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More Stages Equals Better Filtration? The Myth of Reverse Osmosis Filtration

  • More Stages equals better filtration? The myth of reverse osmosis filtration

    The number of stages in a reverse osmosis filtration is becoming an industry jargon in proving the superiority of a filtration, or quality of the water it produces. However, is this really true? Or just another marketing gimmick for companies to sell you more features or add-on that you don’t need. We are here to give you an honest answer.

  • The Function in Each Stage of the Reverse Osmosis Filtration

    A standard RO system is equipped with 3 types of filtration. A sediment filtration, a carbon filtration, and a reverse osmosis filtration. Each filtration plays an important role of their own and also complement one another to achieve the best water filtration possible.

    Sediment filtration removes dust, dirt, particles, and rust in the water. They are normally placed in the first stage of filtration to help remove or trap bigger particles so later stages can work more efficiently in trapping other contaminants that sediment filtration cannot.

    Carbon filtration comes next after the sediment filtration. During this stage of filtration, tastes, odors, cloudiness, colors and various chemicals such as chlorine are absorbed by the carbon before water entering the next stage. Carbon filtration is the most widely used water filtration on the market as they are very effective in removing chlorine in the water and give end user that clean crisp taste. However, they are unable to remove contaminants of smaller molecules size or harmful contaminants such as heavy metals and arsenic.

    Reverse Osmosis is the final stage of filtration process and it is also the core of the entire system. In this stage, the water is pushed through a semipermeable membrane that has a rated pore size of 0.0001 microns. Up to 99% of all impurity such as lead, arsenic, and nitrates are removed from the water.

  • Why Are We Adding More Stages After Reverse Osmosis?

    Water after reverse osmosis is sometimes considered as flavorless water which not a lot of people are accustomed to. By adding a post-taste refining coconut shell carbon filter, it helps remove the residual taste and also add a bit of sweetness back into the water. Some reverse osmosis system is even equipped with remineralization filter to help get that mineral taste back.

  • How Many Stages Are Needed for Reverse Osmosis Filtration?

    Honestly, just reverse osmosis stage itself is needed to remove all the water impurity. However, as mentioned earlier, the stages before it are to help remove all the bigger contaminants so reverse osmosis itself can work more efficiently and increase its longevity. Stages after are to help remove the residual taste of reverse osmosis. The ideal setup in terms of cost and performance perspective is 4 stages, with a sediment filtration stage, a carbon filtration stage, a reverse osmosis stage and the post taste refining stage.

  • Are 5 stages really better than 4?

    Some of the more popular reverse osmosis systems on the market are 5 stages. However, is 5 stages really better than 4? The honest answer is no. Many of the 5 stage RO systems are equipped with two carbon filtration stage before the reverse osmosis. This ideally is to help remove even more contaminants before the water reaches the membrane and help reduce the burden on reverse osmosis.

    However, if we really dive down to the spec of the system, we will notice most of the system offers either a GAC (Granular Active Carbon) filter on the 2nd stage and a carbon block filter in the 3rd stage, or 2 of the same carbon block on 2nd and 3rd stages. GAC carbon is much cheaper and less effective alternatives to a carbon block. The water is pushed through a filter with suspended carbon fines in it, which some of the contaminants do get trapped within the carbon pores, but most just bypass the filter without ever touching the carbon itself. Hence, a need for 3rd stage carbon block filter to really trapped the contaminants.

    Systems with 2 carbon block setup on the 2nd and 3rd stage may be more effective compared to systems with 1 GAC and 1 carbon block in terms of the amount of contaminant removal. However, they encounter the issue with reducing the water pressure to the membrane. The decrease of water pressure to the membrane, the less effective the membrane is going to perform and results in fewer pure water produced. This type of setup sometime might hurt the performance of the system more than it helps.

  • So there’s really no added value in adding more stages of filtration?

    This really depends on what type of filtration we are talking about. If we are adding a UV (Ultra Violet) filtration to help kill and prevent bacteria growth in water that is not pre-treated with chlorination. This is most likely the case for people who live in rural areas or who uses ground or well water as their main sources. Most people who live in the city would not have to worry about bacteria growth as chlorine or chloramine is added to the water to prevent bacteria growth.

  • Conclusion:

    At the end of the day, there’s really no added benefit in offering 2 stages of carbon filters if they are only using the same type of filtration as the one before.