Biological sand water filtration (slow sand water filtration) works mainly because of living things we can’t see with our eyes. If water flows slowly enough through sand, it can be cleansed by these living microscopic organisms with help from gravity and small particles of sand.
To more clearly understand why (and how) biological sand water filters work, know that water is recycled “naturally” by its interaction with living organisms, ultraviolet light (sunlight) , and physical filtration through earth (usually called soil or dirt) and porous rocks. Be aware that this earth is alive all over and that microscopic life exists almost everywhere here on this planet. Lots of these tiny organisms are in water and most of them are harmless to healthy people. Without these organisms, we would not survive. Understand that without water, there would be no life as we know it here on this planet. To be comfortable with the idea that water can flow through “dirt” and come out clean; a new way of thinking may be necessary for lots of people.
To attain this mindset, try to stop thinking of the stuff on the top 15 inches of this earth as “dirt”, or re-define “dirt” (maybe referring to industrial pollution as “dirt” would help); and remember where our food comes from – its NOT the grocery store. All of what we eat depends on land and water – the surface of mother earth. Potatoes, radishes, carrots, garlic, onions, beets, yams, sweet potatoes, shallots, ginger and peanuts, to name just a few, all come right out of the “dirt”. Ok, lets stop calling mother earth “dirt”. All come from mother earth. We may buy them at the “grocery store”, but they all come to us courtesy of mother earth.
In a biological sand water filter, micro-organisms, most of them harmless, grow and cling to the surface of sand particles inside the filter. As water flows slowly through the sand, these organisms form a biofilm that traps and consumes other micro-organisms including those that make people sick. In recent history (the past 150 to 200 years) it has been known (documented) that this type of filtration works in man-made filter systems. In the past 20 years, scientists have discovered a lot about why and how it works.
Some of the details are in the rest of the pages of this blog, and may also be found easily on the internet by doing a search for “slow sand water filter”.
My references can be found here:
http://www.slowsandfilter.org/lit_cited.html
and here:
http://www.shared-source-initiative.com/biosand_filter/references.html
Also included my references:
7 years of experience building and testing slow sand water filters documented here:
http://www.slowsandfilter.org/index.html
http://www.shared-source-initiative.com/biosand_filter/biosand.html
Some filters use charcoal, where does this fit in with the slow sand concept ?
Can charcoal be used in the slow sand process to improve the system.?
Use a charcoal filter following the output of the slow sand filter. In other words, use a charcoal filter on purified water to remove chemical impurities only. Bacteria can grow in charcoal, and cause added contamination.