Ion Exchange Method
Ion exchange method uses spherical resin (ion exchange resin) to filter raw water, and ions in the water will exchange with ions fixed on the resin. The two common ion exchange methods are water softening for hard water and deionization. Water softening is primarily used as a pre-treatment before reverse osmosis (RO) processing to reduce water hardness. In the softening machine, spherical resin softens the water by exchanging two sodium ions for one calcium or magnesium ion. Ion exchange resins use hydrogen ions to exchange with cations and hydroxide ions to exchange with anions. For example, cation exchange resin made from styrene and divinylbenzene with sulfonic acid groups exchanges hydrogen ions with various cations (e.g., Na+, Ca2+, Al3+). Similarly, anion exchange resin made from styrene with quaternary ammonium salt exchanges hydroxide ions with various anions (e.g., Cl-). The hydrogen ions released from cation exchange resin and hydroxide ions from anion exchange resin combine to produce pure water. Cation and anion exchange resins can be separately packaged in different ion exchange beds, forming so-called cation exchange beds and anion exchange beds. They can also be mixed together and placed in the same ion exchange bed. Regardless of the form, after the resin exchanges hydrogen ions and/or hydroxide ions with charged impurities in the water, a “regeneration” process is necessary. The regeneration process is the opposite of the purification process, using hydrogen ions and hydroxide ions to regenerate and remove impurities attached to the ion exchange resin. When ion exchange method is combined with other water purification methods (e.g., reverse osmosis, filtration, and activated carbon adsorption), it plays a crucial role in the entire purification system. While ion exchange effectively removes ions, it is not efficient in removing most organic compounds or microorganisms. Microorganisms can adhere to the resin, using it as a nutrient base, allowing rapid growth and heat generation. Therefore, it needs to be used in conjunction with other purification methods in the system design.
Activated Carbon Adsorption Method
Organic compounds may be cations, anions, or non-ionic substances. Ion exchange resin can remove some soluble organic acids and alkalis (anions and cations) in raw water, but some non-ionic organic compounds may be coated by the resin in a phenomenon called “resin fouling.” This not only reduces the lifespan of the resin but also diminishes 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 organic substances. The adsorption process of activated carbon utilizes the pore size of the activated carbon filter and the permeability of organic substances through the pores. The adsorption rate depends on the molecular weight and size of organic compounds. Some particulate activated carbon is more effective in removing chloramines. Activated carbon can also remove free chlorine in water to protect other purification units in the pure water system. Activated carbon is often used in combination with other treatment methods. In the design of a pure water system, the relevant configuration of activated carbon with other purification units is a crucial aspect.
Microfiltration Method
Microfiltration method includes three types: deep filtration (depth), screen filtration (screen), and surface filtration (surface). Deep filter membranes are made of woven fibers or compressed materials, using random adsorption or capture to retain particles. Screen filter membranes have a consistent structure, like a sieve, trapping particles larger than the pore size on the surface (the pore size of this membrane is very precise). Surface filtration is a multi-layer structure, and when the solution passes through the membrane, particles larger than the internal pores of the membrane will be retained and mainly accumulate on the surface of the membrane.
Due to the different functions of these three membranes, it is crucial to distinguish between them. Deep filtration is a more economical method, capable of removing over 98% of suspended solids while protecting downstream purification units from damage or blockage, making it suitable as a pre-filtration process. Surface filtration can remove over 99.99% of suspended solids, making it suitable for pre-filtration or clarification. Microfiltration membranes (screen filter membranes) are generally placed at the final point of use in the purification system to remove the last traces of resin fragments, carbon debris, colloidal particles, and microorganisms. For example, a 0.22μm microfiltration membrane can filter out all bacteria and is commonly used for sterilization of intravenous injections, sera, and antibiotics.
Ultrafiltration Method
Microfiltration membranes remove particles based on their pore size, while ultrafiltration (UF) membranes act as molecular sieves, separating molecules in a solution based on their size.
UF membranes are strong, thin, and selectively permeable membranes that can retain most molecules above a specific 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 large molecules in the retained liquid, but some large molecules may still leak into the permeate.
UF membranes come in various ranges, and in all cases, the UF membrane will retain most molecules with a molecular weight greater than that defined by its molecular sieve.
Reverse Osmosis Method
Reverse Osmosis (RO) is the most economical method with impurity removal rates ranging from 90% to 99%. The pore structure of RO membranes is denser than that of UF membranes. RO membranes can remove all particles, bacteria, and organic substances with a molecular weight greater than 300, including heat sources.
When two solutions of different concentrations are separated by a semi-permeable membrane, osmosis naturally occurs. Osmotic pressure pushes water through the semi-permeable membrane, diluting the solution with higher concentration, ultimately reaching concentration equilibrium. In water purification systems, pressure is applied to the high-concentration solution to counteract osmotic pressure. This forces pure water to pass through the RO membrane from the high-concentration liquid, which can then be collected. Due to the high density of the RO membrane, the water flow is very slow, and it takes a considerable amount of time for a sufficient amount of water to accumulate in the storage tank.
RO membranes can perform ion exclusion, allowing only water to pass through while retaining and excluding all ions and dissolved molecules (including salts and sugars). RO membranes exclude ions based on charge reactions, with larger charges leading to higher exclusion rates. Therefore, RO membranes can exclude almost all (>99%) strongly ionic high-valence ions. However, for weakly ionic monovalent ions (such as sodium ions), the effectiveness is only around 95%. Different types of RO membranes are required for different feedwater qualities, including those made of cellulose acetate fibers or a mixture of thin-layer polymerization of polysulfone and polyamide substrates.
If the feedwater quality and product water quality are taken as benchmarks and properly designed, RO is the most economically effective method for purifying tap water. RO is also the best pretreatment method for laboratory-grade water systems.
Ultraviolet Irradiation Method
Ultraviolet (UV) irradiation has been widely used in water treatment. The 254nm UV radiation from low-pressure mercury lamps is an effective method of sterilization because the DNA and proteins in bacteria absorb UV light, leading to their death.
Recent advancements in UV lamp manufacturing technology have allowed the production of UV lamps that simultaneously emit wavelengths of 185nm and 254nm. This combination of wavelengths can utilize photooxidation to reduce the total organic carbon concentration in pure water to below 5ppb.
Electrodeionization (EDI) Pure Water Technology
Electrodeionization (EDI) is a technology that combines ion exchange resin and ion-selective permeable membranes with direct current to remove ionized impurities from water. The development of this technology overcomes the limitations of ion exchange resin, especially the release of ion impurities when the ion exchange column is depleted and the work of refilling or regenerating the ion exchange column.
Water passes through one or more chambers filled with ion exchange resin between cationic or anionic selective membranes. Under the influence of an electric field, ions move between the ion exchange resin and enter another chamber, and this process also electrolyzes to generate H+ and OH- needed to maintain the resin in a regenerated state. Ions flowing into the independently chambered sides are flushed away by water.
Distillation Method
The distillation method separates water and contaminants by changing the water’s form from liquid to gas and then back to liquid. Each conversion process provides an opportunity for the separation of pure water and contaminants. In theory, distillation can remove all types of water impurities except substances and azeotropes close to water in vapor pressure.
Like RO, the distillation method produces pure water at a slow rate, so distilled water must be stored for later use. Distillation units are power-intensive, typically consuming 1KW of electricity to produce 1 liter of pure water. Depending on the design of the distillation unit, the resistivity of distilled water can reach approximately 1 MΩ-cm, as CO2 from the air quickly dissolves into distilled water, reducing its conductivity. Freshly distilled water is sterile, but if stored improperly, it may lose its sterility over time.
KDF (Kinetic Degradation Fluxion)
Functions and Benefits of KDF
KDF is a high-purity copper/zinc alloy that works in water treatment through a process known as redox (micro-electrochemical oxidation-reduction) reactions. When in contact with water, the two metals in the alloy form countless small galvanic cell systems at a submicroscopic scale. This material demonstrates powerful reactivity and an extremely fast reaction rate in water, effectively removing up to 99% of chlorine and dissolved metal ions and compounds like lead, mercury, nickel, and chromium. It also exhibits significant inhibitory effects on the growth of bacteria, fungi, algae, and dirt. KDF is used in pretreatment, main treatment, and wastewater treatment equipment. Its implementation or replacement of existing technologies can significantly extend system life, reduce heavy metals, microbes, and dirt, and lower overall costs, simplifying system maintenance.
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Removal of Strong Oxidants (Chlorine):
KDF possesses potent reduction capabilities, effectively removing various strong oxidants in water, particularly chlorine.
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Removal of Heavy Metals:
The KDF treatment medium can eliminate various heavy metal ions in water, including lead, mercury, copper, nickel, cadmium, arsenic, antimony, aluminum, and many other soluble heavy metal ions. Removal occurs through displacement and physical and chemical adsorption reactions.
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Removal of Hydrogen Sulfide:
When using membrane methods for water treatment with groundwater as the water source, hydrogen sulfide may be present. KDF filtration media can remove hydrogen sulfide, and the formed copper sulfide is insoluble in water, which can be flushed away during KDF medium backwashing.
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Reduction of Suspended Solids:
The average particle size of KDF treatment media is approximately 60 mesh, with the smallest particles around 110 mesh. It can also act as a physical filter to effectively remove particles smaller than 50 μm.
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Reduction of Mineral Scaling:
KDF helps reduce mineral scaling in water treatment processes.
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Inhibition of Microbial Growth:
KDF treatment media utilizes several mechanisms to control the growth and reproduction of microorganisms, including changes in redox potential, the formation of hydroxide ions and hydrogen peroxide, and the dissolution of zinc in the medium. When used as a pretreatment for reverse osmosis membranes, KDF can inhibit the reproduction of bacteria, algae, and other microorganisms, preventing damage to the membrane.
