Septic Tank: Mound System

AEX-744
Agriculture and Natural Resources
Date: 
07/26/2019
Karen Mancl, Professor, Food, Agricultural and Biological Engineering
Brian Slater, Associate Professor, Environment and Natural Resources
Peg Cashell, On-site Educator, Logan, Utah

Ohio has large areas of soils that are too shallow for typical septic tank-leach field systems, which require 36 inches of suitable soil before hitting a limiting layer. Limiting layers are bedrock, sand, and gravel, dense and compacted layers, and water tables. About 53 percent of soils in Ohio have limiting layers at shallow depths that do not provide the 36 inches of suitable soil, as shown in Figure 1. In these areas, mound systems are often used to remove pollutants from wastewater to protect the public health and the environment. These systems can be used in soils that have a limiting layer within 12 to 36 inches from the soil surface.

Pie chart with four sections, demonstrating percentage of land area of Ohio with limiting depths

Figure 2 shows a typical mound system. In these systems, specially selected sand is placed on top of natural soil to help treat septic tank effluent. Effluent discharging from a septic tank accumulates in a dosing tank where a pump discharges a predetermined volume of wastewater into the mound.

Mound systems are long and narrow, and must be constructed along the lot contour. For a three-bedroom home, a mound system can be as long as 200 feet, depending on soil and site conditions. For larger homes, up to 30 feet in length per bedroom is added to a mound system.

A mound is constructed in layers of predetermined depths (Figure 3). First, the natural soil depth above the limiting layer is determined (minimum of 12 inches). A layer of specially sized sand is placed on top of the natural soil. Together, the natural soil depth and the added sand equal the minimum depth required to treat wastewater effluent. A layer of gravel or a chamber surrounding the distribution pipes is then placed on top of the sand. Finally, after a final covering with construction fabric, a layer of soil fill is placed over the entire mound to protect the pipes from freezing. A layer of topsoil is also needed to grow grass or other nonwoody plants that control erosion.  

Diagram of a mound system with septic tank and dosing chamber
Diagram of a mound, with gravel, sand fill, construction fabric, pipes and diversion

When building a mound, the site must first be carefully prepared. The grass is mowed and leaves raked away. Trees and shrubs are cut off at ground level, with the roots left in place. The installer will use a chisel-plow to break up the grass and roughen the surface in preparation for the sand layer. The installer will be careful not to compact the soil in the mound area and just downslope of the mound. After construction, the lot will be carefully graded to divert any runoff water around the mound.

For specific information on how to design and construct a mound system, see Ohio State University Extension Bulletin 813, Mound Systems for Onsite Wastewater Treatment and OSU Extension Bulletin 829, Mound Systems: Pressure Distribution of Wastewater. Learn more at setll.osu.edu. Both publications are available online at extensionpubs.osu.edu.

As with all household sewage systems, the homeowner must maintain the system to ensure trouble-free operation. The homeowner should:

  • pump the septic and dosing tanks every one to five years.
  • use water wisely and install water-saving devices in the home.
  • never compact the soil downslope of the mound by paving, constructing a building, or parking cars.
  • avoid clogging pipes with roots by not planting trees or shrubs on the mound.