Sand, effective size and uniformity coefficient have an impact on sand media performance and system longevity
Clean sand is specified as treatment media in many soil treatment areas, such as mounds and media filters. Sand used for these purposes must be clean (typically washed) to prevent system failure.
Dirty sand has silt and clay particles, called fines, which collect and form a restrictive layer, reducing the movement of water through the system. When fines are present in the sand media it can cause moisture to wick up by capillary forces. In addition, pores in the sand can be reduced and limit oxygen transfer. The flow of effluent through the sand can cause the fines to migrate and form a low-permeability layer. This layer reduces the rate of flow and encourages accumulations of biological material, often referred to as a biomat. A biomat that is too thick can cause clogging of the system, resulting in system failure. Another concern is that the sand should not contain too many fine particles. Sand that is on the coarser side will have more pore space and allow for more oxygen transfer.
The granular media must be coarse enough to permit a sufficient flow rate, yet fine enough to provide adequate treatment. Media that is too coarse lowers the wastewater retention time to a point where treatment becomes inadequate. Media with small grain size slow the water movement and increase the chance of clogging. The effective size (D10) and uniformity coefficient (UC) are the principal characteristics of granular media treatment systems and are determined by a sieve analysis. The ideal sand media is a coarse sand with an effective size between 0.3 mm and 0.5 mm (#50 to #35 U.S. Standard sieves). The media sand grains should be relatively uniform in size, having a low UC value (less than 4.0) to promote movement of water and prevent clogging.
Sand is characterized by a sieve analysis. A sieve analysis is performed using a set of stackable mesh trays (sieves) with hole diameters starting large and getting progressively smaller (9.5 to 0.075 mm). The sieves have numbers that correspond to the number of holes per inch in the mesh for sieves #4 and finer. A dry soil sample of known weight is placed on the top sieve and run through a shaker to separate different-size grains. A partial or full set of the sieves can be used, depending on the required specifications of the media.
Material collected from each sieve is weighed and a spreadsheet is set up with the sieve size/number, weight of soil retained on each sieve, cumulative percent retained on each sieve, and cumulative percent passing the sieve size. The cumulative percent passing is the most important part of this spreadsheet, as it tells you how much of each media is smaller than the sieve size. The sand supplier or an independent soil testing lab typically performs the sieve analysis and provides a report. The reports come in varying formats, so be sure to review them carefully. The results then need to be compared to what is required by code, the designer, and what is best for system longevity. Allowable fines range from 1 to 5 percent, with a goal of being as close to 1 percent as possible for system longevity.
Uniformity coefficient and effective size
The data from the sieve analysis can then be further evaluated and characterized by plotting on a grain-size distribution curve. This curve can then be used to determine how uniform the media is in size. The size distribution of sand is represented by the uniformity coefficient. This tells how well graded the sand sample is — that is, whether there is a wide range of particle sizes or whether most of the sample is close to the same size. The effective size of a given sample of sand is the particle size (in millimeters), where 10 percent of the particles in that sample (by weight) are smaller and 90 percent are larger. Usually this is called the D10 and is sometimes written as D10. D60 is when 60 percent of the particles are smaller and 40 percent are larger and can be written as D60. The UC is calculated as D60 divided by D10. A uniformity coefficient of 1.00 means a material has particle grains all the same size; numbers increasingly greater than 1.00 denote increasingly less uniformity.
Sand effective size and uniformity coefficient have an impact on sand media performance. The sand uniformity coefficient has an effect on the time of clogging or longevity of the system. Systems constructed of sand with high uniformity coefficients begin ponding more quickly. Ideal sands are a medium to coarse sand with a D10 between 0.3 and 0.5 mm with a UC of less than 4.0.
The most important feature of granular media is not the grain particles, but rather the pore space in the media. In the figure, the gradation for C33 sand is shown. The material on the coarse end of this gradation (upper limit) has larger particles and soil pores, and reduced surface area. This media provides greater oxygen transfer and water movement, and has reduced clogging potential, but may have reduced treatment capacity compared to sand on the lower limits. The lower limit of the C33 media contains more fine sand particles with greater surface area, which is beneficial for treatment, but with reduced pore space, which can reduce water movement. It is important to realize that not all C33 sand is the same. It is best to stay on the coarse side (right curve with effective size close to 0.3 mm and uniformity coefficient of 4.0) than to be on the fine side (near the left curve).
Field tests for sand quality
A jar test can be conducted on site as a rough field check of the sand cleanliness using the following technique as shown in the figure and described below:
- Place exactly 2 inches of sand in the bottom of a quart jar, and then fill the jar three-fourths full of water.
- Cover the jar and shake the contents vigorously for one minute.
- Allow the jar to stand for 30 minutes, and observe whether there is a layer of silt or clay on top of the sand.
- If the layer of these fine particles is more than 1/8-inch thick, the sand has more than 5 percent fines and is probably not suitable for use in mounds and sand filters (this should be confirmed with sieve analysis).
Too many fine particles may cause the sand media to compact during the construction process and future operation. Also, the long-term acceptance rate of this sand media is slower than the long-term acceptance rate of clean sand, which is the basis for sizing mounds and media filters.
About the author
Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association (MOWA) and the National Onsite Wastewater Recycling Association (NOWRA), and serves on the NSF International Committee on Wastewater Treatment Systems. Send her questions about septic system maintenance and operation by email to firstname.lastname@example.org.