Pure water technology - water purification method

Ion exchange method

The ion exchange method filters raw water with a spherical resin (ion exchange resin), and ions in the water are exchanged with ions fixed on the resin. Two common ion exchange methods are hard water softening and deionization. Hard water softening is primarily a pretreatment procedure that reduces the hardness of water before it is treated by reverse osmosis (RO). The spherical resin in the softening machine softens the water by exchanging two calcium ions with one calcium ion or magnesium ion.

The ion exchange resin exchanges cations with hydrogen ions and exchanges anions with hydroxide ions; the cation exchange resin made of sulfonic acid-containing styrene and divinylbenzene exchanges various cations encountered by hydrogen ion exchange (for example, Na+, Ca2+, Al3+). Similarly, an anion exchange resin made of styrene containing a quaternary ammonium salt exchanges various anions (such as Cl-) encountered by hydroxide ions. The hydrogen ions released from the cation exchange resin are combined with the hydroxide ions released from the anion exchange resin to form pure water.

The anion-cation exchange resins can be packaged separately in different ion exchange beds into so-called anion exchange beds and cation exchange beds. It is also possible to mix the cation exchange resin with the anion exchange resin and place them in the same ion exchange bed. In either form, when the resin exchanges with the charged impurities in the water for hydrogen ions and/or hydroxide ions on the resin, it must be "regenerated." The procedure for regeneration is the reverse of the purification procedure, using hydrogen ions and hydroxide ions for regeneration, and exchanging impurities attached to the ion exchange resin.

If the ion exchange method is combined with other purified water quality 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 can not effectively remove most of the organic matter or microorganisms. Microorganisms can be attached to the resin and the resin can be used as a medium to allow the microorganisms to grow rapidly and generate a heat source. Therefore, it needs to be designed and used in conjunction with other purification methods.

Activated carbon adsorption

The organic matter may be a cationic, anionic or nonionic substance. The ion exchange resin removes some soluble organic acids and organic bases (anions and cations) from the raw water, but some nonionic organic substances are coated with the resin. The phenomenon of "contamination blocking" called resin not only reduces the life of the resin, but also reduces its exchange capacity. To protect the ion exchange resin, an activated carbon filter can be installed in front of the ion exchange resin to remove nonionic organics.

The adsorption process of activated carbon is achieved by utilizing the pore size of the activated carbon filter and the permeability of the organic matter through the pores. The adsorption rate is related to the molecular weight of the organic matter and its molecular size. Some granular activated carbon is more effective in removing chloramine. Activated carbon also removes free chlorine from the water to protect other oxidant-sensitive purification units in the pure water system.

Activated carbon is often used in combination with other treatment methods. The configuration of activated carbon with other relevant purification units is an extremely important project when designing a pure water system.

Microfiltration

Microfiltration methods include three types: depth depth, screen screen, and surface. The deep filter membrane is a matrix made of woven fiber or a compressed material, which is retained by random adsorption or capture. The mesh membrane is basically a uniform structure, like a sieve, which retains particles larger than the pore size on the surface (the pore size of this membrane is very precise), while the surface filtration is multi-layered. Structure, when the solution passes through the filter, particles larger than the pores inside the filter will be retained and mainly accumulated on the surface of the filter.

Due to the different functions of the above three filters, the resolution between the filters is very important. Since depth filtration is a more economical way to remove more than 98% of suspended solids while protecting the downstream purification unit from damage or blockage, it is usually treated as a pre-filtration. Surface filtration removes more than 99.99% of suspended solids, so it can also be used for pre-filtration or clarification. Microporous membranes (mesh membranes) are typically placed at the end point of use in the purification system to remove traces of residual resin fragments, carbonaceous debris, colloidal particles, and microorganisms. For example: 0.22μm microporous membrane, which can filter all bacteria, usually used for sterilization of intravenous fluids, serum and antibiotics.

Ultrafiltration

Microporous membranes remove particles according to their pore size, while ultrafiltration (UF) membranes are molecular sieves that pass the solution through a very fine filter to achieve separation of molecules of different sizes in solution.

Ultrafiltration membranes are tough, thin, and selective membranes that trap most molecules of a certain size, including: gums, microbes, and heat. Smaller molecules, such as water and ions, can pass through the filter. Therefore, the ultrafiltration method can concentrate the macromolecules in the retentate, but some macromolecules will leak into the filtrate.

Ultrafiltration membranes come in several different ranges, and in all cases, the ultrafiltration membrane will remain in a majority of molecules larger than the molecular weight defined by its molecular sieve.

Reverse osmosis

The reverse osmosis (RO) method is the most economical method for achieving 90% to 99% impurity removal rate. The pore structure of the RO membrane is denser than that of the UF membrane, which removes all particles, bacteria, and organic matter (including heat sources) with a molecular weight greater than 300.

When the second different concentrations of the solution are separated by a semipermeable membrane, the infiltration phenomenon occurs naturally. The osmotic pressure presses the water through the semi-permeable membrane, and the water dilutes the solution with a higher concentration, eventually causing a concentration balance. In a water purification system, pressure is applied to a high concentration solution to counter the osmotic pressure. This forces the pure water to pass through the RO membrane from a high concentration of liquid and can be collected. Due to the extremely high density of the RO membrane, the produced water flow is very slow and it takes a considerable amount of time to get enough water in the storage tank.

The RO membrane can perform ion exclusion such that only water can pass through the RO membrane, and all remaining ions and dissolved molecules are trapped and excluded (including salts and sugars). The RO membrane removes ions by charge reaction. The higher the charge, the higher the exclusion, so the RO membrane can almost eliminate all (>99%) strong ionic high-valent ions, but for weak ionic monovalent ions (such as The effect of sodium ion) is only 95%. Different influent waters require different types of RO membranes, including RO acetate esters or a thin layer of polymer blended with polysulfide and polysulfone matrices.

If properly designed, RO is the most cost-effective way to purify tap water based on raw water quality and water quality. RO is also the best pretreatment method for reagent grade pure water systems.

Ultraviolet irradiation

Ultraviolet irradiation has been widely used in water treatment. The 254 nm ultraviolet light emitted by low-pressure mercury lamps is an effective sterilization method because DNA and proteins in bacteria absorb ultraviolet rays and cause death.

Recent advances in UV lamp manufacturing technology have enabled the production of UV lamps that simultaneously produce wavelengths of 185 nm and 254 nm. This combination of wavelengths of light can be used to oxidize organic compounds, followed by the special organic carbon in pure water. The concentration is reduced to below 5 ppb.