Ion exchange method
The ion exchange method uses a spherical resin (ion exchange resin) to filter the raw water, and the ions in the water are exchanged with the ions fixed on the resin. Two common ion exchange methods are hard water softening and deionization. Hard water softening is mainly used in reverse osmosis (RO) treatment to reduce the hardness of the water before a pre-treatment process. The spherical resin in the softener softens the water by exchanging two sodium ions with one calcium ion or magnesium ion.
Ion exchange resins use hydrogen ions to exchange cations and hydroxide ions to exchange anions; cation exchange resins made of sulfonate-containing styrene and divinylbenzene exchange various cations (such as Na+, Ca2+, Al3+). Similarly, an anion exchange resin made of styrene containing quaternary ammonium salt will exchange various anions (such as Cl-) with hydroxide ions. The hydrogen ions released from the cation exchange resin combine with the hydroxide ions released from the anion exchange resin to produce pure water.
The anion and cation exchange resins can be packaged in different ion exchange beds, and divided into so-called anion exchange beds and cation exchange beds. It is also possible to mix the cation exchange resin and anion exchange resin together and place them in the same ion exchange bed. Regardless of the form, when the resin and the charged impurities in the water have exchanged the hydrogen ions and/or hydroxide ions on the resin, it must be “regenerated”. The regeneration procedure is just the opposite of the purification procedure. It uses hydrogen ions and hydroxide ions to regenerate and exchange impurities attached to the ion exchange resin.
If the ion exchange method is combined with other water purification methods (such as reverse osmosis, filtration, and activated carbon adsorption), the ion exchange method will play a very important part in the entire purification system. The ion exchange method can effectively remove ions, but cannot effectively remove most of the organic matter or microorganisms. The microorganisms can be attached to the resin, and the resin is used as a culture medium, so that the microorganisms can grow quickly and generate heat. Therefore, it needs to be designed and used with other purification methods.
Activated carbon adsorption method
Organic matter may be cationic, anionic or non-ionic. Ion exchange resin can remove some soluble organic acids and organic bases (anions and cations) in raw water, but some non-ionic organics will be coated by resin. This process The phenomenon called “pollution blocking” of the resin will not only reduce the life of the resin, but also reduce its exchange capacity. To protect the ion exchange resin, an activated carbon filter can be installed before the ion exchange resin to remove non-ionic organics.
The adsorption process of activated carbon is achieved by using the pore size of the activated carbon filter and the permeability of organic matter through the pores. The adsorption rate is related to the molecular weight and molecular size of the organic matter. Some granular activated carbon can effectively remove chloramine. Activated carbon can also remove free chlorine in the water to protect other purification units in the pure water system that are sensitive to oxidants.
Activated carbon is usually combined with other treatment methods. When designing a pure water system, the configuration of activated carbon and other related purification units is an extremely important item.
Microporous filtration methods include three types: depth, screen, and surface. The depth filter membrane is a matrix made of woven fibers or compressed materials, which uses random adsorption or capture methods to retain particles. The mesh filter membrane basically has a uniform structure, just like a sieve, it will retain particles larger than the pore size on the surface (the pore size of this kind of filter membrane is very precise), while the surface filtration is a multilayer Structure, when the solution passes through the filter membrane, particles with larger pores inside the filter membrane will be retained and mainly accumulated on the surface of the filter membrane.
Due to the different functions of the above three filter membranes, it is very important to distinguish between the filters. Since deep filtration is a more economical way, it can remove more than 98% of the suspended solids while protecting the downstream purification unit from corruption or blockage, so it is usually used as a pre-filtration treatment. Surface filtration can remove more than 99.99% of suspended solids, so it can also be used for pre-filtration or clarification. Microporous membranes (sieve filters) are generally placed at the final point of use in the purification system to remove the last remaining trace resin fragments, carbon chips, colloidal particles and microorganisms. For example: 0.22μm microporous membrane, which can filter all bacteria, is usually used to sterilize intravenous fluids, serum and antibiotics.
The microporous membrane removes particles according to its pore size, while the ultrafiltration (UF) membrane is a molecular sieve, which uses the size as the basis to allow the solution to pass through a very fine filter membrane to achieve the purpose of separating molecules of different sizes in the solution.
Ultrafiltration membrane is a strong, thin, and selective permeable membrane that can retain most of the molecules above a certain size, including colloids, microorganisms, and heat sources. Smaller molecules, such as water and ions, can pass through the membrane. Therefore, the ultrafiltration method can concentrate the macromolecules in the retentate, but some macromolecules will still leak into the filtrate.
There are several different ranges of ultrafiltration membranes. In all cases, the ultrafiltration membrane will retain most of the molecules larger than the molecular weight defined by its molecular sieve.
The reverse osmosis (RO) method is the most economical method that can achieve 90% to 99% impurity removal rate. The filter pore structure of RO membrane is denser than that of UF membrane. RO membrane can remove all particles, bacteria and organic matter with a molecular weight greater than 300 (including heat sources).
When the second solution of different concentration is separated by a semi-permeable membrane, osmosis will occur naturally. Osmotic pressure pushes the water through the semi-permeable membrane, and the water dilutes the solution with higher concentration, finally resulting in a concentration balance. In the water purification system, pressure is applied to a high concentration solution to counter the osmotic pressure. In this way, pure water passes through the RO membrane from a high-concentration liquid and can be collected. Due to the high density of the RO membrane, the output water flow is very slow, and it takes a considerable amount of time before there is enough water in the water storage tank.
RO membrane can perform ion exclusion, so that only water can pass through the RO membrane, and all other ions and dissolved molecules are trapped and eliminated (including salts and sugars). RO membrane eliminates ions by charge reaction. The greater the charge, the higher the rejection. Therefore, the RO membrane can eliminate almost all (>99%) strong ionic high-valent ions, but for weakly ionic monovalent ions (such as Sodium ion) is only 95% effective. Different influents require different types of RO membranes. RO membranes include cellulose acetate or a mixed thin-layer polymer of polythiamine and polysulfone matrix.