Above-ground systems, referred to as mound systems, were developed to compensate for poor-absorbing local soils that cannot support a standard underground septic system.
Simple Waste Management System
All septic systems are designed with two primary components. The first is the septic tank, made of a watertight concrete or fiberglass to collect solid waste called "sludge." The second component incorporates the local soils to absorb the liquid sewage (effluents) through an underground system of perforated pipes, delivering the liquids to a designated area, known as the leach field.
When local soils do not have the capacity to absorb effluents properly, an artificial system is created above ground, known as the mound system.
Up and Out
As with the underground system, the septic tank in a mound system is also buried underground. Depending on the design, there may be two tanks or one tank with two chambers. The first tank, or chamber, holds the solid waste while the second tank (dosing chamber) collects the liquid waste.
This second chamber has a built-in sump pump that pumps these liquid wastes up to the above-ground absorption mound.
Layers of Sand, Gravel and Soil
The first layer of the mound is a tilled-up layer of the soil found on the septic tank site. Next, a layer of sand is spread over the locally tilled soil. Then a layer of gravel and piping is strategically arranged and connected to the dosing chamber below. The gravel is then covered with a construction grade fabric and finally, another layer of soil finishes the absorption mound.
This final layer of soil will be enriched for grass to grow quickly over the mound, to prevent erosion problems. This mound will need to be kept clear of shrubs and trees since the roots may compromise the piping.
Putting It All Together
When the toilet is flushed or the bathtub drained, the waste goes directly into the septic tank. Solids remain in the first tank, while the second chamber collects the liquid waste. This waste (effluent) is then pumped up and spread through the gravel and sand layers. These effluents filter naturally through the soil, being recycled into the environment, mainly as ideal fertilizer for the top layer of soil.
As with an underground septic system, the mound system also needs regular pumping of the sludge tank.