What is a Mound Septic System?

Mary McMahon

A mound septic system is a type of alternative septic system which is used when site conditions do not allow for the installation of a conventional septic system. Because designing and installing septic systems can be a tricky task, it is advisable to let a professional build a mound septic system, to ensure that problems do not develop in the future. People should also be aware that this type of system requires more care and maintenance than conventional systems, and it can be more costly to install.

Woman holding a book
Woman holding a book

There are several reasons why a conventional septic system will not work on a site. One of the most common is poor soil conditions, such as very rocky soil. Another issue can be a high water table, which could cause the septic system to back up, or could lead to contamination of the water table with effluent. In these conditions, a regular septic leach field should not be installed, so a mound septic system may be recommended.

With a mound septic system, effluent from the house is routed to an underground septic tank, as with a regular septic system, with solids settling to the bottom and fluids floating to the top. The fluids are in turn routed to a secondary tank, which triggers a pump to force fluid into the mound when it rises above a certain level. As with a regular septic system, the tank will need to be periodically cleaned out, and maintenance tasks may also need to be performed on the pump used to pressurize the effluent.

The mound is built above grade by layering sand and soil. It effectively acts as a filter for the effluent leached from the septic tank, exactly like a conventional leach field. There are some concerns with a mound septic system installation which have to be taken into account. It's important to use the right kind of materials to fill the mound, and to place the mound in an area which does not get a lot of traffic, because high traffic can compact the soils and make the mound less effective.

Since most people do not want a large mound of dirt in their backyard, it is common to landscape a mound septic system. The mound can actually add texture and visual interest to a garden, and rather than being viewed as an obstacle to overcome, the mound should be thought of as a dynamic addition to the garden. It is not safe to plant trees and shrubs on the mound, as their roots can interfere with the function of the mound, but grasses and plants are fine.

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