There are two basic septic system types — conventional and alternative. Site and soil conditions generally determine the type of system that should be installed. For additional information, contact the onsite program staff at 360-867-2673.

Since your septic system is your responsibility, it is important to understand your particular type of system and how to care for it. The following is a brief explanation of the most common types of septic systems.

Conventional Systems

Gravity System

Basic or standard system, uses gravity to move effluent through the septic tank and into the drainfield. For more information, see Gravity System [PDF] or view the video, Understanding and Maintaining Your Gravity Flow System.

Pressure Distribution System

Has a pump which distributes effluent throughout the drainfield at the same time. Used in most new systems because it makes better use of the entire drainfield. See Pressure Distribution System [PDF] for additional information.

Alternative Systems

Aerobic Treatment Unit (ATU)

Uses oxygen to break down solids, producing cleaner wastewater than conventional types. Often used in environmentally sensitive areas where effluent requires more treatment before entering the drainfield. For additional information, see Aerobic Treatment Units (ATUs).

Mound System

Installed when minimal soil is available for treatment, drainfield is raised above ground level. See Mound System [PDF] for more details.

Sand Filter System

Has a sand filtering system and pump to treat and disburse effluent, used when minimal soil is available for treatment. To find out more, see Sand Filter System [PDF].

Other Systems (Proprietary)

For the most up-to-date information on proprietary sewage products approved by Washington State, see the Washington State Department of Health document titled "List of Registered On-Site Treatment and Distribution Products" [PDF]