A mound system builds the drainfield up above natural grade in an engineered sand fill, rather than placing it in a trench dug into native soil. It is used where a conventional system will not work — typically a shallow soil depth to bedrock or the water table, or soil too tight to meet a conventional application rate — because the raised sand fill creates the vertical separation the site itself does not naturally provide.
Effluent is pressure-dosed up into the sand fill from the tank via a pump chamber, distributed evenly across the mound’s absorption bed, and then treated as it moves down through the engineered sand and into the native soil beneath at a shallower, more controlled rate than a conventional trench requires.
Because the mound is an engineered structure, not a natural landscape feature, it needs sand meeting specific gradation requirements and a design calculated for the site’s actual soil and water table, not a generic size. It is also visibly a mound on the property, typically a few feet high and requiring meaningful lot area, which is a real site-planning consideration beyond the engineering itself.
Mound systems are more expensive to install than a conventional gravity system, largely due to the imported sand fill and the pump system, but they extend septic viability to lots that would otherwise not qualify for any on-site system at all.
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A shallow depth to bedrock or the seasonal high water table, or soil too tight to meet a conventional application rate, can rule out a conventional trench system, and the mound's raised sand fill creates the vertical separation the natural site doesn't provide.
Yes, effluent is pressure-dosed up into the mound from a pump chamber, since gravity alone cannot lift it into the raised sand fill.
It requires imported, gradation-specific sand fill and a pump system, both additional costs a conventional gravity trench does not have.
Guide last reviewed 2026-08-31.