ATU Inspection in 3 Easy Steps

When conducting a general inspection of an ATU, there are three important factors to look for.
ATU Inspection in 3 Easy Steps
This photos shows the surface of an ATU system. The trash tank is where the concrete manhole is, the aeration chamber is the domed tank, and the clarifier where the effluent leaves for the drainfield is the third piece. The controls for the system are in the boxes.

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For an inspection where the expectation is an assessment of the general operation of an aerobic treatment unit, such as for a point-of-sale real estate inspection, three main areas should be evaluated. The three things to assess are: the presence and location of odors, foaming or residuals outside the unit and access to the unit.

Assessment for odors means that you use your nose to check the condition of the system.

Aerobic odors near the system are normal; these odors are mild in nature and really are more of a musty or earthy smell. Also, aerobic odors are usually not objectionable to the facility owner.

Anaerobic odors indicate that the unit is not operating the way it should. They are relatively stronger. Often the smell is like rotten eggs and usually gains the attention of the facility owner, which may be the reason for your inspection. These odors can indicate that oxygen is not being delivered to the unit, so the blower or compressor should be checked to see if it is running.

Locating the source of the odors is critical. If anaerobic odors are exiting the trash tank then the operation of the aerobic unit is probably fine. However, the venting of the air from the trash tank needs to be rerouted or run through a carbon filter to remove the odor.

The site should not have odors around the system. A site that has odors around the system can usually be associated with a design/construction issue or a use/loading issue to the system by the facility. The units need to vent in a proper manner to minimize air moving through the anaerobic compartment. Hydraulic or organic overloading can result in incomplete digestion, which will result in odors. As indicated above, the air source may be turned off or is not operating.

While some units actually have a very active foaming process as part of the treatment system, there should not be any foam or other materials exiting the unit. If there is evidence of this the reason should be investigated. The cause may be as simple as excessive detergent use by the owner, or an indicator of incomplete digestion.

Access to all parts of the system to provide maintenance is important, so in a general inspection there are some basic questions to ask and resolve.

Are the access points located at grade? If they are not, how deep are they buried?

They really shouldn’t be buried, because as far as maintenance visits, any job that starts off with the need to excavate the access is going to involve more money and time. The homeowner may decide that the extra cost is worth not having to see it, but from a service provider standpoint having manhole access to the surface makes taking care of the system much easier.

If the unit is buried too deep, it may be hard to operate and maintain the system. This problem should be noted. It may not be feasible to raise the components, but if it is possible to provide better access through use of additional risers this should be done.

The other main issue with risers is ensuring that they are watertight. Evidence of infiltration in the risers needs to be checked to determine if water is collecting around the system. If so, excess surface water could enter the system, causing hydraulic overloading.

Lids should be securely fastened to restrict access and the lids should be in an operable condition.  

About the Author
Jim Anderson is connected with the University of Minnesota onsite wastewater treatment education program, is an emeritus professor in the university’s Department of Soil Water and Climate, and education coordinator for the National Association of Wastewater Technicians. Send him questions about septic system maintenance and operation by email to


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