The selection of sand fill material is critical to long-term performance of the mound system. The purpose of the sand fill is to accept effluent from the distribution system and partially treat the wastewater before infiltration into the natural soil. A suitable sand is one that can be loaded at a reasonable rate and will provide satisfactory treatment. Generally, the finer the sand the better the treatment and the slower the wastewater infiltration into the absorption bed. Too coarse a sand will allow effluent to pass through the mound with little removal of impurities. Too fine a sand cannot be loaded at an acceptable rate and may cause severe clogging of the sand, which results in failure of the mound system.
Following the USDA Soil Textural Classification, a coarse sand is suitable. However, this is subject to the following two conditions: (1) no more than 20% by weight is gravel (> 2 mm), and (2) no more than 5% by weight is silt and clay (< 0.053 mm). (Note: Request a sieve analysis report on a proposed sand from the aggregate supplier to check these criteria.)
Concrete sand is produced by many sand and gravel quarries in Ohio and generally meets the criteria for the very coarse and very fine fractions. The fine aggregate specified by the Ohio Department of Transportation will meet the mound sand requirements. The specification is detailed in Section 703.02 of Aggregate for Portland Cement Concrete, Office of Construction Administration, 2002 Construction and Material Specifications. Although mason sand is also commonly available, it is a finer sand than concrete sand and is not recommended. Limestone sand is not suited although it may meet size requirements. Limestone sand can dissolve over time, reducing the systemıs useful life.
Sand specifications are also given in terms of effective size and uniformity coefficient. When using these criteria, select a sand with an effective size in the range of 0.15-0.30 mm, and with a uniformity coefficient in the range of 4-6.
When using a sand that meets the guidelines above, the recommended design sand fill loading rate is 1.0 gpd/ft2 if the wastewater is typical domestic septic tank effluent. If the effluent is from a commercial establishment, the wastewater quality should be evaluated and the sand fill loading rate should be adjusted accordingly. When treating higher strength wastewater, the sand fill loading rate should be reduced, or there may need to be additional pretreatment to achieve a waste strength comparable to domestic effluent prior to distribution to the sand fill material.
Sand Fill Loading Rate:
Sand Fill Loading Rate = 1.0 gpd/ft2