How do slow sand filters work?
Slow sand filtration is a biological process that cleans water much the way the sandy bed of a river cleans and recharges an aquifer. A column of water passes through a three-foot layer of fine sand at the rate of 0.1 gallons (0.38 liters) per minute per square foot or less. On the top of the sand, an intense layer of microbes naturally develops. This layer lives by consuming whatever is passing through in the water. In a slow sand filter, this layer, called the schmutzdecke, is responsible for removing up to 99.99% of all bacteria, viruses, Giardia, Cryptosporidium, and parasites through predation. As the water passes through the deeper layers, other processes such as sedimentation, mechanical filtration, and electrical attraction remove still more. The result is that slow sand filters may be the best stand alone water filters known. You can build your own, but… Though SSF is a simple technology, the rules are very, very important and it is vital to you and others that you do the re
