Pre-treatment Methods of Membrane Treatment Technology

In the process of membrane treatment technology widely used in the field of water treatment, pretreatment is crucial. Effective pretreatment can ensure the stable operation of the membrane system, extend the service life of the membrane, and improve the quality of water production. The following will introduce the common pretreatment methods of membrane treatment technology.

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 Filtration pretreatment

Multi-media filtration: Multi-media filters are commonly used filtration pretreatment equipment, filled with a variety of different particle sizes and materials of the filter media, such as quartz sand, anthracite, garnet and so on. Its working principle is based on the deep filtration mechanism, the raw water passes through the filter media layer from top to bottom, the suspended solids, colloids and other particles in the water are intercepted by the filter media and adsorbed on the surface of the filter media or in the internal pore space. Larger particles are first intercepted by the upper layer of coarse filter media, with the depth of the water flow, smaller particles are captured by the lower layer of fine filter media. For example, in the treatment of surface water, multi-media filters can effectively remove sediment, algae and other impurities, reduce the turbidity of the water, for subsequent membrane treatment to provide cleaner feed water. Regular backwashing is the key to the maintenance of multi-media filters, through the combined effect of reverse water flow and compressed air, so that the filter media loosened, the retained impurities discharged, to restore the filter's filtration capacity.

Precision Filtration (Security Filtration): Precision filters usually use filter elements with a filtration precision of 5 - 1μm, such as meltblown filter elements, folded filter elements, etc. It is installed as the last filtration process before the membrane treatment system. It is installed in the membrane treatment system before the last filtration process, the main role is to intercept after multi-media filtration residual particles, colloids and some microorganisms, to prevent these impurities from entering the membrane module, resulting in membrane clogging and damage. Take the reverse osmosis membrane system as an example, the security filter can effectively remove particles with a particle size larger than 5μm to ensure that the water quality entering the reverse osmosis membrane meets the requirements and guarantees the stable operation of the membrane system. The filter element needs to be replaced regularly, when the pressure difference between the filter inlet and outlet reaches the set value, it indicates that the filter element has been blocked by impurities and should be replaced in time to maintain a good filtration effect.

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 Adsorption pretreatment

Activated carbon adsorption: activated carbon has a huge specific surface area and rich microporous structure, which makes it has a very strong adsorption capacity. In membrane pretreatment, activated carbon is mainly used to adsorb organic matter, residual chlorine, odor and some heavy metal ions in water. For the natural organic matter in the water, such as humic acid, fulvic acid, etc., activated carbon removes them through physical adsorption and chemical adsorption, reducing the risk of organic matter contamination of the membrane. At the same time, activated carbon can effectively remove residual chlorine in the water, which has strong oxidizing property and will oxidize and destroy the membrane material. Through the adsorption reaction of activated carbon, the residual chlorine will be converted into harmless substances to protect the membrane module. The operation of the activated carbon adsorption tank requires regular monitoring of import and export water quality indicators, such as organic content, residual chlorine content, etc. When the activated carbon adsorption is saturated, it needs to be replaced or regenerated, and common regeneration methods include thermal regeneration and chemical regeneration.

Ion exchange resin adsorption: Ion exchange resin is a polymer material with exchangeable ion groups, divided into cation exchange resin and anion exchange resin. In the membrane treatment pretreatment, ion exchange resin is mainly used to remove specific ions in water and regulate water quality. For example, strong acidic cation exchange resin can remove calcium and magnesium ions in water, reduce water hardness, and prevent scale formation on the membrane surface. When the raw water contains heavy metal ions, a specific chelating resin can be used for adsorption to remove. The adsorption process of the ion exchange resin is an ion exchange reaction, where the exchangeable ions on the resin are exchanged with the target ions in the water to achieve the purpose of removing or enriching the target ions. Resin failure, regeneration through the regeneration agent to regenerate, restore its adsorption capacity, such as cation exchange resin commonly used hydrochloric acid or sulfuric acid for regeneration.

| Softening pretreatment

Chemical softening method: chemical softening method is to add chemicals to the raw water, such as lime, soda ash, etc., through a chemical reaction to make the water calcium, magnesium ions to form insoluble precipitates, thereby reducing the hardness of water. Lime - soda ash softening method, for example, to add lime to the water (Ca (OH)₂), calcium bicarbonate in the water can be generated with the reaction of calcium carbonate precipitate, to remove the temporary hardness of the water; add soda ash (Na₂CO₃), can be generated with the reaction of calcium chloride in the water, magnesium sulphate precipitation of calcium carbonate and magnesium hydroxide, to remove the hardness of the **. The chemical softening method is suitable for raw water with high hardness and is widely used in large-scale industrial water treatment. However, the method will produce a large amount of sludge, need to be properly handled, and at the same time to pay attention to the control of the amount of chemicals added, to avoid excessive addition to the subsequent membrane treatment has an adverse effect.

Ion exchange softening method: In addition to the above ion exchange resin for specific ion adsorption, ion exchange softening method mainly uses strong acidic cation exchange resin, with sodium ions (Na⁺) as the exchange ions, and calcium and magnesium ions in the water for exchange. The calcium and magnesium ions in the water are adsorbed by the resin, while the sodium ions on the resin are released into the water, thus realizing water softening. Ion exchange softening method has the advantages of good softening effect and stable water quality, which is often used in the pretreatment of membrane treatment system with high water quality requirements, such as the preparation of ultrapure water in the electronic chip manufacturing industry. Ion exchange softening system requires regular regeneration of the resin to maintain its softening capacity.

| Disinfection pretreatment

Chlorine disinfection: Chlorine disinfection is one of the most commonly used disinfection pretreatment methods, by adding chlorine gas (Cl₂), sodium hypochlorite (NaClO) or chlorine dioxide (ClO₂) and other chlorine-containing disinfectants to the raw water, the use of chlorine's strong oxidizing properties to kill bacteria, viruses and other microorganisms in the water. Chlorine in water will be hydrolyzed to generate hypochlorous acid (HClO), hypochlorous acid has a strong bactericidal ability, able to destroy the cell structure of microorganisms and enzyme systems, so as to achieve the purpose of disinfection. Chlorine disinfection has the advantages of good disinfection effect, low cost, easy to use, etc., but will produce disinfection by-products, such as trihalomethanes (THMs), etc., which are potentially harmful to human health. Therefore, when using chlorine disinfection, it is necessary to strictly control the amount of disinfectant dosage and contact time, and monitor the disinfection by-products.

Ultraviolet disinfection: Ultraviolet disinfection uses ultraviolet (UV) radiation to destroy the DNA or RNA structure of microorganisms, causing them to lose their ability to reproduce and survive, thus achieving the purpose of disinfection. UV disinfection has the advantages of fast disinfection speed, no disinfection by-products, no change in the chemical properties of water, etc. It is suitable for membrane treatment systems with high water quality requirements and sensitive to disinfection by-products, such as drinking water treatment. Ultraviolet disinfection equipment usually consists of ultraviolet lamps, quartz casing and reactor, etc. In the operation process, it is necessary to regularly clean the quartz casing to prevent its surface fouling to affect the penetration rate of ultraviolet light, and at the same time, it is necessary to regularly replace the ultraviolet lamps to ensure that disinfection effect.

There are various pretreatment methods of membrane treatment technology, and each method has its unique principle, scope of application, advantages and disadvantages. In practical application, according to the raw water quality, membrane treatment process requirements and treatment costs and other factors, comprehensive selection of suitable pretreatment methods, and reasonable combination and optimization to ensure that the membrane treatment system can operate stably and efficiently, and produce high-quality water to meet the requirements. In the actual engineering cases, the application effect and precautions of different pretreatment methods can be further analyzed in depth by combining specific data and operation experience.