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reverse osmosis02         (Reverse Osmosis)


I wrote the following articles for Koi Magazine.
Therefore they own the copyright but the Editor has given permission for them to be republished here.

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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.

Reverse osmosis 1

Figure 1

Defying 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!

Reverse osmosis 2

Figure 2

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.

Reverse osmosis 3

Figure 3


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

Reverse osmosis 4

Figure 4


Reverse osmosis 5

Figure 5

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.

Reverse osmosis (Koi) fig 6
Figure 6

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.


Why purify water?
As we know, chlorine is put into water supplies to ensure it is safe to drink when it gets to our taps but this disinfectant is harmful to fish. Reverse osmosis purifiers could remove chlorine but since the life of the membrane would be shortened as a result and a carbon pre-filter has to be used to dechlorinate the incoming water first, these purifiers are no better than an ordinary carbon canister filter in this respect. Where reverse osmosis scores very highly over other water purifiers is the degree to which they remove other contaminants. A typical RO unit will remove 98% of nitrate, phosphate and heavy metals such as lead or copper.  Heavy metals in tap-water are in sufficiently low concentrations that they do not pose a health risk to humans but, even at concentrations that are allowable in drinking water, heavy metals pose a risk to koi health.  A cartridge filter could be used tPost it RO 1o remove heavy metals to a large extent but reverse osmosis purifiers do a better job. Where reverse osmosis scores very highly is in removing compounds that koi keepers refer to as general hardness (GH) or carbonate hardness (KH).  This also has the effect of reducing TDS to negligible values.  There will always be different opinions as to the best GH or KH to maintain in a koi pond but RO users are usually trying to provide similar parameters to Japanese mud pond water where hardness is low. It is difficult to prove but there is ample anecdotal evidence to say that skin quality and growth are improved in soft water. Also, if there is any genetic predisposition to shimmies in the koi’s blood-line then hard water will encourage them to appear.  That is not to say that hard water will cause shimmies to spontaneously appear but it will increase the likelihood if the tendency is already present.

Are there any disadvantages?
Reducing the GH in a koi pond poses no risk to koi health but reducing KH must be done with due regard to the fact that it is the KH that prevents the pH from varying unacceptably and even suddenly crashing, possibly to fatal values.  Using only RO water in a pond would involve such constant attention to pH that it would be impractical for all but the most determined.  For this reason, it is usual to mix some dechlorinated tap-water with the RO water to achieve the desired “recipe” and build in a degree of safety as far as pH variations are concerned.  If the incoming tap-water is at, say, KH 6 and it is mixed 50 – 50 with RO water having (virtually) KH 0, then the resulting water mix will be KH 3. It would require more attention to the pH at this value than at KH 6 but if this was acceptable to the pond keeper, there is no reason why a pond could not be successfully managed at this KH or even lower.

Tiny science
The correct name for tiny science is “nanotechnology”. In the simplest of terms, this can be thought of as engineering on a scale that is almost unbelievably small. Typical dimensions are in the region of one thousandth of one millionth of a metre.  Or dimensions as small as one eighty thousandth of the diameter of a typical human hair!  Tiny science, or nanotechnology is what is used to make the current generation of membranes that are used in reverse osmosis filters, but it won’t stop there.  Another branch of this technology is beginning to break new ground.  A new way of making tubes has allowed nanotubes, as they are now being called, to be made.  These are carbon fibre tubes that are so tiny that only three or maybe six molecules can pass through them at a time.  That is not a misprint, three to six molecules at a time!

Enough science, what has this got to do with koi?  Quite a lot actually - one of the problems with the current design of reverse osmosis purifiers is that water has to be forced, under pressure, to pass through the holes in the membranes that are now being used.  This can involve having to use electric pumps to create the kind of pressures necessary.  With the new nanotubes, the inside walls of the tubes are so smooth that there is very little resistance to the flow of water molecules. Even though Post it RO 2the tubes will only allow a few at a time, the molecules will simply slip straight through.  A powerful pump isn’t necessary, and so the carbon footprint of RO water will be reduced. There is even better news. One reason that makes it necessary for a high proportion of the water going into a purifier to be used to wash contaminant molecules away from the current type of membrane is that, although these molecules can’t pass through the holes, the water pressure holds them against the hole.  In an RO unit, these molecules have to be washed away from the holes by a good flow of water.  With the next generation of purifiers that use nanotubes, pressure on the dirty side of the membrane doesn’t have to be so high to force water to pass through to the clean side as it does with membrane filters.  With a much reduced pressure, washing away contaminants will be easier and so far less water will be wasted in this process.  With the combined saving of electricity and also of water, koi keeping, or more exactly koi keeping in RO water, will soon be getting greener.

Reverse osmosis 7The future of reverse osmosis
Little has changed in the design of these units since the early models that used thin film composite membranes.  They are still difficult to manufacture so they have remained expensive. Nanotubes are a cheap solution to many engineering problems, not just water filters, and now that production lines are being set up to mass produce them, the production costs are falling. Reverse osmosis filters will never be as cheap as simple carbon canister filters but if the most expensive component, the membrane, is replaced by nanotubes which are cheap to make, it is reasonable to expect prices to fall.  This, in turn, will allow them to become more popular.

Installing a reverse osmosis unit is not complicated.  They need a water supply to the input, somewhere for the waste water to go and sometimes an electrical supply. They also have to be protected from freezing but are no different to any other purifier in this respect. Since the output is pure RO water and it is usual to have a mix of RO water plus dechlorinated tap-water, they can be used manually to top up a pond whereby the unit is turned on and an appropriate amount of dechlorinated tap-water is added at the same time.  By adjusting the amount of each that is added, the desired GH and KH can be achieved. Attempting to control GH and KH by this method alone would not provide accurately controlled values so final adjustment of the proportions is best done after testing the water parameters.  But then this isn’t an extra chore that has to be undertaken when using RO water because we all test our water parameters anyway, don’t we!?


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