Hello, I am a water treatment specialist with over 10 years of experience in the field. I've worked on various projects, from municipal water treatment plants to industrial water softening systems, and I'm familiar with different methods of removing permanent hardness from water.

Permanent hardness is caused by the presence of dissolved calcium and magnesium salts, primarily calcium sulfate (CaSO4) and calcium chloride (CaCl2). These salts are not easily removed by simple boiling, unlike temporary hardness caused by calcium and magnesium bicarbonates.

Here are the common methods used to remove permanent hardness:

1. Ion Exchange:

* This is one of the most widely used methods for softening water. It involves passing the hard water through a bed of ion exchange resin.
* These resins contain sodium ions (Na+) that are attracted to the calcium (Ca2+) and magnesium (Mg2+) ions in the hard water. The calcium and magnesium ions are then exchanged for the sodium ions, effectively removing the hardness from the water.
* The resin bed needs to be periodically regenerated using a concentrated sodium chloride (NaCl) solution (brine) to replace the sodium ions that were exchanged for the calcium and magnesium ions.

Advantages of Ion Exchange:

* Effective: It effectively removes both calcium and magnesium ions.
* Widely Available: Ion exchange resins are readily available and relatively inexpensive.
* Easy to Operate: Ion exchange systems are generally easy to operate and maintain.

Disadvantages of Ion Exchange:

* Sodium Content: The water treated with ion exchange will have a higher sodium content. This can be a concern for individuals on a low-sodium diet or who have specific health conditions.
* Regeneration Requirements: Regular regeneration is necessary, which requires a supply of brine and can contribute to waste disposal issues.

2. Reverse Osmosis:

* Reverse osmosis is a membrane-based technology that uses pressure to force water molecules through a semi-permeable membrane, leaving behind the dissolved impurities, including calcium and magnesium ions.
* The membrane allows water molecules to pass through but blocks the passage of larger molecules and ions, effectively removing the hardness.

Advantages of Reverse Osmosis:

* High Purity: It produces water with very low mineral content, including hardness.
* Versatile: It can remove a wide range of impurities, including bacteria and viruses.
* Environmentally Friendly: The process doesn't require harsh chemicals and produces minimal waste.

Disadvantages of Reverse Osmosis:

* Higher Cost: Reverse osmosis systems are generally more expensive to purchase and operate than ion exchange systems.
* Water Waste: The process generates a concentrated stream of rejected water ("brine") that needs to be disposed of.

3. Lime-Soda Softening:

* This method uses calcium hydroxide (lime) and sodium carbonate (soda ash) to precipitate out calcium and magnesium ions as insoluble compounds.
* The water is first treated with lime, which reacts with the bicarbonate ions to form calcium carbonate (CaCO3), which precipitates out.
* Then, soda ash is added, which reacts with the remaining calcium and magnesium ions to form more calcium carbonate and magnesium hydroxide (Mg(OH)2), both of which precipitate out, removing the hardness.

Advantages of Lime-Soda Softening:

* Cost-Effective: This method is generally cost-effective, particularly for large-scale applications.
* Removes Other Impurities: It can also remove other impurities like iron and manganese.

Disadvantages of Lime-Soda Softening:

* Complex Process: It requires a complex process with multiple steps and careful chemical control.
* Sludge Generation: The process generates a significant amount of sludge that needs to be disposed of.

4. Electrodialysis:

* This method uses an electric current to remove calcium and magnesium ions from water.
* Water is passed through a membrane stack containing alternating cation and anion exchange membranes.
* The electric current attracts the ions towards the oppositely charged membranes, separating the hard ions from the water.

Advantages of Electrodialysis:

* High Efficiency: It can achieve high levels of hardness removal.
* Low Chemical Usage: It doesn't require the use of chemicals like lime and soda ash.

Disadvantages of Electrodialysis:

* High Cost: Electrodialysis systems are expensive to purchase and operate.
* Scaling Potential: The membranes can be prone to scaling if not properly maintained.

5. Other Methods:

* Other methods, although less commonly used, include zeolite softening and demineralization.
* Zeolite softening uses naturally occurring zeolite minerals to exchange **sodium...read more >>

Temporary hardness is caused by dissolved calcium hydrogencarbonate (which is removed by boiling). Permanent hardness is caused by dissolved calcium sulfate (which is not removed by boiling). Water can be softened using washing soda or an ion-exchange resin.read more >>