requires an initial two-year contract on new systems — and the service provider inspects the unit, typically every four months, and files reports with the county.
- Keep the air pump running. If the hum stops, the unit goes anaerobic within days and starts to smell.
- Keep chlorine tablets in the chlorinator on spray systems — and only tablets made for wastewater. Pool chlorine tablets are a different chemical and can react dangerously with wastewater tablets.
- Have the unit pumped when the service provider's sludge measurement says so, usually every two to five years.
- Don't turn it off when you travel. Bacteria need a constant air supply.
- Keep spray heads clear of plants, people and pets, and never spray onto areas used for vegetables.
Disadvantages of an aerobic septic system
- Higher cost to install and to run than a conventional system.
- It depends on electricity: in a long power cut it stops treating, and some units need reduced water use until power returns.
- More parts to fail — air pump, spray pump, timer, alarm.
- Ongoing service contracts, and in some states regular reports to the county.
- Spray systems put treated water on the lawn, which some owners dislike and which needs chlorine to be safe.
Frequently asked questions
- Do aerobic septic systems need to be pumped?
- Yes, though often less frequently than a conventional tank. The service provider measures the sludge at each visit and tells you when; two to five years is common.
- Which is better, a conventional or aerobic septic system?
- Where the soil allows both, conventional: it's cheaper to install and run and has almost nothing to break. Aerobic systems exist for sites where conventional won't work.
- What is the lifespan of an aerobic septic system?
- The tanks commonly last 20 to 40 years. Air pumps last about 3 to 7 years and are the part most often replaced.
- Can you flush toilet paper in an aerobic septic system?
- Yes, ordinary toilet paper is fine. Avoid wipes, paper towels and anything that doesn't break down quickly.
- Are aerobic septic systems worth it?
- When the site needs one, there's no cheaper legal alternative. Where a conventional system is allowed, the higher running costs usually aren't worth it.