Combined chlorine is chlorine that has already reacted with contaminants in water, meaning it has essentially been “used up” in the sanitation process. This spent chemical compound is the primary reason pool and spa water develops a noticeable, unpleasant odor and causes irritation to the eyes and skin. When this used-up chlorine remains in the water, it indicates a decrease in sanitation effectiveness and a decline in water quality. Managing this specific type of chlorine is fundamental to maintaining a healthy and comfortable aquatic environment.
Free Chlorine Total Chlorine and Combined Chlorine
Chlorine in water exists in three measurable states that collectively determine the overall sanitizing power of the water. The most important form is Free Available Chlorine (FAC), which represents the active disinfectant that destroys bacteria, viruses, and other pathogens. This is the portion of chlorine that has not yet reacted with any contaminants and is actively protecting the water.
Combined Chlorine (CC), also known as chloramines, is the chlorine that has already reacted with pollutants and is no longer an efficient sanitizer. Total Available Chlorine (TAC) is simply the sum of the active and spent forms (FAC + CC = TAC). Therefore, the level of combined chlorine is calculated by subtracting the FAC reading from the TAC reading.
How Combined Chlorine Forms
Combined chlorine forms through a chemical reaction that occurs when the active free chlorine encounters nitrogen-containing compounds in the water. These nitrogenous substances are introduced primarily by swimmers through bodily fluids like sweat and urine, which contain urea and ammonia. Free chlorine readily reacts with this ammonia to create a family of compounds called chloramines.
The initial reaction forms monochloramine, which can then react further to create dichloramine and trichloramine as more chlorine is consumed. These chloramines are much weaker sanitizers than free chlorine, with some being up to 60 times less effective at disinfection. A buildup of these chloramines signals that the free chlorine is being rapidly consumed and the water is struggling to keep up with the contaminant load.
Negative Effects on Water Quality and Health
The accumulation of chloramines has several noticeable consequences for both the water quality and the health of swimmers. The pervasive, strong chemical odor often associated with pools is not the smell of excess chlorine, but rather the gaseous trichloramines escaping into the air. This pungent smell is a clear indicator of high combined chlorine levels and poor water chemistry.
These volatile chloramines are responsible for the physical discomfort experienced by bathers. When chloramines become airborne, they irritate mucous membranes, leading to red, burning eyes and dry, itchy skin. Furthermore, inhaling these compounds can aggravate respiratory issues, contributing to throat irritation and asthma-like symptoms, particularly in indoor environments where ventilation is poor. High combined chlorine levels also mean that the water’s sanitizing capacity is significantly reduced, which leaves the water vulnerable to pathogens.
Eliminating Combined Chlorine
The established method for removing combined chlorine is a targeted process known as breakpoint chlorination, or simply “shocking” the water. This technique involves deliberately adding a large amount of free chlorine to the water to oxidize the chloramines and destroy the chemical bonds. The goal is to reach a precise concentration where the newly added free chlorine breaks down the chloramines into harmless nitrogen gas, which is then vented away from the water.
To successfully achieve this breakpoint, the amount of free chlorine added must be at least ten times the measured combined chlorine level. For example, if the water has a combined chlorine reading of 0.5 parts per million (ppm), the operator must increase the free chlorine to a minimum of 5.0 ppm above the existing free chlorine level. If insufficient chlorine is added, the process will only worsen the issue by converting existing chloramines into more irritating forms. Testing the water both before and after the treatment is necessary to ensure the breakpoint has been reached and the combined chlorine level has returned to near zero. Proper ventilation is also important during this process, especially in indoor pools, to allow the released nitrogen gas to escape the swimming area.