If you’re looking into septic systems for a new home build or replacing an old failing system, you’ve probably heard the terms “aerobic” and “anaerobic.” While both systems are designed to treat your household wastewater, they do it in completely different ways.
The Short Answer: Anaerobic septic systems are the traditional, simple setups that rely on bacteria that do not need oxygen to break down waste. Aerobic systems are more complex, actively pumping oxygen into the tank to support a different type of bacteria that breaks down waste much faster and more thoroughly.
Let’s break down how each system works, the pros and cons of both, and which one is right for your property.
How an Anaerobic Septic System Works
When you think of a “standard” septic system, you are thinking of an anaerobic system. This is what the vast majority of rural and suburban homes use.
The Process:
- Wastewater from your house flows into a large, buried tank (usually concrete or fiberglass).
- Inside the tank, solid waste sinks to the bottom (sludge) and fats/oils float to the top (scum).
- Because the tank is sealed and buried, there is no free oxygen inside.
- Anaerobic bacteria (bacteria that live without oxygen) slowly eat the solid waste, breaking it down over time.
- The relatively clear liquid in the middle (effluent) flows out of the tank and into a traditional drain field, where the soil filters it naturally.
Pros of Anaerobic Systems:
- Simple and Reliable: There are no moving parts, no electricity required, and very little that can mechanically break.
- Lower Cost: They are significantly cheaper to install and maintain than aerobic systems.
Cons of Anaerobic Systems:
- Slower Processing: Anaerobic bacteria are slow eaters. They don’t break down waste as efficiently, meaning the effluent that leaves the tank is not as clean as it would be from an aerobic system.
- Requires Good Soil: Because the effluent isn’t fully treated in the tank, it relies heavily on the soil in the drain field to finish the job. If your property has poor soil (like heavy clay) or a high water table, an anaerobic system might fail or not be permitted at all.
How an Aerobic Septic System Works
An Aerobic Treatment Unit (ATU) is essentially a miniature municipal sewage plant in your backyard. It is used when a property cannot support a traditional drain field.
The Process:
- Wastewater flows into a trash tank where large solids settle.
- The liquid then moves into the aeration chamber. Here, an electric air compressor (aerator) pumps bubbles of oxygen into the wastewater.
- This oxygen fuels aerobic bacteria. These bacteria are incredibly hungry and aggressive, breaking down the organic waste much faster and more completely than anaerobic bacteria.
- The treated liquid moves to a clarification chamber where any remaining solids settle out.
- Finally, the highly treated, nearly clear water is often disinfected (sometimes with chlorine or UV light) and dispersed. Because the water is so clean, it can often be dispersed via shallow drip lines or even surface sprinklers, rather than a deep, traditional drain field.
Pros of Aerobic Systems:
- Highly Efficient: They treat wastewater much more thoroughly, producing a cleaner effluent.
- Flexible Installation: They can be installed on properties with poor soil, high water tables, or limited space where a traditional drain field is impossible.
Cons of Aerobic Systems:
- Expensive: They cost significantly more to install.
- High Maintenance: They require electricity to run the aerator 24/7. They also have moving parts (pumps, alarms, floats) that can break and require frequent professional maintenance and inspections.
Which System is Right for You?
In almost all cases, if your property has the right soil conditions and enough space, a traditional anaerobic system is the best choice. It is cheaper, simpler, and requires far less maintenance.
However, if you are building on a small lot, near a body of water, or on land with heavy clay or rock, local regulations will likely require you to install an aerobic system.
Maintaining Both Systems
Regardless of which system you have, both rely entirely on a healthy bacterial ecosystem to function. If you kill the bacteria, the system fails.
Harsh chemicals, bleach, antibacterial soaps, and excessive grease will destroy both aerobic and anaerobic bacteria.
To protect your investment, you must be mindful of what goes down your drains. For traditional anaerobic systems, using a monthly bacterial treatment like a high-quality bacterial treatment is crucial. It replenishes the bacteria that are inevitably killed off by everyday household use, ensuring the solid waste is broken down efficiently and protecting your drain field from expensive clogs.
The Bottom Line
Anaerobic systems are the simple, cost-effective standard that rely on oxygen-free bacteria and a large drain field. Aerobic systems are complex, expensive alternatives that use pumped oxygen to treat waste highly efficiently, making them ideal for difficult properties. No matter which system you have, protecting the bacteria inside the tank is the key to a long, trouble-free lifespan.
