Water Treatment Methods and Principles: Reverse Osmosis

Reverse osmosis can effectively remove dissolved inorganic matter, organic matter, bacteria, pyrogens and other particles from water, and is the most important part of the treatment of dialysis water. Before understanding the principle of "reverse osmosis," we must first explain the concept of "osmosis." Osmosis refers to the process where two solutions of different concentrations are separated by a semi-permeable membrane. The solute cannot pass through the membrane, so water molecules from the solution with the lower concentration will pass through to the solution with the higher concentration until the concentrations on both sides are equal. Before equilibrium is reached, pressure can be gradually applied to the solution with the higher concentration. This movement of water molecules will temporarily stop; the pressure required is called "osmotic pressure." If the applied force exceeds the osmotic pressure, the movement of water will reverse, flowing from the high-concentration solution to the low-concentration solution. This phenomenon is called "reverse osmosis." Reverse osmosis can purify water down to the ionic level, achieving a rejection rate of 90%-98% for monovalent ions and divalent ions. The ionization rate can reach approximately 95%-99% (preventing substances with a molecular weight greater than 200 Daltons from passing through).

Commonly used semi-permeable membrane materials for reverse osmosis water treatment include cellular membranes, aromatic polyamides, polyimides, and polyfuranes. Their structural shapes include spiral wound, hollow fiber, and tubular. Among these materials, cellulose membranes have the advantage of high chlorine resistance, but under alkaline conditions (pH... The lifespan of polyamide membranes is shortened in environments with temperatures ≥8.0 or in the presence of bacteria. A disadvantage of polyamide is its poor tolerance to chlorine and chloramines. There is currently no definitive conclusion on which material is better.

If pretreatment is not performed before reverse osmosis, contaminants such as calcium, magnesium, and iron ions can easily accumulate on the membrane, causing a decrease in reverse osmosis function. Some membranes (such as polyamide) are easily damaged by chlorine and chloramines; therefore, pretreatment with activated carbon and a softener is necessary before the reverse osmosis membrane. Although reverse osmosis is more expensive, because the pore size of a typical reverse osmosis membrane is approximately 10 Å or less, it can remove bacteria, viruses, pyrogens, and even various dissolved ions. Therefore, it is best to perform this step when preparing water for hemodialysis.