Differences between various process waters

Fresh water is water that has been treated by sedimentation and disinfection； Deionized water is water from which cations and anions have been removed ； Soft water is water from which cations have been removed.

1 Process water, which is fresh water, is supplied from water sources or water treatment plants and is used directly in the production process. 2 Soft water. It refers to water in which the hardness (mainly referring to calcium and magnesium ions in the water) has been removed or reduced to a certain extent. During the softening process of water, only the hardness decreases, while the total salt content remains unchanged. 3 Desalted water. It refers to water in which salts (mainly strong electrolytes soluble in water) have been removed or reduced to a certain level. Its conductivity is generally 1.0–10.0 μS/cm, its resistivity at 25°C is 0.1–1.0×10^6 Ω•cm, and its salt content is 1–5 mg/L. For further reading: Methods of softening water include: (1) Lime-soda method. First, the hardness of the water is determined, after which a fixed amount of calcium hydroxide and sodium carbonate is added; the calcium and magnesium ions in the hard water then precipitate out. Ca(HCO3)2 + Ca(OH)2 = 2CaCO3↓ + 2H2O 　　(2) Phosphate softening method. For boiler water, sodium phosphite (NaPO3) can be added as a water softener; it forms complexes with calcium and magnesium ions, preventing these ions from precipitating out when the water is boiled, and thus avoiding the formation of scale. This method is not suitable for softening drinking water. 　　Mg(HCO3)2 + 2Ca(OH)2 = Mg(OH)2↓ + 2CaCO3↓ + 2H2O 　　(3) Ion exchange method. Although both zeolites and ion exchangers are insoluble in water, the sodium and hydrogen ions contained in them can undergo exchange reactions with the calcium and magnesium ions in hard water, causing these calcium and magnesium ions to be adsorbed by the zeolites, artificial zeolites, and ion exchangers and thus removed. Zeolites and ion exchangers that become ineffective after long-term use can be reused through regeneration; therefore, this method is an economical and advanced softening technique. 　　CaSO4 + Na2CO3 = CaCO3↓ + Na2SO4 　　(4) Chemical addition method: Adding specialized scale inhibitors to water can alter the way calcium and magnesium ions combine with carbonate ions, thereby preventing the formation and deposition of scale. There are many scale inhibitors available for industrial use today. The advantage of this method is that it requires less initial investment and has wide adaptability ； However, the operating costs are high when the water volume is large and soft; due to the presence of chemicals, its use is greatly restricted, and it generally cannot be used for drinking, food processing, industrial production, and other purposes. It is also rarely used in civilian applications. 　　(5) Electromagnetic method: A certain electric or magnetic field is applied to water in order to alter the properties of ions, thereby changing the rate of deposition of calcium carbonate (magnesium carbonate) as well as its physical properties during deposition, and thus preventing the formation of hard water scale. Its features are: low equipment investment, easy installation, and low operating costs ； However, its effectiveness lacks stability, and there are no unified criteria for measurement. Moreover, since its main function is merely to affect the physical properties of scale within a certain range, the duration for which the treated water can be used and the distance over which it can be utilized are limited. It is mainly used for the treatment of circulating cooling water in commercial applications such as central air conditioning, and cannot be applied to the treatment of water used in industrial production or as boiler make-up water (as the mechanism behind this type of equipment has not been fully theoretically proven). 　　(6) Membrane separation method: Both nanofiltration membranes (NF) and reverse osmosis membranes (RO) can remove calcium and magnesium ions from water, thereby significantly reducing water hardness. The advantage of this method is its significant and stable effectiveness, along with the wide range of applications for the treated water ； However, it requires a high inlet water pressure, and both the equipment investment and operating costs are high. It is generally used less often, and is mostly applied in specialized softening treatments. The main methods for producing desalinated water are as follows: ① Distillation, which involves heating salty water to cause it to evaporate, and then condensing the vapor to obtain desalinated water ； ②In the ion exchange method, salt-containing water is passed through an exchange column filled with zeolite or ion exchangers (see ion exchange); ions such as calcium and magnesium remain in the exchange column, while the filtered water is desalinated water ； ③In electrodialysis, thanks to the selective permeability of ion exchange membranes to ions, under the influence of an external electric field, the cations and anions in the water between the two ion exchange membranes move respectively toward the negative and positive electrodes through those membranes. Thus, the area between the membranes becomes a fresh water zone, while the area outside the membranes is a concentrated water zone. Water drawn from freshwater areas is desalinated water. 　　Distillation is commonly used in laboratories to clean containers or prepare solutions, and it is suitable for situations where small quantities are involved and high purity is required. Ion exchange and electrodialysis methods are commonly used in the chemical industry, for example in boiler water, to reduce scaling and corrosion; they are suitable for applications where large volumes of water are required and the purity standard is not very high. This post was last edited by haichuaner on 2009-3-30 22:10]

3# haichuaner explained it very clearly; I now understand. Thank you.

What is the difference between process water, fresh water, and primary water?

I’ve learned it; it was explained very clearly. Thank you:handshake