What Is Water Hardness?

In industrial water treatment, “hardness” refers to dissolved calcium (Ca2+) and magnesium (Mg2+) ions in water. These minerals hinder soap effectiveness, making water difficult to use for washing. Significant levels can cause various issues in industrial systems.

Why is Hardness a Concern?  

Hardness can cause scale formation, reducing heat transfer efficiency, restricting water flow, and promoting corrosion. In industrial settings, these problems increase energy use, damage equipment, and lead to costly downtime.

• Scale Formation: Hardness shows inverse solubility, so hotter water dissolves less. This leads to scale deposits on heat transfer surfaces, pipes, and equipment, forming a hard, insulating layer. Calcium ions can react with carbonate, sulfate, phosphate, and other anions to form low-solubility compounds.

  
  

• Reduced Heat Transfer: Scale insulates surfaces, lowering heat transfer efficiency in boilers and heat exchangers. This increases energy costs and reduces productivity.

  
  

• Restrict Water Flow:  Deposits on pipes, heat exchangers, and other equipment can grow large enough to restrict the flow of water, further impacting efficiency and system performance.

  
  

• Corrosion: Scale deposits can cause localized corrosion beneath the deposit, known as under-deposit corrosion, which may result in equipment failure.

How is Hardness Measured?

• Titration: Hardness is usually measured by titration with ethylenediaminetetraacetic acid (EDTA), which binds calcium and magnesium ions. Indicators show when all hardness ions are bound.

  
  

• Total Hardness:  Analytical tests are available for measuring both Total Hardness and Calcium Hardness.  Since Total Hardness is defined as the sum of calcium and magnesium ions, Magnesium Hardness is simply the difference between Total Hardness and Calcium Hardness.

  
  
  
  

  Total Hardness = Calcium + Magnesium

  
  

  Magnesium Hardness = Total Hardness – Calcium Hardness

• Expressed as CaCO3: Calcium and magnesium concentrations are typically reported in milligrams per liter (mg/L) or parts per million (ppm) as calcium carbonate (CaCO3).

• Hardness ranges: Common ranges to define levels of hardness include:

• Soft for Boiler Feedwater: 0 – 1 mg/L as CaCO3

  • Soft:  0 to 60 mg/L as CaCO3

  • Moderately Hard:  60 and 120 mg/L

  • Hard:  120 and 180 mg/L

  • Very Hard:  Greater than 180 mg/L

How is Hardness Removed?

Several methods are used to remove or reduce hardness in industrial water systems.

Softening: This method involves adding chemicals like lime (calcium hydroxide) and soda ash (sodium carbonate) to precipitate hardness as calcium carbonate and magnesium hydroxide.

• Ion Exchange Softening: Water passes through resin beds that exchange calcium and magnesium ions for sodium ions. Sodium zeolite softeners are commonly used.

• Nanofiltration (NF) & Reverse Osmosis (RO): These membrane filtration processes remove a wide range of impurities from water, including hardness ions, by forcing water through a semi-permeable membrane under pressure.

• Demineralization: This process removes all mineral salts from water, including hardness, through the use of ion exchange resins.

  
  

Why is Hardness Important to Monitor and Control?

Monitoring and controlling hardness is essential in industrial water treatment. Regular testing of makeup and system water helps assess treatment effectiveness and prevent scale buildup and related issues.

• Raw Water Hardness: It is important to know the level of raw water hardness (e.g., city water, well water) entering the facility so pretreatment can be properly sized and the proper chemical treatment program can be selected to prevent scaling in boiler, cooling, and other water systems.

• Pretreatment Performance: Hardness measurement indicates the effectiveness of pretreatment equipment, such as softeners, NF, RO, and demineralizers.

• Boiler System Hardness: Maintaining low hardness in boiler feedwater is crucial, as high levels can cause scale deposits, reduce heat transfer efficiency, and lead to boiler failure.

• Cooling System Hardness: Hardness can lead to scale formation on heat transfer surfaces and within the system’s piping. Careful monitoring and control of hardness levels is important to ensure the system continues to operate efficiently and effectively.  Scaling indices, such as the Langelier Saturation Index (LSI), can be used to predict the potential for scale formation.

Conclusion

In summary, total hardness from calcium and magnesium ions is a key factor in industrial water treatment. Understanding, monitoring, and controlling hardness prevents scale, ensures efficient heat transfer, protects equipment, increases safety, and reduces costs and downtime.