Reverse Osmosis another misnomer

[subhas mohapatra]

The term reverse osmosis is used widely in water purification industry. Although this has become a household word, it has absolutely no relation to Osmosis. Wikeepedia, the Internet encyclopedia, describes it as "opposite of Osmosis", which is scientifically wrong. I offer here some clarifications for readers who have an interest in this area, especially those who are interested in rural potable water supply. 1. Diffusion is a process where gaseous and liquid materials move from a higher gradient (concentration) to a lower gradient. For a solution the term is relative because it has a solvent and a solute.Thus the solvent (e.g. water) will move from its higher gradient to a lower gradient and the solute molecules will move from its higher gradient to lower gradient. When two solution of two different strengths are poured to gather without vigorous mixing, the phenomenon can be observed through unaided eye. In this case, water and solutes move in opposite directions because a Strong solution have higher gradient of solutes whereas a weak solution will have have higher gradient of water. 2. Osmosis is also a process of diffusion except the diffusion takes place across a porous membrane. In this instance, water molecules pass from higher gradient to the lower gradient as stated above. But for solutes, only those solute molecules that can  pass through the pores in the membrane will go from higher to lower gradients. This is called "selective diffusion", just like we filter using a cloth, paper towel  or even a column of sand particles. 3. Note that both diffusion and osmosis are natural processes without man or machine intervention. We can enhance the process of "filtration" by applying pressure on the side where the solvent gradient is higher. This is called high pressure "diffusion" or "filtration". The analytical tool called HPLC (High Pressure Liquid Chromatography or High Performance Liquid Chromatography) works through this principle. The amount of pressure required will depend not only the pore size on each "membrane" or "column", but also the number of layers of such membranes and columns. Because a selective membrane is used for filtration, it is equivalent to osmosis rather than simple diffusion. However, the term "reverse osmosis" would be right only if the solvent flows in the direction of higher gradient. But in reality, the solvent still flows from high gradient to low gradient, except at a much faster rate because of high pressure. Therefore the right term would be "High Pressure Membrane Filtration (HPMF)" or "facilitated filtration" or "active filtration". In science where additional energy is involved, we call it "active diffusion". In biological systems, where extra pressure is usually not available, enzymes carry the solutes across the membrane or they convert the solutes to positive and negative ions which then go across the membrane through the principle of "ion-exchange". In today's domestic water purification systems both "high pressure" and "ion-exchange" principles are used. The high pressure, however, comes from the height of the city water tank instead of a pump. The "ion-exchange" comes from various resins and carbon particles packet into the column.  ------------------------------------------- Did you know that soil-based agriculture is the main cause of world hunger and poverty, and soil-less agriculture is the cheapest, quickest, surest and most sustainable way to not only fight poverty but also to attain prosperity? In addition, soil-less agriculture is also a tool to protect the environment and natural resources. Write to us to learn how this can be accomplished through "Development without government". Dr. Subhas C. Mohapatra, President, IAFF, 1413 Boxwood Lane, Apex, NC 27502, Ph.(919)362-7653. Web site: www.iaff1.org  KINDLY SEND A COPY OF YOUR E-MAIL TO iaf...@gmail.com, as IAFF is gradually moving to the gmail account.