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].
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]
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