Water hardness occurs as an indirect side effect of various chemical compounds. Calcium and magnesium are the two primary minerals that make up hardness in water. Like alkalinity and pH, hardness affects the tendency of the water to be corrosive or scale-forming. (Scale is a deposit that forms on pool walls and equipment when the mineral content of the water is too high.) By maintaining the ideal ranges for hardness and alkalinity, you can keep scale formation to a minimum.

Low hardness levels require immediate attention! They can be very dangerous to your system. Water that is not properly saturated with hardness—water in which the hardness level is too low—will be very corrosive, causing significant damage to metal pipes and fixtures as well as plaster. You must be sure to balance hardness before adding any sanitizer to the water. Otherwise, the water will become even more aggressive (corrosive); it can cause severe damage in a short period of time.

When the hardness level is low, increase the hardness immediately to limit the damage of corrosive water. You can increase the hardness level by adding a chemical like calcium chloride. When the hardness level is too high, excessive scale formation occurs, and the water may become cloudy or discolored. Elevated pH and warmer temperatures will increase scale build-up too. If the hardness level is too high, you can partially drain and refill with fresh water.

The ideal level of hardness for your pool or spa water is from 200 – 400 ppm (mg/L). You should test hardness when adding fresh water, and re-test until you have balanced the water hardness properly. After that, test hardness a minimum of once per month throughout the season. If you use calcium hypochlorite as a sanitizer, you need to test more frequently to ensure that the level has not exceeded the upper limit.

What is Calcium Hardness?

Calcium hardness is the amount of dissolved calcium (plus some other minerals like magnesium) in the water. The word dissolved is important - if you can see calcium scaling up the pipework or the surface of the pool, it is no longer dissolved - it has stolen a march on you. Too much calcium means cloudiness and scaling up, too little could lead to the water satisfying its appetite for calcium by taking it from your grouting.

Understanding Calcium Hardness

The term "water hardness" originated with the use of soap for laundering and cleaning. Certain ions in water combined with the chemicals in soap to form a solid precipitate, or scum, and made it hard to get soap to lather. Thus, water with more than 100 ppm of hardness ions was called hard water. The hardness ions are primarily calcium and magnesium. Sometimes, such others as iron and aluminum are also included but are often neglected because they are easily removed in the water-treatment process. With the development of detergents, the problem of hard water in laundering was greatly reduced, but the term is still commonly used.

Because calcium ions combine with carbonate ions to form the calcium carbonate needed for water saturation, it is important that the calcium level be closely monitored. Therefore, the pool operator needs to measure the calcium hardness to determine the calcium factor (CF) for the Langelier Index. The method for determining calcium hardness is given in the chapter on pool water testing.

Like pH and alkalinity, calcium hardness affects the tendency of water to be corrosive or scaleforming. It also appears to affect the kind of scale formed. When a calcium carbonate precipitate occurs in soft water, the scale particles are large and coarse. This is seen in many tap water pipes where the local water treatment plants soften the water to 100 ppm or less.

Hard water, however, appears to produce a protective scale that has smaller, finer particles that prevent corrosion. Thus, the pool operator should test and maintain calcium hardness at 200 ppm, or higher, both to provide sufficient calcium ion for saturation as calcium carbonate (50 ppm minimum) and to ensure that, if a scale forms at all, it is the less harmful form (200 ppm or more).

The acceptable maximum calcium hardness depends on the amount of total alkalinity needed for pH buffering. If a particular pool tends to change pH rapidly, higher total alkalinity (over 100 ppm) is needed. Calcium hardness should not exceed 400-600 ppm, depending on the pH and temperature of the water. The exact values can be calculated using the Langelier Index.

Some pools tend to have very little pH drift and can use a lower total alkalinity (less than 100 ppm). Under these conditions, calcium hardness may reach 800 ppm, or more, without causing cloudy water or scale formation. Again, all of the factors for water saturation must be considered in determining the proper level to maintain.

Controlling Calcium Hardness

Calcium hardness is increased by the addition of hydrated calcium chloride, a readily available form of calcium salt. Use 10 pounds of calcium chloride (80% CaCI2) for each 10,000 gallons of water to raise the calcium hardness 80 ppm.

The only convenient way to reduce calcium hardness, however, is to remove some of the pool water and replace it with fresh water. Very often, normal splashout by swimmers and filter backwash procedures remove enough water to maintain an acceptable calcium level. With high temperatures and excessive evaporation rates, additional water may have to be drained periodically to lower calcium hardness levels.

Adjusting Calcium Hardness

You want a hardness level between 200 and 400

Raise: ozs calcium chloride = gals x ppm x .00016

Raise: ozs calcium chloride dihydrate = gals x ppm x .0002

To raise hardness in 9,500 gallons of water.
50 ppm with 76 oz's or 4.75 lbs of Calcium Chloride or
50 ppm with 95 oz's or 6 lbs of Calcium Chloride Dihydrate

To lower calcium hardness, drain some water and refill.