Just a reminder. . . .

On this blog, questions and comments relevant to roofwater harvesting / rainwater harvesting, biological sand water filters, and sustainable water filtration practices are encouraged. Also questions regarding the content of slowsandfilter.org, roofwaterharvesting.org, and shared-source-initiative.com/biosand_filter/biosand.html will be answered here. Comments are moderated and may not be posted if they are too far off topic.

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5 Responses to Just a reminder. . . .

  1. Paul St. John says:

    Your work is excellent. What you have documented here and at http://www.slowsandfilter.org is the best I’ve found on the Internet. I have only been reading about these filters for the past couple of days, and I am trying to educate myself as fast as possible because I am very eager to make one. I have a few questions that I haven’t found the answer to yet, and I am hoping that you have a few minutes to help me. I will state my questions as quickly as possible.

    1. How expensive is it to buy the sand from the companies that you recommended here http://www.nsf.org/Certified/Common/Company.asp?Standard=061

    2. You recommend a UV filter right before use of the water. Is there an actual filter you are recommending that we buy? Could we use sunlight?

    3. If so, I am considering a glass chamber that exposes the water to sunlight before recirculating it into the filter. I hope to use the heated water to achieve a convection flow from the bottom of the filter, through a check-valve, thought a glass heating and UV chamber, and back into the top of the filter. The flow would be small, and the water would only flow when there is enough sunlight to achieve convection. Do you think this would be worth pursuing?

    4. Could back flow be achieved safely by raising the water level at the output to 12″ or so above the opening in the filter used for wet farrowing? This would move the water backwards through the filter, but at very slow rate. Could a slow back-flow like this, for a sustained period, clear the filter without damaging the biolayer?

    5. Could tap water kill the biolayer of the filter given that it has Chlorine and fluoride and Lord knows what else in it?

    6. Will anything short of reverse osmosis remove fluoride from water? Does the slow sand filter remove flouride, or chlorine, or any other chemicals or elements added by a typical municipality?

  2. admin says:

    Some very good questions, Paul. Thank you. Hopefully there will be others who can add more info to this post!!!!

    The sand I have purchased locally here, is anywhere from about $7.00 for a 100 pound bag to about $35.00 for a 100 pound bag. The $7.00 sand works just fine. It is .25 mm effective size with at uniformity coefficient of 2, and is ansi/nsf 61 approved. The 35 dollar sand was .35 mm effective size. Since sand is very heavy, the shipping will be horrific unless you can find it locally – or have your local supplier order it for you.

    As for the UV filter, I don’t have a specific model or manufacturer to recommend. It is a well known fact that portions of the ultra-violet radiation spectrum destroy the ability of viruses and bacteria to reproduce, effectively deactivating them. Also glass blocks/attenuates some types of ultra-violet radiation. I would suggest reading this article http://en.wikipedia.org/wiki/Ultraviolet
    on ultra-violet radiation, as it is a very complex topic and far beyond my expertise.

    On the glass chamber idea; it sounds like it might work to cause the water to flow. Keeping in mind that biological sand filters need microscopic life in the water to keep the biolayer alive, you may want to assure that some unfiltered water is added to the input.

    As far as backflow or backwashing, I have not tried it yet. My best guess is that the top few centimeters of sand could be disturbed without destroying the filter, but it would then take 3 or 4 weeks for the filter to start operating; and you would want to be sure to close the output so the supernatant cloudy water could drain off before restarting the filter.

    I have not tried tap water as of yet. The amount of chlorine in the water will, I imagine, vary by location. I would be most concerned about the “Lord knows what else” part. I have had some very nasty chlorinated water in some places here in Washington state; and yet there are other places where there is no odor or taste in the water. Tap water may or may not work. The amount of chlorine residual in the water will determine how much of the “good” microbe life is destroyed. My best guess is that tap water will slowly kill off the life in a biological sand filter and cause it to eventually produce very low quality water, maybe even worse that what you start with. This would be a good project for someone who has access to a public water system; hopefully someone will read this and start an experiment with rain water/roof water and then add tap water to the filter and see what happens by having the water tested.

    As far as the fluoride issue, the test for the presence of fluoride was very encouraging and can be seen near the bottom of the page here:
    Unfortunately, the test was only done on the output water so we don’t know yet how much fluoride was actually removed. An interesting part of the tests done here is that they are done on a filter that has not – yes not – been “cleaned” that is wet harrowed, or raked or deliberately physically disturbed for 3 years. The filter continues to operate and the flow has basically stabilized – in other words – cleaning may not be necessary if the flow rate is not critical. Furthermore, the last test done, when the water was at just above 32 degrees F showed that the filter was still working. They are not supposed to work well at that temp. – but this filter is different – it has not been “cleaned”. Come to think of it wetlands – in their “natural” state – don’t really get “cleaned” regularly every few months either and they purify water also. . . . hhhmmmm – another good experiment would be to take a slow sand filter that has been operating undisturbed for several years and test it before and after for fluoride and THM’s – all we need is some good old “tap” water – five or ten gallons of it!

  3. Paul St. John says:

    Well, I have spent some time trying to find some way to buy the appropriate sand. The closest I have come is a sand that is 0.6mm with a uniformity coefficient of 2.18. However it is not ANSI 61 approved. It is silica sand. Is silica sand an acceptable sand for filtering? Does the ANSI 61 deal with detergents that might have been used to clean the sand?

  4. Rainwater contain small amount of microorganism and rainwater filters are used to make rainwater clear and safe for drinking, cooking for the sake of our health.

  5. admin says:

    Most sand contains silica. When silica sand is wet it is harmless; however when it is dry it can be very harmful if inhaled. The ANSI 61 tests are complex and way beyond my area of knowledge to offer a complete explanation. Generally speaking, the ANSI 61 tests determine whether or not a material is acceptable for use with potable water. Silica sand is used in water filtration regularly.

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