reverse osmosis

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reverse osmosis
reverse osmosis
reverse osmosis
reverse osmosis
Reverse Osmosis Systems
reverse osmosis
Reverse Osmosis Connector Systems
reverse osmosis

reverse osmosis

Things a consumer should know about reverse osmosis

Reverse Osmosis or RO: Is the passing of a greater concentrate under pressure to a lesser concentrate through a semi permeable membrane. Tap water having the higher concentration of impurities than that of cleaner water. The pores of a membrane are very small at around .001 thousands of a micron. To compare the human hair is around 100 microns thick. The effectiveness of a membrane is determined by how the surface of the membrane maintains its integrity against contact with your tap water. Membrane failure can be caused by many different reasons but to keep it simple once the membrane develops a hole there is no saving it.

Pre filter: These range in size from 30 microns down to 1 micron. This filter works on the larger particles in the water. The finer the pre filter the faster it will clog up with impurities. You will want to protect the pre-carbon in a thin film composite or TFC system from fouling by sediment. A 5 to 10 micron pre filter is best suited for residential systems. In a cellulose tri-acetate or CTA system the sediment filter is all that stands between the incoming tap water and your expensive membrane. It works as the first line of defense for both types of systems. If you live in an area that has a lot of sediment than you should change this filter more often, typically it is sufficient to change it every 6 months.

Pre carbon: Should only be used with a TFC system it will remove the chlorine before it comes in contact with your membrane this will extend the life of a TFC membrane. If used on a CTA system it will cause the membrane to fail. The reason is because a CTA needs chlorine in the water to protect it from bio fouling. This filter should be replaced every 6 months.

Membrane: Of coarse is the key to why all RO systems work. There are two types of residential membranes first is the CTA or cellulose tri-acetate membrane, this membrane works well in chlorinated water but has a lesser rejection percentage than the TFC or thin film composite membrane. The CTA also has a limitation with the pH that it can handle it will not survive in pH above 8.5. The TFC on the other hand can survive in feed water with pH of up to 11. However the TFC membrane cannot handle more than 1000 hours of contact with chlorine so this is why all residential TFC systems have a carbon filter ahead of the membrane. Many 4-stage systems are TFC systems because of the pre-carbon filter. If you see a 3 stage system chances are you are looking at a CTA system. Always read the label most membranes are marked TFC or CTA. You should replace a CTA membrane every year and a TFC membrane every 2 years this will ensure optimal performance. In some parts of the country you can get a longer interval for replacement but you will need a total dissolved solids meter to verify when it is time for replacement.

Check valve: A very important part in the RO system. The check valve stops water from running backwards through the membrane after the shut off cuts off the inlet pressure. A non-working check valve can ruin your membrane in less than 1 month if it is left unchanged. It is inexpensive insurance for your expensive membrane.

Flow control: Also very important to the survival of your membrane. A flow control regulates how much water passes across the surface of the membrane and then to drain. You will want to know how much water is running to drain of your new system so that you will know if it decreases you will have to replace it. You can do this with a small cup and wristwatch. Measure how much water runs into the cup from the drain line in 15 seconds and then multiply it by 4 to get 1 minutes worth. A typical 15-gallon per day residential RO system uses 100 to 150 milliliters of water per minute. Mark the cup and save it. When you service your system use the cup to check the flow to drain if the amount of water decreases or increases by any more than half the original amount then it is time for a new flow control.

Post carbon: This is a polishing filter that removes taste and orders. It usually contains smaller more fine carbon than the coarser pre carbon. Most post carbon filters are encapsulated or are in the form of a cartridge. What's inside the filter is simply coconut shells that are burned at 2500 degrees in a furnace. This gives the water a very pleasant taste and should be replaced every 6 months.

Encapsulated filter/membrane: This is a filter that comes sealed inside its own filter housing. The old filter is simply removed and thrown away. The new filter or membrane will not be handled or exposed to outside contamination so it is the best way to ensure a sanitary filter replacement.

Storage tank: This is where the purified water that the RO system produces is stored before use.  The reason most RO systems require a storage tank is to utilize the time when no water is needed to make water.  One draw back to this is that once the water passes through the purifier it has no more chlorine or chlorimine left in it to keep bacteria from colonizing in the storage tank.  This is why all manufacturers of RO systems recommend disinfecting the storage tank with bleach and then rinsing it to drain on a regular basis.  Follow the manufacturer recommendations. RO/CONN connector systems make this a simple operation.

I recommend sanitizing the RO system at least once a year. The best time to do this is while changing your filters. First, shut down the system in the same manner as with a Filter Change. Drain the system, including the tank, and then disconnect the blue tubing from the tank shutoff valve. Put ¼ to ½ teaspoon of household bleach into this tube, using an eyedropper for best results. Reconnect the blue tube to the shutoff valve and follow normal startup procedures, including draining the tank. If you have an icemaker hookup installed, be sure the ball valve in the line to the refrigerator is in the closed position during this procedure. The air pressure of the tank should be checked at this time with the tank empty. The standard 3.2 gallon RO tank will use 8 psi to displace all the water inside. Use a low pressure tire gauge to check pressure and if you need to add air use a bicycle tire pump. Check the storage tank pressure every time you change the membrane.

Ultra Violet: Called UV this is the best way to ensure that the pure water stored in the storage tank stays that way. In this application water comes from the storage tank and before it goes through your faucet it passes through a high intensity ultra violet light. This will kill all forms of bacteria that may or may not exist in the storage tank. Think of it as an insurance system. The UV lamp should be replaced once a year.

Shut off: All new residential systems are now required to have a full tank shut off. This shuts off the inlet pressure to the system when the storage tank becomes full. Older RO systems without this feature should have a full tank shut off installed so that the system does not continue to pass water to drain even though the storage tank has reached its capacity. This device alone will save thousands of gallons of water a year. They do not however last forever. Shut off's should be checked once a year to see if they are functioning this can be bone by turning off the storage tank valve and making sure that no concentrate water is running to drain. If you see water running to drain after 10 minutes in a steady stream you should then check your check valve to make sure that it is functioning. If your check valve is OK then you should replace your shut off.

Twist off filters or Bayonet filters: These are filters that can be removed more easily than conventional filters. By simply turning the filter with your hand it will release from the system. These replacement filters are always encapsulated witch make for more sanitary servicing and require no tools to replace the elements in a system.

Air gap faucet: This protects your entire system from potential contamination from a drain backup. The water running to the drain is vented at the faucet that comes with your system. A proper air gap is a minimum of one inch above the top of your sink. With this gap if your sink backs up and your system is shut off water will not run back up into the drain line of your system contaminating it.

Rejection of impurities: This is what your membrane is removing from your tap water. A new reverse osmosis system will remove about 95% to 99% of all TDS or total dissolved solids. As your membrane gets older its rejection of impurities will fall. Once it falls below 85 % it is time to replace the membrane.

TDS meters: TDS stands for total dissolved solids. The best way to check if your system is operating at its peak performance is to do a water quality check with a TDS meter. Test the tap water and then the purified water. Take note of the readings for example tap water containing 500 ppm of TDS. A typical new TFC RO system will get this number down to around between 5 or 9 ppm. Most of these meters give you a reading in parts per million or ppm of total dissolved solids witch is basically everything in the water that cannot be boiled out of it. A TDS meter can range in price from $45.00 all the way to $500.00. You should keep your TDS meter in good condition and check it against an absolute solution of 100 parts per million.

pH Test: This is especially a good thing to know before your installation. Knowing the feed pH will determine the type of membrane you should use. On high pH conditions that is any thing over 8.5 pH a thin film composite membrane or TFC should be used. If the water is heavily chlorinated and the pH is below 8.5 then a cellulose triacetate membrane may be used. Keep in mind that if the feed water has over 1000 ppm of total dissolved solids than a TFC should be used regardless. Keep your pH meter in good condition by checking it against an absolute solution of pH 7.

Hard water: The hardness of your tap water will greatly affect how long your RO systems membrane will last. If you have hard water than you should also consider using a water softener. However if your water exceeds 1500 ppm of TDS than this will cause your water softener to fail. And you will have to factor the cost.

Reverse osmosis systems are a great way to deal with questionable tap water quality or bad taste that cannot be handled with simple carbon filtration. One must remember that service to these systems is imperative to how long they will last. Following the manufacturer recommendations on filter replacements is the best way to ensure the system you are using is doing what it is suppose to. If you live in an area that has problem water than trial and error will be your guide to keeping your system in optimum condition. You may experience better performance with different types of elements or membranes. Considerations must be made when choosing an RO system to install. Install your system with the idea that maybe someday you may have to get to it and service it. Keep up with the maintenance of your system, it can give you many years of service. The use of reverse osmosis systems around the world has steadily increased over the past 10 years. When this technology was first introduced the systems were clumsy and large much like the first computers were. Now the technology is so advanced some states are electing to process seawater with reverse osmosis to satisfy ever-increasing demands on traditional water sources. Tap water testing now is beginning to be tested into the range of parts per trillion for impurities. As the testing methods improve so must the treatment technologies. We all have a responsibility to keep pollution out of our constantly shrinking fresh water sources.

About the author, David F. Walling has been in the water treatment business for 12 years and has patented products in use with reverse osmosis, water treatment systems. He can be reached through his web site at or 1-800-617-1474, fax: 602-311-1122

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