Another important concept of mound design is absorption width, the width of original soil under the sand layer that is required for the treated effluent to infiltrate into the original soil. If this width is not provided the soil will not be able to absorb the effluent before it reaches the toe of the downslope dike, resulting in effluent surfacing.

Total absorption area for the mound is the product of the length of the bed and the absorption width. Berms at each end of the mound are necessary to finish the mound and tie it into the landscape but are not counted as part of the absorption area.

Required absorption width is dependent on the allowable loading rate for the upper 1 foot of natural original soil under the clean-sand layer. The loading rate of original soil is determined either through soil analysis or relating the percolation rate to the long-term acceptance rate (LTAR).

For example, if the loading rate for the mound bed is 1.0 gpd per square foot and the estimated LTAR for the original soil in the top foot is 0.2 gpd per square foot, the ratio is 5-to-1. This means that it will take 5 feet of absorption width for every foot of bed width for the effluent to infiltrate the soil.

So a 10-foot-wide bed means the absorption width needs to total 50 feet. If the soil LTAR is 0.4 the ratio is 2.5-to-1 and 25 feet of absorption width is needed when the bed is 10 feet wide. As long as sufficient mound width is available so the effluent is accepted into the soil and pressure distribution is used, surfacing at the berm toe should not occur.

On a level site, a slope less than 1 percent, it is assumed that effluent will flow in both directions out of the bed over the soil area. This makes for a symmetrical mound. So on a level site the berms need to extend out from the bed 20 feet in each direction for the first example. The 50 feet of required width is achieved with 20 feet on one side plus the 10 feet under the bed and 20 feet on the other side. This is the area that needs to have the clean sand in contact with the original soil.

The berms can extend farther so the mound can be landscaped in, but that does not factor into necessary absorption width. Typical slope ratios for the mound berms are 4-to-1 or flatter for landscaping and maintenance with the steepest allowed being 3-to-1. For proper infiltration of effluent, slopes may need to be flatter.

On sites sloping more than 1 percent it is assumed all of the effluent will move downslope. Here the absorption width is measured from the upslope edge of the bed downslope 50 feet to provide the necessary width. The upslope berm will need to be finished but does not count in the absorption width.  Very little effluent will move out into the end berms of the mound.

About the Author

Jim Anderson is connected with the University of Minnesota onsite wastewater treatment education program, is an emeritus professor in the university’s Department of Soil Water and Climate, and education coordinator for the National Association of Wastewater Technicians.
Send him questions about septic system maintenance and operation by email to kim.peterson@colepublishing.com.

This article is part of a series on mound design:

• Do We Need Design Guidance for Mound Systems?
• Where Should You Put a Mound System?
• The Importance of Sand Specification
• More Mound Design Tips
• How Do You Determine Absorption Width?