Filter or Purifier -- What's the difference?

A water filter is a device which removes impurities from water by means of a fine physical barrier and/or chemical processes. Filters are used to cleanse water for irrigation, drinking water, aquariums, and swimming pools.

Water filters can be bought at stores to filter drinking water. Major brands include Brita and PUR. These water filters remove substances that are harmful to your health, can increase the risk of developing cancer, and taste or smell bad.

The physics of filters is based on sieving, adsorption, ion exchanges and other processes. Unlike a sieve or screen, a filter can remove particles much smaller than the holes through which the water passes.

The term reverse osmosis comes from the process of osmosis, the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration if no external pressure is applied.

In simple terms, reverse osmosis is the process of pushing a solution through a filter that traps the solute on one side and allows the pure solvent to be obtained from the other side. More formally, it is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. The membrane here is semipermeable, meaning it allows the passage of solvent but not of solute.

The membranes used for reverse osmosis have no pores, the separation takes place in a dense polymer layer of only microscopic thickness. In most cases the membrane is designed to only allow water to pass through. The water goes into solution in the polymer of which the membrane is manufactured, and crosses it by diffusion. This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 5 MPa - 20 MPa (50 bar - 200 bar).

This process is best known for its use in desalination (removing the salt from sea water to get fresh water) and has been used in this way since the early 1970s. (Information provided by Wikipedia)

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