Principle of Reverse Osmosis

In 1748, French scientists first discovered the phenomenon of pigs’ bile membranes being able to permeate water, but due to the technological limitations of the time, they could not exploit it. However, this phenomenon later became an explanation for various natural phenomena over the next two hundred years, such as theories on how trees absorb water. The principle is as follows:

If a semipermeable membrane separates saltwater of different concentrations, water molecules will move from the side with lower concentration to the side with higher concentration due to the difference in potential energy, until the difference in water level and water pressure between the two sides prevents further movement, achieving equilibrium in substance exchange. This is the phenomenon of osmosis.

As the name suggests, in reverse osmosis, if pressure is applied to the high concentration side, the osmosis effect mentioned above can be stopped and reversed, purifying water by forcing water molecules from the high concentration side to the low concentration side.

Simply put, when greater pressure than that of reverse osmosis is applied to the original water, reverse osmosis occurs, during which inorganic salts, heavy metals, organic substances, bacteria, particles, etc., which cannot pass through the semi-permeable membrane, remain concentrated in the original water, while only water molecules and smaller salt molecules permeate through the membrane to the purified water side.

As the original water gradually concentrates, the applied pressure must be gradually increased until it is infinitely large. In practical operation, the concentrated water is discharged, and fresh original water is continuously supplied to maintain a constant pressure, achieving the purpose of reverse osmosis. Speaking of reverse osmosis membranes, they are included in a subset of separation membranes (CROSS-FLOW), which can be further classified as shown in Table 4.16.

• Reverse Osmosis (RO)- Reverse Osmosis (RO)
• Ultra Filtration (UF)- Ultrafiltration (UF)
• Microfiltration (MF) – Microfiltration (MF)
• GAS Separation (GS) – Gas Separation (GS)

In all membrane separation processes, only the water permeability of reverse osmosis membranes is the smallest, ranging between 0.0001 to 0.002 microns. To put this in perspective, the diameter of a human hair is about 10 microns, highlighting the precision of reverse osmosis membrane pores. Besides effectively filtering bacteria and viruses, these membranes can also remove highly toxic heavy metals from water, as illustrated in Figure 4.13 and listed in Table 4.16 for membrane types.