Biosand water filters can be constructed using simple materials and are capable of filtering out up to 99% of sediment, bacteria and viruses. The filter is essentially a column of sand with gravel at the bottom. Contaminated water is poured in the top and filters first through a biological filter, then through the sand and is pushed by gravity up through a tube to a spigot. The biofilter is a layer of naturally occurring micro-organisms very much like those found is salt-water fish tanks. These take a while to grow so filters should be constructed as part of a prep effort rather than on the fly. The system can filter several liters per hour and should be enough to supply a family. While they are very heavy once filled with sand the filter is small enough that it could be placed in a kitchen or laundry room.

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