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.
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).
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).
Created by Derek Baker Last Updated: