Reverse osmosis method can effectively remove inorganic substances, organic matter, bacteria, pyrogens and other particles dissolved in water. The so-called osmosis refers to the separation of two different concentrations of the solution by a semi-permeable membrane, in which the solute cannot pass through the semi-permeable membrane, and the water molecules with the lower concentration will pass through the semi-permeable membrane to reach the other side with higher concentration until the two sides concentration is equal. Before the equilibrium is reached, the pressure can be gradually applied to the higher concentration side, and the above-mentioned water molecule movement state will be temporarily stopped. The pressure required at this time is called “osmotic pressure”, if the applied force is greater than the osmotic pressure, the movement of water move in the opposite direction, that is, from the side of high concentration to the side of low concentration. This phenomenon is called “reverse osmosis”.
The purification effect of reverse osmosis can reach the level of ions. The rejection rate for monovalent ions can reach 90%-98%, while the divalent ions can reach 95%-99%(prevent the passage of substances with a molecular weight greater than 200 Daltons). The semi-permeable membranes commonly used in reverse osmosis water treatment are cellulosic, aromatic polyamides, polyimide or polyfuranes, and its structural shape is spiral wound, hollow fiber type, and tubular. As for the cellulose film in these materials, the advantage is that the chlorine resistance is high, but the service life is shortened under alkaline conditions (pH ≥ 8.0) or in the presence of bacteria. The disadvantage of polyamide is its poor tolerance to chlorine and chloramine.
If the pre-treatment is not done before reverse osmosis, the fouling on the osmosis membrane is easy to accumulate, such as calcium, magnesium, iron, etc., causing a decrease in reverse osmosis function; some membranes (such as polyamide) are easily destroyed by chlorine and chloramine. Therefore, before the reverse osmosis membrane, there must have pre-treatment such as activated carbon and softener.