E9 Septic Tanks and Wastewater Treatment

Topic

In the absence of a sewer network, a septic tank serves as a substitute by retaining wastewater long enough for two simultaneous processes to occur: the settling of solids and the anaerobic digestion of organic matter. This digestion significantly—though partially—reduces the water's organic load before it exits the tank. Consequently, the tank is sized based on the volume of wastewater to be treated and the minimum retention time required for digestion to take place.

The effluent leaving the tank—still only partially treated—is distributed via perforated pipes into an infiltration field. There, it seeps into the surrounding soil and undergoes final purification through the action of soil microorganisms—applying the same principle of infiltration through a porous medium described in the groundwater module, but in reverse: instead of extracting water from the ground, water is introduced into it for purification. The minimum area required for this field depends on the flow rate and the local soil's infiltration capacity, which is determined on-site via a percolation test specific to the terrain; for instance, highly impermeable soil requires a much larger field than permeable soil to achieve the same level of purification.

For sanitary reasons, the infiltration field must be kept at a minimum distance from both drinking water wells and the water table. This is because the very infiltration mechanism that purifies the wastewater could contaminate the drinking water source if the wastewater were to get too close.

Thus, this module—which begins with a tank that retains and digests wastewater—concludes by demonstrating how the entire system operates by linking two stages with opposing yet complementary roles: the tank reduces the organic load without relying on the soil, while the infiltration field completes the process by utilizing the soil, provided it remains sufficiently distant from any drinking water source.

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