Understanding Calcium Hardness in Swimming Pools

What is calcium hardness? It is the measure of the amount of dissolved calcium found in swimming pools and spa water. If this pool parameter which is part of the water chemistry in swimming pools is not right, it can lead to an aggressive pool; thus, causing damage to your pool surfaces and mechanical systems. In most cases this part of your swimming pool water chemistry is not tested very often.

However, you should check it and make any necessary adjustments as needed. Something you should know here. Total hardness is different from calcium hardness.

These are two completely different things but related entities and should be treated as such.

Total hardness is the sum of both calcium and magnesium.

It is sometimes expressed in grains of hardness. The hardness of the calcium in your spa or pool water is measured as calcium carbonate. Whenever this calcium carbonate is not in solution, we refer to it as scale.

How is Calcium Carbonate Recognized?

Calcium carbonate is a salt that is found in nature in the form of:

  • Chalk
  • Calcite
  • Aragonite
  • Limestone

This carbonate is not very soluble and it can easily drop out of solution. This usually occurs if we fail to maintain our swimming pool water balance.

The Effects of Low pH and Low Alkalinity

Whenever you have a low pH level in your swimming pool or spa and a total alkalinity that is also low, a low calcium hardness level can disrupt the equilibrium of the calcium carbonate in your pool water. These poor water parameters will make your pool water very corrosive or aggressive over time. Another healthy pool care concern that we face as pool and spa owners/operators, is the behavior of the calcium in water.

As I mentioned earlier, calcium is not very soluble. This is especially true as the temperature of your swimming pool or spa increases. Because of this problem, managing hot water systems such as hot tubs and spas becomes an exceedingly challenging task.

The Effects of Low Calcium Hardness

Ideally you want the hardness of the calcium level in your swimming pool to be between 200 and 400 ppm (parts per million). Whenever it reaches a level of 1000 ppm, maintaining the other factors in your pool water chemistry will be impossible. These include:

  • Your pool water balance.
  • Lowering swimming pool alkalinity.
  • Maintaining the pH level in your pool.

Low Calcium Hardness Problems

When your swimming pool water has a low calcium content, it tends to dissolve calcium carbonate from other areas.

For example, there are many pools that have finishes that are either made of cement or plaster.

Calcium carbonate is one of the main components in these types of pool surfaces.

Here are some of the problems that these types of swimming pool and spa environment cause:

  • Corrosive water
  • Etching of your pool plaster
  • Pitting of your pool concrete
  • Dissolve grout in your pool
  • Pitting of your pool deck surfaces
  • Staining of your pool surfaces
  • Failure of swimming pool heaters

High Calcium Hardness Level Problems

Elevated levels of this part of your pool water chemistry, could result in several problems which include:

  • Scaling pool water
  • Rough pool and spa surfaces
  • Clogged swimming pool filters
  • Cloudy water in your recreational water facility
  • Clogged swimming pool heater elements
  • Reduction in your pool water circulation
  • Possible skin and eye irritations
  • Lowering your swimming pool's total alkalinity of your pool water chemistry, is very difficult. This process often requires you to partially drain your pool or spa and refill it. When refilling your swimming pool or spa, you would want to use make-up or source water that has a very low level of calcium hardness.

    In some cases, you can pass your make up water through a water softener to reduce its calcium level. In order for you to maintain a balanced saturation index, you can lower both your pH and total alkalinity levels to compensate for the higher hardness of calcium in your water.

    It is especially important that your calcium stay in solution.

    There may be times when you will have to use a sequestering agent to complete this task.

    Using this method will help you prevent any scale formation.

    This is accomplished by interfering with the development crystal formation, due to calcium carbonate.

    Special tip: If you have a leak in your pool or spa, this could lead to low levels of calcium hardness.

    Calcium hardness in a swimming pool

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