The aim is to reduce the amount of water soldiers need to carry. One day's food supply of three meals, weighs 3.5 kilograms but that can be reduced to about 0.4 kilograms with the dehydrated pouches, says spokeswoman Diane Wood. The pouch- containing chicken and rice initially- relies on osmosis to filter the water or urine. When two solutions of different concentrations are separated by a semipermeable membrane, with gaps that allow only water molecules to pass through, the water is drawn to the more concentrated side. The membranes are made of thin sheets of a cellulose-based plastic, with gaps between the fibres that are just 0.5 nanometres across, too small for bacteria to pass through. A hungry soldier pours dirty water into one end of a foil sachet containing two inner pouches separated by the membrane.
The water seeps through the membrane into the dehydrated food on the other side. As it dissolves large molecules in the food, it creates a very high concentration solution. The osmotic pressure created then draws more water through the membrane. Hydration Technology of Albany, Oregon, which makes the membrane, says soldiers should only use urine in an absolute emergency because the membrane is too coarse to filter out urea. The body won't find this toxic over the short term, says Ed Beaudry, an engineer with HTI, but rehydrating food this way in the long term would cause kidney damage.
Author: Duncan Graham-Rowe
This article appears in New Scientist issue: 24 July 2004
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