From Wikipedia:
“Osmosis is the net movement of solvent molecules through a partially permeable membrane into a region of higher solute concentration, in order to equalize the solute concentrations on the two sides.” (Osmosis - Wikipedia)
I argue that “reverse osmosis” is a marketing term that is tantamount to calling jumping in the air “anti-gravity”. My friend argues that it is valid. I counter that he is one of the few people that actually do use only one tenth of his brain capacity and that “reverse osmosis” is really what we we used to call “filtering”.
“However, there are key differences between reverse osmosis and filtration. The predominant removal mechanism in membrane filtration is straining, or size exclusion, so the process can theoretically achieve perfect exclusion of particles regardless of operational parameters such as influent pressure and concentration. Reverse osmosis, however, involves a diffusive mechanism so that separation efficiency is dependent on solute concentration, pressure, and water flux rate.”
(http://en.wikipedia.org/wiki/Reverse_osmosis)
Ok, so… the difference is one uses a semi-permeable membrane and the other does not?
Searches of “reverse osmosis” vs filtration mostly return debates between which one makes water taste better
Is there a tangible difference between “reverse osmosis” and “filtering”?
(A search for “reverse osmosis” returned only one link at SD related to desalinization.)
The most effective method of obtaining high quality, fresh tasting drinking water is multi-stage filtration. These types of purifiers are equipped with two filters: One filters out harmful substances, such as chlorine, out of drinking water, and the other restores proper pH balances to water.
Multi-stage filtration systems, as opposed to residential reverse osmosis systems, also usually feature ion exchange systems. It balances mineral content in the filtered water supply, by removing unneeded calcium and magnesium, while adding sodium and potassium ions, via the resin bed.
Reverse osmosis residential systems simply force a lot of water through a membrane, wasting a lot of excess water. It takes out too many good minerals, adds nothing good, and still let’s things like pesticides through.
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I’m more familiar with these techniques in the laboratory, where the ion-exchange materials Philster describes are useless. In producing purified water, ion-exchange resins which replace dissolved ions with hydrogen or hydroxyl ions are used. Organics are usually removed before this, with carbon filters.
Getting back to RO, this technique is used for smaller water systems, and though it has its similarities to filtering, it’s really not the same. Attempting to filter water through a membrane would quickly block the membrane; RO allows the water to diffuse across the membrane, “seeking” to equalize the pressure, while dissolved ions concentrate in the pre-membrane water. When the membrane does get fouled, it’s by particulates or emulsified contaminants, not by the ions that the membrane is designed to remove.
Yes. An osmosis system can do work. If you take a membrane, put water on one side, and salt solution on the other the water will flow from the water to the saline side with no pressure. This is normal (ie forward) osmosis. Indeed if you put a cylinder and piston around the saline side the flow of water will force the piston out, and you can recover energy from the system. You can measure the pressure exerted, and discover that it is proportional to the molarity of the solution. A filter will not do this. No flow will occur with a filter unless you exert external pressure.
Next, if you apply pressure to the saline side of an osmosis system you can exert enough pressure that the flow slows and then stops. If you do this with a filter, the flow will simply reverse instantly. If you exert more pressure than needed to stop the flow, the saline side will start to flow pure water into the water side. You have reversed the osmotic flow. And guess what - you have reverse osmosis system.