Permanent hardness

Permanent hardness of water, also known as non-carbonate hardness, is a measure of the concentration of calcium and magnesium salts that are not precipitated by boiling. This type of hardness is primarily caused by the presence of sulfate and chloride salts of calcium and magnesium. Unlike temporary hardness, which can be removed by boiling the water, permanent hardness requires more complex treatment methods to be softened.

Causes

Permanent hardness is caused by the dissolution of minerals containing calcium and magnesium into the water supply. These minerals can include gypsum (calcium sulfate), anhydrite (calcium sulfate), and dolomite (calcium magnesium carbonate). The presence of these minerals in the water results in the formation of calcium and magnesium salts that are not removed by heating the water.

Effects

The presence of permanent hardness in water can have several effects on household appliances, industrial processes, and even personal care. In household appliances, such as water heaters and kettles, the hardness can lead to the formation of scale, which reduces the efficiency and lifespan of these appliances. In industrial processes, hard water can cause scaling in boilers and cooling towers, leading to increased energy costs and maintenance. For personal care, hard water can make it difficult to form a lather with soap, leading to increased usage of detergents and soaps.

Treatment

The treatment of permanent hardness typically involves the use of ion exchange resins, reverse osmosis, or electrodialysis. Ion exchange resins are commonly used in water softeners, where calcium and magnesium ions are exchanged for sodium or potassium ions, effectively removing the hardness. Reverse osmosis and electrodialysis are more complex processes that involve the removal of ions from the water through the use of semi-permeable membranes.

Measurement

The measurement of permanent hardness is typically done in terms of milligrams per liter (mg/L) or parts per million (ppm) of calcium carbonate (CaCO3). This measurement is often part of a larger water quality analysis that includes tests for temporary hardness, total hardness, and other water quality parameters.

Conclusion

Permanent hardness is an important water quality parameter that affects both domestic and industrial water use. Understanding the causes and effects of permanent hardness, as well as the methods for its treatment, is essential for managing water resources effectively and ensuring the longevity of water-using appliances and infrastructure.