Gravity Drain Fields

As implied, gravity drainfields work by allowing the effluent to flow from the septic tank into a series of trenches by gravity. To allow this, the drainfield must be below the outlet level of the septic tank.

A conventional gravity system consists of a network of distribution components laid in gravel filled trenches. The trenches are dug in natural, undisturbed soil typically 3 feet above any natural occurring restrictive layer, such as a hardpan or the seasonal high water table.

Diagram of gravity drainfield returning wastewater to the water table

Typical anaerobic gravity drainfield returning wastewater to the water table.

The undisturbed soil filters out the remaining solids in the effluent which are then processed by anaerobic bacteria transported from the septic tank in the effluent.The anaerobic biochemical process cleans the effluent before it reaches the groundwater.

Maximum effectiveness of the final treatment occurs when the soil is dry and permeable. The size of the drainfield depends on the estimated daily wastewater flow (generally based on the number of occupants), soil type, and soil conditions.

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