An aerobic septic system, or ATU, pumps air through the wastewater so oxygen-breathing bacteria treat it far more completely than the anaerobic digestion in a conventional tank. The effluent leaves clean enough that many states allow a much smaller absorption area or surface spray irrigation, which is why an ATU is often the only approved design on a tight or slow-percing lot. Installed cost is $10,000 to $25,000, and the running cost of $400 to $900 a year is the part people underestimate.
Aerobic vs anaerobic: what actually differs
| Anaerobic (conventional) | Aerobic (ATU) | |
|---|---|---|
| Bacteria | Oxygen-free digestion | Oxygen-breathing, far faster |
| Effluent quality | Clarified, high BOD | Treated, low BOD |
| Power | None | Blower running continuously |
| Field size | Full code area | Often reduced, sometimes spray |
| Service | Pump every 3 to 5 yr | 2 to 4 visits a year, often mandatory |
| Install | $6,000 to $15,000 | $10,000 to $25,000 |
| Running | $95 to $345/yr | $400 to $900/yr |
How an ATU is built
Most units are a single tank split into three or four compartments. The first is a trash or pretreatment chamber that works like a normal septic tank, settling the solids. The second is the aeration chamber where a blower on the surface feeds air through a diffuser, keeping the mixture stirred and oxygenated. The third is a clarifier where the biological floc settles out and returns to the aeration chamber. Many units add a fourth chamber for chlorine tablets before discharge.
What it costs to run
- Electricity, $120 to $300 a year. The blower runs continuously or on a long duty cycle.
- Maintenance contract, $200 to $600 a year. Required by law in Texas, Florida and a number of other states, typically two to four inspections.
- Chlorine tablets, $50 to $150 a year where the permit requires disinfection. Use the tablets specified for wastewater, not pool chlorine.
- Blower replacement, $400 to $900 every 5 to 10 years.
- Pumping, $250 to $900 every 2 to 4 years. An ATU still accumulates sludge in the trash chamber.
Several states tie the operating permit for an aerobic system to a current contract with a licensed maintenance provider, and the provider reports to the county. Letting it lapse can mean a violation notice rather than just a neglected system. Check your permit conditions before you assume you can service it yourself.
When an aerobic system is the right choice
- The soil percs too slowly for a conventional field at any reasonable area.
- The lot is too small for a full-size field after setbacks.
- You are close to a well, a lake or a stream and the county requires treated effluent.
- A conventional field has failed and there is no room for a replacement.
- Spray irrigation is permitted locally, which in parts of Texas and the Southwest is the standard rural design.
What goes wrong with them
- The blower fails and nobody notices. Treatment stops within days and the system quietly discharges poorly treated water. This is what the alarm is for, so do not silence it and forget it.
- The alarm is disconnected. Common, and it turns a $400 blower into a field replacement.
- Bleach and heavy cleaner use. An aerobic system depends on a live bacterial culture and is more sensitive to being poisoned than a conventional tank.
- Long vacancies. The culture starves with no incoming waste, and a system that has sat empty for months needs weeks to recover.
- Sprinkler heads mown off on spray systems, which then irrigate one spot.
Is it worth it if a conventional system would fit?
Usually not. Over twenty years an ATU costs $8,000 to $18,000 more to run than a gravity system, has a motor that can fail on a holiday weekend, and delivers a benefit, cleaner effluent, that the soil under a conventional field was already going to provide. Choose an ATU when the site requires it or when a sensitive water body is close by. Choosing one for peace of mind on a lot that percs well buys a service contract you did not need.