Tuesday, September 1, 2015

Water Filtration -- with a Stick!

Water filtration is critical, and is a topic we've discussed at length. But what happens if your filter breaks or is otherwise unavailable? I've discussed how to make a field-expedient filter, but a group of engineers at MIT have demonstrated a nearly free filtration method that works on par with a great many commercially available filters.

Their paper, published in the journal PLoS ONE, is available here. It's written in fairly dry engineering context, so if you want a basic summary for the layman, I'll break it down for you:
  • Filters work by passing water through microscopic holes and passages. 
  • Water molecules are small enough to fit through these openings, but bacteria, protozoa, and other contaminants are not. 
  • The wood of certain trees, such as pine trees, contains passages called xylem that move sap throughout the tree. 
  • These passages are between .07 and .1 micron in size, making them roughly equal to a Sawyer Mini filter element
  • The smallest bacteria are roughly .2 micron in size, and are easily caught in the filter.
Construction of the filter is almost too simple to believe, and virtually free. The researchers stripped the bark off a piece of white pine roughly 1" long and 1/2" in diameter. The wooden plug is then inserted and sealed in to the end of a piece of food-grade tubing. Set up like this, each line can filter roughly of 4 liters of water daily.
Image from http://www.offgridquest.com/813
It really is that simple and easy. The only critical sticking point is that the wood must be fresh cut and must remain moist. When dry, the xylem loses its filtering properties, as cracks appear and contaminants flow freely through. As your wooden filter becomes clogged, it can simply be replaced with a fresh piece, and you're ready to keep filtering.
The wonders of science never cease!


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