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Controlling calcium hardness

Preventing scale

Scale prevention/inhibition is the disruption of the scale formation process. This can be accomplished through various means such as sequestering calcium, binding to newly formed scale clusters (poisoning), or dispersing the scale clusters back into bulk solution. Some products offer one type of inhibition while others are more multi-functional.

A commonly used scale inhibitor is 1-Hydroxyethylidene-1,1-Diphosphonic Acid (HEDP). This particular compound is a great inhibitor as it binds to divalent (+2) cations such as iron and calcium.

Note the end of the molecule has essentially the same structure as carbonate, so the calcium molecule is likely to bind there. Once calcium ions begin to bind to the HEDP it makes it very difficult for the scale clusters’ crystal structure to continue growing. HEDP will prevent scale by sequestering calcium and inhibiting scale cluster growth.

Other non-phosphonated products such as polyacrylic acids (PAAs) or polycarboxylic acids, which act as dispersants, are also available. They do not provide inhibition by sequestering calcium or by poisoning. Rather, they work by dispersing scale clusters back into solution to prevent settling.

Also, compounds exist which distort the surface of the crystal structure of formed scale. By ‘rounding off’ the surface activation sites needed for further scale formation, the heterogeneous nucleation process described above is inhibited. These products can also work as a dispersant to prevent settling of scale clusters.

The best protection will come from using a multi-functional product, which employs a variety of methods (sequestration, poisoning and dispersion) to assist with scale treatment and prevention.



Rigsby_HeadshotKaren Rigsby is the leader of technical services for BioLab, a Chemtura Company. She has been involved with the recreational side of water treatment since 2001, focusing on education, problem resolution and new product development. She began her career in the water treatment industry at BioLab as an analytical chemist in the research and development group. Prior to recreational water, Rigsby was employed by the Georgia Bureau of Investigation as a forensic chemist. Rigsby received her bachelor of science in chemistry from Georgia Tech and is a member of the Association of Pool & Spa Professionals (APSP) Recreational Water Quality Committee and a National Swimming Pool Foundation (NSPF) certified instructor.


Hansen_HeadshotZach Hansen is a new product specialist for BioLab, where he started his career working in automated controller and feeder equipment development. Over the last four years he has focused on new product commercialization and development for the company. Hansen received his bachelor of science degree in chemical engineering from Auburn University in 2004. He can be reached via e-mail at

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2 comments on “Controlling calcium hardness”

  1. We replays trees our neighborhood pool in 2010. The calcium content got too low and the pool is extremely rough now. Now that the water is corrected and we are on the lookout for low calcium can we sand the bottom of the pool so that the kids stop getting blisters ? Or can we sand and paint the bottom of that would help?

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