onsite sewage program

Aerobic Treatment Units

Aerobic treatment units (ATUs) provide aerobic biological decomposition of wastewater constituents by mechanically bringing the wastewater in contact with air. ATUs come in different configurations and sizes, and incorporate a variety of approaches, including air pumps, air injectors, lift pumps and biological-contact surfaces (such as pipes, fabric, grids, gravels, and rotating disks). Other alternative wastewater treatment technologies, such as sand filter systems, may also be described by this characterization, however they are not categorized by the Washington State Department of Health (DOH) as aerobic treatment units or systems.

ATUs are not stand-alone wastewater treatment systems, but provide wastewater treatment prior to disposal in a subsurface drainfield system or provide pre-treatment to other alternative methods of wastewater treatment and/or disposal

This technology is used on sites with shallow soil conditions where treatment must be accomplished before disposal. ATU effluent may be discharged to as little as 12 inches of vertical separation.


To view the complete Recommended Standards and Guidance for Performance, Application, Design, and Operation and Maintenance of the Aerobic Treatment Unit you may follow the following links:


Aerobic Treatment Units
(Effective Date: December 31, 2003) - in Adobe Acrobat Format.