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Chlorine & Other Disinfectants


In the Care & Feeding of Pool Waters we will cover some of the most important chemicals used in pool sanitation and maintenance, why they are used, some testing procedures and just as important, what not to do to maintain a clean sanitized pool that is enjoyable to swim in.

The proper chemical treatment of pool waters depends upon many factors. Some of these factors are inherent to the water used to fill the pool, such as pH and hardness. Other factors are dependent upon what chemicals we add to the pool to maintain it, such as what type of chlorine is used. Also pool use conditions, a heavy swimmer load or light load, can have a dramatic effect upon some chemical characteristics.

Chlorine is the most common chemical used to disinfect pool waters. As most pool operators know, chlorine can be purchased in several different forms. The forms most frequently used in pools are: liquid chlorine (sodium hypochlorite), dry chlorine powder and tablets (calcium hypochlorite), and stabilized chlorine, usually as tablets (chloroiso-cyanurates, also called various triazine compounds). Also used are lithium hypochlorite, a powder and bromine, both tablets and powder. Chlorine gas was used in swimming pool water in the past. We are not aware of any pool currently using chlorine gas. Recenty some locations are using chlorine generated by putting an electric current through salt water. While all of these compounds are used for sanitizing pool water, they can have very different effects on the pool chemistry.

The reasons we use chlorine or bromine are:

1. To kill bacteria and other germs that can be passed through the water and might cause disease. This is the most important reason!

2. To kill algae that can be in the water. While algae are not normally a health hazard, they can discolor and damage a pool.

3. To oxidize and destroy organic matter that is introduced into the pool water (known as superchlorination or shocking the pool).

4. To oxidize and help precipitate (settle out) inorganic compounds, such as iron and manganese, in the water.

The last two reasons help produce a clean sparkling pool without heavy chlorine or other odors.

When chlorine is introduced into water at commonly found pHs, it forms free chlorine. Free chlorine is the common term for the total amount of hypochlorous acid and hypochlorite ions in the water. At pH 7.5 these two chemical species are in a 50:50 mix. This free chlorine is the strong bactericide that we want in the pool water. It can kill disease-causing bacteria in a matter of seconds. It is also a strong oxidizer that can help remove unwanted compounds.

When free chlorine reacts with organic matter some of the free chlorine is converted into combined chlorine. Combined chlorine is the common term for a group of compounds called chloramines. Combined chlorine is bad news. It is not a good sanitizing agent, killing bacteria at only 1/100th the speed of free chlorine. Combined chlorine also irritates the eyes and has a very heavy "chlorine-type" odor. The sum of free chlorine and combined chlorine equals total chlorine.

Many people feel that when they get eye irritation or smell a heavy chlorine odor in their pools that this means there is too much chlorine in the water. This is usually wrong. What they are sensing is combined chlorine. The way to get rid of combined chlorine is to superchlorinate. This means raising the free chlorine level up to or above 10 ppm. Superchlorination causes the combined chlorine to be oxidized (burned up) and leaves the pool with only a free chlorine residual. People should not be using the pool during superchlorination. For those of you in the water treatment business this is also called "breakpoint chlorination".

Pursuant to communications with the New Jersey State Department of Health and Senior Services and based upon their interpretations of the American National Standards Institute (ANSI) free chlorine and bromine levels in public recreational swimming pools, whirlpool, spa and hot tubs Garden State Laboratories has made a change in our reporting procedure.

Free Chlorine levels in both indoor and outdoor swimming pools will be noted as conforming to ANSI standards when equal to or between 1.0 and 10.0 ppm.  ANSI's ideal range is 2.0 to 4.0 ppm.  In whirlpools, spas and hot tubs the free chlorine should be 2.0 to 10.0 ppm with an ideal range of 3.0 to 5.0 ppm.

The ANSI standard for bromine in swimming pools and whirlpools, spas and hot tubs is 2.0 to 10.0 ppm.  The ideal bromine range is 4.0 to 6.0 ppm.

Swimming pool water and whirlpools, spas and hot tubs water should be within acceptable standards for other parameters such as pH, combined chlorine, total alkalinity, stabilizer and pool water clarity.

Chlorine and bromine are tested in pool water using only DPD test kits. They must be tested and the results recorded in a bound logbook every two hours that the pool is in operation.

As pool operators know, chlorine dissipates very rapidly in hot sunny weather. Chlorine reacts with all types of compounds in water, including dirt, sweat, and urine. The ultra-violet (UV) rays of the sun also cause the chlorine in the water to rapidly dissipate. In recent years a chemical called stabilizer (cyanuric acid) has been used to help "stabilize" free chlorine from the UV rays of sun. Cyanuric acid does work, but, like all chemicals, it has to be used properly. When added to water containing free chlorine, the cyanuric acid reacts with the chlorine to form a more stable complex that helps to retain the chlorine in the water helping to cut down on chlorine usage and costs. Since stabilizers only prevent the sun's rays from dissipating the chlorine they are banned from use in indoor pools.

Never use stabilized compounds for superchlorination!

High levels of stabilizer reduces the killing rate of chlorine.

For more information about chlorine stabilization click here to go to our web page on chlorine stabilization and cyanuric acid.

The type of chlorine used in pools can have a dramatic effect on the pH. The recommended pH range is 7.2 to 7.8, ideal 7.4 to 7.6.

Both liquid chlorine (sodium hypochlorite) and most dry powders (calcium hypochlorite) tend to raise the pH through the formation of sodium hydroxide or calcium hydroxide. An acidic compound such as muriatic acid or sodium bisulfite may be needed to lower the pH.

Chlorine gas lowers the pH. When chlorine gas is introduced into water it forms one molecule each of hypochlorous acid and hydrochloric acid. The first is a weak acid and is the compound we want in the water. Hydrochloric acid is a strong acid and lowers the pH. An alkaline material such as soda ash or caustic soda is needed to raise the pH.

Chlorine added to pool water by the electrolysis of salt water tends lowers the pH. It also generates an alkaline solution of sodium hydroxide that can be added to the pool water to raise the pH.

To find out more about pH click here to go to our pH web page.

Other compounds such as ozone and ultraviolet light can also be used to disinfect pool waters. They do kill bacteria, however they do not leave a residual in the water to continue disinfection in the pool itself. While they may be used, a chlorine or bromine compound must be used to maintain disinfection in the pool.

We often get questions about other types of compounds, such as silver and copper ion probes used to disinfect pool waters. These compounds have not been shown to kill bacteria at anywhere near the disinfection rates of chlorine or bromine. In New Jersey, if you do use these compounds you still must maintain the regulatory chlorine or bromine levels. We do not recommend the use of any silver/copper ion probes.

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