Sanitizers
It sounds obvious, but to sanitize a pool, a sanitizer is needed. There are numerous products that claim to kill bacteria or treat bacterial slime, but unless the product is a PMRA/EPA-registered sanitizer as indicated on the label, it is not a sanitizer. In the U.S., the only acceptable alternative to this is the use of an EPA-registered disinfectant. A disinfectant residual is also acceptable for sanitization of pools and spas/hot tubs.
Throughout the rest of this article, the word sanitizer will be used in reference to both sanitizers and disinfectants. Chlorine and bromine (Br) are the only chemicals registered by the PMRA for sanitizing pools and spas/hot tubs. In addition to chlorine and bromine, polyhexamethylene biguanide (PHMB) and some metal systems are registered for use in the U.S. by the EPA.
There are various forms of these chemicals, but these are the only sanitizers recognized by each agency for providing sufficient sanitization.
That said, each of these sanitizing chemicals have requirements for optimum performance. Those requirements are discussed below.
Chlorine
Chlorine activity is affected by pH, ammonia (NH3), cyanuric acid (CNOH)3, and other nitrogen (N)-containing contaminants. The effect of pH on chlorine can be seen in Figure 1 below.
To understand this graph, one needs to know the different forms of chlorine (e.g. chlorine gas, calcium hypochlorite [Ca(ClO)2], trichloroisocyanuric acid [C3Cl3N3O3], etc.), release hypochlorous acid (HOCl) when added to water. Hypochlorous acid is the active sanitizer that kills bacteria in the pool. At low pH, all of the free chlorine in the pool is present as hypochlorous acid; however, at high pH, it splits into hydrogen ions (H+) and hypochlorite ions (OCl–). The latter is not as effective at killing bacteria as hypochlorous acid. At low pH, the activity of chlorine is maximized, but low pH can also cause corrosion of pool surfaces and irritate bather’s eyes and/or skin. The recommended pH range of 7.2 to 7.8 is a compromise to maintain chlorine efficacy without causing other problems.
When bathers urinate or sweat in the pool, a variety of different chemical contaminants are contributed to the water. One of the primary components of sweat and urine is urea CO(NH2)2. When chlorine reacts with urea, it forms combined chlorine. The three main types of combined chlorine are monochloramine (NH2Cl), dichloramine (NHCl2), and trichloramine (NCl3). Free chlorine is much more effective than combined chlorine. This is another reason why it is important for bathers to shower (removing any dried sweat from their bodies) and use the washroom before they enter the pool. People can still sweat as they swim, so it is important to know how to get rid of chloramines once they are formed.
The most common way of doing this is to perform breakpoint chlorination. To achieve breakpoint chlorination, chlorine is simply added at a dosage that is 10 times the combined chlorine concentration. This dosage is usually sufficient to fully oxidize the nitrogen in the chloramines to nitrogen gas.
The third factor that has a large impact on chlorine efficacy is cyanuric acid (CYA). This stabilizer is added to pools to help prevent sunlight from destroying chlorine. When chlorine is bound to cyanuric acid, it can be released quickly to perform its job of sanitizing. However, as the cyanuric acid concentration increases, chlorine effectiveness slows down.
Over the years, many studies have been performed demonstrating the adverse impact of cyanuric acid on chlorine effectiveness in pure distilled water as well as samples of water taken from pools. The effect of cyanuric acid on chlorine effectiveness has been seen with bacteria, viruses, and protozoa such as Cryptosporidium (Crypto).
The CDC recently published an article where it examined the effect of cyanuric acid on kill rates for Crypto.4 It found with 50 parts per million (ppm) cyanuric acid, it was unable to achieve a targeted 99.9 per cent reduction of the parasite. With 20 ppm cyanuric acid, the amount of time needed to kill Crypto was approximately 10 to 20 times longer than without cyanuric acid. Since cyanuric acid can affect the ability of chlorine to sanitize pools and spas/hot tubs, its concentration should be monitored and controlled. The ANSI/APSP-11 standard and the MAHC currently set the limit at 100 ppm. This is a hotly debated topic, however, and some public health authorities are lowering their cyanuric acid limits below 100 ppm in an effort to help avoid bather illnesses.
Bromine
Bromine sanitizers are much less susceptible to high pH values than chlorine. The active form of bromine (HOBr) (shown in Figure 2 above) is present at much higher pH values than chlorine. This is one of the reasons why bromine is so popular in spas/hot tubs, where the pH tends to drift up more quickly due to the high temperatures and aeration of the water.
Like chlorine, bromine will react with bather waste to produce bromamines, but bromamines are almost as effective in killing bacteria as free bromine.
A disadvantage of bromine, however, is it cannot be stabilized to prevent destruction by sunlight. It is not at all stabilized by cyanuric acid, and a molecule called hydantoin (C3H4N2O2), which is used to provide bromine is only slightly effective in stabilizing it.
