The water filter installed in the kitchen advertises to work by the method of "reverse osmosis".

Can someone explain this concept without using technical terms or marketing mumbo jumbo?

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    The simple answer is that it uses pressure to push water from the side with high concentration of contaminants through a filter to an area with low concentrations. Osmosis works the other way around. – SDsolar Jan 10 '17 at 14:49

A RO filter filters water by pushing water it through a semi-permeable membrane (think of it as a piece of plastic that can let water (but not much else) slowly ooze through).

Normally, if you separate dirty water and clean water with such a membrane, natural forces will cause the clean water to slowly move over to the dirty side. This process is called Osmosis. (this is an oversimplification, not all impurities cause osmotic pressure).

In a Reverse Osmosis filter, a pump is used to pressurize the water on the dirty side, which forces the water to go the opposite direction...from the dirty side to the clean side. i.e. it's Reversing the Osmosis. This clean water is then collected and that's what you drink.

Eventually the water on the dirty side becomes so full of impurities that it's hard to push any more water through the membrane, so that water is drained away and replaced with fresh water. This is why RO filters use so much water -- they waste 4 to 20 gallons of water for each gallon of purified water they produce.

Most RO filters also have a conventional pre-filter (or multiple filters) to remove a lot of the contaminants before they reach the RO membrane so the RO membrane is only removing the dissolved impurities that are hard to remove with a filter.

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    I've often thought that a well-designed RO system would have a big holding tank for the "waste" water, and that would be plumbed to supply the toilets/water-closets with flushing water - plenty clean enough for that job. Never seen it, though - all small-system isolated designs. – Ecnerwal Jan 10 '17 at 2:46
  • @Ecnerwal - not all RO systems waste enough water to make salvaging like that worthwhile. In my own experience it is possible to achieve ratios of 1 part permeate product to 2 parts waste (softened and pre-filtered water). – user39367 Jan 10 '17 at 3:30
  • "Normally, if you separate dirty water and clean water with such a membrane, natural forces will cause the clean water to slowly move over to the dirty side." Why so? There isn't a way to separate the dirty and clean water, without pressurizing the water? – Fil Jan 12 '17 at 9:36
  • @Fil - that's hard to explain without the scientific jargon you're trying to avoid, though a good start to be to read the Wikipedia page I linked to. That question is probably not a good fit for this DIY site, it would probably be better answered on chemistry.stackexchange.com, though the question has already been asked – Johnny Jan 12 '17 at 16:35

"Reverse Osmosis" water is really just highly filtered water.

The filter membrane has such tiny holes that most dissolved molecules like salt, heavy metals, and other contaminants cannot pass through. It should be noted that not all contaminants are removed effectively by reverse osmosis.

Many under-the-counter RO systems use the water utility's line pressure to force the water through the membrane. Other systems have auxiliary pumps. I have had good luck with a booster device called a "permeate pump."

The systems require heavy maintenance compared to most consumer plumbing:

  • Pre-filter changes are every 6 months or so.
  • Membrane changes are every year or two.
  • Tubing and cartridges must be fitted and inspected carefully so unexpected flooding does not occur.
  • Quick connects are convenient but the seals can develop leaks.

Maintenance must be kept up or the units can waste a lot of water or produce poor quality water. Because service is messy and requires working under the sink, it is one of my least favorite jobs in my house.

Here is a functional diagram:

enter image description here

Bonus question and answer:

  1. Is an under-the-sink RO system really necessary?

  2. Usually not. In my case our local water utility had issues with E. coli contamination and my system also includes UV sterilization. Also the supplied water is very hard and I use a whole house water softener. Softened water does not taste great. I set up the RO system with a good finishing filter so the water would taste better than the tap water and now I cannot go back.

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  • Not true @JimStewart when incoming water pressure is great enough there is no pump. If you've been to a home or remodeling show anywhere in the country you will see several companies hawking RO systems for the kitchen, whether needed or not. To determine if it's RO system or not the better indicator is whether there is a waste water connection from the filter. – Tyson Jan 10 '17 at 14:17
  • You can have the RO system installed in the basement close to the floor drain and run 1/4" flexible tubing to the faucet. It makes the maintenance much easier. And it won't cause flood when it leaks. – cuteCAT Jan 10 '17 at 15:21
  • @Sherwood. Nice idea. Sadly, no basement and no floor drain in my case. – user39367 Jan 10 '17 at 15:23
  • @Tyson and chris I removed my incorrect comment. As you pointed out part of it was wrong (and I'm glad to be corrected). When I reread it I saw another part made no sense. Yikes! However as far as arsenic removal it does appear that there are other options than RO. However, I defer to your knowledge: public.health.oregon.gov/HealthyEnvironments/DrinkingWater/… – Jim Stewart Jan 10 '17 at 17:39
  • @Jim Stewart - I revised my answer to include that not all contaminants are removed effectively by RO (one notable example being As III). – user39367 Jan 11 '17 at 6:44

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