is diffusion through a semi-permiable membrane across a concentration gradient. ie some types of membranes, like cell wall and such, will allow certain molecules to flow accross them until the ionic concentrations between the two sides of the membranes are equalized.
So if you stick a ballon that is made out of a semi permeable memberane and it is filled with fresh water in salt water
. The fresh water will migrate accross the membrane until the concentration of salts are equal. I n this case until the ballon is empty.
is, well, the reverse. You apply enough pressure across the membrane and instead of fresh water leaching into the salt water
, you end up with fresh water being pressured to the area of less concentration.
There are several types of filters, TFT and such, but they all work on similar principles. the membrane is spiral wrapped around a core
water is placed under pressure within the core
. On the outer layers fresh water is the product. These membranes don't per se have small holes in them, unless you consider a hole to be the size that will let a water molecule through and not a salt
The membranes are sensitive to certain chemical polutants. Chlorines, chloromines, and petrochemicals. They will destroy the membrane. What is ususally done is the inclussion of pre filters to remove these contaminates. Usually you'll have something on the order of a 5 micron filter to remove most particulates then and additional filter of activated charcol to remove the chemical polutants. If you keep these two filters serviced and excercise proper care, the membrane can last a year or more.
I try never to use my when I can't see through the water. This helps. Oh, and by the way, the product water is VERY pure. Almost equivalent to distilled water in some instances it is more pure! So you may have to do things like take mineral supplements to get back some of the trace minerals you'll no longer get from your drinking water