Reverse osmosis simplified. (Rewritten and expanded from the short description in “Manky Sanke Explains”)
In order to understand how reverse osmosis water purifiers work, it will first be necessary to understand what osmosis actually is and how it works. Osmosis is the flow of liquid through “semi permeable” membranes. These materials differ from permeable or impermeable materials as follows:
An impermeable, or waterproof material, is one that will not allow liquids to pass through it. Glass is impermeable and it can be used to store liquids. If water is put into a glass container, the water will stay inside it indefinitely. A permeable material is one that will easily allow liquids to pass through it. Cotton cloth is permeable. Cloth bags cannot be used to store liquids, they would run straight through!
There is another type of material that cannot be described as either permeable or impermeable. These materials are called “semi permeable”, and it is this type of material that is necessary for osmosis. Our skin is an example of a semi permeable membrane. It is waterproof enough to keep our blood and body fluids inside, but it will allow water to pass through when we perspire. Also, if we sit in a bath for too long, our skin will absorb water and become puffy.
Originally, pig’s bladders were used as semi permeable membranes but advances in nanotechnology now make it possible for semi permeable membranes to be manufactured artificially. The first semi permeable membrane that was suitable for use in reverse osmosis purifiers was patented in 1960. Currently, the membranes used are called TFC or thin film composite membranes and they can defy gravity.
Figure 1 shows two examples of a double container with both halves connected via a semi permeable membrane. One double container has both sides filled to identical levels with pure water. The second has both sides filled to identical levels with water that contains impurities. In both cases, it is not surprising to find that there is no movement of water through the membrane from one side of the container to the other. Gravity causes water to “find its own level”, so if the levels in each half are initially the same, both halves are in balance, and there will be no water flow. Gravity has not been defied – yet!
However, if each half is initially filled to identical levels, but with water that contains different concentrations of impurities, as in figure 2, something surprising happens. Water flows through the membrane, in the direction of the arrow, leaving the impurities behind. What happens is that water from the lower concentration of impurities, on the right hand side, is defying gravity and diluting the higher concentration on the left hand side. This force is called osmosis or osmotic pressure.
In figure 3, as the concentration of the impurities on the left hand side is being diluted, the loss of water from the right hand side means that the concentration of the impurities on that side is increasing. This process will attempt to continue until the relative concentration of impurities in both halves is exactly the same. The force that drives this process is called “osmotic pressure”, but since there are limits to its strength, the process will stop when the downward force of gravity on the raised liquid is equal to the osmotic pressure that is trying to drive more water into the left hand side and raise it further.
A piece of cake
understood how osmosis works, understanding reverse osmosis is a piece of cake. The fact that osmotic pressure has limits can be exploited to make water purifiers. In figure 4, if the left hand side is filled with water containing impurities and pressure is applied to it, the water can be forced to flow through the membrane in what osmosis would call the “wrong direction”. Only pure water can flow through the membrane and therefore the impurities are left behind as in figure 5. If the left hand side is constantly refilled with impure water, and the pure water in the right hand side is constantly drawn off, this will form the basis of a continuous flow reverse osmosis purifier.
In practice, it is necessary to regularly back flush the membrane as it would quickly tend to scale up with the accumulation of impurities that it has filtered out and the amount of water that is used to do this is significant. Reverse osmosis purifiers are very good at removing contaminants but they waste a lot of water in doing so. Figure 6 shows a highly simplified schematic diagram of a reverse osmosis purifier. The membrane can be thought of as a sieve with holes just large enough to allow pure water molecules to pass through it. Larger molecules such as the typical contaminants in ordinary tap water cannot squeeze through these holes so the water that leaves through the “purified water out” port will be almost 100% pure. The contaminants that are left behind would soon clog up the membrane and the output of RO water would reduce to almost nothing.
The diagram shows a purifier that uses the “cross flow” principle to solve this problem. By this method, water that is to be purified enters at one edge of the “dirty side” of the membrane and waste water leaves at the opposite edge. This causes water to wash across the membrane and keep it clean. Some of the water molecules entering the purifier will squeeze through the holes in the membrane and will leave through the “pure water out” port. The contaminants, being much larger in size, cannot get through such small holes but they will not be able to build up and clog the membrane because the flow of water across the dirty side will wash them away and out through the waste port. Beautifully simple isn’t it?
What a waste
Actually it’s a bit more complicated than that in practice but figure 6 shows the basic principle involved. One complication is the sheer amount of waste. As water washes over the dirty side of the membrane, only a proportion of it manages to squeeze through the holes and emerge as purified water. The rest, apart from washing away the contaminants, is wasted. Figures from a typical manufacturer show that the waste can be as much as 75%. In other words, only a quarter of the water entering the purifier is actually purified, the rest is lost. This problem is addressed in some designs by giving the waste water a second chance. It is passed to a second similar arrangement where more water succeeds in passing through a second membrane reducing the amount that is lost but the process will always be wasteful as far as usage of water is concerned.
Figure 6 shows chlorine as one of the contaminants in the tap-water fed to the unit because it is the most dangerous one as far as koi are concerned, and therefore the most vital one to be removed. If ordinary tap-water was directly fed to a reverse osmosis purifier it would successfully remove all the chlorine from it but, in doing so, the life of the membrane would be shortened. To avoid this, a carbon pre-filter is used to remove chlorine so that a membrane life of 25,000 gallons is easily achievable before it has to be replaced.