Finding sand and mesh size for a slow sand filter

This is an update of the previous post on slow sand filter sand sources. Instead of trying to find specialized sand sizes from a manufacturer, it can be more convenient to sift easily available sand to get the right grain size distribution (uniformity coefficient) that will work most effectively in a slow sand filter. Below is an image of six different sand products we compared here. Three of them (the .15 mm effective size, the .25 mm effective size,  and the Target filter sand)  are sand that is already working in successful tested slow sand filters we have in operation here.  The other three are readily available sand products that can be found just about anywhere. We ran 1 liter of each type of sand through successively smaller wire mesh sizes to determine, more precisely,  the breakdown on the variation in size of the individual grains of sand in each type of sand. Each mound of sand shows how much of a specific size group is in each type of sand. Of particular interest are the first mounds and the last mounds. The wire mesh sizes and a manufacturer source are here:

McNichols has the wire mesh available from an online catalog
At their page on the left side of the page, click the “see more” link in red below the “mesh size 1″ link in red, to expand the list and show all the available sizes.

3860753610 McNICHOLS Quality Wire Mesh, Square Weave, Stainless Steel Type 304,
Woven Construction, 60 Mesh, 0.0075″ Wire, .0092″ Opening, 36.0000″ Width

3860754810 McNICHOLS Quality Wire Mesh, Square Weave, Stainless Steel Type 304,
Woven Construction, 60 Mesh, 0.0075″ Wire, .0092″ Opening, 48.0000″ width

3880554810 McNICHOLS Quality Wire Mesh, Square Weave, Stainless Steel Type 304,
Plain Weave Construction, 80 Mesh, 0.0055″ Wire, .007″ Opening, 48.0000″ width

3850093610 McNICHOLS Quality Wire Mesh, Square Weave, Stainless Steel Type 304,
Woven Construction, 50 Mesh, 0.0090″ Wire, .011″ Opening, 36.0000″ Width

3850094810 McNICHOLS Quality Wire Mesh, Square Weave, Stainless Steel Type 304,
Woven Construction, 50 Mesh, 0.0090″ Wire, .011″ Opening, 48.0000″ Width

3140104810 McNICHOLS Quality Wire Mesh, Square Weave, Stainless Steel Type 316,
Woven Construction, 40 Mesh, 0.0100″ Wire, .015″ Opening, 48.0000″ Width

3840103610 McNICHOLS Quality Wire Mesh, Square Weave, Stainless Steel Type 304,
Woven Construction, 40 Mesh, 0.0100″ Wire, .015″ Opening, 36.0000″ Width

3840104810 McNICHOLS Quality Wire Mesh, Square Weave, Stainless Steel Type 304,
Woven Construction, 40 Mesh, 0.0100″ Wire, .015″ Opening, 48.0000″ Width

3830123610 McNICHOLS Quality Wire Mesh, Square Weave, Stainless Steel Type 304,
Woven Construction, 30 Mesh, 0.0120″ Wire, .0213″ Opening, 36.0000″

3830124810 McNICHOLS Quality Wire Mesh, Square Weave, Stainless Steel Type 304,
Woven Construction, 30 Mesh, 0.0120″ Wire, .0213″ Opening, 48.0000” Width

The first mounds, in row 1 of the play sand, mortar sand and mason’s sand columns; are the very coarse pieces mixed with smaller pieces that really prevent a slow sand filter from working. This is because too many large pieces of material mixed with small pieces, prevent the biofilm from forming in, and on the top layer. The biofilm at the top layer is where most of the biological purification action takes place.

The last mounds of sand, in row 6 of all but the “Target filter sand” column, are all the very small powder like pieces of sand that will make the output water highly turbid (cloudy) and undesirable.

sifted sand comparisons

This shows sand sifted into specific particle sizes

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12 Responses to Finding sand and mesh size for a slow sand filter

  1. Steve Bavis says:

    Hi,
    I am thinking of setting up a slow sand filter….however I mostly want something to filter out the heavy iron in my well water.
    Do you have any experience with this type of application ?

    Thanks,
    Steve

  2. Orpheus says:

    I have not tested specifically for iron removal in any of the filters in operation here. My guess (and it is just a guess) is that a functioning slow sand filter will remove some iron from water that flows through it, but I cannot be 100 percent sure of that. If anyone has experience with iron removal using a slow sand filter, hopefully they will post their test results, or a link to test results, here.

    A slow sand filter will remove 99.999 percent of biological contamination from water which makes the use of a uv filter (as a safety measure) in combination with a carbon filter quite practical on the purified output. A carbon filter may be effective at removing iron from water. Note that using a carbon filter device on water that has not been purified is asking for trouble. Bacteria will grow in a carbon filter and eventually produce biologically contaminated output water much worse than the input water.

  3. Cam says:

    Hi, I’ve been following your work for a while now, and it has been the basis for my own designs & water catchment plans. I’ve been scouting for a supplier of the taller black 40″ barrels you’ve been using in your plans, but the one source that usually carries them hasn’t had them in lately. Do you have a source in the area that doesn’t charge a huge premium on these barrels?

    If I can’t track down the taller, skinnier ones, what are your recommendations as far as building a sand filter with the standard 34-35″ tall barrels? Have your tests shown adequate filtration with the shorter barrels? Thanks so much!!

  4. Orpheus says:

    Good questions. Thank you.
    For barrels, try the ReStore
    or Skagit Building Salvage in Mount Vernon

    My experience has been that either size will work. The shorter wider barrels should use a fine sand such as .15 mm effective size. The taller barrels (the 40 inch tall) can have .25 mm sand as the top level. There is a filter here, (filter 4) in the 35 inch tall size that does use the .25 mm effective size sand; and it is working, but the water clarity is better in filter 1 which has .15 mm sand as the filter sand.
    I’ve just finished some tests on the newest filter (filter 5) which uses the 40 inch barrel with .15 mm effective sand, and the output of filter 4 (the 35 inch tall with .25 mm effective size sand). That info will go up as soon as time permits. These new tests (done here on site) illustrate the situation with water testing quite well. It is amazing how much bacteria there is in water. Specific tests for e coli and coliform don’t show the true nature of the situation. These two types of bacteria are just indicators – not the actual condition of the water. So, when we say a filter is “working” we actually mean it is taking out e coli and coliform successfully – however, there are multitudes of harmless bacteria still there. These tests demonstrate that fact clearly. Another thing these tests show is how it “looks” when 99.99 percent of the bacteria is removed from water. I’ll have the images and explanations up here as soon as time permits.

    All that said, still; a uv filter, or some kind of secondary filtration is an absolute must on the output of any filter where the water will be used for anything other than irrigation, or utility work.

  5. Cam says:

    Thanks so much for your quick reply and all of the help. Your website & research has spawned quite an in-depth project to water our garden 🙂

    I will check out the suppliers you provided. Also, anyone who is reading this and looking for rain barrels in Kitsap County should know that Kitsap Lumber in West Bremerton carries a good stock of them for around $25 a piece.

    Once more question – I am less concerned with the over-all flow rate, and more concerned with the level of bacterial filtration achieved by the 2 parallel sand filter’s I’m building. (2 filters, so I can take on down for cleaning, and have the other still working as the cleaned one starts back up again over 3-4 weeks). Would you recommend .15mm effective sized silica sand for the entire filter? Or is there an obvious benefit to the 2 different sized sands working together in the system?

    Thank you so much! You’ve inspired me to take on this this massive water system project, and I’ve turned on several friends & family members to building their own designs. 🙂

  6. Orpheus says:

    It’s good to hear that other people are taking slow sand filtration seriously. Thank you for posting your information here, Cam. Anything you can contribute will be greatly appreciated.

    My humble apologies for the lack of more detailed information in my postings here. As this project continues, it has become nearly impossible to keep up with posting the results of each experiment. This is the 6th year of the study, and there are currently eight different filters running here continuously, documented on 3 websites and this blog. Two pdf’s and a small book are in the process of being created here that hopefully will contain a better combination of all information. The problem is that this is an ongoing study, and an online blog is the best way to document each learning experience – as learning is dynamic and not static. A pdf or a book cannot change daily as new information is discovered. As time goes on the amount of data here is becoming tremendous. I am glad to answer all questions, but do encourage reading through the 3 websites:
    slowsandfilter.org
    roofwaterharvesting.org
    shared-source-initiative.com/biosand_filter/biosand.html
    and this blog as it is becoming very difficult to remember everything that has happened in the past 6 years – good thing its written down!

    I would use .15 mm sand in both filters, with maybe 3 or 4 inches of .35 mm sand at the bottom on top of the gravel. It has been my experience that finer sand results in better filtration overall – but much slower flow rate. The slower flow rate allows more time for purification. On the downside; finer sand tends to clog up very quickly if the turbidity of the input water is too high (above 10 ntu’s) . A pre-filter of some sort might be a good idea unless your input water is non-turbid. The filter here, called “filter 1” uses 2 settling barrels in series before the storage barrel, and the water for the filter is drawn from about 5 inches above the bottom in the storage barrel. The pre-filtering is done by just storing the water in 3 barrels before it goes into the filter.

    To the best of my knowledge, the main reason for using a coarser sand on the bottom is that most of the biological action takes place in the top 5 or 6 inches of sand. So a coarser sand on the bottom where (in theory) smaller sand size is less critical, may offer less resistance to the overall flow rate; while not detracting from maximum purification capability – thereby (in theory) allowing the best of both flow rate, and purification. This appears to be the consensus of opinion in the research that I have read. The filters here have shown that to be mostly true; however the best functioning filter still is “filter 1” which has mostly .15 mm effective size sand and a very slow flow rate.

  7. Cam says:

    Excellent! Thank you.

    Once other though crossed my mind during the planning of this, that I thought may warrant some consideration. As I was planning, I thought about supplying a backup water source (we’re in the city, on city water) to feed the filters slowly during non-rain periods… and the thought occurred to me. City water should have 3-6ppm chlorination. I imagine this would kill the sand filters on contact. I understand that chlorine eventually breaks down after interacting with bacteria, but the city would have to make sure we had adequate ppm to keep it clean as it entered our house. So this may be something for us “city folks” to consider as we’re planning to build these. I imagine that even using water from the tap in the initial building of the sand filter would slow down (or stop completely) the process of the bacteria forming. Your thoughts?

    So to combat it, I’ve built the first-flush diverter, and it’s ready to supply water to the sand filter barrels (once I can track them down) and I’ll install the sand after they’re filled.

    Did you ever consider using landscape filter paper between the layer of sand and the bottom gravel to keep the silt from building up in the drain lines? I guess it would need to be some sort of potable water-rated filter paper…. Just thinking out loud. 🙂

  8. Orpheus says:

    Wow, thanks again for the contribution. You’ve got me thinking. I’m not sure about the chlorine thing. To the best of my knowledge, chlorine that is dissolved in water does break down and/or combine with other compounds readily. It could be that there might not be enough residual chlorine left in the water to adversely affect your filters. . . but I have no experience with that situation. We are not on city water here so there is no practical way to check that hypothesis from this location. I think if you used chlorinated water initially , it might take a very long time to get any beneficial microbes growing in the schmutzdecke. In a well established filter, a very small percentage of chlorine may not harm it significantly. . . but I don’t know. It would be very interesting to find out, though. I suppose knowing the concentration of chlorine per unit volume of input water first, then running it through the filter for a given length of time, while concurrently doing a coliform test before chlorine and after chlorine several times would give an answer. That may be very helpful to a lot of people considering a slow sand filter.

    As far as the landscape filter paper – I have not tried it, but have considered it – particularly after shoveling about 900 pounds of sand back out one time (on filter 3) because the drain was clogging up with sand!!! I believe some filter designs I have seen show either fine stainless steel screens, or some type of cloth combined with charcoal at the bottom of the sand right on top of the gravel. I have not tried it – it might work. You would definitely want something that would not clog up too much though, because shoveling that sand out of a barrel is hard work. Yes, if one ever wants to use a filter for potable water, then whatever goes inside should be carefully considered.

  9. Cam says:

    Ya, that would be a real interesting test. I figured you were on a well, so didn’t have to deal with it. Around here (and supposedly with most municipal water), there should be 3-6 parts per million of residual chlorine. They adjust the starting ppm it at their plants based on the initial amount of bacteria/etc in the water, but from what I understand, they shoot for somewhere in that range after bacteria has been killed to keep the water from getting contaminated during transit to the individual households. My bet is that the biological life -both good and bad – would be dead…. there’s an interesting idea….The sand filters the water clean, but the chlorine does the job for the schmutzdecke. Anyway, I’m thinking out loud again. I bet it’s too much chlorine to support life, since that’s exactly what it’s there for. 🙂

    I hadn’t seen your 3rd website till today (roofwaterharvesting.org), but now I see that you’ve built a cistern for storing the clean/filtered water! I love it. That’s exactly the route I went. Filtered water will dump into a temporary food-grade 5 gallon bucket ($7 thanks to Winco food’s bulk food section) and then into the storage cistern to be later pumped up into a pressurized side of the system (pump, pressure tank, etc) for running sprinkler heads in the garden. The 5 gal bucket acts as a holding tank that’s big enough for a backup little 12v pump to automatically fill up the 2 filter’s supply water during non-rain events. (so I dont have to use the city water). It’s all been automated by a couple of float valves. I’m still in the process, but very excited to see all fully functioning. For now, only the first-flush is active.

    You mentioned that you aren’t having issues with algae growth in your cistern since the lid was on. Are you having any contamination issues thus far with the longer-term storage in the cistern? Since I plan to store the 1500 gallons through the dry months to keep the garden growing, I’ve actually done a lot of research into the chlorine issue. I will be proportionally chlorinating the filtered water as it enters my cistern. Since I know the ppm needed for longer-term storage, it will be an easy calculation. If you’re interested, the proportional pump I’m using (in theory) looks very promising. It’s called the Chemilizer HN55 (best price was QC supply online). The pump runs entirely off of water flow, so no power is required!! The one I picked up does a 1:100 ratio of chemical to water. So, all that’s needed is to take the 5% household bleach concentration and water it down to the right level (math yet to be figured out. haha!). Knowing the starting contamination of the water will be a must, but once I’ve got the filters up and running and can run some lab tests on it, I should be ready to start preparing the water for long-term and contaminate-free storage.

    Next is buying pool-shock and learning to create bleach at home…. For very cheap, you can have a seemingly endless supply of chlorine stored that doesn’t go bad and is ready to purify your water. Like you said… in a long-term water shortage emergency, that water coming from the filters is going to look pretty tempting. 🙂

    All this to water the garden for free….

  10. Orpheus says:

    Very interesting. Your system sounds very well thought out. The idea of having additional treatment for the output water is really a good idea. Also, thank you for the input on the chlorination of the public water system. That should be very helpful to others; particularly those who are just starting, or thinking about starting, a slow sand filter system.

    We’ve had some contamination in the cistern, but not much, mostly only on the surface of the water (the top 1/10 inch). The water we use from the cistern goes through the two slow sand filters as a re circulation system in the summer and as a first filter system in the fall, winter, and spring. The only critters that get in the cistern are insects, and the spiders keep most of them out. The arachnids are smart enough to keep out of the water. After drinking water from a hole in the ground for 38 years, having a few bugs on the surface of the water doesn’t bother me at all – still having some kind of back-up water treatment does sound prudent. The cistern has an overflow pipe that allows the surface contamination to drain off during the winter months. During the rainy season, the water from the filters flows into the cistern continuously at about 50 liters per hour and then out through the overflow into the fish pond below, then out of the fish pond back into the ground. So the water in the cistern is continually refreshed. During the summer months, the water is recirculated from the cistern, through the filters, and back into the cistern, adding small amounts of rain when possible through the tops of the filter barrels. All the pumps work on 12 volts dc. No solar panels hooked up yet. . . . . The racsoons here have, unfortunately, kept the fish pond free of fish; but we call it a fish pond anyway – and have stopped putting fish in the pond – for now.

    Water testing can be done with kits availabe from Micrology laboratories. We have done some of the tests here and I am in the process of putting together a detailed post with images, etc. . The lab tests here (at the epa certified lab) cost about 30 dollars each, just for he coliform test, so the testing can get very expensive. Doing your own tests is probably not as accurate, but better than nothing, and way less expensive, so more tests can be done. One thing I have learned so far, it is super easy, and I mean super easy, to get contamination in the water without even realizing it, and there are multitudes of microbes in all water – its just the amount and type that are dangerous.

    All that said, we have been using water from the slow sand filters for our vegetable garden for 4 years now, and have not run out yet.

  11. Cam says:

    I really look forward to your updated posts on the water testing! Also, thanks for taking the time to respond in such detail to all of my messages. 🙂

    I did a similar thing with my overflow. built a nice little holding pond filled with 7/8″ drain rock that will slowly let the overflow seep back into the ground without drowning my neighbors down the hill!

    I laughed pretty hard at your comment about drinking water out of a hole in the ground. LOL!

    I’ll be sure to send you some photos of the project once its in operation and let you know how well its working with the extra systems being added on to. The solar panels haven’t been ordered yet, but soon enough! We’ll be running a 7gpm 12v pump. Anxious to see what kind of bumps in the road we hit as I get everything assembled and running.

    Thanks again!

  12. Orpheus says:

    Thank you, Cam. I really appreciate your input!

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