OK, I did a search for reverse osmosis, and came up with this thread and some other threads that aren’t directly related.
My question is: What is the difference between reverse osmosis and filtration?
And doesn’t filtration also use pressure (gravity or a pump) to force water through a filter (can’t that be considered a membrane?)?
I’d suggest it’s a question of scales. Typically, filtration involves the separation of solid from liquid by means of some discriminating porous surface whilst osmosis (normal or reverse) involves movement of solvent across a membrane which discriminates against any (or almost any) solute. Both could be considered filtration of a sort, although with osmosis, particular size is not necessarily the key to movement across the membrane as hydrophilicity/hydrophobicity also play an important part, particularly in vivo.
Reverse osmosis works by countering natural osmosis. Filtration just catches stuff. More technically speaking…
Reverse Osmosis (RO) is used to reduce the concentration of dissolved solids, both organic and inorganic, by use of a semi-permeable membrane under hydrostatic pressure. The primary objective of the RO process is to remove ionic species. Osmosis is the process where the solvent (i.e., water) flows from a high concentration solution through a semipermeable membrane to a low concentration solution (thereby concentrating the contaminants). The RO process separates ions from water by opposing the natural osmotic movement through the use of an applied pressure that is greater than the osmotic pressure.
Filtration of suspended solids is a standard pretreatment process to remove particles which would clog subsequent treatment systems and shorten their useful life. The most common method of filtration is through a sand or multimedia filter. Granular media filtration is typically used for treating aqueous waste streams. The filter media consists of a bed of granular particles. The bed is contained within a basin and is supported by an underdrain system which allows the filtered liquid to be drawn off while retaining the filter media in place. As water laden with suspended solids passes through the bed of the filter medium, the particles become entrapped on top of, and within, the bed. This either reduces the filtration rate at a constant pressure or increases the amount of pressure needed to force the water through the filter. In order to prevent plugging, the filter is occasionally backwashed at a high velocity to dislodge the particles.
“Osmosis is the process where the solvent (i.e., water) flows from a high concentration solution through a semipermeable membrane to a low concentration solution (thereby concentrating the contaminants).”
Phobos, you got this backwards. Osmosis is the movement of solution from an area of low concentration across a membrane to an area of high concentration.
I kind of agree with wevets. RO is basically filtering with a superfine membrane.
John
I can’t add any real worthwhile info to this thread, but I can say this. Our house had a Reverse Osmosis system installed in it when we bought it. After I did the maintence in it (if memory serves, I replaced 2 filters and the membrane) I was the proud owner of the freshest, best tasting water I have ever had. I have drank out of several regular “filter” systems and man, I am not going back after having this system. Next house we buy gets one too.
Eric
PS - Does anyone know if RO systems remove flouride too?
IANAD, but it’s my understanding that drinking undiluted RO water is bad for you: most of the minerals have been removed from it and so it osmotically removes minerals from your body as it passes through. This is what I was told when I installed an RO system for my aquariums, but I never verified it. Does anyone know if this is true?
RO is used primarily to make sea water potable.
Yes, but is it a one-step process? Are some of the minerals replenished? Or am I mistaken about how much the mineral content of water is reduced in the RO process? I know that Chicago’s extremely “hard” (=high mineral content) water was much “softened” (=reduced mineral content) by RO treatment. Exactly how much was it softened? How does it compare to distilled water?
Most drinking water is processed using RO systems. The machines out in front of supermarkets are RO systems hooked up to ordinary tap water. Drinking RO water is fine for you, but I think DI (distilled & deionized)water could be harmful. Many cities locally are installing large scale RO plants to meet federal water guidelines for mineral levels. There are different type of RO membranes, and if the process is designed for drinking water then it is unlikely it will be pure enough to be harmful.
Back to the OP. There are lots of different types of membrane filtration systems. Micro-, Ultra-, RO-, Nano-, and different types of dialysis, just for a start. The diferences have to do with the membranes, the desired particle size, shape, polarity, etc. to be removed, the needed pressures, and a bunch of other stuff that I don’t have a solid grasp of. Current classification has RO systems removing partilces down to between 1 and 10 Angstroms, Ultrafiltration does 10 and 200 Angstroms, Micro- does 200 to 100,000 A, etc.
As I understand it, the variety of membranes now available allow for all sort of different processes to take place. The prices of the systems are becoming more attractive, and the technology is improving significantly. Whole home RO systems should be available within a few years to replace ion exchange (salt based) water softeners.
You sure? Why would you move the water (the thing you’re trying to clean) to a more contaminated place?
Quoth the Water Quality and Treatment Handbook - 4th edition (by the American Water Works Association)…
This paragraph does not specifically point out what’s on the other side of the membrane (where the water goes), but my question still stands…the purpose is to produce cleaner water.
Except for places like Saudi Arabia, most drinking water from the tap is not RO’d (most drinking water treatment facilities use coagulation/sedimentaion and filtration). RO is too expensive for large volumes. But you’re right that a lot of non-spring bottled water is RO’d.
Most US cities don’t use RO, but filtration to purify their water. However, I don’t know about it being too expensive. The town of Mt. Pleasant, SC, with a population of over 50,000 uses RO. I really don’t know the expense issue.
I think I understand the somewhat fine, but clear distinction now.
Thanks folks!
[QUOTE]
*Originally posted by Phobos *
**
You sure? Why would you move the water (the thing you’re trying to clean) to a more contaminated place?
Quoth the Water Quality and Treatment Handbook - 4th edition (by the American Water Works Association)…
Osmosis IS the movement of the solvent from a low concentration solution (which can be pure solvent, e.g. water) to a high concentration solution through a semipermeable membrane.
This is analagous to diffusion - as far as the solvent is concerned, it is diffusing from a high concentration region of solvent (the low concentration solution) to a low concentration region of solvent (the high concentration solution).
That’s why you can’t use osmosis for purifying water - you use REVERSE osmosis. You have to artifically apply a back pressure which exceeds the osmotic pressure, as your quote describes.
Phobos - The RO systems I am seeing go in locally are the least expensive option available to them. Basically, if a city has a number of wells, and some of them are really mineral heavy, then they feed the most mineral heavy water through an RO system and blend it with the rest of the wells. The result is water with a low enough mineral content to meet federal drinking water guidelines. I agree that if they tried to run the entire city’s water supply through RO it would be wildly expensive, but when they just run a quarter of it or so through it works out better than other methods of removing minerals. I know of 4 different systems in construction locally, and the largest is designed for 6 million gallons per day, expanded in two years to 10. It should be on-line in about 4 months.
I also know they have done this successfully in other places around the SouthWest U.S., because the water district people spoke about visiting other large scale RO plants before agreeing to the design of theirs. As I understand it, improved manufacturing techniques have dropped the price and improved the durability of the membranes.