Septic Tank: Mound System

Agriculture and Natural Resources
Karen Mancl, Professor, Food, Agricultural and Biological Engineering
Brian Slater, Associate Professor, Environment and Natural Resources

Ohio has large areas of soils that are shallow to a limiting condition. Therefore, mound systems are used to remove pollutants from wastewater to protect the public health and the environment. Mound systems are best suited for lots with soils from 36 inches to 12 inches in depth to a limiting condition.

Table 1. Minimum soil depth requirements for septic and mound systems.
Limiting Condition Mound Subsurface Soil Treatment (in inches)
Fractured/Karst Bedrock 12 54
Groundwater or Aquafer 12 54
Highly Weathered Soils on the Low-Lime Till Plains 12* 42
Other Limiting Conditions 12* 36
*Eight inches are permitted under the Ohio Household Sewage Treatment rules. However, research at The Ohio State University shows pathogen removal requires at least 12 inches of unsaturated soil. Application of undisinfected wastewater to very shallow soil is not recommended. 

In a mound system, specially selected sand is placed on top of the natural soil to help treat septic tank effluent. The sand depth is determined by the natural soil depth above a limiting layer. Limiting layers are bedrock, sand and gravel; dense and compacted layers; and water tables. The sand depth plus the natural soil must equal 1½ feet to 3 feet to meet Ohio requirements.

In a mound system, effluent discharging from a septic tank accumulates in a dosing tank until a pump moves a predetermined volume of wastewater into the mound. Figure 1 shows a mound system.

A mound is constructed in layers of predetermined depths (Figure 2). First, the natural soil above the limiting layer is determined. The natural soil depth above a limiting layer should be a minimum of 12 inches. A layer of specially sized sand is placed on top of the natural soil, so together, they equal the minimum required depth. A layer of gravel surrounding the distribution system pipes is then placed on top of the sand. Finally, after covering the gravel with construction fabric, a layer of soil fill is placed over the entire mound to protect the mound from freezing. A layer of topsoil is also needed to grow grass or other nonwoody plants that control erosion.  Trees and shrubs should not be planted on a mound, because their roots tend to clog pipes in the distribution system. The sides of the mound are sloped to make the mound convenient and safe to mow.

Figure 1. Components of a mound system.
Figure 2. Details of a 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.

Appropriate techniques are necessary for an efficient mound construction project. The site for the mound must 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 area of the mound is then chisel-plowed to break up the grass and roughen the surface in preparation for the sand layer. Avoid compacting the soil under and just downslope of the mound as the sand, gravel, pipes and soil are put into place. Work should always be done from the upslope side (or from just one side for flat lots) using lightweight, tracked vehicles. Following construction, a diversion ditch may be needed on the upslope side to divert surface runoff around the mound. For specific information on how to design and construct a mound system, check Ohio State University Extension Bulletin 813, Mound Systems for Onsite Wastewater Treatment and OSU Extension Bulletin 829, Mound Systems: Pressure Distribution of Wastewater. Both publications are available online or at Ohio county Extension offices.

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

  • pump the septic and dosing tanks every 1 to 5 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.

Depending on the cost of sand and the amount of labor involved, mound systems are usually twice the cost of a comparably sized leach field system. The higher cost is necessary, however, if the site and soil conditions make treatment of the wastewater impossible in a subsurface system. A mound system permits a home to be occupied in a rural area while still protecting the environment.