The Importance of Sand Specification

The size and quality of the sand used in a mound system can make a big difference in long-term operation

The Importance of Sand Specification

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One of the critical factors in mound design is the sand specification for the clean sand layer. This will determine the dimensions of the infiltration bed.

In Minnesota, most of our mounds are designed assuming a soil-sizing factor of 0.83 square feet per gallon per day, which is equivalent to a loading rate of 1.2 gallons per square foot per day. This number is based on early mound research conducted in Wisconsin where they determined that if a clean sand was loaded no more than 1.2 gallons per day per square foot, pathogens would be removed in less than 2 feet in a sand column. 

Some engineers have argued for specifications of less-clean sand to slow movement of the water and provide better treatment. However, using less-clean sand makes it more difficult to predict the long-term acceptance rate. Clean sand combined with the loading rate ensures treatment.

In mounds we control the loading rate by how we dose the system using a pump and pressure distribution. We can also be certain with clean sand that it will handle the desired loading rate long-term. The more fines brought in, the less certain we are about what the actual long-term acceptance rate of the material will be.

In terms of the sand specification itself, it is important from a treatment standpoint that the sand particles consist of a variety of sizes, not uniformly sized particles. Over the years I have seen instances in Minnesota where a material consisting of more than 50 percent fine sand particles has had problems with formation of a biomat, leading to hydraulic failure because the long-term acceptance rate was less than 1.2 gallons per day per square foot.

The sand should have less than 5 percent of particles smaller than sand; so less than 5 percent passing a size 200 sieve. A 200 sieve has an opening of 0.075 mm. In this case less is better, though back when I started we allowed up to 10 percent fine particles. Again as we learned more about the long-term operation of these systems (20-plus years) those with fewer fines performed the longest.

When ordering sand at the quarry, the sand uniformity and sieve sizes can be specified. To check if it meets the fine requirement there is a simple test you can do when the sand arrives at the site. Place 2 inches of sand in the bottom of a quart jar; fill the jar three-fourths full with water and shake the contents vigorously. Allow the jar to stand for 30 minutes and then observe whether there is a layer formed on top of the sand. If the layer of these fine particles is more than 1/8-inch thick the sand is not suitable for use in mounds. In addition, if the water above the layer is still cloudy this means there is a significant amount of fine clay particles and the material should not be used.

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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