A conventional gravity system relies on natural slope to move settled effluent from the tank into a drainfield of perforated pipe laid in gravel trenches, with no pump required. It is the simplest and usually least expensive design, and is used whenever the site’s slope, soil, and water table allow it.
Effluent leaves the tank and flows by gravity through distribution pipe into a series of trenches, where it seeps out through the perforations into surrounding gravel and then into the native soil beneath. No pump, float switch, or electrical component is required to move it, which is also why gravity systems need less ongoing mechanical maintenance than pumped alternatives.
What rules a gravity system in or out is entirely site-dependent: adequate slope to carry flow to the field without a pump, soil that meets the application-rate and depth requirements, and enough separation from the seasonal high water table and any restrictive layer. Where any of those is missing, a pressure-dosed, mound, or other alternative system takes its place.
Because it is the default, lowest-complexity design, a gravity system is generally what a designer specifies first and only moves away from when the soil evaluation rules it out.
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It relies on natural slope to move effluent from the tank to the drainfield, without a pump, which makes it the simplest and typically least expensive design where site conditions allow it.
When the site lacks adequate slope, the soil doesn't meet application-rate or depth requirements, or the seasonal high water table or a restrictive layer is too close to the surface, an alternative like a pressure-dosed or mound system is used instead.
Guide last reviewed 2026-08-31.