What is a Scaling Environment?

Calcium carbonate scaling is a type of precipitation and occurs when calcium (Ca2+) and carbonate (CO32-) ions dissolved in a water react to form solid calcium carbonate (CaCO3).
When the product of the calcium ([Ca2+]) and carbonate ion ([CO32-]) concentrations is greater than the calcium carbonate solubility product (KS), the water is supersaturated with respect to calcium carbonate and precipitation can occur. Conversely, when the product of the calcium and carbonate ion concentrations is less than the calcium carbonate solubility product, the water is undersaturated with respect to calcium carbonate and the water can dissolve solid calcium carbonate; thus scaling will not occur.

[Ca2+][CO32-] > KS, Water Supersaturated,
CaCO3 Can Precipitate

[Ca2+][CO32-] < KS, Water Undersaturated,
CaCO3 Dissolves


The calcium carbonate solubility product KS is not constant, but decreases with increasing temperature. Therefore, a water that is undersaturated at a low temperature may become supersaturated when heated to a higher temperature. In general, a water's tendency to scale increases with increasing temperature. This is why, in areas with hard water, scaling tends to be worst in appliances that heat water, such as solar hot water systems, electric water heaters, coffee makers, etc... Mathematical functions that relate KS to temperature can be found in the literature (AHPA, 1989).

In general, the more a water is supersaturated with respect to calcium carbonate, the faster scaling will occur. Therefore, the most important predictor of a water's tendency to scale is the degree to which it is supersatured with respect to calcium carbonate; estimating the a water's degree of supersaturation requires knowledge of the water's chemistry.

When a water is analyzed, typically the calcium ion concentration, [Ca2+], and/or the total hardness is measured. Total hardness is the sum of the dissolved calcium and magnesium ion concentrations.

Total Hardness = [Ca2+] + [Mg2+]

The total hardness of most domestic waters is predominately due to calcium. A water's tendency to scale increases with increasing calcium concentration and hardness.

When a water is analyzed, the carbonate ion concentration is not measured directly, but must usually be estimated from a pH and total alkalinity measurement. The carbonate ion concentration, and therefore a water's tendency to scale, increases with both pH and total alkalinity.

Finally, due to the competing effects of temperature on pH, alkalinity, and the calcium carbonate solubility product, the tendency of lower pH waters (pH ~7.5) to scale increases much more rapidly with temperature than for higher pHs waters (pH ~ 8.5) (Baker, 2000).

In Summary, Scaling Mainly Varies with Water Chemistry and Temperature:

  • Water chemistry: Scaling increases with increasing Calcium Concentration, Hardness, Alkalinity and pH.
  • Temperature: Scaling increases with increasing temperature. Scaling rates increase much more rapidly with temperature for waters with lower pHs (~7.5) than for those with higher pHs (~8.5).

When describing a water, it is important to make a distinction between the qualitative term hard and the quantitative measure hardness. A water is described as hard if it tends to produce scale, irrespective of its hardness. For a given hardness, a water may or may not produce scale, depending on its alkalinity, pH and temperature. Therefore, knowing a water's hardness in absence of its alkalinity and in particular its pH does not indicate if the water will tend to scale.

For more detailed information on water chemistry and scaling refer to Baker (2000), Snoeyink and Jenkins (1980), and Loewenthal and Marais (1976).

Next: Where does Scaling Commonly Occur in a SHW System?
Previous: What is Scaling?
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Created by Derek Baker
Last Updated: 8/16/2001