the calcium-carbonate balance

general: natural water

As discussed in the sections water’s chemistry, terms used by water analysts and natural water, it is the geological structure of the ground that collects, drains and stores water and that helps to determine the water’s "appearance".

Low solubility granite structures are associated with water that has a low salt content but that is likely to contain a lot of CO₂ (depending on the quality of the upper layers); conversely, alluvial or karstic areas will produce water with high ion concentrations, especially Ca²⁺, Mg²⁺ and HCO₃^– ions. As it flows on the surface (rivers-lakes), water enters into an equilibrium with the atmosphere, receives pollution… all of which contributes to the gradual change in its original properties.

soft water/hard water

As a consequence of the above, water hardness, measured by its TH (in F°) will vary widely from one area to the next (tables 43 and 44).

For example, water that has a TH < 5 °F will be termed "soft", and "hard" from a TH of between 15 and 20°C.

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Throughout the world:

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aggressive water/scaling water

Before we even embark on a theoretical interpretation of calcium carbonate dissolution/precipitation phenomena, it is quite logical to deduce that water with low salt contents will have retained a dissolution capacity with regard to the materials they come into contact with (recipients, pipes …).

Conversely, water with high salt contents and especially salts belonging to the alkaline earths, can deposit the less soluble of these salts; they will tend to create deposits that result in the formation of crystals at the solid-liquid interface.

A particular feature of calcium bicarbonate is that it only exists in the dissolved form; when it precipitates, it does so in the calcium carbonate form, one that is only slightly soluble and it can only remain in water when it is balanced out by available CO₂:

As the CO₂ content has an influence on pH, we can conclude that, for a given mineralisation (fixed CaH and M-alk.), there will be a pH value for the equilibrium between this water and the calcium carbonate: this value is termed the Langelier pHor saturation pH hence the designation "pH_S" (the corresponding CO₂ content then being know as the balancing CO₂, a concept that will be quantified further on).

Therefore, we can "classify" water on the basis of its pH compared with this pH_S:

• if pH > pH_S, water will tend to deposit CaCO₃ and is then designated as scale-forming;
• if pH < pH_S, water will tend to dissolve CaCO₃ and is then termed aggressive.

Note: later on, we shall see that the pH_s value of water rises as its M-alk. (and, therefore, its mineralisation) decreases. E.g. (figure 99) for a pH that is systematically set to 8 (discharge into a mains system), lightly mineralised water will have a higher pH_s and will, therefore, still be aggressive; on the other hand, a more mineralised water will have a lower pH_s and will, therefore, be scale-forming; this explains the degree of "correlation" that links soft water with aggressive water, or hard water with scale-forming water, despite different meanings.

Carbonate calcium potential precipitation (CCPP) is an interesting parameter for quantifying what may be scale-forming or may have dissolving properties (pipes made from concrete) knowing that the CCPP is a level of concentration expressed in CaCO₃ (positive if water can cause scaling or negative if water is aggressive).

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