Mound System
Also called: raised septic system, elevated drain field system
A mound system places the drain field in an elevated, engineered bed of fill material constructed above the native soil surface. It is used on sites where shallow soil depth, high seasonal water tables, or low-permeability soil make a conventional in-ground drain field impractical or prohibited. The elevation creates the separation distance between the bottom of the drain field and limiting layers in the native soil that is required for effective treatment.
Mound system designs, fill material specifications, setback requirements, and maintenance obligations vary by site conditions and county regulations. In some Washington counties, mound systems require an O&M agreement with a service company that meets county program requirements and periodic county-required inspections. Consult your system records and county health department for requirements specific to your installation.
How It Generally Works
1Primary treatment in the septic tank
Wastewater enters the septic tank for standard settling and separation. The effluent filter at the tank outlet is particularly important on mound systems because solids that escape the tank can damage the engineered mound media, which is typically more difficult to rehabilitate than a conventional field.
2Effluent is pumped to the mound
Because the mound sits above the surrounding grade, gravity alone cannot deliver effluent to it. A pump chamber houses a submersible pump that delivers timed doses of effluent uphill through a force main to the distribution network inside the mound.
3Distribution across the mound bed
Inside the mound, pressurized laterals distribute effluent evenly across a bed of clean sand or other approved fill material. The engineered media provides the treatment zone that the native soil cannot supply at adequate depth.
4Downward percolation through fill and native soil
Effluent percolates down through the mound media, receiving biological and physical treatment along the way. By the time it reaches the native soil surface, it has been partially treated. It then continues downward through the native soil to complete treatment before reaching groundwater.
Common Maintenance Considerations
Pump and dosing system
The pump, floats, and timer require periodic inspection - as with any pump-based system. Mound systems cannot function without the pump; failure stops effluent delivery and fills the pump chamber.
Mound surface and vegetation
The mound surface must be maintained with shallow-rooted grass cover that allows evapotranspiration without allowing roots to penetrate the distribution network. Deep-rooted plants, shrubs, and trees should not be planted on or near the mound. The mound should not be covered with impermeable materials such as plastic sheeting, gravel mulch, or hardscape.
Controlling water on and around the mound
Surface water should drain away from the mound, not pond on it. Irrigation systems should not direct water onto the mound surface. Excessive surface water saturation reduces the mound's ability to absorb effluent.
Tank pumping and filter service
The septic tank requires periodic pumping regardless of the mound system's performance. The effluent filter - critical for protecting the mound media - should be cleaned at every pump-out.
Observable Conditions to Share for Service Planning
The following are observable conditions, not remote diagnoses. General Septic can assess the system on site as part of the requested service.
Wet areas or effluent surfacing on or around the mound
Saturated areas on the mound surface, effluent seeping from the mound sides, or odor emanating from the mound may indicate the system is not dispersing effluent as designed. Keep people and pets away, reduce water use, and describe the observation to General Septic or the applicable health authority.
Pump alarm active
An active alarm at the control panel means the pump chamber level is above normal. Reduce water use and describe the alarm to General Septic. An unaddressed alarm on a mound system can lead to effluent surfacing on the mound.
Mound settling, erosion, or berm damage
If the mound surface has settled unevenly, developed bare eroded areas, or had the surrounding berm disturbed - by vehicles, landscaping, or other activities - the system's structural integrity may be affected and should be assessed.
System Details to Include in a Request
Having this information ready helps General Septic plan the requested service:
- Whether the alarm is currently active
- Any visible wet areas on or around the mound
- When the tank was last pumped
- Pump brand and control panel model, if visible
- Any recent changes on or around the mound (landscaping, vehicles, construction)
Related Services
Describe a request in the greater Seattle area
General Septic directly provides septic services across King, Pierce, and Snohomish counties with its staff and equipment. Describe your system and situation in the request form so the team can plan the work.