Water treatment methods and principles(2)

 5. Ultraviolet disinfection method of water treatment method and principle

Ultraviolet disinfection is one of the commonly used methods at present. Its sterilization machine is to destroy the life 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, it is artificial ultraviolet energy with a wavelength of 253.7nm using a low-pressure mercury discharge lamp. The principle of the ultraviolet sterilization lamp is the same as that of the fluorescent lamp, except that the fluorescent tube is not coated with fluorescent material. The material of the lamp tube is quartz glass with high ultraviolet penetration rate. General ultraviolet devices are divided into irradiation type, immersion type and running water type according to the purpose.

Ultraviolet light used in hemodialysis dilution water is placed on the pipeline between the water storage tank and the dialysis machine, that is, all dialysis water must be irradiated with ultraviolet light before use to achieve a complete sterilization effect. Pseudomonas aeruginosa and Escherichia coli are the most susceptible to ultraviolet rays; on the contrary, Bacillus subtilis spores are more tolerant. Because ultraviolet disinfection is safe, economical, less selective for bacteria, and water quality will not change, this method has been widely used in recent years. For example, this method is often used for drinking water on ships.

6. Water treatment methods and principles of distillation

Distillation is an ancient but effective water treatment method. It can remove any non-volatile impurities, but it cannot remove volatile contaminants. It requires a large water storage tank for storage. Although it is an important cause of pollution, hemodialysis water is currently not treated in this way.

7. Ultrafiltration method of water treatment method and principle

Ultra filtration (ultra filtration) is similar to reverse osmosis, it also uses a semi-permeable membrane, but it can not control the removal of ions, because the pore size of the membrane is large, about 10-200A. Only bacteria, viruses, pyrogens and particulates can be excluded, and water-soluble ions cannot be filtered. 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 last step of water treatment to prevent the 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 determine whether the ultrafiltration membrane is effective. Similar to activated carbon, the backwash method is usually used to remove impurities attached to it.

8. Reverse osmosis method of water treatment method and principle

Reverse osmosis 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. To understand 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, the lower concentration One water molecule will pass through the semi-permeable membrane to the other with a higher concentration until the concentration on both sides is equal. Before equilibrium is reached, pressure can be gradually applied on the side with a higher concentration. Temporarily stop, the pressure required at this time is called “osmotic pressure”. If the applied force is greater than the osmotic pressure, the movement of water will go in the opposite direction, that is, from an example of high concentration to a low concentration On the other hand, this phenomenon is called “reverse osmosis.” The purification effect of reverse osmosis can reach the level of ions, and the rejection rate (rejection rate) for monovalent ions can reach 90% -98%, while the double Valence ions (divalent ions) can reach about 95% -99% (can prevent substances with molecular weight greater than 200 Daltons from passing).

Commonly used semi-permeable membrane materials for reverse osmosis water treatment are cellulosic, aromatic polyamides, polyimide or polyfuranes, etc. As for its structure, there are spiral wound and hollow fiber. (hollow fiber) and tubular type (tubular) etc. The advantage of cellulose membranes in these materials is that they have high chlorine resistance, but under alkaline conditions (pH ≥ 8.0) or in 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 final conclusion.

If the pre-treatment is not done before reverse osmosis, dirt will accumulate on the permeable membrane, such as calcium, magnesium, iron, etc., which will cause the decline of reverse osmosis function; some membranes (such as polyamide) are easily damaged by chlorine and ammonia, Therefore, pre-treatments such as activated carbon and softener are required before the reverse osmosis membrane. Although the price of reverse osmosis is relatively high, because the pore size of the reverse osmosis membrane is generally below 10A, 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 analysis.

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