Rain water harvesting and slow sand water filters

Update April 16, 2017:

The 5 gallon filters, and the smaller 6 inch filter mentioned in this post DO NOT WORK. They either add biological contamination, or do not remove any biological contamination. Do not use them.  

I am writing this to summarize what has been learned here in the past 7 years of designing, building and operating small slow sand water filters – thousands of hours and hundreds of pages of documentation. The details can be found by searching this blog, or accessing each of the water filter websites  that are associated with this blog. This activity has had far more failures that successes. We’ve learned more from the failures.

The most important issues to consider when putting together a DIY small slow sand filter that will work to purify water follow:

1. Sand: Fine sand will provide better quality output at the expense of a very slow flow. Coarse sand will provide marginal quality output and a higher rate of flow than fine sand. A good compromise is to use .15 mm effective size sand and then provide storage for the output water. Never use “beach sand” from an ocean beach, there could be all kinds of contamination in it, including large amounts of salt and anything that can be tracked in by foot traffic. Know where the sand comes from to be sure there is no contamination. Mason’s sand, “play sand”, or commercial bagged sand is a good choice, but must be sifted first, and then washed until the water runs clear. The best, and the most expensive, is filter sand; processed specifically for water filtration and rated for potable water. The sand depth is critical. Very small diameter, shallow filters (4 to 5 inch diameter, and less than 12 inches deep) will not remove much contamination.

2. Container: We have used 55 gallon plastic barrels, 65 gallon plastic barrels, and 5 gallon plastic buckets. We also have one very small filter about 5 inches in diameter, and about 9 inches deep running here as an experiment. The diameter of a diy small slow sand filter is critical. As diameter decreases, flow rate must also decrease. See number 3 below for more on flow rate. Always be sure to rough up the inside of the container with sand paper. Make the lines of abrasion horizontal or random but NOT vertical. A smooth surface inside the container will allow un-purified water to slip down between the sand and the container surface. The 5 gallon filters we have tested here work marginally, but they do take out significant amounts of particulate matter. The biological tests done here on these small filters are field tests only; and are inconclusive at worst, and show some bacterial removal at best. The 55 gallon containers, and the 65 gallon containers work very well and the output of these filters has been tested by EPA certified laboratories with over 60 individual tests, repeatedly showing complete or nearly complete removal of ecoli, fecal coliform and coliform bacteria. The turbidiy of the output on the larger container filters is excellent, meeting or usually exceeding drinking water standards. These larger filters also remove hydrocarbons from harvested roofwater runoff from an asphalt roof.

3. Flow rate: A slow flow rate is absolutely necessary to maintain maximum purification. The slower the flow, the better the output. It is better to put up with a slow flow rate and have lots of storage, than to have a rapid flow rate just because it is convenient. These filters do not turn on and off like typical suburban homeowner’s plumbing fixtures supplied by public water systems. If a small diameter container is used (something like 5 or 6 inches), then the flow must be very, very slow like maybe 1 or 1.5 cups per hour; or there will not be enough contact time between the water and the grains of sand. This contact time is what facilitates the purification.

There are two ways to describe “flow rate”. One is by using numbers to indicate how fast the water moves through the sand bed (sometimes called “hydraulic loading rate”). The other way is to simply state how much volume of water flows out of the filter in a given amount of time. These are two very different actions. A basic explanation of flow rate is here. For the 5 gallon filter with .15 mm effective size sand, the flow should be no more than a drip like a leaky faucet about 1/2 to 3/4 of a gallon per hour and that is absolute maximum. For the 55 gallon, or the 65 gallon barrel filter, with .15 mm effective size sand, The flow should be about 7 gallons per hour (these flow rates are for a “ripened” filter) Note that all slow sand filters will start out with a much greater flow rate than they will have after running for a while. This is because the schmutzdecke gets thicker and presents more resistance to water flow.

4. Turbidity (the cloudiness of input water):
Water that is cloudy, muddy, or water that has lots of particulate matter in it will clog a slow sand filter within days, or hours. You can’t just dump mud puddle water into a slow sand filter and have it magically come out pure; and maintain a usable flow rate. Keep the water that goes in reasonably clear, pre-filter it with coarse sand first if your source water is turbid. A more detailed explanation of turbidity issue can be found here. If your input water looks like unfiltered lemon juice, it will clog your filter within a few days or weeks.

5. “Cleaning a slow sand filter: 
Clean your filter by “wet harrowing” it. There is no need to replace the sand under normal operating situations. Even if you clog it up you can still use wet harrowing to “clean” it. To wet harrow your slow sand filter, temporarily plug the output pipe so water does not flow,  then gently agitate the water inside the filter just above the top of the sand to stir up the excess accumulation of substance on the top inch or so of the sand. Do this until the water on top is filled with muck,  do not forget to plug the output pipe or the mucky water will just flow right down and contaminate your filter (we made this mistake once) and then carefully drain off the mucky water and add fresh water. Do this without excessively disturbing the sand below about 1/2 inch. The less the sand is disturbed the better. Do this until the water is clear. Then open the output pipe and let the filter run for several days to restore the biological layer.

Backwashing a water filter:

Do not ever “backwash” your slow sand filter to “clean” it. Backwashing is running water forcefully backwards through the filter. Backwashing is for rapid sand filtration only. If you backwash your slow sand filter, you will mix the gravel at the bottom with the sand and the layers of sand associated biological layers will be destroyed. The filter will then be destroyed. 

6. Recirculation:
During dry periods where no input water is available, always have a reserve of unfiltered input water, and some filtered water to mix and recirculate through your filter until the rainy season returns. Water must run through these filters continuously, or they will become dormant and will not filter water until they are run for several weeks or longer. If they are left un-attended too long you may have to start over with them.

7. Freezing weather:
These filters will not work if they are frozen. They work marginally at 32 degrees F if the water keeps circulating through them. Don’t glue the pipes, they will crack if they freeze. If the pipes are not glued, they will just be pushed apart by the expanding water, much like a freeze plug in an automobile engine, instead of cracking the pipes. The expanding water has to have some place to go. I’ve been through this for the past 7 years. The first year I made the mistake of gluing the pipes. They were destroyed.

8. Add the sand to the water:
When putting one of these filters together, put water in the container first, then add the sand as you go, keeping the sand under water at all times. This is to prevent air pockets from forming in the sand, and to allow for testing the container for leaks. It is easier to drain out water, than it is to shovel the sand out of the filter. If air pockets form, the filter will not work until the sand is removed, cleaned, sterilized; and then put back into the container.  Don’t just fill the container with sand and then put water into it. Even if you have wet sand, air pockets can still form.

9. Wash the sand and gravel completely:
The more you wash the sand and gravel, the faster you will see a “clear” output from your filter. Wash the sand and gravel (before you put it into the filter) until the water comes out clear. This may take lots of water.

10. Don’t disturb the top of the sand:
The water input must not disturb the top of the sand ever. Keep the water flowing in very gentle. We use a “baffle” pipe assembly to do that.

11. Location:
A slow sand filter in a 55 gallon barrel is extremely heavy, over 700 pounds when it is full of water and operating. Be sure to locate and level the filter in a spot that will be its permanent location. It is nearly impossible to move this type of filter, without emptying it and restarting it, once it is set up.
Be sure the filter sits in a place where it is secure. The supports must be able to handle at least 2000 pounds safely. Moving a filter will disrupt the sand layers and cause the filter to stop functioning.

12. Know the basic reasons these filters work:
A slow sand filter works 3 ways: Biological action, Physical straining, and adsorption.

Biological (action) predation: All water, with the exception of medically sterilized water, and distilled water, has microscopic life in it. These microbes will grow in the slow sand filter if they are kept under water and oxygen and food are available. There is a mini-ecosystem that lives in these filters. This system results in the formation of a biological film, called a Schmutzedecke (German for dirt blanket) on top of the sand and to a lesser amount, further down in the sand. Disease causing bacteria, and viruses (bad bugs) are literally eaten by this collection of microbes in the filter. All that is left is harmless minerals. Most, if not all, of the small number of bad bugs that happen to slip by the schmutzedecke will die off as they move further down in the sand layer due to lack of available nutrients.

Physical straining: Particulate matter is strained out by the sand and the biofilm on the top of the sand.

Adsorption: This is a fancy way of saying that the sand grains actually can attract small particles and cause them to stick to individual grains of sand. This is similar to but, not exactly  like , the way water actually sticks to itself due to  cohesion and adhesion.

This filter (we call it a “micro” filter) uses a 2 liter soft drink bottle, a rubber band a plastic straw, some plumbers putty, and coarse sand and fine sand. The fine sand on the top is .15 mm effective size, and the coarse sand is small pea gravel  approximately 2.5 mm effective size. The “baffle” consists of 2 re-used yogurt plastic containers one inside the other with small holes (approximately 1/16 inch in diameter) punched in each. The total cost was under a dollar. This filter has been used to filter water for a vaporizer. The well water here has lots of iron (we assume it is iron, the well is cast iron pipe, and does rust on the outside; and there are constantly accumulating reddish brown stains on the washtub) and sediment that builds up in a vaporizer and ruins the machine in a few weeks of daily use. This filter has been is use for 2 years now. The previous post shows how effective it is in removing  particulate matter. If you look closely at the images below you can see the schmutzdecke on top of the sand. This filter has never been cleaned, and still keeps working. We will probably never “clean” it. The flow from it is extremely slow, very very slow – less than 1 cup in an hour. Most people would consider this rate of flow unusable. We are willing to wait for water to flow through the sand.  We use the filtered water from it in the vaporizer and there is virtually no build up visible yet after 6 months of use. We have not checked it for biological contaminate removal yet, however we are drinking the water from the well and have been for close to 40 years, and will not hesitate to drink the water from this filter. We do know that there are no pathogens in the well water, but there are likely other microorganisms in the well water because we do not use chlorine and never have. This filter would probably not be practical for any other use. Its pathogen removal capabilities are unknown. Since the cost is under 1 dollar, (actually all of the material we used was “recycled” so, really, our cost was 0, if you had to buy the stuff outright it would probably be about 1 dollar) it would be easier to make a new one if this one stops functioning.  Images below:

This is a 2 liter countertop slow sand filter. It filters out iron, and particulate matter from the well water we use here.

This is another view of the small slow sand filter. Notice the “schmutzdecke” on the top of the .15 mm effective size sand.

This is the filter in action. It removes iron, and particulate matter from the well water.

In September of last year a 5 gallon slow sand filter (filter 10a)  was set up. The orginal post is here. Since then 2 more filters have been added to the experiment. We call them filter 10b and 10c. They have been running for about 4 months, however this winter has been very cold and all the filters here were frozen solid for nearly 45 days total. The first flow rate test this year was just done today. No dependable tests could be done in January, or February because the temps were below freezing too often.

The flow rate chart for filter 10a (from last years post) is here:

flow rates for filter 10b and 10c:
:

If you “backwash” a slow sand filter, you will destroy it.  Backwashing is for rapid sand filters. I posted a bit of info on this about a year ago. I did not mention why backwashing will ruin a slow sand filter. Here is why:

A slow sand filter works by allowing layers of living microscopic organisms to actually grow in the sand. As water flows down through the sand, these organisms form layers, the most dense being on the top. There are layers further down in the sand that never need to be disturbed when the filter is “cleaned”. Also, most slow sand filters use a layer of gravel at the bottom to prevent the sand from clogging up the drainage pipes. Backwashing (forcing water back up through the bottom of the filter) will mix the sand and gravel, and mix the biological layers. This will destroy the filters effectiveness and ruin it. Don’t backwash a slow sand filter.

It is now February 7, 2014. At this location its been below freezing at night for all of February, and for the past 4 days not above freezing at all. On the 6th of Feb. at about 4:00 am, the low here was 13.5 degrees F (-10 deg C).  All of the filters are frozen solid with the exception of the larger filters 2 and 3. They are being fed with water from the shallow well that is heated by the un-frozen ground here and the pump house has a very small heater to keep the pump from freezing; the water must run continuously or it will freeze in a few minutes at 14 degrees F.  All of the ponds are frozen. It will likely be at least another week before we can continue running water through the filters, and then at least another 3 weeks until we can do another test.  Probably the first part of March at the earliest, weather permitting.

Water is becoming an issue here in the U.S. for a number of reasons :

http://www.marketwatch.com/story/us-drought-hurts-cattle-crops-prices-heat-up-2014-02-07

http://www.opb.org/news/article/council-recommends-drought-declaration-for-4-ore-counties/

http://bigislandnow.com/2014/01/16/big-isle-drought-declaration-triggers-usda-loan-assistance/

http://www.usda.gov/wps/portal/usda/usdahome?contentidonly=true&contentid=drought_news.html

Rain water harvesting is a way to store water. Even if it is only used for non-potable purposes, it still helps save potable water. A 1500 square foot roof can catch 3700 gallons of water from 5 inches of rain (allowing 20 percent loss due to evaporation, first flow diversion, and non-smooth roof surfaces). There are 231 cubic inches per gallon.  A 1500 square foot roof has  1500 X 144 = 216000 square inches of surface. Five inches of rain on a 216000 square inch surface is 1080000 cubic inches. Divide that by 231 and you have 4675 gallons of water. Take away 20 percent of that due to surface conditions, evaporation, and needed first flow diversion and you have 4675 – 935 = 3740 gallons. divide that by 5 = 748 gallons from 1 inch of rain. add another 20 percent loss and you have 600 (598.4)  gallons. You can wash a lot of clothes with 600 gallons of water. Think about it. Its not against state law to set up a few rain barrels. Check out the rest of this blog and its accompanying websites for lots of info. 

The runoff from the average house with 6 downspouts could easily fill 6 55 gallon rain barrels from just 1 inch of rain. A slow sand water filter, combined with a first flow diverter, will purify the water so it can be used without the danger of serious contamination.

This is an update from a previous post that has become so huge I will have to do something, not sure what yet. Anyway, here is the info on Ohio:

Update, February 1, 2014 regarding Ohio:

After checking the Ohio gov’t website, I found nothing stating that rain water harvesting (setting up rain barrels) is against the law. It is encouraged. See see the numerous links that follow:

A rain barrel store in Westlake Ohio sells rain barrels

A rain barrel store in Lima, Ohio sells them

This link lists partnering communities in a rain water harvesting related effort where rain barrels are encouraged

Clermont county, Ohio recommends rain barrels

The city of Cleveland, Ohio recommends rain barrels

The Ohio EPA recommends and funds rain barrels

Ohio has lots of rules about  water. Be sure to read up before you start your rainwater harvesting project. It is highly likely that its not against the law to harvest rain water in Ohio, but watch the regulations and read the definitions.

There are places in the US where water is rapidly becoming an issue. California is in the midst of a severe drought. In other places, there are problems with water polluted from hydraulic fracturing  otherwise known as “fracking” and chemical spills. For some people, rain water harvesting (roof water harvesting) may be a viable solution to water shortages. If your’re on a well and it goes dry you’ve got a real problem. If your drinking water gets polluted, you’ve got a problem. Check the pages of this blog for some suggestions on how to harvest water from the surface of your roof. Contrary to some people’s media caused fear, it is not against the law (with only a few exceptions) for individuals to harvest small amounts (under 10,000 gallons per day) of rain water for personal use. 

The weather has warmed up here. For the past 2 weeks it has been above freezing most of the time. The 5 gallon filters have been running for 2 weeks. The output on all;  10a, 10b, and 10c; has cleared up considerably. If the above freezing weather holds for another 2 weeks, we will do another test on the outputs of each.

Jan. 10, 2014 post filter water

Jan 10, 2014 pre-filter water

October 14, 2013 post filter water

October 14, 2013 pre-filter water

Although the outputs have cleared up, and it looks as though these small shallow filters may work in a somewhat limited fashion, it must be realized that the flow rate on these filters is very, very, very slow. Much like a dripping faucet. They will not function at all with a rapid flow. Do not confuse these filters with the type of device you might see used with pressurized water from a kitchen faucet, or filtered water that flows from a refrigerator dispenser. You can’t just pour water in a slow sand filter and have it come “flowing” out magically cleaned and pure to fill a glass in just a few seconds. We are talking 3 or 4 hours here to provide even one gallon of water.  Please see the extensive post we have on these small filters for more information. We have been working with these 5 gallon filters for nearly 4 months now, and with larger versions for 7 and a half years. There is a tremendous amount of information.