Kidneys of humans filter impurities by use of a dual membrane system.

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"Using a system based on the human body's kidneys - the ultimate in water recycling technology - Singapore and Orange County, CA have developed schemes that will use a dual membrane process to recycle domestic waste water (sewage) to levels that approach the quality of distilled water. Like the kidney, these recycling plants use two membranes, one with larger holes to remove micro-organisms such as protozoa and bacteria that cause infection, while the second separates salt from water." (Courtesy of the Biomimicry Guild)
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