Biological Systems Engineering


Date of this Version



WASTE MANAGEMENT D-5, Home Waste Systems Issued June 1981, 15,000


Several areas in Nebraska are not appropriate for onsite home sewage treatment using conventional septic tank absorption field systems. Site factors that limit the use of conventional absorption fields include slowly permeable soils, shallow or gravelly soils and high water tables. The septic tank-mound system, which consists of a septic tank for primary sewage treatment and a constructed mound for final waste treatment was developed to overcome these site limitations (Figure 1).

Although mound systems differ only slightly in principle from conventional sewage systems, they allow the user to select suitable materials to build an absorption field that relies on evapotranspiration as well as the absorption capacity of existing soil. A pump located in a chamber is generally recommended to distribute the effluent evenly in the mound.

Mound location, size and shape, construction procedures, distribution of effluent, and dosing quantity all interact in determining how well the mound will function. Each component must be properly designed and constructed to prevent premature failure of the system .

Mound Location

The septic tank-mound system can be constructed on soils having percolation rates from 0 to 120 minutes per inch (0 to 48 min/ em); the traditional septic tank-absorption field system is limited to soils with percolation rates from 5 to 60 minutes per inch (2 to 24 min/em). Ideally, the mound should be constructed on the flat area of a hill crest. Other flat areas that do not receive drainage water from surrounding land are also suitable.

Gently sloping areas can be used if the soils have minimum percolation rates shown in Table 1. Do not locate the mound on a natural slope exceeding 12 percent under any soil condition, and do not move or grade the soil to change the ground slope.

A mound system cannot be used in areas subject to frequent flooding because of possible downstream water contamination. Areas where a seasonal water table, rock formation or impervious stratum occurs within 24 inches (61 em) of the soil surface should also be avoided, since ground water close to the soil surface could be contaminated. Greater distances will further reduce the possibility of contamination. The mound absorption area should be at least 100 feet (30 m) down slope from any well or spring, 50 feet (15 m) from streams, 5 feet (1.5 m) from property lines, and 10 feet (3 m) from the drip area of trees.