Osmotic Pressure is a kind of fluid pressure that develops when solutions of different strength are separated by a semi-permeable mernbrane-such as the membranes inside our bodies and surrounding all of our cells. [Interestingly, osmotic pressure is one of the few phenomena that depends only on the total number of particles (including atoms, ions and molecules) in solution, and not their size, or type, or chemistry. A tiny hydrogen ion exerts the same effect as a huge protein molecule or microscopic particle of scale. The only other phenomena with this characteristic are freezing point, boiling point, and vapor pressure.] A semi-permeable membrane may be of the osmosis type (which allows only water to pass through) or the dialysis type (which allows only dissolved ions to pass through), or it may actually be a living membrane able to choose specific materials. All are based on diffusion, which is a fundamental physical process in which atoms and molecules that are concentrated in one place tend to spread out and equalize the concentration-like a drop of perfume permeating a room. The “drive” to equalize concentrations is fundamental in nature, and this is the “force” behind osmotic pressure.
When solutions of different strength are separated by an osmotic membrane, the system “attempts” to equalize the concentrations, but the only mechanism permitted is to allow water to move one way or the other (that’s the definition of an osmotic membrane). The only way movement of water can equalize the concentrations is by moving through the membrane if) the direction of the more concentrated solution, as if in an attempt to dilute it. (If water went the other way the concentrated side would only get more concentrated.) Thus, to look at it another way, concentrated solutions have the ability to draw water across membranes toward and into them. A concentrated solution exerts a real force upon a dilute one if they’re connected by a semi-permeable membrane, and the magnitude of the force is -a direct function of the TDS. Therefore, if we drink something that is more concentrated than our bodily fluids, like sea water, it draws moisture out of our tissues and into the intestines, causing diarrhea and making us more dehydrated than before. The only way to reverse the process is to apply pressure to the concentrated side – a pressure greater than the osmotic pressure the solution already has – and that is reverse osmosis. Water with a TDS of 2000 ppm exerts an osmotic pressure of about 20 psi, and that is therefore the usual limit of water quality for RO systems that use only the 40-60 psi line pressure provided by the water works. Water with more than 2000 ppm TDS requires pumps and high-pressure housings for RO to be cost-effective.
The ion exchange process for reducing TDS is called demineralization. It requires two types of ion exchange media: a cationic resin or zeolite in “hydrogen form” (regenerated with acid, so that the exchange sites are loaded with H+ ions); and an anionic resin in “hydroxide form” (regenerated with strong base, so that the exchange sites are loaded with OH- ions). Put them together and they exchange everything with an electrical charge for either H+ or OH-, which then combine to form water. However, molecules that are not ionized will not be removed. Examples: organic solvents classed as volatile organic chemicals (VOCs) such as trichloroethylene and benzene, most insecticides and herbicides, most of the organic taste & odor compounds, and a host of other organic chemicals both natural and synthetic. Demineralization leaves in a lot that both reverse osmosis and distillation can remove.