Summary of common water treatment methods for different water qualities

Summary of common water treatment methods for different water qualities
In the environmental water treatment industry, the commonly used water treatment methods generally include: 


(1) Sedimentation 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 
(9) Biochemical methods, etc. 


Now, let's introduce the principles and functions of these water treatment methods here. 

1. Sediment Filtration Method
The purpose of the sediment filtration method is to remove all suspended particles or colloidal substances from the water source. If these particles are not removed, they can cause damage to other precision filtration membranes in the dialysis water or even block the water channels. This is the oldest and simplest water purification method, so this step is commonly used in the initial treatment of water purification or when necessary, several more filters will be added to the pipeline to remove larger impurities. 
There are many types of filters used for filtering suspended particulate matter. For example, there are mesh filters, sand filters (such as quartz sand), or membrane filters, etc. As long as the particle size is larger than the size of these holes, it will be blocked. 
There is another issue to note regarding the sedimentation filtration method. As the particulate matter keeps being blocked and accumulates, these substances may have bacteria growing on them, which release toxic substances through the filter, causing a pyrogen reaction. Therefore, the filter needs to be replaced frequently. In principle, when the pressure difference between the inlet and outlet water reaches five times the original value, the filter should be replaced. 

II. Hard Water Softening Method (Soft Water Equipment) 
The softening of hard water requires the use of ion exchange method. The purpose is to use an anion exchange resin to exchange calcium and magnesium ions in hard water with sodium ions, thereby reducing the concentration of calcium and magnesium ions in the water source. The reaction equation for softening is as follows: 
Ca2++2Na-EX→Ca-EX2+2Na+1

Mg2++2Na-EX→Mg-EX2+2Na+1

The ion exchange resins currently available on the market are spherical synthetic organic polymer electrolytes. 
If there is no cation softening during the water treatment process, not only will calcium and magnesium deposits accumulate on the reverse osmosis membrane, reducing its efficiency and even damaging the membrane, but patients will also be prone to developing hard water syndrome. Hard water softeners can also cause problems with bacterial proliferation. Therefore, the equipment needs to have a reverse flushing function. After a certain period of time, it is necessary to perform a reverse flushing to prevent too many impurities from adhering to it. 

III. Activated Carbon Filter 
Activated carbon is produced by dry distillation and carbonization of substances such as wood, residual wood chips, fruit pits, coconut shells, coal or petroleum residues at high temperatures. After being made, it needs to be activated with hot air or steam. Its main function is to remove chlorine and chlorine amine and other soluble organic substances with molecular weights ranging from 60 to 300 daltons. The surface of activated carbon is granular, and the interior is porous. There are many capillary tubes with diameters of approximately 10nm to 1A inside the pores, and the internal surface area of 1 gram of activated carbon can reach 700-1400 m2. These capillary tube inner surfaces and particle surfaces are where the adsorption occurs. 

IV. Deionization Method 
The purpose of the deionization method is to remove the inorganic ions dissolved in water, just like in a water softener, it also utilizes the principle of ion exchange resins. Here, two types of resins are used - cation exchange resins and anion exchange resins. Cation exchange resins use hydrogen ions (H+) to exchange cations; while anion exchange resins use hydroxide ions (OH-) to exchange anions. 
Once the adsorption capacity of these resins is exhausted, they also need to be restored. Cation exchange resins require strong acid for restoration; conversely, anions require strong alkali for restoration. If the anion exchange resins are exhausted without being restored, the weakest adsorption force, fluorine, will gradually appear in the dialysis water, causing osteomalacia, osteoporosis and other bone disorders; if the cation exchange resins are exhausted, hydrogen ions will also appear in the dialysis water, causing an increase in water acidity. Therefore, the effectiveness of the deionization function needs to be monitored frequently. 
Generally, the resistivity or conductivity of the water quality is used as the criterion. The ion-exchange resins used in the deionization process can also cause the reproduction of bacteria, leading to bacteremia. This is an important point to note. 

V. Reverse Osmosis Method 
Reverse osmosis technology can effectively remove inorganic substances, organic compounds, bacteria, endotoxins and other particles dissolved in water. It is the most crucial step in the treatment of drinking water for dialysis. 
The purification effect of reverse osmosis can reach the ionic level. The commonly used semi-permeable membrane materials for reverse osmosis water treatment include fiber membranes, aromatic polyurea type, etc. As for its structural shapes, there are helical type, hollow fiber type and tubular type, etc. 
If no pre-treatment is carried out before reverse osmosis, impurities such as calcium, magnesium, iron, etc. are likely to accumulate on the reverse osmosis membrane, resulting in a decline in the reverse osmosis function. Some membranes are easily damaged by chlorine and chlorine amine. Therefore, before the reverse osmosis membrane, there should be pre-treatment such as activated carbon and softeners. So it is best to include this step when preparing the water for hemodialysis dialysis. 

VI. Post-filtration Method 


The ultrafiltration method is similar to reverse osmosis. It also uses semi-permeable membranes. However, it cannot control the removal of ions because the pore size of the membrane is relatively large, ranging from 10 to 200 A. It can only remove bacteria, viruses, endotoxins, and particulate matter, but it cannot filter water-soluble ions. 


The main function of the ultrafiltration method is to act as a pre-treatment step for reverse osmosis 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 bacteria from contaminating the water in the pipeline upstream. Generally, the effectiveness of the ultrafiltration membrane is determined by the pressure difference between the inlet and outlet water, similar to activated carbon. Usually, the reverse flushing method is used to remove the impurities attached to it. 

VII. Distillation Method 
Distillation is an ancient yet effective water treatment method. It can remove any non-volatile impurities, but it cannot eliminate volatile pollutants. It requires a large water storage tank for storage, and this storage tank along with the delivery pipes are significant causes of pollution. Currently, blood dialysis water does not undergo treatment in this way. 
VIII. Ultraviolet Disinfection Method 
Ultraviolet disinfection method is one of the commonly used methods at present. Its sterilization mechanism is to destroy the genetic material of bacteria - the nucleic acid, preventing them from reproducing. The most significant reaction is that the pyrimidine bases within the nucleic acid molecules transform into dimers. Generally, low-pressure mercury discharge lamps (sterilization lamps) with artificial ultraviolet energy of 253.7nm wavelength are used. The principle of ultraviolet sterilization lamps is the same as that of fluorescent lamps, except that the lamp tube does not contain fluorescent substances and the material of the lamp tube is quartz glass with high ultraviolet penetration rate. Generally, ultraviolet devices are classified according to their uses as irradiation type, immersion type and flow-through type. 
IX. Biochemical Method 
The biochemical water treatment method utilizes various bacteria and microorganisms existing in nature to decompose and convert organic substances in wastewater into harmless substances, thereby purifying the wastewater. The biochemical water treatment methods can be classified as activated sludge method, biofilm method, biological oxidation tower, land treatment system, and anaerobic biological water treatment method. 
The process of the biochemical water treatment method is as follows:
Raw water → Grate → Equalization tank → Contact oxidation tank → Sedimentation area → Filtration → Disinfection → Discharged water. 
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