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How Does an RO Filter Work?

Reverse Osmosis (RO) was developed over 40 years ago to purify sea water. As the capabilities of reverse osmosis to desalinize water were recognized, it soon became a favorite product for businesses and homes. Beginning in the 1970s, reverse osmosis systems became available to the public. Reverse osmosis produces mineral-free water and captures a variety of harmful contaminants through a series of filters. To understand how the filters work, first the process of reverse osmosis must be explained.

  1. Reverse Osmosis

    • Osmosis is the term given for the tendency of water to move from a strong saline solution into a weak saline solution, achieving a balance of salt in both solutions. Drinking seawater is an example of osmosis as the body pulls water from certain areas to dilute the salt in the stomach, causing dehydration. Reverse osmosis is the opposite of osmosis, meaning water is forced into a stronger solution from a weaker solution through a semi-permeable membrane. Salts cannot pass through the membrane because of their size, and the end result is desalinated water.

    Prefilters

    • The feed water, or tap water, coming into a reverse system is first sent into a prefilter stage. The prefilter is also known as a sediment filter and removes suspended solids, which are small solid particles in the water, such as soil. Prefilters are used to extend the life of the semipermeable membrane.

    Activated Carbon Filter

    • The activated carbon filter can be placed either before or after the semipermeable membrane. These filters consists of carbon in either powdered, granular or pellet form. Activated means the carbon is steamed at 1,800 degrees, causing the pore sizes to shrink. Water is passed through the carbon filter and pollutants are absorbed to the carbon. Activated carbon removes chemicals and organic compounds, such as chlorine, pesticides and some types of carcinogens. Furthermore, some types of activated carbon filters are certified to remove microorganisms and heavy metals.

    Semipermeable Membrane

    • A semipermeable membrane is a cell membrane that permits various molecules to pass through but not others. Membranes are a pliable sheet measured by pore size, or size of microscopic holes, and given a unit of microns. The size of the molecule is what dictates whether it can pass through the membrane or not. Reverse osmosis membranes are constructed out of cellulose acetate, polysulfonate or polyamide. The skin of the membrane is the active barrier that allows water to pass through and consists of .25 microns with a support layer of 100 microns. At .25 microns only water can pass through, leaving behind large salts and some contaminates.

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