Why Do Reverse Osmosis Systems Have Pre-Filters?

On of the most common questions that I answer is why a reverse osmosis system has a sediment filter and carbon filters prior to the membrane.

The answer is that it is to protect the more expensive membrane. The membrane is typically made of cellulose acetate. This filter, though physically strong, does have some weaknesses.

The sediment filter is used to remove dirt and other particles which can coat the membrane, which will reduce the efficiency. Think of the sediment drain filter as an air conditioning filter. When you first put it in, it is clean and captures a lot of dirt. As the filter gets loaded, less waster can pass through due to plugging of the pores. This situation occurs at the membrane as well. What will happen is that the membrane becomes less efficient and will actually treat less water over time.

As for the carbon filters, most folks know that they will remove organic contaminants. The organic contaminants include hydrocarbons, chlorine complexes and other similar chemicals, which can have a detrimental effect to your health.

But here is the kicker – the chlorine that your water company puts into the system to kill bacteria and other organisms will destroy the membrane, making it more porous. Over time, the chlorine dissolves the membrane and makes the pores larger. Pretty soon, the membrane is not doing its job as a filter, which allows water to pass through. Next thing you know, the water you drink has increased levels of the contaminants that you do not want.

It is important to change out your sediment and carbon filters more frequently than what most manufacturers “suggest”. Just like a car air filter, you need to change it more than once a year if you drive your car on dusty roads to keep your fuel efficiency up. The same is true with your Reverse Osmosis system. Most public water companies start increasing the chlorine additions to the water to fight bacterial growth as the water temperature increases. I usually recommend that the sediment filters get changed every three months and the carbon filters every six months so that the Reverse Osmosis system is operating a most efficiently.

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