Controlling calcium hardness

By Karen Rigsby and Zach Hansen

Maintaining appropriate levels of calcium hardness is a key balance factor in basic recreational water care. Typically, water in a vinyl-lined or fibreglass swimming pool should contain a calcium hardness level of 175 to 225 parts per million (ppm), while the recommended range for plaster-surfaced pools is a little higher at 200 to 275 ppm. In addition, the overall balance of calcium hardness is directly related to other key balancing factors, total alkalinity (TA) and pH. Managing calcium hardness is extremely important due to the problems that can occur if it is not properly controlled.

Low calcium hardness

If the calcium level in swimming pool water is too low, it can cause plaster and grout to deteriorate and metal to corrode. The composition of plaster and grout includes calcium-based minerals such as silicates (Si), hydroxides (OH⁻), carbonates (CO₃²⁻), and sulfates (SO₄²⁻₄). If the water in contact with these surfaces does not have sufficient calcium hardness, these calcium-based minerals can slowly dissolve, leaving voids in the surface. This is a result of material erosion called surface etch. Etching can be uniform or irregular and can cause a roughened outward appearance. Calcium (Ca) can also leach from areas below the surface by diffusion, which can result in loss of structural strength and, possibly, breaking away of significant amounts of underlying material.

Alternatively, although the metal corrosion process can be complex, it is known that low levels of calcium hardness can accelerate corrosion of certain metal alloys—particularly on hot surfaces such as heat exchangers. Thus, insufficient levels of calcium hardness can reduce the service life of components made from such metal alloys and contaminate the water with dissolved metals that can ultimately cause staining.

It is important for a pool operator to understand the cost of operating a pool with low calcium hardness far outweighs the cost to treat it, as low calcium hardness can be easily corrected. For example, a calcium hardness increaser can be used as a cost-effective method to boost calcium levels and appropriately balance pool water. However, an effective method of water testing is also critical for an operator to measure calcium hardness levels in a pool, and then adjust accordingly.

High calcium hardness

On the other hand, if calcium hardness is too high, precipitation (the formation of a suspension of an insoluble compound when mixing two solutions) can occur, which can cause cloudy water or scale on surfaces. The upper limit recommendations for calcium hardness exist to prevent the pool from becoming oversaturated with calcium-based minerals, leading to mineral precipitating from solution. The most commonly formed calcium-based mineral is calcium carbonate (CaCO₃), which is how management of calcium hardness directly describes the concentration of bicarbonate (HCO₃⁻) and carbonate (CO₃⁻²) ions in the water, where pH determines their subsequent ion ratio: bicarbonate to carbonate (HCO⁻:CO₃⁻²). At higher pH, a greater concentration of carbonate ions will exist in the water.

It is simple to understand how calcium reacts with carbonate to form insoluble calcium carbonate by looking at the following reaction depicted in Figure 1.

It is important that pH is maintained to minimize the concentration of carbonate. However, calcium carbonate can precipitate even though most of the alkalinity (A_T) is in the form of bicarbonate, if either calcium hardness or alkalinity levels are too high. This process is illustrated in the reactions below. First calcium combines with bicarbonate to produce soluble calcium bicarbonate. Then, calcium bicarbonate reacts to create precipitated calcium carbonate, carbon dioxide and water (see Figure 2).

When calcium carbonate precipitates it can cause cloudy water and sediment to settle on the pool or spa floor. This places a demand on the filtration system and can create a need for more frequent vacuuming. In more severe cases, calcium carbonate can attach to surfaces, called scale. It is important to prevent scaling on heaters as it impedes the efficiency of heat transfer, wastes energy, promotes localized conditions of under-deposit corrosion and shortens heater life. On other surfaces such as pool walls, scale can create a dull unattractive appearance and cause the surface to feel rough. It could also clog pipes and filters. Ideally, calcium levels should never reach more than 400 ppm. However, in hard source water areas, where calcium hardness could be higher than 400 ppm straight from the tap, scale formation can be managed through the use of scale-inhibiting products.