Aerobic treatment units are nothing more than small, modified packaged plants modeled after large wastewater treatment plants that are used to treat the wastewater in the backyard. 

The main objective of an ATU is to remove a substantial part of the organic matter as well as the suspended solids that are in the wastewater. This provides for a “cleaner” effluent from the standpoint of organic matter. When the effluent is delivered to a soil treatment unit for final treatment and dispersal this provides the advantage of reducing biomat development. When the unit is used in front of a soil dispersal unit for final treatment the effluent is usually delivered through a pressure distribution network. Cleaner effluent provides the additional advantage of reducing orifice plugging and the need for maintenance.

As the name of the units imply, the treatment process is aerobic and relies on the presence of aerobic bacteria to supply the treatment. Ammonia in the wastewater is converted to the nitrate form of nitrogen. While this conversion is good from the standpoint of eliminating the ammonia, which is toxic to fish and other aquatic organisms, elevated nitrates in surface or groundwater used for drinking water can cause problems. In coastal estuary environments nitrate can contribute to harmful algal blooms which cause environmental problems.

Recently in areas where there are nitrate concerns, ATUs are operated to send the nitrified effluent through a sequence where oxygen is limited. The effluent is then in an anaerobic state to allow for denitrification to occur to reduce the total amount of nitrogen delivered to the environment. In my view, if nitrogen reduction is the goal, use of recirculating media filters is a more efficient way to provide this treatment.

The aerobic treatment process does not impact phosphorus levels, and treatment or fixing of phosphorus takes place in the final soil dispersal component of the individual sewage treatment system.

Another aspect to be aware of is that with ATUs you will get only a reduction of the pathogenic organisms. You will not get as high a reduction of pathogenic organisms as when using media filters. If the effluent is to be surface discharged, a disinfection component is needed following the unit. Otherwise final soil dispersal is where final treatment for pathogens would take place. 

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 kim.peterson@colepublishing.com.

This article is part of a series on ATUs:

• An Introduction to ATUs
• Wastewater Loads and the ATU
• How Well Do You Know Your ATUs?
• ATU Components You Should Know
• ATU Inspection in 3 Easy Steps