General water treatment methods and principles

Commonly used water treatment methods are: (1) sediment filtration method, (2) hard water softening method, (3) activated carbon adsorption method, (4) deionization method, (5) reverse osmosis method, (6) ultrafiltration method, (7) Distillation method, (8) Ultraviolet disinfection method, etc. Now the principles and functions of these treatment methods are explained here.

The purpose of the deionization method is to remove the inorganic ions dissolved in water. Like the hard water softener, it also uses the principle of ion exchange resin. Two resins are used here-cation exchange resin and anion exchange resin. Cation exchange resins use hydrogen ions (H+) to exchange cations; and anion exchange resins use hydroxide ions (OH-) to exchange anions. Hydrogen ions and hydroxide ions combine to form neutral water. The reaction equation is as follows:
M+x+xH-Re→M-M-Rex+xH+1
A-z+zOH-Re→A-Rez+zOH-1
In the above formula, M+x means cation, x means electricity valence, M+x cation exchanges hydrogen ion with H-Re on the cation resin, Az means anion, z means electricity valence, after Az is combined with anion exchange resin, Release OH- ions. The combination of H+ ions and OH- ions becomes neutral water.
These resins also need to be reduced after their adsorption capacity is exhausted. Cation exchange resins need strong acids to reduce; on the contrary, anions need strong alkalis to reduce. Cation exchange resins have different adsorption capacities for various cations. Their strength and relative relationship are as follows:
Ba2+>Pb2+>Sr2+>Ca2+>Ni2+>Cd2+>CU2+>Co2+>Zn2+>Mg2+>Ag1+>Cs1+>K1+>NH41+>Na1+>H1+
The affinity strength of the anion exchange resin and each anion is as follows:
S02-4+>I->NO3->NO2->Cl->HCO3->OH->F-
If the anion exchange resin is exhausted without reduction, the weakest fluorine will gradually appear in the dialysis water, causing rickets, osteoporosis and other bone diseases; if the cation exchange resin is exhausted, hydrogen ion It will also appear in the dialysis water, causing the acidity of the water to increase, so whether the deionization function is effective requires frequent monitoring. It is generally judged by the resistance coefficient (resistivity) or conductivity (conductivity) of the water quality. The ion exchange resin used in the deionization method can also cause bacteria to multiply and cause bacteremia, which is worth noting.

Reverse osmosis method can effectively remove inorganic substances, organic substances, bacteria, pyrogens and other particles dissolved in water. It 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. The so-called osmosis refers to the separation of two solutions of different concentrations by a semi-permeable membrane, where the solute cannot penetrate the semi-permeable membrane, and the lower concentration One side of the water molecules will pass through the semi-permeable membrane to reach the other with a higher concentration until the concentrations on both sides are equal. Before reaching equilibrium, you can gradually apply pressure on the higher concentration side, and the aforementioned water molecule movement state will be Stop temporarily, the pressure required at this time is called “osmotic pressure”. If the applied force is greater than the osmotic pressure, the water will move in the opposite direction, that is, from a high concentration to a low concentration. On the one hand, this phenomenon is called “reverse osmosis.” The purification effect of reverse osmosis can reach the level of ions, and the rejection rate of monovalent ions can reach 90%-98%, and Divalent ions can reach about 95%-99% (it can prevent substances with a molecular weight greater than 200 daltons from passing).
The semi-permeable membrane materials commonly used in reverse osmosis water treatment include cellulosic, aromatic polyamides, polyimide or polyfuranes, etc. As for its structure and shape, there are spiral wound and hollow fiber types. (hollow fiber) and tubular (tubular), etc. As for the advantage of cellulose membrane in these materials, it has high chlorine resistance, but under alkaline conditions (pH ≥8.0) or the presence of bacteria, the service life will be shortened. The disadvantage of polyamide is its poor tolerance to chlorine and chloramines. As for which material is better, there is no conclusion yet.
If the pre-treatment is not done before reverse osmosis, dirt will easily accumulate on the osmosis membrane, such as calcium, magnesium, iron plasma, which will cause the decline of reverse osmosis function; some membranes (such as polyamide) are easily damaged by chlorine and chloramine. Therefore, activated carbon and softener must be pre-treated before the reverse osmosis membrane. Although the price of reverse osmosis is relatively high, because the pore size of the general reverse osmosis membrane is about 10A or less, it can exclude bacteria, viruses, pyrogens, and even various soluble ions. Therefore, it is best to prepare this step when preparing the water for hemodialysis.

The ultrafiltration method is similar to the reverse osmosis method. It also uses a semi-permeable membrane, but it cannot control the removal of ions, because the pore size of the membrane is larger, about 10-200A. It can only exclude bacteria, viruses, pyrogens and particulate matter, but cannot filter water-soluble ions. The main function of the ultrafiltration method is to act as a pretreatment of the reverse osmosis method to prevent the reverse osmosis membrane from being contaminated by bacteria. It can also be used in the final step of water treatment to prevent upstream water from being contaminated by bacteria in the pipeline. Generally, the difference between the inlet water pressure and the outlet water pressure is used to judge whether the ultrafiltration membrane is effective. Similar to activated carbon, the reverse flushing method is usually used to remove impurities attached to it.

7. distillation

Distillation is an ancient but also effective water treatment method. It can remove any non-volatile impurities, but cannot eliminate volatile pollutants. It requires a large water storage tank for storage. This water storage tank and delivery pipe are An important cause of pollution, the current hemodialysis water does not need to be treated in this way.

Ultraviolet disinfection is one of the commonly used methods at present. Its sterilization mechanism is to destroy the biological genetic material of bacterial nucleic acid, making it unable to reproduce. The most important reaction is that the pyrimidine base in the nucleic acid molecule becomes a dimer. . Generally, the artificial 253.7nm wavelength ultraviolet energy of low-pressure mercury discharge lamp (germicidal lamp) is used. The principle of the ultraviolet germicidal lamp is the same as that of the fluorescent lamp, except that the inside of the lamp tube is not coated with fluorescent material, and the material of the lamp tube is quartz glass with high ultraviolet transmittance. Generally, UV devices are divided into irradiation type, immersion type and flowing water type according to their purpose.
The ultraviolet light used by the water treatment ultraviolet germicidal lamp in the hemodialysis dilution water is installed in the pipeline between the water storage tank and the dialysis machine, that is, all the dialysis water must be irradiated with ultraviolet light before use to achieve complete sterilization. effect. Pseudomonas aeruginosa and Escherichia coli are the most sensitive to ultraviolet rays; on the contrary, subtilis spores are more tolerant. Because the ultraviolet disinfection method is safe, economical, has little selection of bacteria, and the water quality will not change, this method has been widely used in recent years. For example, this kind of disinfection method is often used for drinking water on ships. The Igolamella, Basraella, Salmonella, etc. in the water are completely killed, and it can dive into the center of the water to sterilize 360 ​​degrees, and the effect is equal to three times that of the surface germicidal lamp. It can eliminate the algae in the water, the effect is obvious, and it is convenient to use. The ultraviolet germicidal lamp is suitable for: filtration of fisheries of various sizes, water treatment, large and small pools, swimming pools, and hot springs. The sterilization efficiency can reach 99%-99.99%.

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