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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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.
"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:
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:
Bonus question and answer:
Is an under-the-sink RO system really necessary?
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.