October 2021

· 7 min read

Learn How Reverse Osmosis Works Straight From the Experts

Looking for a water filter? Our expert gives you the lowdown on what makes reverse osmosis systems unique.
Woman filling glass with water from a faucet - Learn How Reverse Osmosis Works Straight From The Experts | NSF International

If there is one silver lining to the COVID-19 pandemic, it’s that we have all learned not to take anything for granted, especially our health. And we’ve learned that simple things are vital to our well-being, like drinking enough water. Many of us have added or considered adding water filters to our kitchens to filter out harmful contaminants like lead, chlorine and pesticides.

But how do you know if you need a water filtration system? What’s the best kind for you, your family and your lifestyle?

At NSF, we’re safety champions about all things water-related, and I often get questions from consumers who want to do their homework before buying a water filter. One thing I’m frequently asked is, “What’s the difference between a reverse osmosis system and a water filter system?” It sounds complicated. That’s why I turned to Rick Andrew, NSF International’s water treatment expert, to explain the differences between these two powerful and advanced treatment systems.

Both reverse osmosis (commonly called RO) and filtration systems remove specific contaminants in your water. Both use a treatment process that separates impurities from the water, although they use different ways to achieve this.

In a filtration system, the home’s water passes through a filter. This filter attracts, traps and retains contaminants, pulling some out of the water like a magnet and binding them together. After passing through the filtration system, your water has fewer contaminants because they are kept inside the filter.

Unlike a filtration system that only uses a filter to attract and retain contaminants, a reverse osmosis (RO) system uses a semi-permeable membrane to remove impurities.

The reverse pressure used with an RO system forces water through a semi-permeable membrane. Specific contaminants can’t pass through the membrane and are, ultimately, washed away through a secondary stream of water known as the “reject” stream. Most RO systems also incorporate one or more additional filters on either side of the membrane.

Because a semi-permeable membrane can’t distinguish between a contaminant and a mineral, an RO system will reject the minerals along with the contaminants. Some RO systems have remineralization where these minerals are added back to the water after treatment. The potassium, calcium, sodium and other minerals add flavor to the water. It’s a matter of personal health and preference whether you need these minerals in your drinking water. For information on your health and minerals in the water, you should contact your health care provider.

A traditional RO system lives under the sink with a storage tank. However, you can also consider a tankless RO system or a countertop RO system. An RO system piped to the cold water line comes with its own faucet. You can often also choose the faucet’s color or finish, such as brushed nickel or stainless.

Rick offers this caveat: No matter which system you select, it is no good unless you frequently change your filter according to the manufacturer’s directions.

Unlike a filtration system that only uses a filter to attract and retain contaminants, a reverse osmosis (RO) system uses a semi-permeable membrane to remove impurities.

The reverse pressure used with an RO system forces water through a semi-permeable membrane. Specific contaminants can’t pass through the membrane and are, ultimately, washed away through a secondary stream of water known as the “reject” stream. Most RO systems also incorporate one or more additional filters on either side of the membrane.

Because a semi-permeable membrane can’t distinguish between a contaminant and a mineral, an RO system will reject the minerals along with the contaminants. Some RO systems have remineralization where these minerals are added back to the water after treatment. The potassium, calcium, sodium and other minerals add flavor to the water. It’s a matter of personal health and preference whether you need these minerals in your drinking water. For information on your health and minerals in the water, you should contact your health care provider.

A traditional RO system lives under the sink with a storage tank. However, you can also consider a tankless RO system or a countertop RO system. An RO system piped to the cold water line comes with its own faucet. You can often also choose the faucet’s color or finish, such as brushed nickel or stainless.

Rick offers this caveat: No matter which system you select, it is no good unless you frequently change your filter according to the manufacturer’s directions.

Definitions

  • Osmosis: the spontaneous passage or diffusion of water through a semi-permeable membrane
  • Semi-permeable membrane: a type of biological or synthetic polymeric membrane that will allow specific molecules or ions to pass through it by osmosis
  • Remineralization: the addition of minerals such as potassium, calcium, magnesium and sodium to water after it is treated or filtered
  • Filtration: the process of separating dissolved or suspended substances from water using various types of filtration media

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