MadSci Network: Biochemistry

Re: Is it possible to create a simple water filter that uses reverse osmosis?

Date: Tue Feb 10 07:30:48 1998
Posted By: Chris Taylor, Student, chemistry, University of Wales, Swansea
Area of science: Biochemistry
ID: 884306167.Bc

First of all, for the benifit of those who don't know what we're going to talk about, I'll briefly explain what a reverse osmosis water filter does.

It is a way of taking salty water and using high pressure to remove that salt so it is suitable for drinking. It consists of two solutions either side of a special semi-porous membrane. The membrane allows water to pass through, but not the large, solvated, ions that make up salt. One side of the membrane has pure water and the other side has salty water. Left to it's own devices, the pure water would pass through the membrane until each side was equally salty. This, of course, is no use, as it just leaves us with a lot of slightly salty water. To prevent this, pressure is applied to the salty side. The pressure required to exactly cancel out the tendancy of the pure water to move into the salty is called the osmotic pressure. If a greater pressure is applied, water will actually move from the salty side to the pure side. If you remember that the salt cannot pass through you will realise that this gives us more pure water on on side and a more concentrated salt solution on the other.

Now, I'm not the most technically minded of people, but I'll do what I can to help with the difficult part of the problem. I would consider the really hard bit to be making, or getting hold of a suitable membrane. You need a semiporous membrane witht the correct pore size. It should let water molecules through, but not the large solvated sodium and chlorine ions of the salt.

It has been suggested to me that you could try contacting Dupont, the company who make the commercial membranes. It is possible that they would let you have a sample. The other possibility is to ask at a hospital lab as they may have something suitable. Perhaps someone else as Mad Scientist could suggest exactly what to ask for? If you want to make your own membrane (not recomended!) I would suggest asking another question about that, as I don't have that kind of expertise.

My other suggestion is that to make the operation of the filter easier you use a dilute salt solution. I say this because the pressure you need is proportional to the concentration of the salt ions, so you'll need less pressure with a dilute solution.

Apart from that, you just need to make some apparatus that gives you two soltions, joined only by your membrane, and a method of applying pressure to the salty side. I would suggest a syringe or similar if you are keeping it cheap. Commercially, powerful pumps are used, but that would be expensive.

I hope this helps!

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