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 Permit Assistance Center at 360-786-5490.
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.
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,
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.
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).
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)
This alternative system consists of
different layers of sand and gravel placed
in a watertight box. Sewage effluent is
pumped into the bottom of the filter and
then works its way up through the sand and
out into the soil.
In this system, the effluent is filtered through a bed of engineering textile material that is
enclosed in a watertight container. This allows treatment of a large amount of wastewater in a small space and is clean enough for subsurface irrigation.
This is a shallow, pressure-dosed system that equally distributes pretreated wastewater at a slow rate over the entire drainfield, preventing saturation of the soil.