It is a process of water quality regulation such as sedimentation, filtration, coagulation, flocculation, corrosion inhibition and scale inhibition for a specific purpose.
Because social production and life are closely related to water, the field of water treatment involves a wide range of applications, which constitutes a huge industrial application.
Water treatment usually includes sewage treatment and drinking water treatment. Commonly used water treatment agents are: polyaluminum chloride, polyaluminum ferric chloride, basic aluminum chloride, polyacrylamide, activated carbon and various filter materials. Common water treatment methods are: (1) sediment filtration, (2) hard water softening, (3) activated carbon adsorption, (4) deionization, (5) reverse osmosis, (6) ultrafiltration, (7) distillation and (8) ultraviolet disinfection. Now, here will explain the principles and functions of these treatments one by one.
1. The purpose of sediment filtration is to remove suspended particulate matter or colloidal matter from water sources. If these particles are not removed, it will damage other precision filtration membranes of dialysis water and even block the waterway. This is the oldest and simplest water purification method, so this step is often used in the pre-treatment of water purification, or if necessary, more filters will be added to the pipeline to remove large impurities. There are many kinds of filters used to filter suspended particles, such as screen filter, sand filter (such as quartz sand) or membrane filter. As long as the particle size is larger than that of these holes, it will be blocked. It is impossible to stop water-soluble ions. If the filter element is not replaced or cleaned for a long time, more and more particles will accumulate on the filter element, and the water flow and water pressure will gradually decrease. People use the difference between inlet pressure and outlet pressure to judge the degree of filter blockage. Therefore, the filter should be backwashed regularly to eliminate the impurities accumulated on it, and the filter should be replaced within a fixed time.
There is another problem worthy of attention in sediment filtration method, because particles are constantly blocked and accumulated, bacteria may breed here on the surface of these substances, and release toxic substances through the filter, causing pyrogen reaction, so the filter should be replaced frequently. In principle, when the pressure difference between water inlet and water outlet rises to 5 times, the filter element needs to be replaced. Second, the hard water softening method
The softening of hard water requires ion exchange method, which aims to exchange calcium and magnesium ions with sodium ions in hard water by cation exchange resin, thus reducing the concentration of calcium and magnesium ions in water source. The softening reaction formula is as follows:
Ca2++2Na-EX→Ca-EX2+2Na+ 1
Mg2++2Na-EX→Mg-EX2+2Na+ 1
In the formula, EX stands for ion exchange resin, which combines Ca2+ and Mg2+ to release Na+ ions originally contained in it.
The ion exchange resin currently on the market is a spherical synthetic organic polymer electrolyte. Sodium chloride is hidden in the resin matrix. In the softening process of hard water, sodium ions will be gradually consumed, and the softening effect of exchange resin will gradually decrease. At this time, it is necessary to regenerate, that is, add a specific concentration of physiological saline every once in a while, which is generally 10%. The reaction mode is as follows:
Ca-EX2+2Na+ (strong brine) → 2Na-EX+Ca2+
Mg-EX2+2Na+ (strong brine) → 2Na-EX+Mg2+
If there is no cation softening in the water treatment process, not only calcium and magnesium will be deposited on the reverse osmosis membrane, but also the curative effect will be reduced and even the reverse osmosis membrane will be destroyed, and patients will easily get hard water syndrome. Hard water softener can also cause the problem of bacterial reproduction, so the equipment needs to have the function of backwashing, and it should be backwashed once after a period of time to prevent it from absorbing too many impurities. Another noteworthy problem is hypernatremia, because the softening and reduction process of dialysis water is controlled by a timer. Under normal circumstances, the reduction mostly occurs at midnight, which is controlled by a valve. If something goes wrong, a lot of salt water will flood into the water source, which will cause hypernatremia in patients.
Three. activated carbon
Activated carbon is made of wood, sawdust, stone, coconut shell, coal or petroleum residue by high temperature carbonization, and it needs to be activated by hot air or steam after being made. Its main function is to remove chlorine, chloramines and other soluble organic substances with molecular weight of 60 to 300 daltons. The surface of activated carbon is granular and the inside is porous. There are many capillaries with the size of 10n m ~ lA in the pores. The inner surface area of 1g activated carbon is as high as 700- 1400m2, and the inner surface of these capillaries and the surface of particles are adsorption. The factors affecting the ability of activated carbon to remove organic matter are the area of activated carbon itself, the size of pores, the molecular weight and polarity of organic matter to be removed, and it mainly removes impurities through physical adsorption ability. When the adsorption capacity reaches saturation, too many impurities will fall down and pollute the downstream water quality, so it is necessary to remove the impurities adsorbed on it by backwashing regularly.
If the adsorption capacity of this activated carbon filter is obviously reduced, it must be updated. Determining the TOC concentration difference (or bacterial number difference) between inlet and outlet water is one of the bases for considering replacing activated carbon. Some reverse osmosis membranes have poor tolerance to chlorine, so activated carbon should be treated before reverse osmosis to make chlorine effectively adsorbed by activated carbon. However, the bacteria adsorbed on the pores of activated carbon are easy to breed and grow, and the removal effect of activated carbon on macromolecular organic matter is limited, so the reverse osmosis membrane behind it needs to be strengthened.
Four. Deionization method
The purpose of deionization is to remove inorganic ions dissolved in water. Like the hard water softener, it also uses the principle of ion exchange resin. Two kinds of resins are used here-cation exchange resin and anion exchange resin. Cation exchange resin uses hydrogen ions (H+) to exchange cations; Anion exchange resin uses hydroxyl ions (OH-) to exchange anions, and hydrogen ions and hydroxyl ions combine with each other 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 represents cation, x represents electricity price, M+x cation exchanges with hydrogen ions of H-Re on cation resin, A-z represents anion, and z represents electricity price. After combining with anion exchange resin, A-z releases OH- ions. H+ ions combine with OH- ions to form neutral water.
After the adsorption capacity of these resins is exhausted, they need to be reduced again, and the cation exchange resins need to be reduced by strong acid; On the contrary, anions need a strong base to be reduced. The adsorption capacity of cation exchange resin for various cations is different, and its strength and relative relationship are as follows:
ba2+>； Pb2+>； sr2+& gt； Ca2+>； Ni2+>； Cd2+>； CU2+& gt； CO2+>； Zn2+>； Mg2+>； ag 1+>； cs 1+>； k 1+>； NH 4 1+>； na 1+>； H 1+
The affinity strength of anion exchange resin for each anion is as follows:
s02-4+>； I-> NO3->； NO2- & gt； cl->； HCO 3-& gt； Oh-> F-
If anion exchange resin is exhausted and not reduced, fluorine with the weakest adsorption will gradually appear in dialysis water, causing rickets, osteoporosis and other bone diseases; If the cation exchange resin is exhausted, hydrogen ions will also appear in the dialysis water, which will increase the acidity of the water, so whether the deionization function is effective needs to be monitored from time to time. Generally, it is judged by the resistivity or conductivity of water quality. It is worth noting that the ion exchange resin used in deionization can also cause bacterial reproduction and bacteremia.
Verb (abbreviation of verb) reverse osmosis method
Reverse osmosis can effectively remove dissolved inorganic, organic, bacterial, pyrogen and other particles in water, which is the most important part of dialysis water treatment. Before understanding the principle of "reverse osmosis", we must first explain the concept of "infiltration". Permeation means that two solutions with different concentrations are separated by a semipermeable membrane, and the solute in them cannot penetrate the semipermeable membrane, so the water molecules with lower concentration will pass through the semipermeable membrane to reach the other side with higher concentration until the concentrations on both sides are equal. Before reaching equilibrium, pressure can be gradually applied on the side with higher concentration to temporarily stop the movement of the water molecules. The pressure required at this time is called "osmotic pressure". If the applied force is greater than the osmotic pressure, water will move in the opposite direction, that is, from the example with high concentration to the side with low concentration. This phenomenon is called "reverse osmosis". The purification effect of reverse osmosis can reach ion level, and the rejection rate of monovalent ions can reach 90%-98%, and the rejection rate of divalent ions can reach about 95%-99% (which can prevent substances with molecular weight greater than 200 Daltons from passing through).
Semi-permeable membrane materials commonly used in reverse osmosis water treatment include fiber membrane, aromatic polyamide, polyimide or polyfuran. As far as its structural shape is concerned, there are spiral winding, hollow fiber and tubular. For these materials, cellulose membrane has the advantage of high chlorine resistance, but its service life will be 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. As for which material is better, there is no conclusion yet.
If pretreatment is not done before reverse osmosis, dirt such as calcium, magnesium and iron plasma will easily accumulate on the osmotic membrane, leading to the decline of reverse osmosis function; Some membranes (such as polyamide) are easily damaged by chlorine and chloramine, so activated carbon and softener should be pretreated before reverse osmosis membrane. Although reverse osmosis is expensive, because the pore size of reverse osmosis membrane is generally below 10A, it can eliminate bacteria, viruses, pyrogens and even various dissolved ions, so it is best to prepare this step when preparing dialysis water for hemodialysis.
Six, ultrafiltration method
Ultrafiltration is similar to reverse osmosis, but it also uses semi-permeable membrane. However, due to the large pore size of the membrane, about 10-200 A, it is impossible to control the removal of ions, only bacteria, viruses, pyrogens and particles can be excluded, and water-soluble ions cannot be filtered. The main function of ultrafiltration is as pretreatment of reverse osmosis to prevent reverse osmosis membrane from being contaminated by bacteria. It can also be used in the last step of water treatment to prevent 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 filtration membrane is effective or not. Similar to activated carbon, backwashing is usually used to remove impurities attached to it.
Seven, distillation method
Distillation is an ancient and effective water treatment method. It can remove any non-volatile impurities, but it cannot eliminate volatile pollutants. It needs a large storage tank for storage, but this storage tank and pipeline are the important causes of pollution. At present, hemodialysis water is not treated in this way.
Eight, ultraviolet disinfection method
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 most important reaction is that pyrimidine bases in nucleic acid molecules become dimers. Low-pressure mercury discharge lamp (germicidal lamp) is generally used for artificial ultraviolet energy, and the wavelength is 253.7nm. The principle of ultraviolet germicidal lamp is the same as that of fluorescent lamp, except that fluorescent substances are not coated inside the lamp tube, and the material of the lamp tube is timely glass with high ultraviolet transmittance. General ultraviolet devices are divided into irradiation type, immersion type and running water type according to their uses.
Water treatment ultraviolet sterilization lamp is used to dilute the ultraviolet rays of hemodialysis water and put them on the pipeline between the water storage tank and the dialyzer, that is, all dialysis water should be irradiated with ultraviolet rays once before use, so as to achieve the effect of thorough sterilization. Pseudomonas aeruginosa and Escherichia coli are the most sensitive to ultraviolet rays; On the contrary, spores of Bacillus subtilis are more tolerant. Ultraviolet disinfection has been widely used in recent years, such as drinking water on board, because it is safe and economical, has little selectivity to bacteria and does not change the water quality. Kill all Igora, Basila and Salmonella in the water, and it can dive into the water center to sterilize at 360 degrees, and the effect is three times that of the water surface germicidal lamp. It can eliminate algae in water with obvious effect and convenient use. Ultraviolet germicidal lamp is suitable for filtration, water treatment, large and small pools, swimming pools and hot springs in various fishing grounds. The sterilization efficiency can reach 99%-99.99%.
Nine, biochemical method
[1] The biochemical water treatment method uses all kinds of bacteria and microorganisms existing in nature to decompose the organic matter in wastewater into harmless substances and purify the wastewater. 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 → grille → regulating tank → contact oxidation tank → sedimentation area → filtration → disinfection → effluent.
1, activated sludge treatment method
(1) Blast aeration: that is, exhaust aeration, in which compressed air is continuously blown into the wastewater to ensure a certain amount of dissolved oxygen in the water, so as to maintain the life activities of microorganisms, decompose the organic matter in the water, and achieve the purification effect of water treatment.
(2) Mechanical aeration: that is, surface aeration, which uses the mechanical impeller installed in the aeration tank to rotate and violently stir the water surface, so that oxygen in the air can be dissolved in the water for microbial life activities and biochemical effects, and the purification effect of water treatment can be achieved.
(3) Pure oxygen aeration: pure oxygen is blown into the water according to the method of blast aeration, so as to improve the oxygenation efficiency and accelerate the purification speed of water treatment.
2, biofilm water treatment method
(1) biofilter: The wastewater flows through the biofilm growing on the surface of the filter material, and the organic matter in the wastewater is degraded through the material exchange and biochemical action between the two surfaces, thus achieving the purpose of purifying water.
(2) Biological turntable: It is composed of several closely spaced disks fixed on a horizontal axis, and a biofilm grows on the surface of the rotating disks to achieve the effect of water treatment and purification.
(3) Bio-contact oxidation: all the fillers for microorganisms to inhabit are immersed in the wastewater, and air is filled into the wastewater through mechanical equipment to degrade the organic matter in the wastewater, thus purifying the wastewater.
3. Land treatment system (1) Land infiltration: domestic sewage is treated by microorganisms in soil film and the purification ability of plant roots to pollutants, and the water and fertilizer in sewage are used to promote the growth of crops, pastures and trees.
(2) Sewage irrigation: The main purpose of this water treatment method is irrigation to make full use of the purified sewage.
4. Anaerobic biological water treatment method: the organic matter in sewage is decomposed by anaerobic microorganisms to achieve the purpose of water treatment and purification, and at the same time, methane gas, CO2 and other gases are produced.