Introduction to the method and equipment of softening water

The process of reducing the content of calcium and magnesium ions in water to turn hard water into soft water is called water softening. The main methods are as follows:

1. Boiling method (only for temporary hard water)

Reaction when boiling temporarily hard water:

Ca(HCO3)2=CaCO3↓+H2O+CO2↑

Mg(HCO3)2=MgCO3↓+H2O+CO2↑

Since CaCO3 is insoluble and MgCO3 is slightly soluble, magnesium carbonate can also react with water to form more insoluble magnesium hydroxide under further heating conditions:

MgCO3+H2O=Mg(OH)2↓+CO2↑

It can be seen that the main components of scale are CaCO3 and Mg(OH)2

2. Medicament softening method

The classic industrial water treatment method is chemical softening method, such as adding lime (CaO), sodium phosphate and so on. Adding lime can make the carbon dioxide, calcium bicarbonate and magnesium bicarbonate in the water generate precipitation of calcium carbonate and magnesium hydroxide. For hard water with high permanent hardness, add appropriate amount of soda ash. The amount of lime added when softening, according to experience, every time the temporary hardness of one thousand liters of water is reduced, 10 grams of pure calcium oxide should be added. During the reaction, magnesium is precipitated in the form of magnesium hydroxide, and calcium is precipitated in the form of calcium carbonate.

Ca2+(aq)–lime-soda method–>CaCO3(s)

Mg2+(aq)–lime-soda method–>Mg(OH)2(s)

3. Ion exchange method

It uses ion exchangers to exchange ions in the water with diffusible ions in the ion exchanger to soften the water. Most beverage water uses organic synthetic ion exchange resins as ion exchangers. When treating water, let the water pass through the cation column from top to bottom, so that the metal ions in the water are absorbed by the cation exchange resin, and the hydrogen ions in the cation exchange resin are exchanged into the water; then pass through the anode column to make the water The anions are absorbed by the anion resin, and the anion resin exchanges the hydroxide ions into the water and ionizes the hydrogen to synthesize water to purify the water.

4. Electrodialysis and ultrafiltration technology

The electrodialysis method uses the selective permeability of the anion and cation exchange membranes to the ions in the water under the action of an external direct current electric field, so that the anions and cations in the water move to the anode and the cathode through the anion and cation exchange membranes respectively to achieve purification. This technology is often used to prepare primary pure water from tap water.

5. Reverse osmosis method (ultrafiltration technology)

It uses pressure as the driving force to increase the water pressure to overcome the osmotic pressure, and make the water pass through the functional semi-permeable membrane for desalination and purification. The reverse osmosis method can also remove colloidal substances, and the water utilization rate can reach more than 75%; the reverse osmosis method has a large water production capacity and simple operation, which can effectively purify the water to meet national standards.

The choice of equipment depends entirely on your treatment water volume, water quality and required water quality.

The ion exchanger is usually referred to as a softener. Generally speaking, the jar is filled with ion exchange resin, and the hardness ions (calcium and magnesium ions) are exchanged by substitution (such as sodium ions). Generally speaking, it is used for occasions where the water volume is not very large (below 100 tons) and the hardness is not particularly high. Otherwise, the amount of salt used for resin regeneration is quite large, and the replacement wastewater treatment is also a troublesome matter.

After softening, the hardness of the effluent water can generally be controlled at about 0.03, but it only reduces the hardness and does not improve the TDS in the water.

No matter how large the amount of water or the hardness is (>10), it can be considered that the agent is softened and the equipment investment is low. Or reverse osmosis, high equipment investment.

The desalination rate of the other three methods is UF<RO<EDI. Generally speaking, UF can be used as RO pretreatment, and RO effluent can be used as EDI inlet water.

UF ultrafiltration is mainly used to remove suspended solids.
RO reverse osmosis can fully remove various ions in the water body (desalination rate>99%), but the pretreatment needs to be set up, and the investment cost is relatively high. It is used in occasions that have certain requirements for the conductivity of the water body. It is often used for drinking water treatment or boiler make-up water softening treatment in power plants (boiler make-up water not only needs to remove hardness to prevent scaling, but also consider the corrosion of pipes due to the level of salt content).
EDI is used in occasions that require high water resistivity, such as ultrapure water in the electronics industry.

Ion exchangers can also be used. Ion exchange is one of the main methods of softening and desalination in water treatment. The essence of ion exchange is the exchange reaction of insoluble ions (ion exchangers) with other same ions in the solution, usually Reversible chemical adsorption. The choice of exchange resin has a lot to do with the pH of the water to be treated. Generally speaking, the ionization of strong acid and strong base active groups is basically independent of pH. Only weakly basic exchange resins have strong exchange capacity in acidic solutions. The acid exchange resin has a strong exchange capacity in the alkaline solution, so it depends on the nature of your influent water. If it is tap water, you can just check it a little. Follow this idea to find it. That’s right. Make this equipment There are many manufacturers, if you are interested, you can also make your own. The technical threshold is not very high! !
RO, UF, and electrodialysis are indeed very good desalination methods, but considering that they are used in the textile industry instead of producing ultrapure water, the cost is also a big problem, so it is more practical to consider ion exchange in a comprehensive way.

Pentair Water Softener Tank

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