A biosand filter is a simple type of water purification system that uses sand, gravel, gravity and some simple engineering to purify water contaminated with biologics and some chemicals. Its structure is generally made of concrete or plastic and is filled with layers of gravel and sand that encourage the growth of good microbes that are naturally present in water, just like in the ground in nature. These good microbes develop into what's called a "biolayer" or a "biofilm," which destroys pathogenic (i.e., disease-causing) microbes to create clean drinking water. The sand also causes pathogenic microbes to become trapped, adsorbed (i.e., stuck to the sand particles) or die from lack of food or oxygen.

The biosand filter constructed in this Instructable was made using materials commonly available at any hardware store (Home Depot in this case). Two were built to test their effectiveness for a real-world application in the community of Chakicherla Pedda Patapu Palem in Andhra Pradesh, India for an Engineers Without Borders project run by the San Diego Professional Chapter's James Harper, PE. Check out this Engineers Without Borders project at the EWB San Diego Professional Chapter website.

Many types of biosand filters are out there, but the standard one that is currently in use all of Africa and other parts of the world is made out of concrete. The Centre for Affordable Water and Sanitation Technology (CAWST) developed a great biosand filter construction guide that has lots of useful information. I've attached a PDF copy of this document to this Instructable for your knowledge and reading pleasure.

A few other interesting biosand filters out there have been made by Surfers Without Borders and the BioAusable Project.

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This project is ongoing and not complete but should be completed within the next two weeks. Please check back soon for more to come!