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Eventually the polyphosphonates break down to orthophosphate, which then bonds with calcium to form calcium phosphate scale.

As preventive actions for calcium phosphate scaling have become commonplace for commercial boiler water, the problem of calcium phosphate scale has had to be addressed, through proper management of calcium phosphate levels in make-up water, as well as in the amount of usage of phosphate-based scale products.

What is it?

Calcium phosphate is found in phosphate rock and is largely mined for the production of certain cleaners and detergents. It also makes up about 20 percent of our skeletal system, and is the majority material within our teeth. It is vital to healthy growth of humans and plants.

But while it may be good for living things, calcium phosphate scale can be detrimental to pool and hot tub systems.

Scale is formed in recreational water by precipitation and crystal growth on surfaces in contact with water. Three of the common mineral compounds in water are calcium carbonate, calcium phosphate and magnesium silicate. These three compounds remain soluble in lower-temperature water. In higher temperatures they supersaturate and precipitate on surfaces in the form of scale.

Calcium phosphate scale is formed in water when minerals containing calcium ions combine with orthophosphate. The result in pools and spas is an insoluble white scale on the surfaces of pools and equipment. Since calcium phosphate precipitates at higher temperatures, it tends to be more of a problem in heated pools or spas. Precipitated calcium phosphate can lead to the destruction of heat exchangers by coating the heating elements — preventing heat from being dispersed and potentially burning the elements out.

What causes it?

One of the possible reasons for the increased appearance of calcium phosphate scale in swimming pools could be the increased use of metal and scale remover products, which contain phosphates such as polyphosphonates (HEDP) or phosphoric and phosphonic acid. When these products are used, the phosphates eventually end up in the orthophosphate form.

In pools where orthophosphate levels climb and high calcium is present, calcium phosphate could precipitate in higher-temperature water. In the orthophosphate calcium scenario, tri-calcium phosphate is formed from the reaction of orthophosphate with calcium at the heat surface. 

Why must it be prevented?

Phosphate removal has become a standard in the swimming pool industry to remove the orthophosphate prime nutrient, which algae can use to resist typical chlorine and algaecide treatments as well. If a test determines that phosphates are present in the pool’s water, phosphate removers are recommended to keep resistant algae strains from flourishing in the absence of a chlorine sanitizer or an EPA-registered algaecide. 

There is another reason to be concerned about high levels of orthophosphate in swimming pool and hot tub water — especially in areas with high water hardness, or those where calcium hypochlorite and calcium chloride are used: High levels of phosphate combined with high calcium hardness can lead to the formation of calcium phosphate scale, particularly on heat exchangers, and permanent equipment damage or failure may be the result.

Most metal removal and prevention products are either polyphosphonates or blends of phosphoric or phosphonic acid. While these are effective in dealing with certain metal stains and scale, they also eventually break down to orthophosphate, which will accumulate over time.

In one recent case, a commercial aquatic facility in Arizona used a phosphate-based chemical on a weekly basis. The pool was heated, and also had high levels of calcium.

The facility itself went through several heat exchangers in a short time-frame. Each of the heat exchangers were covered with a white “scale-like” material. After laboratory testing, the white material was determined to be calcium phosphate scale. A phosphate removal treatment was then conducted, and the phosphate-based chemical was switched to a nonphosphate-based product.

As a result, the problems associated with the failing heat exchangers at this facility are gone.

Preventative action is particularly important for pools in which phosphate-based metal removal or sequestering products are being used regularly, as well as in heated pools where calcium is present. This latter category would include pools in areas whose water has high calcium hardness, such as Arizona, as well as pools that regularly use calcium hypochlorite.

Another possible treatment is ethylenediaminetetraacetic acid (EDTA), a non-phosphate anti-scale chemical.