What Doesn’t Reverse Osmosis Remove?

Reverse osmosis filters are often touted as one of the best types of filters on the market. From promising great tasting water to impurity removal, you’d be forgiven for thinking there wasn’t much an RO filter couldn’t do.

And while reverse osmosis filters are a very effective method for filtering many kinds of contaminants and impurities from water, they have situations where they shine, and others where different types of filtration might provide better overall water results.

How Does Reverse Osmosis Work?

To understand what is not removed by reverse osmosis, and the general benefits and limitations of an RO system, it’s important to understand how this kind of filtration system works.

One of the advantages of RO is that they work at the molecular level — meaning they include a filter membrane that’s so small only water molecules can pass through it. This process generally means you end up with some of the cleanest, purest water available. However, this does not always make it a fool-proof water treatment solution.

What is Not Removed by Reverse Osmosis?

Because there are some contaminants that are molecularly smaller than water, Reverse Osmosis isn’t always the silver bullet many people expect when it comes to providing water that’s completely free of impurities. For example, some common contaminants that can slip through the average RO filter are:

  • Pesticides
  • Herbicides
  • Many other agricultural treatment products like fungicides
  • Some dissolved gasses, like hydrogen sulfide
  • Certain organic compounds
  • Chlorine — RO can remove various quantities of chlorine, but there is a possibility that the average home RO filter may not have the capacity to capture all the chlorine present in water, though this will largely depend on the chemical’s concentrations in the water supply.

Does Reverse Osmosis Remove Bacteria?

Reverse Osmosis technology is effective at filtering out most forms of  bacteria. As with all filtration systems however, in order for RO to effectively remove bacteria, the filter must be replaced regularly and installed correctly.

Does Reverse Osmosis Remove Fluoride?

Because fluoride molecules are larger than water molecules, reverse osmosis will filter out fluoride effectively. Again, this effectiveness still depends on proper care and maintenance of the RO filter.

Benefits and Limitations of RO Filtration

Reverse Osmosis water treatment is a great first step in residential home water treatment. And, for general municipal water supplies, RO can filter out the most concerning contaminants like lead, arsenic, and other heavy metals, as well as troublesome dissolved hard minerals.

It’s important to note however, that due to the nature of the treatment (water being forced through multiple membrane layers in a multi-step process) RO treatment can be somewhat time consuming. For large homes or businesses, this may not be a reliable or dependable solution for the long term. Capacity is one of the important limitations to look at when considering RO filtration for home or office.

In all cases, to get the best possible solution for your water treatment needs, it’s important to start with a water test to determine what’s in your water. For reverse osmosis systems especially, it’s important to understand not only what is your water (iron, calcium, chlorine, etc.) but how much of those contaminants may be present. Once you know what kinds of contaminants are a problem for your specific home, and their average levels in your water supply, you’ll be in a better position to determine the right water filter for you.


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