Though calcium phosphate scale is a newly discovered problem for the swimming pool industry, it has long been recognized in the boiler industry, where the practice of using polyphosphonates to control calcium carbonate scale is often used.
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
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
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
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
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.