A recent paper on arXiv.org (also reported on the MIT technology review) suggests an interesting approach to water filtering.
The water filter suggested by Lee, Boutilier, Chambers, Venkatesh, and Karnik
The vessels of plant xylem consist of tubular cells which undergo programmed cell death, leaving long thin tubes which conduct water to the top of even the tallest tree, through capillary action. As illustrated below, the individual xylem vessel elements include perforation plates at their ends; it is these which can act as water filters.
Lee, Boutilier, Chambers, Venkatesh, and Karnik found that the water filter illustrated above could remove particles larger than 0.1 µm – enough to filter out bacteria, though not viruses. Epoxy glue is needed to seal the filtering wood into tubing, however, and the flow rate is a low 180 ml per hour, even under pressure. Still, this is a very interesting low-cost water-purification technique.
Xylem cells (image by Kelvinsong)
“Water, water, every where; and all the boards did shrink. Water, water, every where; nor any drop to drink.” – The Rime of the Ancient Mariner
The maps below, from the Gravity Recovery and Climate Experiment satellites, show US underground water in surface soil, in the “root zone,” and in deeper groundwater aquifers (amazing that they can detect underground water from space, isn’t it?).
As of September 2012, it was still very dry down there, especially in Texas, and the drought continues in the central USA. Let’s hope for some good rains there. Click here for more information and an animated time series of the data.