A Reverse Osmosis Membrane Is Not A Filter

Reverse osmosis (RO) membranes are often confused with filters, but RO membranes do not filter. They do not trap unwanted material and load up with it until becoming full thus needing replacing. Rather RO membranes separate the unwanted material from the good material and channel them separate ways. The good material or product water, will either be held in a pressurized storage vessel until needed or in some cases directed straight out of the RO faucet. The unwanted or rejected material will be channeled down a drain. This reject water is sometimes referred to as brine. It is the stuff in your tap water that you do not want to drink if you can help it.

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Reverse osmosis systems all make use of some type of filtration for the most part, to protect the membrane. A sediment pre-filter of 5-microns is commonly used, and because RO membranes are susceptible to deterioration from chlorine, a carbon impregnated sediment pre-filter can be used. Some other RO systems use a separate carbon filter cartridge to pre-treat the water before it reaches the membrane. Once the reject water is separated from the product water, the product water might have a very bland or even plastic taste to it. That is why most RO systems make use of a carbon block filter to refresh the taste of the water before it goes to the storage tank or out the faucet. So while pre and post filtration and filters are important to the proper function of the reverse osmosis system as a whole, the membrane is not a filter.

Because the goal of the sediment and carbon filters is to protect the membrane, it is important to change these filters on a regular basis. The owner’s manual for your RO will give suggested time periods for switching the sediment, pre and post carbon filters. The inline carbon block filter is usually changed when the membrane is switched out but can be changed more frequently if desired. The carbon block cartridge is more of a “personal preference” filter anyway. If you keep up with the suggested filter replacement intervals, you should get 3 to 4 years out of the life of your RO membrane, which is good because it is the most expensive part of your RO system. An alternate method to monitoring your reverse osmosis system’s performance is to measure the input and output water with a TDS meter. Once your output water reaches 30 parts-per-million or milligrams-per-liter (ppm mg/L), you will know that one, or in some cases both, of your pre-filters needs attention. It should also be pointed out that the input water requirement for most reverse osmosis systems is a water hardness of less than 10 grains-per-gallon (GPG) or around 200 mg/L.

With these bits of knowledge you can not only rest assured that you are consuming the healthiest and best tasting water available for you and your family, but you can also keep the costs down by greatly extending the life of your reverse osmosis system’s most expensive component, the RO membrane.