Pure water refers to water that is generally sourced from urban tap water and can be filtered through multiple layers to remove harmful substances such as microorganisms. However, it also removes minerals such as fluorine, potassium, calcium, and magnesium that the human body needs.
With the rapid development of modern technology and industry, and the relatively lagging environmental governance, the current situation of water pollution in China is severe. Due to the uncontrolled discharge of industrial and domestic wastewater, as well as agricultural pollution, surface water now contains not only mud and sand, but also animal and plant decays. There are also a large number of substances that pose a threat to human health, such as bleach, pesticides, heavy metals, limestone, and iron. These pollutants accumulate in the human body for a long time and pose great harm to human health. They can cause cancer, mutation, and distortion, making them invisible killers. However, traditional tap water production processes not only fail to remove organic compounds from it, but if chlorine is added to tap water production, new and more severe organic pollutants such as trichloromethane will be generated, resulting in stronger mutagenicity of tap water compared to natural water. In addition, tap water needs to go through a long water supply pipeline system after leaving the factory, especially the roof water tank of high-rise residential buildings, which has serious "secondary pollution". This type of water, of course, cannot be consumed raw. Even if boiled, it can only kill bacteria and cannot remove harmful chemicals. Furthermore, drinking purified water not only eliminates harmful effects on health, but also benefits longevity. Because the purer the water, the more it can function as a carrier, the stronger its ability to dissolve various metabolic substances in the body, and the easier it is to be absorbed by the human body, which is beneficial for generating fluids, quenching thirst, and relieving fatigue. Therefore, in order to maintain health, improve people's health level, develop the pure water industry, and produce high-quality drinking water, pure water treatment involves secondary purification of tap water, further filtering out harmful substances such as chlorides and bacteria in tap water, achieving sterilization and disinfection effects.
2. Methods for pure water treatment
1. Membrane microporous filtration (MF) for pure water treatment
The membrane microporous filtration method includes three forms: deep filtration, sieve filtration, and surface filtration. Deep filtration is a matrix made of woven fibers or compressed materials, which uses adsorption or capture methods to retain particles, such as commonly used multimedia filtration or sand filtration; Deep filtration is a relatively economical method that can remove over 98% of suspended solids while protecting downstream purification units from blockage, and is therefore commonly used as pretreatment.
Surface filtration is a multi-layer structure. When the solution passes through the filter membrane, particles with larger pores inside the membrane will be left behind and mainly accumulate on the surface of the filter membrane, such as commonly used PP fiber filtration. Surface filtration can remove over 99.9% of suspended solids, so it can also be used for pretreatment or clarification.
The screen mesh filter membrane is basically a consistent structure, like a sieve, which leaves particles larger than the pore size on the surface (the pore size of this filter membrane is very precise), such as the point security filter used at the end of the ultrapure water machine; Mesh filtration and microporous filtration are generally placed at the final point of use in purification systems to remove residual trace amounts of resin flakes, carbon shavings, colloids, and microorganisms.
2. Activated carbon adsorption for pure water treatment
Activated carbon adsorption is a method of removing one or more harmful substances from water by utilizing the porous properties of activated carbon. Activated carbon adsorption has a good effect on removing organic matter, colloids, microorganisms, residual chlorine, odor, etc. in water. Meanwhile, due to its certain reducing effect, activated carbon also has a good removal effect on oxidants in water.
Due to the saturation value of the adsorption function of activated carbon, when the saturation adsorption capacity is reached, the adsorption function of the activated carbon filter will be greatly reduced. Therefore, it is necessary to pay attention to analyzing the adsorption capacity of activated carbon, replace it in a timely manner, or disinfect and restore it through high-pressure steam. However, at the same time, the organic matter adsorbed on the surface of activated carbon may become a nutrient source or a breeding ground for bacterial growth, so the issue of microbial growth in activated carbon filters is also worth paying attention to. Regular disinfection is necessary for controlling bacterial growth. It is worth noting that in the early stages of using activated carbon (or during the operation of newly replaced activated carbon), a small amount of extremely fine powder activated carbon may enter the reverse osmosis system with water flow, causing fouling and blockage of the reverse osmosis membrane flow channel, resulting in an increase in operating pressure, a decrease in water production, and an increase in system pressure drop. Moreover, this destructive effect is difficult to recover with conventional cleaning methods. So it is necessary to rinse the activated carbon clean and remove the fine powder before sending the filtered water to the subsequent RO system. Activated carbon plays a significant role, but during use, attention should also be paid to disinfection and new activated carbon must be washed clean.
3. Reverse Osmosis (RO) Pure Water Treatment
Reverse osmosis refers to the process where a pressure greater than osmotic pressure is applied on the concentrated solution side, causing the solvent in the concentrated solution to flow towards the dilute solution. The flow direction of this solvent is opposite to the original direction of permeation, and this process is called reverse osmosis; This principle is used in the field of liquid separation for purification, impurity removal, and treatment of liquid substances.
The working principle of reverse osmosis membrane: A thin film that has selectivity for penetrating substances is called a semi permeable membrane. Generally, a thin film that can only penetrate solvents but cannot penetrate solutes is called an ideal semi permeable membrane. When the same volume of dilute solution (such as fresh water) and concentrated solution (such as saline water) are placed on both sides of the semi permeable membrane, the solvent in the dilute solution will naturally pass through the semi permeable membrane and spontaneously flow towards the side of the concentrated solution. This phenomenon is called permeation. When permeation reaches equilibrium, the liquid level on one side of the concentrated solution will be higher than that of the dilute solution by a certain height, forming a pressure difference, which is called osmotic pressure. Reverse osmosis is a reverse migration movement of osmosis, which is a separation method that separates solutes and solvents in a solvent through the selective interception effect of a semi permeable membrane under pressure. It has been widely used in the purification and concentration of various solutions. The most common application example is in water treatment processes, where reverse osmosis technology is used to remove impurities such as inorganic ions, bacteria, viruses, organic matter, and colloids from the raw water, in order to obtain high-quality pure water.
4. Ion exchange (IX) pure water treatment
Ion exchange pure water equipment is a traditional water treatment process that uses anion and cation exchange resins to replace various anions and cations in water. The combination of anion and cation exchange resins in different proportions can form an ion exchange cation bed system, an ion exchange anion bed system, and an ion exchange mixed bed (compound bed) system. The mixed bed (compound bed) system is usually used after water treatment processes such as reverse osmosis to produce ultrapure water and high-purity water. It is currently one of the irreplaceable means for preparing ultrapure water and high-purity water. Its effluent conductivity can be below 1uS/cm, and the effluent resistivity can reach over 1M Ω. cm. Depending on different water quality and usage requirements, the effluent resistivity can be controlled between 1-18M Ω. cm. It is widely used in the preparation of ultra pure water and high-purity water in industries such as electronics, power, chemical industry, electroplating, boiler feed water, and pharmaceutical ultra pure water.
The salts contained in the raw water, such as calcium magnesium sodium salts such as Ca (HCO3) 2 and MgSO4, are replaced by the active groups of the cation resin when passing through the exchange resin layer, while the anions HCO3- and SO42- are replaced by the active groups of the anion resin, resulting in ultrapurification of the water. If the content of bicarbonate in the raw water is high, a degassing tower should be installed between the anion and cation exchange columns to remove CO2 gas and reduce the load on the anion bed.
5. Ultraviolet (UV) ultrapure water treatment
The main process of cell proliferation is: the long strands of DNA are opened, and after opening, each long chain of adenine unit searches for thymine unit connections. Each long strand can replicate the same chain as the newly separated long strand, restoring the original intact DNA before division, and becoming the new cell foundation. Ultraviolet radiation with a wavelength of 240-280nm can disrupt DNA's ability to produce proteins and replicate, with ultraviolet radiation with a wavelength of 265nm having the strongest killing effect on bacteria and viruses. After the DNA and RNA of bacteria and viruses are damaged, their ability to produce proteins and reproduce has been lost. Due to the short lifespan of bacteria and viruses, those that cannot reproduce will quickly die. Ultraviolet radiation is used to prevent the survival of microorganisms in tap water and achieve the effect of sterilization and disinfection.
At present, only artificial mercury (alloy) light sources can output sufficient UV intensity (UVC) for engineering disinfection. The ultraviolet sterilization lamp tube is made of quartz glass, and the mercury lamp is divided into three types based on the different mercury vapor pressure inside the lamp tube and the different UV output intensity after being lit: low pressure low intensity mercury lamp, medium pressure high intensity mercury lamp, and low pressure high intensity mercury lamp.
The bactericidal effect is determined by the radiation dose received by microorganisms, as well as the output energy of ultraviolet radiation, which is related to the type, intensity, and usage time of the lamp. As the lamp ages, it will lose 30% -50% of its intensity.
UV irradiation dose refers to the amount of specific wavelength ultraviolet radiation required to achieve a certain bacterial inactivation rate: irradiation dose (J/m2)=irradiation time (s) x UVC intensity (W/m2). The higher the irradiation dose, the higher the disinfection efficiency. Due to the size requirements of the equipment, the general irradiation time is only a few seconds. Therefore, the UVC output intensity of the lamp tube becomes the most important parameter to measure the performance of UV disinfection equipment.
6. Ultra filtration (UF) pure water treatment
Ultrafiltration technology is a high-tech widely used in the fields of water purification, solution separation, concentration, and extraction of useful substances from wastewater, as well as wastewater purification and reuse. The characteristics are simple usage process, no need for heating, energy conservation, low-pressure operation, and small device footprint.
Ultrafiltration (UF) pure water treatment principle: UF is a membrane separation process that uses screening as the separation principle and pressure as the driving force, with a filtration accuracy of 0.005-0.01 μ Within the range of m, it can effectively remove particles, colloids, bacterial cushion layers, and high molecular weight organic substances from water. It can be widely used for the separation, concentration, and purification of substances. The ultrafiltration process has no phase conversion and operates at room temperature, which is particularly suitable for the separation of thermosensitive substances. It has good temperature resistance, acid and alkali resistance, and oxidation resistance. It can be used continuously for a long time under conditions below 60 ℃ and pH 2-11.
Hollow fiber ultrafiltration membrane is the most mature and advanced form of ultrafiltration technology. Hollow fibers have an outer diameter of 0.5-2.0mm and an inner diameter of 0.3-1.4mm. The wall of the hollow fiber tube is covered with micropores, and the pore size is expressed in terms of the molecular weight that can intercept substances, with a molecular weight that can reach several thousand to several hundred thousand. The raw water flows under pressure outside or inside the hollow fiber, forming an external pressure type and an internal pressure type, respectively. Ultrafiltration is a dynamic filtration process, in which the intercepted substances can be reduced and eliminated with concentration, without clogging the membrane surface, and can operate continuously for a long time.
7. EDI Pure Water Treatment
The working principle of EDI ultra pure water treatment equipment: The electrodeionization (EDI) system is mainly a scientific water treatment technology that uses the direct current electric field to cause directional movement of ions in the water dielectric through the separator, and uses the selective permeation effect of exchange membranes to purify water quality. Between a pair of electrodes in an electrodialysis machine, there are usually multiple sets of alternating arrangement of negative membranes, positive membranes, and separators (A, B), forming a concentration chamber and a dilution chamber (i.e. cations can pass through the positive membrane, and anions can pass through the negative membrane). Cations in the dilute room water migrate towards the negative electrode and pass through the positive membrane, which is intercepted by the negative membrane in the concentrated room; Anions in water migrate towards the positive electrode direction through the negative membrane, which is intercepted by the positive membrane in the concentration chamber. As a result, the number of ions in the water passing through the dilution chamber gradually decreases and becomes fresh water. In contrast, the water in the concentration chamber, due to the continuous influx of negative and positive ions in the concentration chamber, increases the concentration of dielectric ions and becomes concentrated water, achieving the purpose of desalination, purification, concentration or refining.
