ATU Components You Should Know

Manufacturer training is important, but knowing the five basic components of any ATU will help you with initial inspections
ATU Components You Should Know
This photo shows the aeration unit of an operating ATU.

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I have been told by others in the industry that there are approximately 200 different kinds of aerobic treatment units that are marketed in the U.S. For service providers this is a challenge because in my experience most manufacturers will have their reasons for why their unit is the best one and have their own particular operation and maintenance requirements.

No matter who the manufacturer is, the ATU will have five components. As a part of any inspection or maintenance activity the service provider should be able to identify each of these components.

The five components are: a trash tank to remove larger solids and inorganic plastics; the aerobic treatment component where the wastewater is brought in contact with aerobic microorganisms and where the organic material is broken down; the air supply system to provide oxygen for the treatment process; a clarification component that is a quiet area allowing any excess material to settle before the liquid is delivered for final treatment and dispersal; and finally a sludge return process where material is returned to the aeration chamber or pretreatment tank for additional treatment.

With the wide variety of units on the market, identifying components and determining access to each component can be difficult. This is why manufacturers require service providers to complete their training course before working on most units. The problem is that as a service provider you never know what unit you will find when you get the service call; so it is important at the very least to be able to identify the parts and the manufacturer as a start to any assessment of the system.

There are units on the market that have readily identifiable components. These are what I call “three tank units,” where the first tank in the series is the trash tank; the second tank is the aerobic treatment unit; and the third tank is the clarifier which has a sludge return pump back to the pretreatment trash (septic) tank and the piping to the final treatment and dispersal area. This type of setup and operation provides some opportunity for denitrification as a part of the process.

Most units combine some of the components or functions together; so there are units where all of these functions can be in a single tank. An example is that some units do not require a trash tank and you would see that this function actually takes place in an aeration chamber.

When troubleshooting or inspecting a unit it is important that each component can be accessed. Unfortunately there are units that have been installed where access is limited or not possible. In these cases access will need to be added or possibly the entire unit replaced. These treatment units require continued maintenance to operate the way they are supposed to. Without that access they will not provide the expected performance, which can create problems for final soil dispersal or cause the unit not to meet expected permit levels for surface discharge.

Trash removal and anaerobic treatment are generally conducted in a trash tank, which really operates as a septic tank. These components can remove the plastic objects and other non-degradable items from the wastewater. It is important to make sure these materials are captured to prevent damage or plugging of the aeration system. As mentioned above, this component can also be used in the denitrification process by returning a part of the aerobic effluent to anaerobic conditions. The size of the trash tank is also important. Use a smaller tank than you would for a normal septic tank. If the septic tank removes too much BOD, then the microorganisms in the ATU will have little to eat and performance in terms of BOD removal will actually be reduced. 

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

This article is part of a series on ATUs:


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