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Filter Performance

  • If you have a filter or RO system in your home, you need to check the water it produces periodically to make sure it's working properly. The performance of RO systems and filters are measured by the amount of TDS Reduced by the filters and membranes. The reduction of TDS indicates the reduction of microorganisms and harmful non-solids such as chlorine and fluorine.
  • So be sure to check the quality of your water every month to make sure your filters or membranes are working well!

    Carbon Filters
  • Granular activated carbon filtration is the most common technology used in home filter systems. Unfortunately, these home systems are often poorly maintained. In many cases, filters are not cleaned properly, or filter elements are not changed at appropriate intervals. Over time, effectiveness declines, and in some cases the contaminants in the overloaded filter actually begin to discharge back into the water.

    RO Percentage Rejection
  • The effectiveness of an RO unit is characterized by the rejection rate or rejection percentage.
  • The rejection rate is the percent of a contaminant that does not move through, or is rejected by the membrane. Rejection rates need to be high enough to reduce the contaminant level in the untreated water to a safe level.
  • To determine the needed rejection rate, it is necessary to consider the initial concentration. For example, when the feed water contains 300 ppm total dissolved solids (TDS), the product water may have 15 to 30 ppm (95% and 90% rejection ratio respectively).
  • An RO system design is based on a certain range of feed water TDS, the percentage of rejection and percentage of recovery desired. For a given system, the higher the percentage of recovery or the lower the percentage of rejection, the poorer the quality of product water becomes. (US FDA)


With a TDS monitor or meter, you can easily determine your RO system's performance by calculating the percent rejection. Once you have obtained the TDS levels of your feed water (IN) and product water (OUT), use the calculator below to determine the percent rejection of your RO system.
Enter the TDS level of the product water
Enter the TDS level of the feed water

Percent Rejection = %


RO Monitors and Controllers: Pros and Cons

Read the article from the November 2006 issue of Water Technology Magazine, by HM Digital's Director of Sales and Marketing, Rob Samborn

Is low TDS water beneficial to the human body?

Read the Water Quality Association's Science Advisory Committee's report on "Consumption of Low TDS Water"


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