Updated October 2026. Checked against U.S. EPA and Texas A&M AgriLife Extension guidance.
An anaerobic septic system is the standard septic tank and drainfield most homes have: bacteria that work without oxygen break down waste in the tank, and the soil finishes treatment. An aerobic system adds an air pump so oxygen-using bacteria treat the water more completely before it leaves. Anaerobic systems are simpler and cheaper; aerobic systems cost more and need power and regular maintenance but fit sites a conventional system cannot.
Aerobic vs anaerobic septic systems compared
| Anaerobic (conventional) | Aerobic (ATU) | |
|---|---|---|
| Bacteria | Work without oxygen | Need oxygen from an air pump |
| Treatment level | Partial in the tank; the soil finishes it | More complete before the water leaves the unit |
| Power | None for a gravity system | Air pump runs continuously |
| Drainfield | Larger; needs suitable soil | Smaller field, spray heads or drip lines |
| Maintenance | Pumping every 3 to 5 years (EPA) | Regular lifetime maintenance (EPA), often a service contract |
| Cost | Lower to install and run | Higher to install and run |
| Best for | Lots with good soil and space | Small lots, poor soil, high water table, sensitive water nearby |
What is an anaerobic septic system?
An anaerobic septic system is a conventional septic tank and drainfield. Wastewater flows into the tank, solids sink to form sludge, grease and oils float to form scum, and anaerobic bacteria slowly break down the waste. The liquid in the middle flows out to the drainfield, where the soil filters and treats it. The EPA describes a conventional system as a septic tank and a trench or bed, typically installed at a single-family home.
For the full picture, see how a septic system works.
What is an aerobic septic system?
An aerobic septic system, or aerobic treatment unit, pumps air into a treatment chamber. Texas A&M AgriLife Extension describes the stages as removing solids in a pretreatment tank, aerating the wastewater so microbes can break it down, settling the remaining solids in a clarifier, disinfecting with chlorine or UV light, and then spraying or dripping the treated water onto the yard. More detail: what is an aerobic septic system.
Which system is right for your property?
- Good soil and enough space: a conventional anaerobic system is usually the simpler, cheaper choice.
- Small lot, shallow soil or a high water table: the EPA says an aerobic unit can work where a conventional system cannot.
- Near a lake, stream or well that is sensitive to nutrients: an aerobic unit may be required or recommended.
- You want the least upkeep: conventional. Aerobic systems need ongoing service and power.
The final choice is set by your soil test and your county or state permit rules, not by preference alone.
Not sure which system your lot needs?
A soil test and site evaluation decide what is allowed. Talk to a local septic company about options and a quote.
Calls are answered by a pay-per-call partner network that connects you with a local septic company. Calling is free; we may be paid for referrals.
Can you convert an anaerobic system to aerobic?
Often, yes. A septic company can add an aerobic unit after the existing tank or replace the tank with an aerobic system. InspectAPedia notes that adding aeration to a tank will almost certainly improve treatment, but retrofits in systems not designed for it need testing. Conversions usually need a permit. See aerobic septic system cost for what to budget.
Pros and cons of each system
| System | Pros | Cons |
|---|---|---|
| Anaerobic | Simple, no power, low running cost, long life with pumping | Needs good soil and space; less complete treatment in the tank |
| Aerobic | Cleaner effluent, smaller field, works on difficult sites | Higher cost, electricity, more parts, required maintenance |
Quick answers
What type of bacteria is in a septic tank?
A conventional septic tank relies mainly on anaerobic bacteria, which do not need oxygen. Aerobic units add oxygen so aerobic bacteria do most of the work.
Are anaerobic septic systems still used?
Yes. The conventional septic tank and drainfield is still the most common household system.
Is an aerobic septic system better?
It treats water more completely, but it is not better for every home. Where a conventional system is allowed, it is usually cheaper and simpler to own.
Do septic additives help aerobic or anaerobic systems?
Neither type needs them for normal operation. See our review of septic tank treatments for what the evidence says.
Sources
- U.S. EPA, Types of septic systems
- Texas A&M AgriLife Extension, Living with an aerobic treatment unit and spray field (B-6234)
- U.S. EPA, How to care for your septic system
- InspectAPedia, Drainfield restoration methods