Reverse Osmosis vs. Selective Filtration: What's the Difference?

A crash course on filtration types

There are a few household things that we use every day that are semi-permeable, and selectively permeable. 

Real-world materials that are semi-permeable are egg whites and parchment paper. Semi-permeable membranes are a thick, sometimes viscous filter that sifts out particles down to a certain size micron. This process is commonly used in reverse osmosis filters. 

Real-world materials that are selectively permeable are the phospholipid plasma membrane, an outer layer surrounding every cell in our bodies; they’re more actively selective when it comes to choosing or targeting what passes through.

Both of these methods are used in water filtration systems, too. Let’s break down how they’ll give you clean, filtered water to match your needs.


Reverse osmosis (RO) is a process of separation that uses water pressure to force the source water through a semipermeable membrane that retains the solid contaminants on one side while allowing water to pass on to the other side.  ROvSelective_Templatebody2

In English? It’s like a mosquito net. It lets the water flow through the membrane while blocking out objects that are too big to pass through. That’s why this process is effective at removing anything bigger than a water molecule.

Want more reasons to drink up? Reverse osmosis is famous for its ability to reduce fluoride, lead, mercury, arsenic, and a multitude of other contaminants. 

So, what’s the downside? 

During the process, RO systems produce quite a bit of wastewater; a typical system has a ratio of about 4:1. That’s four gallons wasted for every gallon of clean water produced. 

There are “zero waste” systems – these tend to use electricity or power to force the water through the RO membrane or recycle wastewater back through the original process. This can put more wear-and-tear on both your filters and the RO membrane. But all that really means is that when you’re using an RO system, you’ll need to replace filters more often.

Because RO systems are effective at removing solids from your water, they also tend to demineralize your already-treated water. So, when you’re looking for a system, find one that uses reverse osmosis processes while also retaining the healthy minerals that your body needs like calcium, potassium and magnesium. NSF certified to remove 95% of fluoride plus bacteria and cysts from your water, the Aquasana Reverse Osmosis + Claryum Filter does just that. You’ll feel better, and your water will taste better, too.

In English? Reverse Osmosis is like a mosquito net. It lets the water flow through the membrane while blocking out objects that are too big to pass through.


Most filters on the market use carbon filtration technology to retain contaminants inside your filter, preventing them from flowing through your tap. The process of selective absorption (selectively permeable), acts like a magnet, allowing contaminants to collect on a large surface area where molecules exert force for other molecules to attract to. The longer the water stays in contact with the carbon media, the more contaminants can be absorbed from the water.

There are many carbon filters on the market that are selectively permeable, including:WHFilltration-Templatefeature

  • Activated carbon – reduces organic chemicals while the catalytic carbon targets chlorine and chloramines.
  • Ion exchange – filters out heavy metals like lead and mercury while capturing asbestos and chlorine-resistant cysts like cryptosporidium and giardia, making water safe to drink.

There are also methods like catalytic carbon, standard gravity pitchers and micron filters

There’s a positive and a negative to everything, however. As previously mentioned, the downside to Reverse Osmosis is that there is plenty of water wasted in the filtration process. For selectively permeable filters, the downside is that it’s not necessarily as powerful at removing contaminants as filtration systems that use semi-permeable filtration. So it really all just depends on what your contaminant removal needs are.


Some of the most harmful contaminants are the ones we can’t see, smell or taste. That’s where the NSF Certification comes in. 

As we’ve previously written, the NSF is a third-party, nonprofit organization that develops standards for and certifies goods including clean water, food and consumer products. Their goal is to help consumers identify which products are up to their standards, and which ones aren’t.

Before buying any water filter, find one that passes NSF/ANSI standards (including NSF #42 for aesthetic effects and NSF #53 for health effects).  

    • NSF Standard 42 – establishes a minimum requirement for drinking or under-counter systems to remove a total of three contaminants such as chlorine taste and odor, as well as particulates that may be present in drinking water. 
    • NSF Standard 53 – establishes a minimum requirement for drinking or under-counter systems to remove a total of 57 contaminants, including lead, mercury, cryptosporidium, giardia, volatile organic chemicals (VOCs), and other harmful contaminants from drinking water. 
  • Note: This certification does not mean that all these contaminants are tested – manufacturers can choose any of them to test to get this certification, so be sure your shopping list includes filters that sift out lead, mercury and any other elements for the healthiest drinking water. 

Looking for the NSF Certification on Aquasana’s Whole Home Systems? We’ve got you covered. We were the first on the market with an NSF Certification for systems designed to remove PFOA/PFOS so you can drink up, knowing your water is clean, healthy, and safe for the whole family.



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