A SUMMARY OF THIS POST:
For a slow sand filter to work we need to use “graded” sand. “Graded” sand means sand that has already been processed (sifted) so that the grains of sand it contains are mostly the same size. “Un-graded” sand means un-sifted sand that has many different sizes of grains of sand. Here, we try to make our own “graded” sand from “un-graded” “mortar” sand. First, we sift “graded” sand and “un-graded” sand through a series of wire screens to determine if we can duplicate the sizes of the “graded” sand product that we already know will work in a slow sand filter. We have found that it is possible to very closely duplicate the grain sizes we need by sifting the “un-graded” “mortar” sand. The image directly below is the result of comparing the “graded” sand to sifted “un-graded sand” (in this case “mortar” sand) after sifting each through a series of wire screens.
MORE ABOUT SAND GRAIN SIZE (THE PROCEDURE WE USED TO COMPARE TWO PRODUCTS):
We compared two sand products (shown in the large image above). One is a “graded sand” product from Unimin, specified as .15mm effective size for filtration (the bottom row in the above image). This is the sand we have used successfully in 3 of the filters running here.
The other product is sand from a local sand and gravel supplier. They call it “mortar sand”. It contains very little dust. The less dust, the better.
To sift the sand for comparison purposes, we used 3 different stainless steel wire mesh sizes available from Grainger and McNichols (both suppliers have extensive online catalogs) the links I have here may change slightly so here are the size specifications of the wire mesh I have used. If you order, make sure it is stainless steel wire mesh. It may be necessary to search the website online catalogs or actually call the suppliers:
30 mesh stainless steel wire screen: opening size: .54 mm (.0213 inches) plain weave
40 mesh stainless steel wire screen: opening size: .381 mm (.015 inches) plain weave
60 mesh stainless steel wire screen: opening size: .259 mm (.0102 inches) plain weave
We took random samples in equal quantities of each type of sand and ran them through the 30 mesh screen. Be aware that both types of sand are angular in shape (not round). We took what went through the 30 mesh screen and put it through the 40 mesh screen. Then we took that which went through the 40 mesh screen and put it through the 60 mesh screen.
We ended up with six samples of sand:
The first sample is what is retained on the 30 mesh screen (R30). This sample consists of grains that will not pass through the .54 mm openings in the wire mesh.
The second sample is what passed through the 30 mesh screen (T30). This sample consists of sand grains that fit through the .54 mm openings and anything smaller.
The third sample is what was retained on the 40 mesh screen (R40).This sample consists of sand grains that will not fit through the .381 mm openings in the wire mesh.
The fourth sample is what passed through the 40 mesh screen (T40). This sample consists of sand grains that will fit through the .381 mm openings in the wire mesh and anything smaller.
The fifth sample is what was retained on the 60 mesh screen (R60).This sample consists of sand grains that will not fit through the .259 mm openings in the wire mesh.
The sixth sample is what passed through the 60 mesh screen (T60).This sample consists of sand grains that will fit through the .259 mm openings in the wire mesh screen and anything smaller.
Note (Sept. 10, 2012) We have tried using a 50 mesh stainless steel wire screen in place of the 60 mesh screen. The openings in the 50 mesh are square weave at .2794 mm (.011 inches) using .2286 mm (.0090 inches) wire diameter supplied by McNichols. The slightly larger openings in the 50 mesh screen allow more of the excessively fine particles and dust to be sorted out, and make the sifting faster and easier. This, of course depends on the nature of the sand being used.
We are, in essence, sorting the grains of sand in a given sample to get a better idea of the sizes of the grains. As it turns out, the most significant difference between the “graded” sand and the “mortar” sand is that the mortar sand has more of the much larger pieces throughout a non-sifted sample than does the “graded” sand. These large pieces are the ones you DON’T want mixed up in the sand you use in the upper regions (and on the top region) of your filter. Both samples show a significant amount of very fine powder-like residual material after passing through the 60 mesh screen. This fine powder-like substance is what becomes a problem. It takes a long time (weeks to months) for it to wash out as the filter ripens. This means the output water does not “clear” up for a significant time period. Sifting out this dust requires wearing a dust mask and is quite labor intensive. The other alternative is to use lots of water to wash this fine material out at the last stage of sifting on the 60 mesh screen.
It does appear that by sifting the “mortar sand” through the 30 mesh wire and then washing what has passed through the 30 mesh (use the 60 mesh wire to hold the sand while you wash it) it is possible to “manufacture” your own “graded” sand without having to search for a “commercially manufactured” “brand”. It is important to note that both sand products are randomly shaped angular sand (not round) so the way the grains fit through the openings may not be perfectly consistant in each instance of sifting. The “mortar sand” we used here is 24 dollars for a half yard – more than enough to put together a small slow sand filter. A layer of two or 3 inches of the coarse sand that does not pass through the 30 mesh wire can be used on the very bottom above the pea gravel for added insurance that the fine sand won’t overwhelm the drain pipes. The stainless steel wire screen material costs anywhere from about 15 dollars to 35 dollars depending on the size you get (the number of lineal feet). The screen material is available as 3 feet wide or 4 feet wide, and you can specify the length you want. It comes as just the wire screen, you will need to build a frame for the screen, at a cost of about 5 or 6 dollars. We used 2X4’s .
Although we have not tried all possible types of sand, it is very likely that this procedure will work with any sand because the sizes of the screens stay the same. The amount of sand you end up with that does not pass through the 30 mesh, and the amount you end up with that does not pass through the 60 mesh may vary depending on the various sizes of the grains of sand you have on hand. “Mortar” sand usually is already somewhat “graded”. “Builders” sand is less likely to be “graded” and will probably have bigger grains, and lots of dust. The best sand is that which is dug from a quarry or directly out of the ground in an area where there is not likely to be contamination. The “masons” sand from Lowes will work, but it is very dusty. The “play sand” from Lowes also may work but it, too, is very dusty. When you sift sand; wear a dust mask, ALWAYS!!!!!!