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Information for window cleaners about reverse osmosis (RO) systems

Information for window cleaners about reverse osmosis systems

Window cleaners have been using deionised water for cleaning glass for many years and have made good use of reverse osmosis technology.

What many window cleaners may not realise is that reverse osmosis membranes (with the exception of small residential membranes) are not designed to work off tap pressure. Even the much loved HF5-4040 "Ultra Low Pressure" membrane is designed to be used in engineered reverse osmosis systems running at about 80 psi. Normal Extra low energy membarnes are designed to work at about 100 psi. If you study the data sheets you will also note that the manufacturers recomend that they should be run with 85% waste. This is the reason why we seldom reach the advertised 99% or 98.5% rejection rates nor the high flow rates shown on the data sheets.

We have found through empirical data gathered from hundreds of window cleaners over many years that results really do vary! The HF5-4040 was a great advancement and has gone a long way to making it possible to make water at even the lowest pressures where the HF4, XLE, ESPA4 or AK membranes would fail. Before the HF5 you would have been forced to use a pumped system which in itself is something of a mine field. The Extra low energy membranes still have their place however - where water pressure is good (eg 60 psi) they will reject better, i.e. give you a better tds however they won't beat the HF5 for product flow. If you are using a booster pump then the extra low energy membranes tend to give a better tds than the HF5. This is because most booster pumpes used by window cleaners do not give enough flow for the HF5-4040 to work properly - high flow membranes need a high flow input which can be an issue with many mains booster pumps as flow is reduced as pressure is raised.

GE have recently upgraded the AK4040TF with a new high rejection membrane - the new code is AK90. We haven't had much feed back on this one yet as its relatively new but looking at the stats it should give a better tds. We can't comment on how well it will cope at sub 40 psi pressures as the old AK4040 was a little hit and miss at low pressures but if you have a good 60 psi this membrane might be worth your consideration to get more life out of your resin.

The good water to waste water ratio is critical in getting the most out of your membrane. We've found that 70% waste to 30% product water is about the right balance. As we have said previously the manufactures actually recomend a ratio of 15% product to 85% waste but this is achieved in a controlled environment with the correct pressure and flow feeding the membrane to give optimum results. When you are relying on tap pressure you need to create more back pressure by closing the waste a little more. The more you close your waste the higher your inlet pressure goes and whilst pressure is important so is making sure enogh water goes down the drain or the result is either a clogged membrane with no flow or the pores in the membrane are made bigger and that drives the tds up. The best way to get the best results is to try different positions on your waste valve starting with it fully open. Use your tds meter to measure the water quality at each position. At the position where you get the lowest tds your membrane is happiest - this is called the sweet spot. You'll probably find as you close your valve the the tds will go down because you are increasing your inlet pressure but after a certian point the tds will start to climb because not enough water is going down the drain.
If you have 2 membranes then you should be able to achieve nearer to 50/50 ratios as the second membrane uses the waste from the first..

Once you have set up your membrane it is important to check it frequently because your mains water pressure can vary. Also if you check frequently you can spot problems with your membrane early and take action to stop further deteriation. Often window cleaners do not check their membrane until their resin exhausts quickly and by then too much damage has been done to the membrane to remedy the problem and a new membrane is needed.

Emphasis should also be given to pre-filtration. Reverse osmosis membranes cannot tolerate chlorine so a carbon filter is needed to remove the chlorine. Carbon blocks are the most effective but GAC (granular activated carbon) have less of a pressure drop. We usually advise using a carbon block filter but if your pressure is particularly low you could use a GAC filter although it is advised to change it more frequently and to use a good quality filter. The membrane also requires that the water be filtered to a nominal 5 micron. Most carbon blocks are 5 micron so could be used on their own however they tend to clog up fast and GAC filters will only filter to 20 micron so in both cases pre-filtration with a good quality depth filter is necessary. If your pressure is very low you could use a pleated 5 micron filter instead as there is less pressure drop however they cost more and last less so depth filtration is prefered. Another option is to use the new fiberdyne technology which is both an effective chlorine remover and retainer of sediment. Fibredyne filters have a low pressure drop and only one filter is needed instead of two lowering your set up costs and reducing your maintenance.

Most of us are aware of the dangers of frost to the membrane (and filter housings) but less consideration is given to biological growth that can ruin a membrane. This is most likely to happen when the membrane is not used for a long period - once there is growth on the membrane it is difficult to recover from so in most cases the membrane must be replaced and the system disinfected before installing new membranes and filters. In industrial systems it is possible to dose the system with chemicals to kill off any biological fouling however on a small scale this is not viable. Sometimes growth is very obvious as you'll see black, brown or green spotting on the end of the membrane or inside the membrane housing, other times its harder to see as the build up is inside the membrane and disection would be needed to see it, usually though you should be able to feel a slimy/soapy residue on the membrane and/or housing. Your pre-filters may also show signs of biological fouling. If you use clear filter housings you should try to ensure that they are not exposed to sunlight as this will precipitate growth on the filters that may end up in your membrane.

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