In the environmental protection water treatment industry, the commonly used water treatment methods generally include:
（1） Sediment filtration method
(2) hard water softening method
（3） Activated carbon adsorption method
(4) deionization method
（5） Reverse osmosis method
(6) excess filtration method
（7） Distillation method,
(8) ultraviolet disinfection method
（9） Biochemical method, etc
Now, the principles and functions of these water treatment methods are introduced here. According to different water quality, the commonly used water treatment methods are summarized as follows:
1、 Sediment filtration
The purpose of sediment filtration method is to remove suspended particulate matter or colloidal matter from the water source. If these particles are not removed, they will damage other precision filtration membranes of dialysis water or even block the waterway. This is an ancient and simple water purification method, so this step is often used in the preliminary treatment of water purification, or if necessary, several filters will be added to the pipeline to remove large impurities.
There are many kinds of filters used to filter suspended particulate matter, such as mesh filter, sand filter (such as quartz sand) or membrane filter. As long as the particle size is larger than the size of these holes, it will be blocked.
There is another problem worth noting in the sediment filtration method, because the particulate matter is constantly blocked and accumulated. There may be bacteria on the surface of these substances to reproduce here and release toxic substances through the filter, resulting in pyrogen reaction. Therefore, the filter should be replaced frequently. In principle, when the pressure drop between inlet and outlet increases to five times of the original, the filter needs to be replaced.
2、 Hard water softening method (softened water equipment)
The softening of hard water needs to use ion exchange method. Its purpose is to use cation exchange resin to exchange calcium and magnesium ions in hard water with sodium ions, so as to reduce the concentration of calcium and magnesium ions in water source.
If there is no softening of cations in the process of water treatment, it is not only the deposition of calcium and magnesium on the reverse osmosis membrane, which will reduce the efficiency and even destroy the reverse osmosis membrane. At the same time, patients are also easy to get hard water syndrome. The hard water softener will also cause the problem of bacterial reproduction, so the equipment needs to have the function of back flushing. After a period of time, it needs to be back flushed once to prevent too many impurities from adsorbing it.
3、 Activated carbon filter
Activated carbon is made of wood, sawdust, fruit core, coconut shell, coal or oil residue, which is carbonized by dry distillation at high temperature. After being made, it needs to be activated by hot air or steam. Its main function is to remove chlorine, ammonia chloride and other dissolved organic substances with molecular weight of 60 to 300 daltons. The surface of activated carbon is granular and the interior is porous. There are many capillaries about 1onm ~ La in the pores. The internal surface area of 1G activated carbon is as high as 700-1400m2, and the inner surface of these capillaries and particle surface are the place of adsorption.
4、 Deionization method
The purpose of deionization method is to eliminate inorganic ions dissolved in water. Like hard water softener, it also uses the principle of ion exchange resin. In this, two resins are used – cation exchange resin and anion exchange resin. Cation exchange resin uses hydrogen ion (H +) to exchange cations; Anion exchange resin uses hydroxyl ions (OH -) to exchange anions.
After the adsorption capacity of these resins is exhausted, they also need to be reduced, and cation exchange resins need strong acid to be reduced; On the contrary, anions need strong bases to reduce. If the anion exchange resin is exhausted without reduction, fluorine with weak adsorption will gradually appear in the dialysis water, resulting in chondropathy, osteoporosis and other bone diseases; If the cation exchange resin is exhausted, hydrogen ions will also appear in the dialysis water, resulting in the increase of water acidity. Therefore, whether the deionization function is effective needs to be monitored from time to time.
It is generally judged by the resistance coefficient or conductivity of water quality. The ion exchange resin used in deionization method will also cause bacterial reproduction and bacteremia, which is worthy of attention.
5、 Reverse osmosis
Reverse osmosis can effectively remove inorganic matter, organic matter, bacteria, pyrogen and other particles dissolved in water. It is an important link in the treatment of dialysis water.
The purification effect of reverse osmosis can reach the level of ions. The commonly used semi permeable membrane materials for reverse osmosis water treatment include fibrous membrane, aromatic polyamides, polyimide or polyfuranes, and its structural shapes include spiral wound, hollow fiber and tubular.
If no pretreatment is done before reverse osmosis, dirt is easy to accumulate on the osmosis membrane, such as calcium, magnesium and iron plasma, resulting in the decline of reverse osmosis function; Some membranes (such as polyamide) are easy to be damaged by chlorine and ammonia chloride. Therefore, activated carbon and softener should be pretreated before reverse osmosis membrane. Therefore, the preparation of water for hemodialysis analysis and release is a step.
6、 Excess filtration
Similar to reverse osmosis, ultrafiltration uses semi permeable membrane, but it cannot control the removal of ions because the pore size of the membrane is large, about 10-200a. Only bacteria, viruses, pyrogens and particles can be excluded, while water-soluble ions cannot be filtered.
The main function of ultrafiltration method is to act as the pretreatment of reverse osmosis method to prevent the reverse osmosis membrane from being polluted by bacteria. It can also be used in the next step of water treatment to prevent the upstream water from being polluted by bacteria in the pipeline. Generally, the difference between inlet pressure and outlet pressure is used to judge whether the excess filter membrane is effective. Similar to activated carbon, it usually uses the back flushing method to remove the impurities attached to it.
7、 Distillation method
Distillation is an ancient but effective water treatment method. It can remove any nonvolatile impurities, but it can not eliminate volatile pollutants. It needs a large water storage tank to store. This water storage tank and delivery pipe are an important cause of pollution. At present, the water for hemodialysis is not treated in this way.
8、 Ultraviolet disinfection
Ultraviolet disinfection is one of the commonly used methods at present. Its sterilization mechanism is to destroy the life genetic material of bacterial nucleic acid and make it unable to reproduce. The major reaction is that the pyrimidine base in the nucleic acid molecule becomes a dimer. Generally, the artificial ultraviolet energy of 253.7 nm wavelength of low-pressure mercury discharge lamp (sterilization lamp) is used. The principle of ultraviolet sterilization lamp is the same as that of fluorescent lamp, except that the interior of the lamp tube is not coated with fluorescent substances. The material of the lamp tube is quartz glass with high ultraviolet penetration rate. General ultraviolet devices can be divided into irradiation type, immersion type and running water type according to their purposes.
9、 Biochemical method
Biochemical water treatment method uses various bacteria and microorganisms existing in nature to decompose the organic matter in the wastewater into harmless substances, so that the wastewater can be purified. Biochemical water treatment methods can be divided into activated sludge method, biofilm method, biological oxidation tower, land treatment system and anaerobic biological water treatment method.
Biochemical water treatment process:
Raw water → grid → regulating tank → contact oxidation tank → sedimentation area → filtration → disinfection → effluent.
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