Science Buddies: "Ask an Expert"

What is reverse osmosis mean? I need to understand this for my project on removing salt from water

Hi Ashek,

Welcome to Scibuddies! We will be happy to help you create a great science project.

Reverse osmosis like regular osmosis uses a special membrane that has tiny pores. The holes allow some molecules, like water to pass through freely but keep larger molecules from going through. In reverse osmosis pressure is applied to the solution on one side of the membrane and this causes the water molecules to go out rather come in as they normally would do. This is how drinkable water is prepared from salt water. The pressure forces water molecules through the holes in the membrane but does not allow the salt molecules to pass.

These references on reverse osmosis explain the process pretty clearly:
http://en.wikipedia.org/wiki/Reverse_osmosis
http://science.howstuffworks.com/reverse-osmosis.htm
http://www.youtube.com/watch?v=mZ7bgkFgqJQ

Let us know if you have more questions. Please include details of your project design so we can answer you better.

Good luck!

Sybee

Thank you Sybee. The you tube link was very useful.

I am Planningto build an apparatus to seperate salt from water. Can anyone give me an suggestion on building apparatus. I have seen the one in science buddies.
wanted to try this with different types of apparatus.

Hi Ashek,

I think your idea of building an easily used desalination device out of inexpensive materials is wonderful! In the U.S. we take clean water for granted, but in many places around the world people struggle to get drinkable water, which could be easily supplied by seawater if they just had a way to remove the salt.

I looked at the Scibuddies desal project and it is simply a still that heats seawater with sunlight and lets it condense. Drinkable water could also be produced from seawater by reverse osmosis (RO) but the device is a little more complicated. First you need a membrane that is permeable to water but not salt and second you need a way to apply pressure to the liquid on one side of the membrane to force water molecules through against the diffusion gradient.

You can buy RO cartridges, but they are relatively expensive (average cost about $40US). Also, if seawater is being purified there needs to be a pre-filter to remove particles that will foul the RO membrane. A pre-filter can be something as simple as a column of sand in a drain pipe that can easily be replaced when it becomes clogged.

The second requirement is pressure. In the case of large RO membranes used for desal the pressure needs to be in the 600-1200 psi range, but personal desal units can work with a pressure as low as 40 psi (http://en.wikipedia.org/wiki/Reverse_osmosis). An air pump powered by a 12 V battery charged by a photovoltaic panel would probably work well enough for a small setup.

I hope this information gives you an idea what you would have to do to build a working RO unit to purify seawater. I don’t know where to get a cheap, functional RO membrane, but maybe one of the other experts has some ideas. It would be great if you could just use something common and inexpensive like cling wrap or plastic grocery bags for the membrane but I doubt if that plastic is permeable to water molecules. If you could come up with a cheap, reusable RO membrane material you would provide a great service to millions of people.

Good luck with your project!

Sybee

Thank you sybee!
1. can I use --- http://www.hometrainingtools.com/dialys ... E-MEMBRLG/ as semi permiable membrane.
2. will my plastic container be able to handle 40 Psi of air pressure.?
Let me know I am just trying to build a model

Hi Ashek,

I took a look at the page you sent for the membrane but this one won't work. In the description it says that the membrane is permeable to water and most ions. That means the sodium chloride from saltwater would pass through it. You need a reverse osmosis membrane not a dialysis membrane. I wish I could tell you where to get an inexpensive RO membrane but the only ones I saw were in the form of a cartridge and cost $50-100. You could go online and find companies that sell RO membranes and call them. Tell them you are high school student on a budget and ask them if they could sell you a small piece of RO membrane for your project. If they can't do it they might be able to tell you who could.

I may have missed it but what container are you talking about? How are you going to pressurize it? I would think the difficulty would be how to support the membrane so it doesn't blow out. These are sort of engineering details so you might want to post a query on the Physical Science forum asking how to build a pressurized RO device.

Good luck!

Sybee

Thank you so very much for giving me ideas. I will post pressurized containers.