Safe water

Maintaining safe water for the bathers.


Chlorine for water disinfection.

Chlorine is used for continuous disinfection of the water in the swimming pool. It kills harmful microorganisms that can cause health problems like gastroenteritis, Legionnaires disease, ear infections and athlete's foot. In case of heavy usage of the pool, the test of the chlorine levels should be performed more frequently.

There are 3 types of measurement of chlorine levels in the pool water.

1. Free Available Chlorine (FAC).

Free Available Chlorine (FAC) is the amount of total remaining chlorine in the water that has not reacted with the contaminants and is “free” to go and kill other contaminants.

There is a link between pH level and the activity of chlorine. The lower the pH level the more active free available chlorine there is to kill the viruses and bacteria.

Free Chlorine is the sum of HOCl and OCl-. Depending on the pH of the water (see the graph below), the hypochlorous acid (HOCl) will partially dissociate to the hypochlorite ion (OCl-). Both hypochlorous acid (HOCl) and hypochlorite (OCl-) disinfect water but hypochlorous acid (HOCl) is a more effective disinfectant. HOCl is more active free chlorine where OCI- is available free chlorine that acts slower. The lower the pH, the higher the proportion of HOCl, as free available chlorine is more active in acid water.

Illustration of the relationship between free active chlorine and pH



Therefore, when algae are present, or when shock treatment is needed, it is recommended to lower the pH and add liquid chlorine or calcium hypochlorite to kill the algae and disinfect the water (shock treatment). Liquid chlorine (Norlex Hypochlorite) or calcium hypochlorite (Norlex Granulate) are recommended as these products do not contain cyanuric acid (stabilizer). Please look under cyanuric acid for more information here.

It is important to have a test that can measure FAC (free available chlorine). We recommend you use our proposal of Photometers, please look under Test Tools & Accessories

2. Combined Available Chlorine (CAC).

The amount of chlorine in the water that has reacted and combined with ammonia, nitrogen, urine, and other waste from swimmers. This type of chlorine has no disinfecting properties as it has been used. 

3. Total Chlorine.

The sum of the Free Available Chlorine (FAC) and Combined Available Chlorine (CAC). 

To maintain good water quality in the indoor pools (where you don’t use stabilized chlorine), it is advised to keep FAC levels between 0.5 and 1.5 ppm, depending on the pH value.

Regarding the outdoor pools the FAC level will depend on the pH level and the maximum level of cyanuric acid (stabilizer) allowed.  It is advised to maintain chlorine level between 0.1-1.5 ppm. Please read further on the role of cyanuric acid below.

Influence of cyanuric acid on the activity of chlorine in the outdoor pools.

Cyanuric acid is used mainly in the outdoor pools to stabilize the chlorine.

Sun’s ultraviolet can cause loss of chlorine in the water. When a swimming pool is exposed to a continuous sunlight radiation, 90-95% of the free chlorine will have evaporated within 2 hours. Therefore, chlorine used for shock treatment should be applied after dark. Adding stabilizer or cyanuric acid helps to reduce excess loos of chlorine by the ultraviolet light.

However, high levels of cyanuric acid will make the chlorine less active, thus reduce the disinfection effect, and the water may become over stabilized. When at the end of the season, the pool water turns green, but your chlorine level readings are within range, it is because the levels of cyanuric acid are too high, and the water is over stabilized.

When water contains cyanuric acid, the reading of the chlorine concentration with the DPD1 reagent, shows the available chlorine level and not the free available chlorine.

Therefore, at the beginning of the season to treat the algae and when shock treatment is needed, we recommend using calcium chlorine (Norlex Granulate) and liquid chlorine (Norlex Hypochlorite), as these products do not contain cyanuric acid.

Keep in mind that liquid chlorine will increase the pH of the water faster where the calcium chlorine will also increase the pH but not as high.

Norlex Multi Tabs 4 in 1 have an added stabilizer. Sun Guard is a separate stabilizer, but it should NOT be used in combination with stabilized disinfectants or bromine.

If you are continuously using stabilized chlorine, the cyanuric acid will build up faster. To remove some of the content of cyanuric acid at the end of the season, you will need to refresh some of the water. For example, if the content of the cyanuric acid reads 5 % above the recommended dosage, you will have to refresh 5% of the water. The same applies when the level of Total Dissolved Solids is above the recommended range. You cannot remove the TDS by chemicals as that would be adding more minerals that are also part of TDS.

When measuring the levels of the chemical components in the pool water, always look first at the content of cyanuric acid.


Shock treatment.

The strong smell of chlorine is not a sign that too much chlorine has been used but it can be an indicator that more chlorine is needed to sanitize the water. Shock treatment is based on adding larger amount then normal dosage of oxidizing chemicals into the water to destroy organic contaminants and oxidize the ammonia and nitrogen compounds, thus clear the area of irritating chloramine odor.

For this purpose, you can use Norlex Granulate or Hypochlorite, please follow the instructions and read the safety data sheets before application.

While treating the pool water with shock treatment, it is important to turn the circulation pump on.


Any questions about water balance?

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