Reverse Osmosis is not Filtration


Learning Center | Filtration | Oct 11, 2017

ro filter We regularly receive questions about whether a reverse osmosis (RO) system can eliminate a specific contaminant or not. 99% of the time, the root of the question is a misunderstanding of how a RO system differs from a filter. The best way we’ve been able to differentiate the two is to think of a filter like a sponge and RO like a wall.

Filters in one form or another capture the toxins suspended in the water as it passes through the filter media. Sometimes it’s an absorption, like with carbon where the carbon holds onto the chlorine until it runs out of space. In others it’s more like a net where particles are trapped and not allowed through. With sediment filters or carbon blocks, the smaller the micron rating, the smaller the particles it can capture. At least to a point, when the rating gets too small or if there are higher levels of contaminants in the source water than expected, the filters can clog up very quickly.

This tendency to clog is a problem because in order to filter many of the toxins we need to eliminate, the micron rating would have to be so small the filter wouldn’t work for more than a few days.

Which is why you need RO.

All ROs are built around the membrane. It’s what does the heavy lifting and really cleans the water. But it’s fragile, so to keep it in good working order, it needs to be protected. Which is why all ROs also have filters as a pre-membrane stage. In our case with the MAXX, we use a 10 micron sediment cartridge first to get the largest particles out. If not, these could quickly plug up the membrane. From there the water hits a very high-end wood carbon to take care of chloramines and chlorine. Chloramines are particularly important because if they are not eliminated, the membranes can essentially be eaten away and become ineffective.

It’s at that point the water encounters the membrane and it’s .0001 micron rating. Depending on the filter rating, that’s an improvement factor of at least 100,000x. Think of them like a wall that only lets the cleanest water molecules through, not the junk hanging on to them. Because the membranes rely on an osmotic process instead of filtration, that junk we don’t want to drink heads down the drain tube instead of piling up like a filter would. You still need to change the membrane every few years, but it’s a much longer timeline than a filter would provide, even at 100,000x better performance.

If you’re more visual, here’s a spectrum to illustrate the vast differences between what a RO eliminates and your typical filter. {click image for a larger view}


ro particle


Notice the size differences. It’s enormous. This is why it’s important to remineralize RO water. It strips nearly everything out, including any beneficial minerals. The membrane is highly effective and unless you are getting a cheapy one from China, they are all made by reputable companies (DOW, Filmtec, etc) and work nearly identically. A membrane is a membrane.

Variations in brand names or even stages of filtration are fairly miniscule when it comes to performance. The only features that have significant impact on the overall rating of a RO system are permeate pumps to quickly the production time and lessen waste and the quality of post-mineral cartridges.