I. Description

A mound system is a pressure-dosed absorption system in a sand fill that is elevated above the natural soil surface. The design overcomes certain site restrictions such as slowly permeable soils, shallow permeable soils over porous bedrock, and permeable soils with water tables higher than is otherwise allowable by local codes.

The design of the mound system is based on expected daily wastewater volume and natural soil/subsurface characteristics. Effluent from the septic tank or other pretreatment unit process(es) is pumped or siphoned to the elevated absorption area and distributed through a distribution network located in the coarse aggregate at the top of the mound. The effluent then passes through the aggregate and infiltrates the sand fill. The sand and the biological mat which develops treats the wastewater and permits the spread of the filtrate over a large area of native soil called the "basal area".

The basal area required for infiltration beneath the sand mound is determined and controlled by the hydraulic capacity of the underlying soil. In some cases, particularly for larger mound systems serving small communities, it may be necessary to estimate ground water mounding beneath the system. During construction of mound systems, special attention should be given to ensuring that the basal area of the system is properly scarified, and that compaction of the basal area is minimized.

II. Common Modifications

Shapes and dimensions of mounds may vary significantly depending upon slopes, water tables and percolation rates at a particular site.

Geotextile fabric is preferred over straw or untreated building paper between the top of the stone bed/trench and the cover material to prevent clogging of the stone in the mound.

A typical mound system provides both a treatment and disposal function with discharge from the mound percolating directly into the in-situ soil. Where enhanced removal of nitrogen or other wastewater constituents is required, the bottom of the mound is lined and a separate soil disposal system is used when sufficient treatment is completed.

III. Technology Status

Mound systems have been in common use in the U.S. for at least 30 years, and for about 10 to 12 years in the Austin area. Systems which are designed and constructed as described above have been in use for over twenty years.

IV. Applications

Mound systems may provide an appropriate alternative disposal method in areas which are unsuitable for conventional onsite disposal systems. Site restrictions which may be overcome by mound systems include porous or karstic bedrock, very shallow soils overlying bedrock, slowly permeable soils and high groundwater tables. Properly designed and constructed mound systems should require minimal maintenance, and can operate in all climates.

V. Limitations

Land availability on the site may limit the applicability of mound systems since elevated mound systems require more space than conventional systems because of sand fill requirements. Mound systems may not function properly on soils with a percolation rate over 120 minutes per inch. Slope limitations are more restrictive than for conventional soil absorption systems. Pumped systems require a reliable power source, increased maintenance and periodic repair or replacement of pumps. Mound systems must be designed by a licensed professional engineer.

VI. Typical Equipment/Number of Manufacturers

All of the materials used for constructing mound systems are available from local sources.

VII. Performance

The performance of mound systems is a function of the site conditions, and the design, construction and maintenance procedures used for the system. Significant removal of BOD, TSS, bacteria and viruses may be provided by a properly designed, constructed and maintained mound system. This type of system can provide for excellent nitrification, but nitrate removal will be limited in most cases.

VIII. Overall Reliability

Properly designed, constructed and maintained mound systems have been demonstrated to provide an economical alternative to conventional on-site systems in many non-sewered suburban and rural areas. System life for a mound which is properly sited, designed, installed and maintained may equal or exceed 20 years.

IX. Operation and Maintenance Requirements

Pretreatment Units: If a septic tank is used to pretreat the wastewater prior to applying the effluent to the mound, the tank should be pumped periodically to remove sludge/solids (see fact sheet for Septic Tanks) Other types of pretreatment units and processes will require maintenance of some type.

Mound: Mechanical and electrical components must be maintained and repaired/replaced as needed for systems utilizing low-pressure distribution. Vegetation must be adequately maintained over the mound disposal system.

X. Potential Environmental Impacts

Aesthetic issues may arise for mounds due to their elevation above the natural grade, and their size and shape. Improperly sited, designed or constructed mounds may have problems such as odors and effluent surfacing, with the potential for public health risks and surface or ground water contamination.

XI. Energy Consumption

It is assumed here that the energy consumption for a mound system is similar to a low-pressure dosing disposal system. It is estimated that the average pump operating in an onsite pumped disposal system (average of 20' of head and about 30 gpm, at an overall efficiency of about 30%) operating approximately 8 minutes per day, will use approximately 0.38 KW of power. This would be about 0.051 KWH of energy per day.

XII. Costs

XIII. Aesthetic Considerations

The size and shape of mound systems, and their elevation above the natural grade, may present some concerns related to grading, landscaping and aesthetics for the site. Some odors may be emitted from the mound, although a properly designed, constructed and functioning system should produce very little if any odor.

XIV. State and Local Acceptance

Mound systems have been permitted and installed in Central and other portions of Texas, and in many other areas of the U.S. They have been used for at least 30 years in the U.S.

XV. References

1. U.S. EPA, "Onsite Wastewater Treatment and Disposal Systems Design Manual", EPA/625/l-80-012, October 1980.

2. "Guide to Wastewater Facilities Planning in Unsewered Areas, Volume II: Engineer's Guide", SSWMP Publication #14.9, J.E. Kleinschmidt, R.J. Otis, University of Wisconsin, Madison, 1981.

3. "Wisconsin Mound Soil Absorption System Siting, Design and Construction Manual", J.C. Converse and E.J. Tyler; Small Scale Waste Management Project, University of Wisconsin - Madison; January, 1994.

4. National Small Flows Clearinghouse, West Virginia University, Morgantown, West Virginia; Publication WWPCGN28.

5. "Technical Summary of Appropriate Technologies for Small Community Wastewater Treatment Systems", M.E. Barrett and J.F. Malina; Technical Report submitted to the Texas Water Development Board, 1991.