What’s the Difference Between Water Purification and Water Filtration?

The other day a woman wrote me and said she wanted the purest water possible at every faucet in her house. “No you don’t”, I said.

What’s the difference between purified water vs filtered water? Words have certain technical meanings in the water treatment world but of course consumers don’t know this. And not knowing you can be asking for the wrong things when you start your search for a water treatment system for your home. This can cost you money. More importantly it can simply lead you astray as you waste time searching for the wrong thing.

Filtration, as such, involves the process or processes of separating suspended matter from a liquid. The filtration process is perhaps the most the most readily understood technique in water treatment….

As part of the hydrologic cycle filtration takes places as water seeps and percolates down through layers of the earth. By the time the water has reached the underground aquifer, it is free of any particulate and some of its absorbed gases.” Source: page 1 Water Processing, Third Edition, Wes McGowan, Water Quality Association.

Even though this definition sounds like it means simply removing sediment from water the term is now also used to mean removing chemicals, like chlorine, from water. Filtered water is generally thought of as water that has had solids and chemicals removed but retains its natural level of dissolved solids.

My Kitchen Defender and Urban Defender whole house filter are examples of filtration.

Purified water is water that has been mechanically filtered or processed to remove impurities and make it suitable for use. Distilled water has been the most common form of purified water, but, in recent years, water is more frequently purified by other processes including Capacitive deionization, reverse osmosis, carbon filtering, microfiltration, ultrafiltration, ultraviolet oxidation, or electrodeionization. Combinations of a number of these processes have come into use to produce water of such high purity that its trace contaminants are measured in parts per billion (ppb) or parts per trillion (ppt). Source: Wikipedia, Purified Water, August 1, 2015

When you look at these two definitions it’s hard to see a difference.  That difference is pretty subtle and a matter of degree. And to some extent I disagree with the inclusion of carbon filtering except as one component in a process.

Basically purified water refers to water with very little left in it. I think of purified water as having been subject to a process that reduces or removed dissolved solids. Other chemicals, such as pharmaceuticals, not normally removed by filtration may also be removed.

Purified water is not generally used in a whole house application because the water would be aggressive and tend to dissolve pipes and fixtures. Since water is the universal solvent when dissolved solids have been removed it will naturally seek to replace them.

My custom reverse osmosis system is an example of purification.

Or at least that’s how I think of the difference between purified water vs filtered water.


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About James McMahon

Studied ecology at the University of Illinois, mountain survival at Eastern Washing University, Deep Ecology at Naropa, River Ecology with The Nature Conservancy and Luna Leopold
This entry was posted in Reverse Osmosis, Sweetwater LLC, Water Purification, Well Water Purification, Whole House Water Filtration and Purification. Bookmark the permalink.

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