Rain water harvesting and slow sand water filters

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.

Just a thought about catching rain water to use in an emergency (I have to have a rant once in a while). . . .

It could easily happen. An emergency situation due to an earthquake, a power outage, or the “100 year floods” that have been happening every 10 years or so. What would you do if you had no dependable water for, say; 4 weeks? Rush to the store and buy bottled water – like several hundred thousand others? How long before its gone?  Water is critical to survival, but its always there when you turn on your faucet. . .    .  If there is an extended power outage, or there has been an earthquake there will likely be no water available for a VERY LONG TIME (speaking of quakes, we just had one off the coast of Oregon yesterday April 11 – a 5.9; and one today April 12 in the Gulf of California a 6.2 ). If there has been massive flooding and all the water is contaminated, everyone else will be looking for water – 4 days without water and your in very big danger of meeting your maker; think about it, and children really need water – what do you do? Drink from a mud puddle? Drink flood water? Can you even imagine what would be in that water? Of course these two options are crazy. Now; that slow sand filtered roofwater starts looking really good, doesn’t it? – even if there are a few coliforms in it, and you’re going to be really really careful about it; right? You’re going to make sure the filter is taken care of, right?  My point here is that each person is responsible for the operation of their roofwater filter and each person makes their own decision about how to use it – but it should be an informed decision based on knowledge. Each person makes their own decisions about what to do – I am not responsible for others actions or decisions  and all of the information I provide still does not make water 100 percent safe, but in an emergency this 55 gallon barrel of filtered roof water sitting by my garage is going to look really, really good. Think carefully. Several months without water supplied to your suburban house will be a horrific situation for you. As long as it rains, you will have water – if you have set up a slow sand filter; that is.