Water filters

Instructions for making a water filter from gravel, sand, grass and a plastic bottle: [1]

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See also Living Machines

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UV LEDs can be used to kill pathogens and are cheap.

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Slow sand filter with layers of fine sand at the top, coarse sand in the middle and gravel at the bottom.

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According to research by Micha Hummler:

Original Plans from Josh Kearns and Aqueous Solutions: *http://www.aqsolutions.org/

Here is a $90 DIY water filter for purifying rainwater to drinking quality. Bill of materials and instructions are included (CC-SA license):

http://www.aqsolutions.org/images/2008/05/bucket-filter.pdf