Aerobic Septic Systems: How They Work, What They Cost, and What They Need

An aerobic septic system is a small wastewater treatment plant buried in your yard. It pumps air into the wastewater so bacteria that need oxygen can break down waste faster than they do in a conventional tank. The cleaner water is then disinfected and sent to a spray field, drip lines or a drainfield. If you own one, you have moving parts, a power bill and probably a service contract. This guide covers what that means in practice.

What an aerobic treatment unit is, and why a site gets one

An aerobic treatment unit, or ATU, treats wastewater with air. A conventional system relies on the soil under the drainfield to do most of the cleanup. An ATU does more of that work in the tank, so the water that reaches the ground is cleaner.

That matters when the soil cannot do the job alone. Common reasons a site gets an ATU:

• Poor or shallow soils. Clay, rock or a thin layer of soil over bedrock will not absorb and filter wastewater well.
• A high water table. There is too little dry soil between the field and groundwater.
• A small lot. There may be no room for a full-size drainfield.
• A failed conventional field. Replacing it with the same design may not be allowed.
• Local rules. Some counties require advanced treatment near lakes, wells or other sensitive areas.

Rules vary by state and county, and the permit often sets the design and the maintenance requirement. Find your state to see how licensing works where you live.

The parts and the flow through them

Designs differ by manufacturer, but most follow the same path:

• Trash or pretreatment tank. Wastewater leaves the house and settles here. Solids and grease stay behind.
• Aeration chamber. An aerator or air pump bubbles air through the water. The bacteria that grow on the oxygen digest the waste.
• Clarifier. The water settles again so the bacteria and leftover solids drop out and clear water moves on.
• Pump tank. Treated water collects here, and a pump with floats moves it onward on a schedule or when the level rises.
• Disinfection. Many systems pass the water over calcium hypochlorite tablets in a feeder, or under an ultraviolet (UV) light, to kill remaining germs.
• Dispersal. The water goes out through spray heads, drip tubing or a drainfield, depending on the permit and the site.

A control panel ties it together. It runs the timers and sounds the alarm when something goes wrong.

What an aerobic system costs to install and to run

Alternative systems, which include aerobic units and mound systems, typically cost $10,000 to $20,000 installed. A conventional system typically costs $3,600 to $12,500. Soil conditions, how far the dispersal area sits from the house, tank size, the dispersal type and permit fees all move the price. Our septic installation cost guide breaks down what drives each part.

The purchase price is not the whole cost. Budget for these running costs:

• Electricity. The aerator runs all day and every day, and the pump and panel add to it. The bill depends on the equipment and your local rates.
• Service contract. The price depends on how many visits the permit requires, your distance from the provider and what the contract covers.
• Disinfection. Chlorine tablets are used up and need refilling, or a UV lamp needs replacing on a schedule.
• Parts. Aerators, pumps, floats and diffusers wear out. Warranty terms and labor drive what a repair costs.
• Pumping. Routine pumping runs $250 to $600, on the provider’s schedule.

Maintenance and service contracts

Many permits for aerobic systems require an ongoing contract with a licensed maintenance provider, and several inspection visits a year are common. The permit usually sets the requirement, and it varies by state and county. Skipping the contract can put the permit out of compliance, even when the system looks fine.

A visit usually covers:

• Checking that the aerator or air pump is running and the air is reaching the chamber.
• Testing the pump, floats and alarm.
• Measuring sludge and scum, and cleaning filters.
• Checking and refilling the chlorine feeder, or checking the UV light.
• Looking over the spray heads, drip lines or drainfield.
• Writing a report for you, and in many places for the local health department.

Pumping follows the provider’s measurements, not a fixed interval. Our pumping frequency guide explains why measurements beat a calendar for any system. Keep every service report with your permit. Buyers and inspectors will ask for them.

The alarm and what to do when it sounds

The alarm tells you something needs attention now. The common causes are high water in a tank, an aerator or air pump that has stopped, and a failed effluent pump or float. Do not ignore it, and do not assume it will clear itself.

• Cut water use. Hold off on laundry, dishwasher loads and long showers.
• Check the breaker. A tripped breaker is a common and easy fix.
• Silence the horn if the panel has a button. The light usually stays on until the fault is fixed.
• Call your service provider. Say what the panel shows and how long it has been on.

Do not open tanks to look around. Tanks can hold dangerous gas. High water that keeps rising, with sewage coming up in the yard or the house, is urgent, and the same causes show up in our guide to drain field failure signs.

The chlorine rule

Use only calcium hypochlorite tablets made for septic systems. Do not use pool tablets. Most pool tablets are trichlor, which can release dangerous gas, especially if different chlorine products mix, and which can damage the unit and its parts. Store septic tablets away from other chemicals, keep them dry, and ask your provider which product fits your feeder. If your system uses UV instead, you do not need tablets at all.

Our what not to flush list applies to aerobic systems as much as any other.

Power outages

Treatment stops when the aerator stops. A short outage is usually fine if you use less water until power returns. After a long outage, the bacteria may need time to recover, and the pump tank may have filled. Reset any tripped breakers, watch for the alarm and call your provider if it sounds or if the water smells bad. If outages are common where you live, ask the provider whether a generator hookup makes sense.

What fails and how long parts last

Aerobic systems fail at the moving parts, not the tanks. Typical ranges, which depend on the brand, the water quality and how well the system is maintained:

Regular service finds a weak part before it fails. A system that has not been serviced for years often needs more than a quick fix.

Conventional versus aerobic

A conventional system is a tank and a drainfield, with no moving parts and no electricity. Our guide to what a septic tank is covers the basics. An aerobic system adds treatment, power and service.

• Installed cost: conventional $3,600 to $12,500; alternative systems, including aerobic, $10,000 to $20,000.
• Service: conventional systems need pumping and occasional inspection; aerobic systems usually need a contract and regular visits.
• Site: conventional systems need good soil and room; aerobic systems work where conventional ones cannot.

Who is licensed to install and service them

Installers and maintenance providers are licensed or certified by the state or county, and the requirements vary. Many places also require manufacturer training for particular brands. Ask for the license number and check it with the agency that issued it. Our guide on how to choose a septic company lists the questions to ask. When you are ready, request free quotes for aerobic septic work, or find your state and browse licensed contractors in your area.

Common questions