Question:
You mention using clean sand in a lot of your columns. What do you mean when you say “clean sand?” I thought sand was sand. And why is clean sand so important?
Answer:
Your question is timely, because a series of mound systems have been reported failing in our area recently. Mound systems have had an excellent performance record in my home state, Minnesota, as well as many other states. So it was unusual that a series of mounds should start failing. The interesting fact was all these failing mounds had been installed by the same contractor under a local grant program for a small city without municipal sewer.
With the extensive number of failures, an investigation had to be conducted. The conclusion was that sand placed under the rock layer of the mounds did not meet required specifications. The sand was not “clean sand” as had been specified. Apparently the contractor did not know about clean sand and apparently the inspector did not adequately inspect the construction materials for the mounds.
What is clean sand and why is it so necessary in the construction and successful operation of a mound sewage treatment system?
DEFINITION IMPORTANT
First of all, clean sand, as described by soil scientists through a sieve analysis, is a soil texture composed by weight of at least 25 percent very coarse, coarse and medium sand varying in size from 2.0 to 0.25 mm (millimeters), less than 50 percent of fine or very fine sand ranging in size between 0.25 and 0.05 mm, and no more than 10 percent of particles smaller than 0.05 mm. This is technical, but is the definition soil scientists use to describe clean sand.
A sieve analysis is not really needed on the job, as the fruit jar test is a simple way for both the installer and the inspector to test the quality of the sand. Clean sand can easily be determined by placing exactly 2 inches of the sand in the bottom of a quart fruit jar. Then fill the jar three-fourths full of water. Place the cover on the jar and shake the contents vigorously.
Allow the jar to stand for about an hour and observe if 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 is not suitable for use in mound construction.
What is the reason for this? Sand having too many fine particles will be compacted during mound construction. This slightly compacted sand will not have the open pores necessary for the successful operation of the mound.
Also, the long term acceptance rate, or LTAR, of the unclean sand will be less than that of clean sand. The acceptance rate of clean sand is the value used to size the rock layer area of the mound.
SOIL SIZING FORMULA
The size of the rock layer of the mound is based on 0.83 square feet per gallon of effluent per day, which is 1.2 gallons per day per square foot of sewage tank effluent. But this is a valid number if, and only if, clean sand is used. This is also the soil sizing factor for soil having a percolation rate in the range of 0.1 to 5 minutes per inch.
Using unclean sand results in a mound with an actual capacity that is less than designed. The LTAR will be less than 1.2 gallons per day per square foot. Sewage effluent will begin to surface along the side slopes of the improperly constructed mound. And this is apparently what happened in the example I mentioned above.
Another example of improper mound construction occurred in another state. The officials who developed the code for mound construction in that state did not understand the function of the clean sand layer in a sewage treatment mound. They apparently had the opinion the sand layer under the rock layer would not provide adequate treatment, so they specified a layer of loam soil for “better” treatment.
The soil sizing factor for clean sand was used and when the effluent was discharged onto the loam soil under the rock layer of the mound, the biomat quickly formed and the mounds failed hydraulically.
Sewage tank effluent is applied as a dose with pressure distribution to the rock layer of the mound. The effluent percolates through the rock to the sand layer. As the liquid percolates downward, air is drawn into the sand layer. If the application rate remains less than 1.2 gallons per day per square foot, conditions in the sand layer alternate between anaerobic and aerobic.
The result of these alternating conditions causes a reduction of the nitrates in the percolating liquid. Research has shown nitrate reductions of up to 44 percent.
Sand can be useful in another application for onsite sewage treatment systems. Soils having a high clay content easily compact or the texture breaks down under wet conditions.
A trench constructed in a clay loam soil should have a 4-6 inch layer of clean sand placed on the bottom of the trench. The purpose of the sand layer is to protect the compaction of the clay loam soil from the rock placed in the trench.
PREVENTS COMPACTION
The soil may be relatively dry when the trench is constructed. When effluent enters the trench and percolates downward to the soil in the trench bottom, the weight of the trench rock will tend to compact the soil. A layer of clean sand between the soil and the trench rock will prevent this compaction. The same procedure should be used for a seepage bed installed in clay loam soil.
Clean sand is an essential material required in the construction of a sewage treatment mound. Sand can also be used in other soil absorption systems to make them more effective. Some sand pits naturally contain sand that is clean and suitable for use in mound construction. Other sand pits need to have the sand washed to be suitable.
