There are two fundamentally different approaches to onsite wastewater treatment. Most homes have the older, simpler one. A growing minority have the newer, more complex one. Knowing which you have — or which you should choose if you're installing — matters for cost, maintenance, and longevity.
Anaerobic systems (conventional)
The standard septic system. Wastewater flows into a sealed tank, where bacteria that thrive without oxygen break down organic matter slowly over weeks. Solids settle to the bottom, scum floats to the top, and clearer effluent in the middle flows out to a drain field, where soil bacteria complete the treatment.
How it works
- Single sealed tank, no electricity
- No moving parts (other than mechanical baffles)
- Bacteria operate without oxygen (anaerobic)
- Effluent quality is moderate; relies on the drain field for final treatment
- Drain field needs to be sized generously
Cost
$5,000-$15,000 installed for a typical residential system. Almost no operating cost. Pumping every 3-5 years runs $300-$650.
Lifespan
Concrete tank: 30-40 years. Drain field: 20-30 years.
Aerobic systems (advanced treatment)
An aerobic treatment unit (ATU) actively pumps oxygen into a treatment chamber, supporting aerobic bacteria that break down waste much faster and more thoroughly than anaerobic bacteria. The result is a higher-quality effluent that needs less soil treatment downstream.
How it works
- Multi-chamber tank with a treatment compartment and a separation compartment
- Air compressor pumps oxygen into the treatment chamber 24/7
- Aerobic bacteria break down waste much more thoroughly
- Effluent quality is much higher than a conventional tank
- Drain field can be smaller, or in some cases replaced by surface irrigation
- Often required where conventional systems aren't feasible (small lots, poor soils, near water bodies)
Cost
$14,000-$25,000+ installed. Operating cost: electricity for the compressor (~$15-$25/month) plus ongoing maintenance. Many jurisdictions (including parts of New Mexico) require annual maintenance contracts: $200-$500/year.
Lifespan
Tank: 30-40 years. Compressor and internals: 10-15 years before replacement. Diffusers and air-supply parts: 5-10 years.
When you'd choose each
Choose conventional anaerobic when:
- Soil conditions are good (perc test passes for conventional design)
- Lot is large enough for a properly-sized drain field
- No proximity restrictions to wells, streams, or property lines
- Power reliability is questionable (no compressor to fail)
Choose aerobic when:
- Soils don't support conventional treatment alone
- Lot size is too small for a conventional drain field
- Proximity to surface water or wells requires higher effluent quality
- Local jurisdiction requires it for new installs
- Replacing a failed system where conventional won't fit
Maintenance differences
Conventional: pump every 3-5 years, that's it. Aerobic: pump every 2-3 years (more frequent because the smaller chambers fill faster), plus annual professional maintenance, plus monitoring of the air compressor and alarm system.