Swimming Pool Blog

Calcium Hardness in Swimming Pools


by Rob Cox, January 5, 2010

Calcium Hardness in Swimming Pools

calcium chloride CaCL2
Discovered in 1603 by a fellow named Homberg, who noticed that when he fused this salt, it became phosphorescent, and for a long time it was known as Homberg's Phosphorous. Now known as Calcium Chloride, or CaCl2, it is found naturally in solution in sea water and in many mineral springs as a crystalline soluble salt. A close cousin of table salt, or sodium chloride, it is also found in the bloodstream of humans and animals.

Calcium Chloride is a very useful compound. Once taken as a tonic for the sick and infirmed, it also serves as an excellant de-icer. Many commercial ice and snow melting chemicals are a form of Calcium Chloride. Pellets tend to work better than flakes for this purpose. If you live in the snowbelt, that snowplow truck may be spreading Calcium Chloride instead of Sodium Chloride these days. Not quite as caustic, or damaging to asphalt and steel automobiles, a bit more viscous which keeps it on the road longer, and several dollars cheaper per pound than "regular" salt, make Calcium Chloride the preferred choice for snow melting.

You may have heard of the terms "soft water" or "hard water". This refers to the level of Calcium and other salts dissolved in water. Soft water, with a lower level of salts, will lather excessively with soap in the tub or shower, but is also harder to rinse off completely, leaving a bather feeling a little "slick". Hard water, with higher levels of dissolved salts, is more difficult to produce a soap lather, but rinses off quite well. The reason for this is the reaction of the salt ions with the molecules of sodium sterate found in most soaps. If your bath water leaves you feeling less than "zestfully clean", your soft water can be hardened by using household water treatment equipment to add salts to the water. More common is a household water softening system that removes salts and minerals from the water.

However, the purpose of this article is to discuss how Calcium Chloride is used in swimming pools. Why do we care about how hard or soft pool water is? It's not for the "feel" of the water, and I hope that you're not concerned with how well your soap lathers up in your pool.

When your pool water is too soft, under 150ppm, it becomes "agressive" and will try to increase it's own hardness level by actually pulling calcium out of pool plaster or grout between the tiles. In vinyl pools, there is very little calcium in the structure, but over time, such agressive or corrosive water will cause the liner to lose some of it's plasticity, and fail prematurely. Fiberglass pools can become etched and lose some of the gelcoat gloss when Calcium Hardness levels are too low. This is known as a Corrosive water condition.

When your pool water is too hard, over 400ppm, it becomes what is known as a Scaling water condition. When the Calcium levels are too high in the pool, they can precipitate out of solution, and deposit their crystals on pool surfaces, inside pipes or equipment, and/or create cloudy / hazy pool water. These deposits are sometimes known as "Scale". Those pool owners who live in hard water areas know what I am talking about. There are service companies that specialize in removing calcium deposits from tile lines and other pool surfaces with a bead blasting process. In extreme cases, pool filters can become clogged with calcium requiring a filter media change.

When your pool water is too soft, you can raise it by adding Calcium Chloride, or the less common Calcium Carbonate. Commonly, this is sold under many brand names - commonly referred to as "Calcium Hardness Increaser". You could also use a snow melt product, but make sure it is 100% Calcium Chloride or Carbonate. Always read the package instructions, but generally speaking, adding at a rate of 1 pound per 10,000 gals of water will raise your pool Calcium Hardness level by 10ppm. Calcium creates a good amount of heat when mixed with water, so it is recommended to pour the calcium into a bucket of water, stirring with something other than your hand, and then pour the mixture into the pool water.

Calcium levels do not deplete like Chlorine does; that is, once raised, it will stay fairly constant for a while. After months of backwashing and rain, it may begin to drop again if you are in a soft water area. For most pools, a once or twice per year addition of Calcium Hardness Increaser is sufficient. Make sure you are using a good pool test kit to accurately determine your Hardness level.

When your pool water is too hard, the general method is to lower by dilution. This means to drain some of the water, and refill with softer water. Not the simplest or cheapest method for some people. There is a product that will lower some of the Calcium Hardness levels in pools, this is known as Super Sequa Sol, and help to keep Calcium sequestered, or tied up in solution. Control of your pH and Alkalinity levels in the pool water is also important to help control high Calcium levels. The use of Calcium based sanitizers in your pool will contribute to Calcium Hardness levels, raising the level - Calcium Hypochlorite shock, or calcium based chlorine tablets.

Keeping your Calcium Hardness levels in your pool at the proper level, around 200-400 ppm, is one of the important facets of water balance. Next time we will look at the history and importance of Total Alkalinity in swimming pools.