Rain water harvesting and slow sand water filters

A SUMMARY OF THIS POST:

For a slow sand filter to work we need to use “uniformly graded” sand. “Uniformly 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 “uniformly graded” sand from “un-graded” “mortar” sand. First, we sift “uniformly graded” sand and “un-graded” sand through a series of wire screens to determine if we can duplicate the sizes of the “uniformly 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 “uniformly graded” sand to sifted “un-graded sand” (in this case “mortar” sand) after sifting each through a series of wire screens.

graded and ungraded sand that has been sifted using wire mesh

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 “uniformly 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 “uniformly 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.

CONCLUSIONS:

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 “uniformly 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 “uniformly graded”. “Builders” sand is less likely to be “uniformly 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!!!!!!

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.

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.

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!)

PARTS:
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)

SAND AND GRAVEL:
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

TOOLS:
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)
pliers
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

The answer to this question is no; with maybe a couple of exceptions; but these exceptions are rare cases that probably don’t apply to most situations. If the “beach” in question is a river bed where fresh, non polluted water flows, then the sand there may be ok; but it is a big risk to use it. For one, most pristine river beds are protected and removing sand may be harmful to the ecosystem and against the law. There could be any kind of bacteria present in the sand there, and it would take disinfection and testing to be sure the sand was clean.

If the “beach” is on a fresh water lake with no public use and limited access to human use, then maybe the sand would work. There would still be the danger of contamination from zoonotic organisms (this also applies to the river bed sand mentioned above).

If the “beach” is an ocean beach, then there will be salt in the sand which is totally unacceptable, and furthermore if the beach is a place where there is lots of “use” then there could be anything in the sand – anything – including salt. Think about it: dogs, horses, people, all who have walked almost anywhere . . .   need I say more? Do not EVER use sand from a public ocean beach in a water filter.

Another problem with “beach” sand is lack of uniform size of the individual grains.

See this previous post for sand source suggestions.

Water can be tested by using kits available from this source:  Micrology Laboratories. Their website has the information necessary to understand how to use the test kits. Tests can be done for E coli, Coliform, and Aeromonas bacteria. Their website has a large amount of information so be sure to read it all. There are many more tests that can be done and they have downloadable instructions for all.

Here is some interesting information about slow sand filtration and rainwater harvesting:

A study documented by the University of Cincinnati has shown that the Mayan civilization knew about using sand for water filtration.

In Washington state, King county officials have considered “allowing” people to harvest rainwater for drinking purposes. (this is info from last year)

The first link is new information just published, however the second link is about information from last year. The connections here to slow sand water filtration and rainwater harvesting may not be blindingly obvious to some at first glance but read through the articles completely and then decide.

The first link points to information about another civilization from several thousand years ago using sand to filter water. These sand filters did undoubtedly acquire some biological action as there were likely no electric motors and petroleum powered high pressure pumps to “backwash” the filters they built.

Regarding the second link, rain water harvesting will need some kind of filtration to go along with just capturing water from a roof or other impervious surface. Small slow sand filters are well suited to this task, particularly for individual water systems supplying one household. This is the focus of the information at the second link – rain water harvesting for individuals.

This is a summary of 6 years of experience building operating and testing small slow sand water filters. There have been a significant number of failures which have been good learning experiences, subsequently resulting in astounding successes. The failures are listed with the solution resulting in success as the “Fix”. Eight different large units have been built modified, studied; and all are currently in operation. Each of the filters are slightly different but all based on the same operating principles. One experimental small unit is also operating indoors on the counter top. The larger units are all built with mostly recycled containers made of food grade plastic. Sixty five gallon, 45 gallon and 55 gallon capacities have been used.

Failures and successes have been due to the following:

1. Sand size too large and not evenly sized:”construction” grade sand does not work well – coliform not removed completely:

Fix: The sand must be at least .25 mm effective size with a uniformity coefficient of less than 2. (the sand must be evenly sized and fine grained).  The best combination is .15 mm effective size on top half and .25 mm effective size on the bottom half.

2. Freezing weather; if the filter freezes it will cease to function and pipe damage may occur.

Fix: Keep the filters from freezing solid. Here, in western Washington state this really is not a big problem, however there are places where freezing will destroy a small sand filter. Several weeks of below 20 degree weather is survivable if the filter is kept running with water supplied from a non-freezing source. Several months of below 10 degree weather might ruin the filter.

3. Highly turbid water will clog the filter very quickly, all of the filters running on roofwater have at one time or another become clogged resulting in very slow output flow.

Fix: Supply non turbid water to the filter input. A pre-filter or first flush diverter is necessary. If the water is above 10 ntu’s it will drastically slow down the flow.

4. Air pockets forming in the sand during construction, adding water to barrel full of sand is a guarantee for disaster. This happened once here Never again. DO NOT JUST PUT SAND IN THE BARREL AND THEN ADD WATER. This will not work and you will end up with smelly toxic water output: that happened here and 1000 pounds of sand had to be shoveled out – wet and smelly – and completely cleaned and then shoveled back in.

Fix: When the filter is built, add the water first; then put the sand in slowly stirring it to be sure all air bubbles are gone.

5. Insufficient drain system.

Fix: Make sure there is adequate drainage at the bottom of the inside of the barrel under the gravel; a good size is quarter inch holes drilled in 1/2 inch pvc pipes with at least 6 pipes covering the entire bottom of the barrel. Put the drain system in and make sure the flow out is just as if it was flowing out of a 1/2 inch hole in the bottom of the barrel.

6. Sand not washed thoroughly enough causing cloudy water output.

Fix: Wash the sand until the water flowing from it is CLEAR. If you don’t it will take several months or more for the water to clear up.

7. The use of a barrel that held onions.

Fix: The most recent filter put together here used a barrel that had held onions. That odor does not go away and just gets worse. The water output is clear and most likely bacteria free, but very undesirable until the water sits for several days. If you have a barrel that had onions in it make sure you clean it out somehow before using it. I don’t know how to get that odor out. Update July 17 2012: The filter output is now free of the onion odor. I have been running water through the filter manually for the past week at maximum flow, apparently enough water through the filter will slowly remove the odor. Water dissolves most things quite well.

8. Improper first flush diverter function causing turbid water to clog the filter.

Fix: Make sure the first flush divereter is functional. The divereter here was neglected for several weeks and the filter clogged up from non-turbid water spilling over from the diverter. Make sure the “slow drain” is on and functional so the diverter empties after each rain event.

The small unit in the house is just that – very small. A 2 litre clear plastic pop bottle with the top 2 inches cut off is filled with 5 inches of  .15 mm effective size sand with 1 inch of small gravel (1/4 inch diameter pebbles) in the bottom. A plastic straw is used as the output pipe sealed with plumbers putty at the output located at the bottom of the bottle. A 1 quart plastic yogurt container with small holes in the bottom is used as the baffle – it fits snugly into the bottle. The only cost was the straw and the plumbers putty, and the sand at the most 25 cents. The rest is recycled.  We use it to filter well water. So far it is working and shows the schmutzdecke forming at the top. The water out is clearer and tastes better than the water straight from the faucet. I never thought this could work. . . .  hmmmm. Tests will go in when time and money allow.

This is an update on the operation of Filter 4 (see details of construction in the post on November 13, 2011). It has been in operation for six months now. At this point in time it is producing very clear water and the flow rate has stabilized at about 30 litres per hour (.130 m/hr). Another water test will go in as time and finances permit. Over 4000 liters (about 1000 gallons) of water have been filtered in the past 3 months as a result of only rainwater flowing from the 100 square foot roof of the adjoining building.  A YouTube video on the construction of this filter is in the works and I will have it up (hopefully) within the next couple of weeks. More later . . . .   .

The pond filter works, quite well. It has up to this point been fairly “biomatic” that is to say, the filter has not needed cleaning (although I have wet-harrowed it several times) and has worked with naturally occurring biological removal of pathogens – it is biomatic, so in my writing I will call these pond filters “biomatic pond filters”, or in the case of rainwater filters, “biomatic water filters”. These filters do not need backwashing – ever; in fact “backwashing” (forcing water backwards through the output to “break up” the “fouled” sand) will ruin them. For some reason there is lots of misunderstanding about the difference between a rapid sand filter and a slow sand filter. People need to get over this idea that all sand filters need to be backwashed. Perhaps some of the confusion comes from lack of knowledge, and the (deliberate?) lack of available information. For the past 40 or 50 years, sand filters have been designed to work using chemicals like chlorine or ozone to kill pathogens in the output water. This is old, non-sustainable, profit centered technology. And furthermore, chlorine and ozone are, for all intents and purposes, ineffective in preventing disease from cryptosporidium (cryptosporidiosis), or giardia lamblia (Giardiasis often called “Beaver fever”) acquired from the cyst form of these pathogens. These are two different diseases and Beaver Fever is NOT caused by cryptosporidium. The beneficial microbes in Slow sand filters (biomatic filters) remove these pathogens from water – by not just killing them, but by actually removing them in the process of consuming them and breaking them down into harmless elements.