In the realm of industrial water treatment, the term "hardness" is a fundamental concept that refers to the presence of dissolved calcium (Ca2+) and magnesium (Mg2+) ions in water. These minerals are called “hardness” because their presence keeps soap from “sudsing” and thus makes water “hard” to wash with. When present in significant quantities, they can cause a variety of problems in industrial water systems.
Why is Hardness a Concern?
Hardness can lead to scale formation, which can decrease the efficiency of heat transfer, restrict water flow, and promote corrosion. In industrial settings, these issues can result in increased energy consumption, equipment damage, and costly downtime.
- Scale Formation: Hardness exhibits retrograde (or inverse) solubility, which means the hotter the water, the less soluble it is. This can lead to scale deposits on heat transfer surfaces, pipes, and other equipment, forming a hard, insulating layer of scale. Hardness ions, particularly calcium, can react with carbonate, sulfate, phosphate, and other anions, forming low solubility compounds.
- Reduced Heat Transfer: Scale acts as an insulator, reducing the efficiency of heat transfer in boilers and heat exchangers. This can lead to higher energy costs and reduced 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 also create areas of localized corrosion, particularly under the deposit. This is called under-deposit corrosion and can lead to equipment failure.
How is Hardness Measured?
- Titration: Hardness is typically measured by titration using a solution of ethylenediaminetetraacetic acid (EDTA), a chelating agent that binds to calcium and magnesium ions. Indicators are used to determine when all the hardness ions have been bound by EDTA.
- 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: The concentration of calcium and magnesium is usually expressed 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:
- Precipitation 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: This process involves passing water through a bed of resin that exchanges hardness ions (calcium and magnesium) for sodium ions. Sodium zeolite softeners are a common example.
- 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 a critical aspect of industrial water treatment. Regular testing of hardness levels in both the makeup water and system water helps to determine the effectiveness of treatment programs and prevent scale buildup and related problems.
- 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: Measuring hardness can be an indicator of how well pretreatment equipment is performing (e.g., softeners, NF, RO, and demineralizers).
- Boiler System Hardness: Maintaining low levels of hardness in boiler feedwater is crucial. Hardness in boiler feedwater can cause scale deposits, decreased 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.
Conclusions
In summary, total hardness, caused by calcium and magnesium ions, is a key parameter in industrial water treatment. Understanding, monitoring, and controlling hardness levels are essential for preventing scale formation, ensuring efficient heat transfer, protecting equipment from corrosion, increasing safety, and ultimately, minimizing costs and downtime.
Written by: James McDonald, PE, CWT