A brief guide to how RO (Reverse Osmosis) Works

Discussion in 'Water Parameters and Additives' started by Neil H, 18 Nov 2010.

  1. Neil H

    Neil H Moderator MASA Contributor

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    I did some research on reverse osmosis in a bid to understand what is happening in our RO machines, I thought some people may find this and interesting read and have tried to summarise what I have found in the text below which has various internet sources...

    To understand how the process of reverse osmosis works, it helps to understand the process of osmosis which is everywhere in nature. When two solutions having different dissolved mineral concentrations are separated by a semi-permeable membrane, water flows from the less concentrated solution to the more concentrated solution until an equilibrium is reached. For example is we have 2 salt solutions separated by a semi permeable membrane, one solution at 1020 and the other at 1024, water molecules would move from one solution to another until the solutions have the same salinity.

    Osmotic pressure is a measure of how badly the water wants to go from the "clean" side (in our example the 1020 salinity) to the "dirty side" (the 1024 side)(low mineral to high mineral content side of the membrane) and that is governed by the mineral concentration differential (i.e. in our case the number of salt crystals present). This pressure can be surprisingly high and accounts for one mechanism used by trees to move water from the deepest root to the tallest limb, frequently a vertical distance of 30m or more.

    As water moves through the membrane, most minerals it contains are left behind. The mechanisms which enable the water molecules to pass through the membrane leaving most of the dissolved minerals (ions) behind are not fully understood but it is definitely much more complex than simple filtration.


    ­in the above image the left is a beaker filled with water, and a tube has been half-submerged in the water. As one would expect, the water level in the tube is the same as the water level in the beaker. In the middle figure, the end of the tube has been sealed with a "semipermeable membrane" (same as is found in an RO machine) and the tube has been half-filled with a salt water solution and submerged. Initially, the level of the salt solution and the water are equal, but over time, something strange happens -- the water in the tube actually rises. The rise is attributed to "osmotic pressure."

    A semi permeable membrane is a membrane that will pass some molecules (atoms) but not others. The best common example of a semi permeable membrane would be the lining of your intestines, or a cell wall. Which allows some molecules to pass into our bodies and keeps other molecules out.
    [​IMG]
    In the figure above, the membrane allows passage of water molecules but not salt molecules. On the salty side, some of the pores get plugged with salt atoms, but on the pure-water side that does not happen. Therefore, more water passes from the pure-water side to the salty side, as there are more pores on the pure-water side for the water molecules to pass through. The water on the salty side rises until one of two things occurs:
    • The salt concentration becomes the same on both sides of the membrane (which isn't going to happen in this case since there is pure water on one side and salty water on the other and the membrane prevents the salt from passing to the clean side).
    • The water pressure rises as the height of the column of salty water rises, until it is equal to the osmotic pressure. At that point, osmosis will stop.
    Osmosis, by the way, is why drinking salty water (like ocean water) will kill you. When you put salty water in your stomach, osmotic pressure begins drawing water out of your body to try to dilute the salt in your stomach. Eventually, you dehydrate and die.
    In reverse osmosis, the idea is to use the membrane to act like an extremely fine filter to create drinkable water from contaminated water. The salty water is put on one side of the membrane and pressure is applied to stop, and then reverse, the osmotic process. It generally takes a lot of pressure and is fairly slow, but it works.
    And that ladies and gents is how RO works and why it is so important !
     
    leslie hempel and chas84 like this.
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  3. 459b

    459b Moderator MASA Contributor

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    Some good info there!

    Two things i noticed...

    water moves up trees by capillary action, not osmosis. Trees are packed with tiny tubes that take advantage of waters forces of cohesion to transprt water t othe top of the tree.

    are you talking bout trees or RO membranes? Trees membranes are selectively permeable and actively decide which molecules are transpoted through and which are left behind.
    RO mwembranes on the other hand rely on the pore size. So any molecule the same size as water and smaller will be forced through. Hence the need for di-ionizing resin after the membrane to remove everything.
     
  4. Neil H

    Neil H Thread Starter Moderator MASA Contributor

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    Hi 459b,

    completely see where you are coming from, and like most things in nature if one tries to simplify things too much there are always problems .... perhaps i oversimplified, but i think the jist of the article provides some interesting information for the uninformed.

     
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