Many people in charge of swimming pools with high bathing load complain about not being able to reduce the level of combined chlorine (CC) below 50% of free chlorine (FC) level, i.e. 0.5-1ppm CC for 1-2 ppm FC. Values above these can create problems with lowering CC level, even if shock treatment (super chlorination) is applied. One thing to keep in mind is that there are two different types of chloramines, and their treatment requires two different approaches.
When hypochlorite or chlorine gas is added to the water, it forms hypochlorous acid (HOCl), which acts as a disinfectant (kills bacteria, algae etc) as well as oxidiser (removes organic and inorganic impurities). This is the desired effect. The side effect, which is impossible to avoid, is reaction of hypochlorous acid with nitrogen-containing contaminants, introduced to the pool by bathers (sweat, traces of soap) and surrounding environment (tree leaves). The products of this reaction form a group called “combined chlorine” or “chloramines”. For the purposes of this article we will split combined chlorine in 2 groups: organic and inorganic chloramines.
Inorganic chloramines are a product of chemical reaction between hypochlorous acid (HOCl) and ammonia (NH3). The most common types of chloramines found in a swimming pool are monochloramine (NH2Cl), dichloramine (NHCl2), and trichloramine (NCl3). The reactions looks like this:
Monochloramine may then react with more hypochlorous acid to form a dichloramine:
Finally, the dichloramine may react with hypochlorous acid to form a trichloramine:
The type of chloramines produced will depend on pH.
Organic chloramines are a product of HOCl reacting with nitrogen-containing organic compounds like proteins for example. Reaction is described as follows:
where R –> organic radical.
Signs of organic chloramines
Organic chloramines appear to be a problem only in big indoor pools with high bathing load. They don’t appear to be a problem in outdoor pools, possibly because of the effects of sunlight on chloramines, both preventative and destroying.
Main sign of organic chloramines in a swimming pool is that combined chlorine stays at the same level even after repeated shock treatment, however there is no problem with “chlorine smell”, breathing or skin/eyes irritation.
Impact on water quality
Inorganic chloramines are usually the reason for so called “chlorine smell”, breathing problems and skin/eyes irritation, reported by some pool users. This is usually found when the concentrations of combined chlorine are around 0.3 ppm. On the other hand, while organic chloramines can be a reason behind bad taste or smell, usually they can be present in water in much greater concentrations (3 ppm) without causing significant change in water quality. Because they are often mistakenly identified as toxic inorganic chloramines, they are usually referred to as “nuisance residuals”.
Testing for combined chlorine
The standard free chlorine test using DPD tester measures the amount of HOCl. This is performed with one agent. Then another test is done for “total chlorine” (TC), using different agent, which detects the presence of inorganic chloramines. Combined chlorine is then calculated by subtracting free chlorine from total chlorine.
However the presence of organic chloramines can create a problem. They often react with second agent and are interpreted as inorganic chloramines. This often leads to a false conclusion about high levels of mono-, di- and trichloramines.
Breakpoint chlorination (shock treatment)
To remove chloramines the process called “breakpoint chlorination” is used. During this process chlorine is added until the level of HOCl becomes high enough to convert all chloramines into nitrogen gas (N2), HCl and water. Usually the level required is 10:1. Breakpoint chlorination removes inorganic chloramines pretty easily. For a pool with up to 0.5 ppm of inorganic chloramines, dose of 5ppm of chlorine should be quite sufficient. The effect will be disappearance of problems related to “chlorine smell” and irritation.
Unfortunately a lot of organic chloramines are very resistant to oxidation by chlorine. Consequently shock treatment doesn’t work for them.
Removal of organic chloramines
Removal of organic chloramines is not an easy task, because as said previously they are resistant to oxidation by chlorine. So breakpoint chlorination isn’t much help here.
The best method, although usually unwanted due to economical reasons, is dilution with fresh water.
Another method, which is effective in some degree, is active carbon filter. These filters, which remove chloramines, can be installed in parallel to the main filtration system, so that a small amount of water is diverted through them. However these filters require periodic regeneration.
Other equipment like salt water chlorinators, UV water treatment systems and ozone generators is also known to prevent the build-up and/or reduce the level of both organic and inorganic chloramines.
Organic chloramines are a product of reaction of chlorine with different amines. Their presence is virtually invisible, but when testing with DPD test they can affect the results. Shock treatment is effective in removal of toxic inorganic chloramines (responsible for “chlorine smell”, breathing problems and skin and eye irritations), however doesn’t work for organic chloramines The best way to remove organic chloramines is to dilute the water.