How does a slow sand water filter work?

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.

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4 Responses to How does a slow sand water filter work?

  1. Water Purifiers says:

    Is this something that I could make a home?? If so could you send me more information on where I could find how to make or put together. If you knew a website that would be great.
    Thanks for the great info.

  2. AquasafeSystems says:

    I understand how it works, but does this method really remove all the harmful impurities in the water, or just the bacteria?

  3. Orpheus says:

    The water quality test results on the filters referred to by the author of this blog can be found on slow sand filter dot org
    and on roof water harvesting dot org

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