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An industry expert answers the question: How large should the bed be?

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One question that comes into play when designing mound systems is, how large should the bed be? In Minnesota we have continued to use a soil-sizing factor for the sand bed in the mound of 0.83 square feet per gallon per day, which is equivalent to a loading rate of 1.2 gallons per day per square foot.

Some states have adopted a soil-sizing factor of 1 square foot per gpd. This lengthens out the mound and reduces the linear loading rate along the mound from 12 gallons per linear foot to 10 gallons per linear foot. This is consistent with the concept that longer and narrower on the contour is better.

To provide the context for this: If we use Minnesota numbers for a typical four-bedroom house, 600 gpd x 0.83 square foot per gpd = 498 or approximately 500 square feet of bed area is needed. If the soil-sizing factor of 1 is used, approximately 600 square feet of area is needed. The maximum width recommended for the bed is 10 feet.

This means the Minnesota bed would have dimensions of 10 feet by 50 feet and in other states 10 feet by 60 feet. For very challenging sites it may be desirable to further reduce the linear loading rate by reducing the bed width and extending the mound farther along the contour. This needs to be considered especially for larger mounds serving commercial or other establishments.

For single residences a mound generally has one bed. Early designs incorporated two 10-foot-wide beds separated by at least 4 feet of sand. This still is appropriate where the site limits the length of the mound and for larger flow mounds.

This is only appropriate though if there is 2 feet of original soil with a long-term acceptance rate between 0.2 and 0.8 gpd per square foot. The reason for the 4-foot separation between beds and the soil requirement is to provide adequate absorption width and a sufficient depth of permeable soil to allow the effluent to move away laterally.

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:



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