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Slow sand water filter sand source


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.

sand grain sizes
graded and ungraded sand that has been sifted using wire mesh


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 .

These are the frames we made for the wire screen
use small pieces of trim to hold the wire screens in place

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!!!!!!

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water filter sand

This is an update on the post about sifting sand for a slow sand water filter.

I wrote about this in response to a question from a reader.  I recommended using stainless steel wire cloth to sift “play sand” so it will be of uniform size and therefore work in a slow sand water filter. This has turned out to be somewhat time consuming, however once you get the stainless steel wire cloth set up on a frame the sifting is actually quite easy. The most recent effort uses 30 mesh stainless steel wire cloth to separate out the coarse sand, and then 60 mesh to allow the silt and very fine sand to be removed, as it will fall through the 60 mesh openings leaving the larger grains to sit on top. This works very well and is much better than starting out with the 40 mesh. What is left is very close to .15 mm effective size sand. Washing the sand then is quite easy and does not require lots of water. Absolutely wear a dust mask if you do this when the sand is dry. The dust from any sand is very harmful. I am in the process of locating sand that does not have as much silt (dust). Possibly “masonry sand” ???? Masonry sand must be free of organic material and silt.

Here in the Seattle area, it was fairly easy to locate  a source for the  sand from a manufacturer who supplies sifted, pre-packaged, graded sand; but this is not always possible in all locations.

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How does a slow sand water filter work?

A slow sand water filter (sometimes called a biological sand water filter) works like this:

Because of the pull of gravity, water flows down through a sand bed in a container about 3 feet deep from the top to the bottom. After 2 or 3 weeks, a biological film forms on (and in) the top 1 or 2 inches of the sand because all water with the exception of distilled water actually has live bacteria in it. The top of the sand must not be disturbed and must be covered by water all the time. At first this biological film is not visible however, after 5 or 6 months the sand surface may start to look darker as organic material builds up in the biological layer. This layer contains non-harmful (good) microscopic bugs that live on harmful (bad) microscopic bugs. The bad bugs are the ones that make people sick. These “bugs” are too small to see without a microscope. The good bugs eat the bad bugs and all that is left is harmless purified water with some naturally occurring chemicals. The good bugs cannot live without oxygen. The “good bugs” are called “aerobic” because they need oxygen dissolved in water to survive. Most water has enough oxygen dissolved in it for the “good” bugs to stay alive. From the top, the water then flows very slowly through the rest of the sand and gets cleaned more by physical filtering action and some biological action. The water at the bottom of the sand layer is very clean and biologically purified, so it is allowed to flow out into a storage container. The sand does not need to be replaced, but sometimes filter operators decide to change the top 2 inches of sand rather than just clean it.

The information above is an extremely simplified explanation, but should give anyone the basic idea of how a slow sand water filter works.  See slowsandfilter dot org for lots more detail about how these filters work.

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Slow sand water filter DIY video

The new video for filter 4 is up on YouTube as of today Aug. 3, 2012.

Below is a parts list: (this may have some errors and omissions – occasionally, I do this documentation in a hurry in my spare time if it needs to be – so please let me know if there are any questions and i will fix it!)

55 gallon barrel (see the image on the video for type – there are different types of 55 gallon barrels) (complete with top) (food grade plastic)
You will need a source of clean water (lots of it)
Hydrogen peroxide 3 percent solution
Teflon tape (double strength Pink) (wraps on the flexible hose in the drain system at the bottom inside the barrel)
liquid pipe sealer “T plus 2″ Rectorseal
5 gallon bucket for the top reservoir (hooks up to the baffle assembly)

12ea 50 pound bags of “Target” (brand name – not from a Target store) filter sand .45 mm effective size
80 pounds of .25 mm effective size filter sand
90 pounds of pea gravel 1/2 inch effective size

PVC FITTINGS AND PIPE: (All the pvc fittings are 1/2 inch.)
1/2 inch pvc pipe (10 foot section).
8 each 90 degree coupler
90 degree coupler female threaded adapter
4 each male threaded adapter
1 each female threaded adapter (will go inside the reservoir bucket on top)
1 cross connector (for the drain pipe assembly)
2 T connectors
If you use a diverter, you will need a 2 inch section of pvc to connect the diverter to the 5 gallon reservoir

3/4 inch spade bit
#45 drill bit
5/32 inch drill bit
electric hand drill, or a “brace and bit” hand drill, or a drill press
A hack saw (or some way to cut the pvc pipe)
you will need a tool to tighten the pipe clamps (hose clamps) a straight slot screwdriver or socket or pliersA socket works best – avoid having the screwdriver slip off and gouging your hand!!!!
A “screen” to hold the gravel while you wash it
A cloth to hold the sand on the screen while you wash it
A container to set the screen on while you wash the sand and gravel