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Common water treatment methods include: sediment filtration, hard water softening, activated carbon adsorption, deionization, reverse osmosis, ultrafiltration, distillation, ultraviolet disinfection, and biochemical methods.
01 Precipitate Filtration Method
The purpose of the precipitate filtration method is to remove suspended particles or colloidal substances from the water source. If these particles are not removed, they can damage other fine filtration membranes used for dialysis water or even block the water pathways. 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 additional filters may be added in the pipelines to remove larger impurities. There are many types of filters used to filter suspended particles, such as mesh filters, sand filters (like quartz sand), or membrane filters. As long as the particle size is larger than the size of the filter openings, it will be blocked. Dissolved ions in the water, however, cannot be blocked.
02 Hard Water Softening Method
The softening of hard water requires the use of ion exchange method. Its purpose is to use cation exchange resin to exchange the calcium and magnesium ions in hard water with sodium ions, in order to reduce the concentration of calcium and magnesium ions in the water source.
03 Activated Carbon
Activated carbon is made from materials such as wood, wood residues, fruit pits, coconut shells, coal, or petroleum residues through carbonization by dry distillation at high temperatures. After production, it needs to be activated with hot air or steam. Its main function is to remove chlorine and chloramine, as well as other soluble organic substances with molecular weights from 60 to 0 Dalton. The surface of activated carbon is granular, and the interior is porous, with many capillaries about 10 nm to 1 Å in size. The internal surface area of 1 g of activated carbon reaches 700-1400 m², and the inner surfaces of these capillaries and the surfaces of the particles are where adsorption occurs.
04 Deionization Method
The purpose of the deionization method is to remove inorganic ions dissolved in water. Similar to a water softener for hard water, it also utilizes the principle of ion exchange resins. Two types of resins are used here – cation exchange resin and anion exchange resin. The cation exchange resin uses hydrogen ions (H⁺) to exchange cations, while the anion exchange resin uses hydroxide ions (OH⁻) to exchange anions. Hydrogen ions and hydroxide ions combine with each other to form neutral water.
05 Reverse Osmosis Method
The reverse osmosis method can effectively remove inorganic substances, organic substances, bacteria, pyrogens, and other particles dissolved in water, making it an important step in the preparation of dialysis water. Osmosis refers to the phenomenon where, with a semipermeable membrane separating two solutions of different concentrations, if the solute cannot pass through the membrane, the water molecules on the side of lower concentration will move through the membrane to the side of higher concentration until the concentrations on both sides are equal. Before equilibrium is reached, pressure can gradually be applied to the higher concentration side. In this state, the movement of water molecules temporarily stops, and the required pressure is called 'osmotic pressure.' If the applied pressure exceeds the osmotic pressure, the movement of water molecules will reverse, flowing from the higher concentration side to the lower concentration side. This phenomenon is called 'reverse osmosis.'
06 Ultrafiltration Method
The ultrafiltration method is similar to the reverse osmosis method and also uses a semipermeable membrane. However, it cannot control the removal of ions because the membrane has larger pores, about 10-200 Å in diameter. It can only remove bacteria, viruses, pyrogens, and particulate matter, but cannot filter water-soluble ions. The main function of the ultrafiltration method is to serve as a pre-treatment for reverse osmosis to prevent reverse osmosis membranes from being contaminated by bacteria. It can also be used as a post-treatment step in water treatment to prevent water upstream from being contaminated by bacteria in the pipelines. Generally, the effectiveness of the ultrafiltration membrane is determined by the pressure difference between the inlet and outlet water. Similar to activated carbon, impurities attached to it are normally removed using backwashing.
07 Distillation Method
The distillation method is an ancient yet effective water treatment method. It can remove any non-volatile impurities but cannot eliminate volatile pollutants. It requires a large water storage tank, and this storage tank and the delivery pipes are major sources of contamination. Water used for hemodialysis is not treated using this method.
08 Ultraviolet Disinfection Method
The ultraviolet disinfection method is one of the commonly used methods. Ultraviolet disinfection does not produce any secondary pollutants and is considered a new generation of disinfection technology. With the advantages of broad-spectrum activity, low cost, long lifespan, high water capacity, and no pollution compared to other disinfection methods, it has gradually become a mainstream disinfection method in developed Western countries. Its bactericidal mechanism is to destroy the genetic material of bacterial nucleic acids, preventing reproduction, with a significant reaction being the formation of pyrimidine dimers within nucleic acid molecules. Typically, low-pressure mercury discharge lamps (germicidal lamps) are used to produce artificial ultraviolet energy at a wavelength of 253.7 nm. The principle of ultraviolet germicidal lamps is the same as that of fluorescent lamps, except that the inside of the tube is not coated with fluorescent material, and the tube is made of quartz glass with high ultraviolet transmittance. Generally, ultraviolet devices are classified according to their use into irradiation type, immersion type, and flowing water type.
09 Biochemical Method
The biochemical water treatment method uses various bacteria and microorganisms that exist in nature to decompose and convert organic matter in wastewater into harmless substances, thereby purifying the wastewater. The 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. The process of biochemical water treatment is: raw water → grille → regulation tank → contact oxidation tank → sedimentation tank → filtration → disinfection → effluent.
