Environmental Health
Septic Systems
  Aerobic Treatment Units (ATU)  
  How Does It Work?  l  Maintenance  l  Advantages and Disadvantages  

Aerobic Treatment Units (ATUs) are similar to standard septic systems in that they use natural processes to treat wastewater. But unlike conventional systems, ATUs also use oxygen to break down organic matter, much the same as municipal wastewater treatment systems, but in a scaled-down version.

Because ATUs decompose organic solids quickly, the wastewater leaving the system is cleaner. ATUs are useful in environmentally sensitive areas or locations that are less suitable for conventional or gravity flow septic systems, such as inappropriate soil conditions where the water table is too high to allow the drainfield to operate effectively.

How Does It Work?

There are many types of ATUS, but the most common household ATUs use a process called suspended growth. These units have a main compartment (aeration chamber) in which air is forced and mixed with the wastewater. This creates an environment where bacteria are free-floating in the liquid and grow as they digest the solids (suspended growth).

Many units include a second chamber where solids, that the bacteria are unable to digest, settle. The two chambers are connected, so these undigested solids can be returned to the aeration chamber, either by gravity or a pump. It is this process of return and mixing that is important for effective operation.


Because ATUs are more complicated than conventional septic systems, maintenance is key to their life and performance. There are more than 20 brands of ATUs approved for use in Washington state and each has individual requirements specified by the manufacturer that must be performed by an approved specialist. Because of their complexity, Thurston County requires the following be adhered to for the life of the system:

Operational Certificate

Renewable each year with Thurston County Environmental Health (see Operational Certificates for more information).

Maintenance Contract

A current and continuous service contract with a County approved manufacturer's certified monitoring specialist [PDF] (CMS), proof of which is required annually with the renewal of the Operational Certificate.

Monitoring and Maintenance

Monitoring and maintenance are important to the performance and longevity of ATUs. Some of the requirements specified by the manufacturer and performed by the CMS are:

  • Check electrical panel for functioning alarms
  • Check and clean filters on aerator
  • Check odor — should be “musty”
  • Check clarity of effluent — should be clear, brownish color; gray/black means proper oxygenation is not occurring
  • Record sludge level in pretreatment tank — pump as needed and refill with water
  • Record sludge level in aeration tank — pump as needed

Additionally, if the ATU system has a disinfection unit, quarterly fecal coliform testing is required. Your specific requirements should be listed on the Operational Certificate.

Because it is important to maintain an active population of bacteria in the system to break down solids, homeowners should be aware of the following:

  • Keep electricity going to the ATU — the aeration system needs a continuous supply of power (see Special Conditions for additional information).
  • Too much water or wastewater in the system dilutes the bacteria food source.
  • Adding too much organic matter (like fat, grease, or garbage disposal debris) results in more solids than the bacteria can consume. The more organic matter, the more frequently the system must be pumped.
  • Too many toxic or cleaning products can kill the microbes, reducing the amount of solids that are consumed.
  • Infrequent use (such as vacation homes) may not keep enough waste in the system to give the bacteria enough food to sustain themselves. Use the toilet a few times to allow the microbes enough time to rebuild themselves before doing laundry or other high-water usage activities.

Advantages and Disadvantages

There are many advantages and disadvantages to ATUs, the following is adapted from U.S. Environmental Protection Agency Aerobic Treatment Fact Sheet[PDF].


  • Higher level of treatment than a septic tank
  • Helps protect water resources in areas of failing septic systems
  • Alternative for sites unsuitable for septic systems
  • May extend life of drainfield
  • Reduces ammonia discharged to receiving waters


  • More expensive to operate than a septic system
  • Requires electricity
  • Includes mechanical parts that can break down
  • Requires more maintenance than a septic tank system
  • Subject to upsets under sudden heavy loads or when neglected
  • May release more nitrates into groundwater than a septic system
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  • Inspect and pump tank as necessary, generally every 3-5 years
  • Minimize water use
  • Keep solid wastes to a minimum
  • Don't put toxic chemicals down sinks or toilets
  • Plant only grass on or near the drainfield
  • Don't drive on the drainfield
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This page last updated: 07/09/21