Designing a DIY slow sand filter: what to consider

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. Do not ever “backwash” your filter. You will destroy it. 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. This is a very stupid thing to do.

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.

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23 Responses to Designing a DIY slow sand filter: what to consider

  1. PB says:

    thanks for posting all of this information, and the videos. I have a question of scale that I’d appreciate your views on: I plan to capture the runoff from a 2,500 sq ft catchment. You indicate in para 3 to expect about 7gph flow rate for a 55gal barrel filter. At that rate, it will take roughly 5 days to filter the harvest from one inch of rainfall on my catchment. I need to double or triple the flow rate to handle the water in a reasonable time. Query whether using a larger vessel (say 6 or 7 ft diameter x 4 ft tall) or using multiple 55gal filters would be more effective? I plan for pre-filter storage with a small transfer pump to the filter in any event, to handle torrential downpours or protracted rainfall. I appreciate any advice! PB

  2. filter_guy says:

    If you use a single larger vessel for the filter, then your water supply will be effectively stopped when (or if) it becomes necessary to clean (wet harrow) the filter. If you have 3 or 4 separate barrels started about 3 weeks apart, then each one can be cleaned ( wet harrowed ) at different times, leaving the others still running. Be sure they are all on equal elevations, or water may flow backwards into a filter that sits lower than the others. Alternatively, you could have all filters empty through seperate outputs that flow into a larger storage container – just be sure that all the outputs remain above the water level in the storage container. Water can do very strange things as it seeks its own level!! All this assumes that the input water to your filter(s) is of consistent turbidity all the time. It has been our experience here, over and over again, that excessively turbid water will clog a slow sand filter very quickly; within days or even hours. Pre-filtering is a very good idea. The most effective “pre-filtering” we have used here consists of a first flow diverter that has its output piped into two successive 55 gallon “settling” barrels. That water, in turn flows into a 55 gallon storage container where water is drawn from about 4 inches above the very bottom and subsequently flows into the slow sand filter (filter 1). A better solution (and possibly more expensive) is a float device that sits on top of the water and allows output to be drawn from about 3 or 4 inches below the surface of the water. This filter (filter 1) has been running for 6 years and has not been cleaned (wet harrowed) at all for 5 years. It still produces very clean water.

  3. PB says:

    thank you. is your whole system (other than the circulation pump that keeps the filter alive in dry spells) gravity fed? I was thinking of pumping from pre-filter storage to the filter(s) in order to make sure the filters don’t get flooded in a big rain, to maximize quality of input water, and to maintain a steady feedrate to the filters. Have you found that a gravity system accomplishes that well enough to avoid the hassle of setting up a pump?

  4. filter_guy says:

    Gravity feed is used on filter 1, filter 4, and filter 5. Filter 2 and 3 are fed by a small Direct current 12 volt pump that feeds water to them through a float valve system. Both ways work as long as the surface of the sand remains undisturbed.

    An overflow will allow the unfiltered water that flows in too fast to simply flow into your unfiltered storage tank. This way you don’t loose any water. The filtered output can be allowed to flow into a separate container. Then during dry spells, a small dc pump can be used to move the unfiltered overflow water back through your filter via the use of a float valve, or the re-circulation idea with the output just flowing into your filtered water container.

    We use this “overflow” idea on the pond filters here. The pond then becomes the overflow container, and the filtered water container. Since the pond water is only for frogs, fish, and water cress, it really does not matter if some of the overflow does not go through the filter right away. Because the pond is basically a “closed” system, all of the water eventually gets filtered, and we do not need to worry about flow rate slowing down, and causing the pond water to spill out over the top of the filter. No need to regulate the flow into the pond filter. And, the splashing of the water tends to mix oxygen into the water in the dry hot summer time when it really needs all the oxygen it can get.

  5. BR says:

    Would this work to recirculate water from an indoor heated swimming pool? We live in a cold climate 30 miles from the nearest swimming pool and medical rehab and long term health requires swimming 3-5 days per week. The drive and chlorine are taking a toll on me and I would like to build a small indoor pool with a biological filter that we can make ourself. Any thoughts on this would be greatly appreciated.

  6. filter_guy says:

    Probably would work. I have not tried it, however; We do have a 2500 gallon cistern here which could be used as a very small swimming pool, although we have not used it as such. The water in it is recirculated through two 65 gallon slow sand filters. You would want to know what to test for and do it on a regular basis. Also you would need to have an adequate flow to be sure all the water in the pool went through the filters at least once between uses. The size of your filters will depend on the size of your pool. The water could not be too hot. 75 degrees maximum comes to mind. There must be enough oxygen dissolved in the water to keep the microbes in the schmutzdecke alive. Figure your flow in gallons per hour, and compare that to the size of your pool. You might get away with .45 mm effective size sand in the filters which would give a good flow rate, as compared to the .25 mm effective size used for non-swimming pool water.

  7. BR says:

    Thank you for the advice!

  8. M says:

    Hey there,
    Really awesome information you have here. So I’m building a slow sand filter as part of a science project. I’m wondering, instead of a 55 gallon barrel, what if i had several smaller bottles, that added up to 55 gallons, and then I attached them using a tube so the water systematically runs through each bottle. Would this be effective or no, do you think?

  9. filter_guy says:

    M: “Would this be effective or no, do you think?” I am sorry, but to be honest, there is not a simple yes or no answer. We have tried what you describe here, with 5 gallon buckets. It did not work as well as a single 55 gallon container. Instead of re-posting, in this response to your question, all the information we have already posted, go to our table of contents page and read all about the 5 gallon filter experiments we did here at this location. There is over two years of work on the 5 gallon filter design, including testing and observations documented on this blog. At the table of contents page look for the topic of “DIY 5 gallon slow sand filter”. There are links to 8 posts with information on the type of filter systems you describe. You may be able to come up with a design that works for a very small slow sand filter; our success was very limited.

  10. M says:

    Ok will do, thank you so much!

  11. shatha says:

    good evening, thanks for this information ,can you help me with my experience!
    I want to design a small slow sand filter with dimension of ( dia=8 cm and height =27 cm ) I want to know how can I calculate the depth of sand bed, gravel layer and water depth above the sand bed ?

  12. filter_guy says:

    We are sorry, but our experience with very small slow sand filters of the type you describe, is limited, and we do not have enough data to help you with your specific request. It would probably be a good idea to do some research about the theory of slow sand filtration, and then do your own pilot test.

  13. Michael Ray says:

    What are the recommended thickness for each layer in a 55 gallon drum

  14. Lynn Jensen says:

    I’m looking at making a sand filter to re-filter stored tap water for removing possible contamination from sitting for a prolonged period. Would this be a satisfactory method?

  15. filter_guy says:

    Yes. Make sure there’s no chlorine in the water. If it sits for a “long” time and there is a vent, the chlorine will most likely evaporate, and not be of any concern. Keep in mind that the filter must run for 3 or 4 weeks before any real filtering can be accomplished. Also, keep in mind how a slow sand filter (biological sand filter) works. There’s lots of info on this blog about that.

  16. msk says:

    I have a 2500 gallon slow sand filter. I have a question about “wet harrowing.” I have read some sites that say to rake the top layer of sand. You state that the sand should be gently stirred and not raked. We have to chlorinate the water before use, does that make any difference to the process? I was also told that we should wet harrow every couple of months. Is that accurate?

  17. filter_guy says:

    Either wet harrowing, or removing some of the top layer will work. If you wet harrow, you will not have to add sand as often as you will if you remove sand every time the filter needs to be “cleaned”. The frequency of cleaning is dependent, mainly, on the condition of the input water, the ambient temperature, and the effective size of the sand in the filter. There could be other factors that influence the frequency of cleaning. It is necessary to monitor the water quality to determine when the filter needs to be cleaned. Most filter systems should include two separate filters running at different start times on the same input water. That way, each can be cleaned at different intervals without the loss of output water.

  18. msk says:

    Thank you for your response. I am not very well versed on these system but I am trying to get up to speed on them. Because of that your response was not as clear as I hoped but it was clearly well thought out and appreciated.
    We actually have three 2500 gallon filters so I will be able to do them at different times.

  19. greg says:

    thanks! this is kickbutt info! your own private water treatment plant!

    so other than coarse sand filtering, what you suggest for the mud water source pre-treatment if that’s all you have?


  20. sohel maniyar says:

    hello, sir
    I’m looking at making a water filter by using charcoal, sand, gravel, tulsi,ice cream stick cilantro . Would this be a satisfactory method?

  21. filter_guy says:

    I’m assuming you are serious. Ocimum tenuiflorum (Tulsi) is an herb, as is cilantro. To the best of my knowledge, neither have a use in a slow sand filter. I am not familiar with “Ice cream stick cilantro”. This blog has information that will answer your questions regarding charcoal, sand, and gravel used in a slow sand filter.

  22. Tushar says:

    You have done an outstanding work plus the way you have document is easy to understand, specially the diagrams.

    I was thinking of installing a biofilter for cleaning my 250 litre capacity aquarium. Was wondering whether I can do away with a smaller biofilter as same water will be recirculated continuously. If yes than what size you think would be appropriate?

  23. Karen says:

    Hi. If I just want to filter my shower water, can I use a pool sand filter? I don’t want a 55 gallon barrel of sand in my home.

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