For most pools and spas, chlorine really is the king of water quality maintenance.

It’s true that for aesthetic reasons and preventative maintenance, we must also pay attention to water balance. And filtration is also essential for providing clean, clear water, removing the contaminants that can promote the growth of bacteria and algae.

But an argument could be made that if there’s one thing we can’t do without, it is disinfection. For most swimming pools and spas, that is achieved with chlorine.

For infrequently used residential pools, oftentimes the main function of chlorine is algae prevention and remediation.

But for frequently used residential pools, and certainly in the case of public pools and spas, the essential purpose of chlorine is disinfection, which is crucial to preventing waterborne diseases.

Previous issues of Service Industry News have focused on the use of chlorine for algae prevention. It has been proposed, based on both empirical and scientific research, that the best way to prevent algae is to maintain the chlorine at a level proportional to the cyanuric acid. Experts say that keeping the free chlorine concentration at a level corresponding to 7.5 percent of the cyanuric acid level will prevent common green algae, regardless of other issues the pool might have, such as high phosphates.

But which chlorine levels are truly necessary for disease prevention? And knowing the huge role that cyanuric acid plays in algae control, what role does it play in pathogen control?

Getting full disinfection out of chlorine is what this issue of Service Industry News is about.

Every year, millions of Americans get sick from diseases spread in water. The Centers for Disease Control and Prevention (CDC) reports that 7.2 million Americans get sick from the water we use for drinking, bathing, swimming, and even cooling highrise buildings.

And the numbers specific to pool and spa water represent a not insignificant portion of those cases.

According to the most recent data from the CDC, for the period 2015-2019, a total of 208 outbreaks were associated with poorly treated recreational water. Those outbreaks resulted in at least 3,646 cases of illness, 286 hospitalizations, and 13 deaths.

Almost all — 96 percent — of the outbreaks were associated with public (non-backyard) pools, hot tubs, or water playgrounds. Cryptosporidium accounted for 2,492 of the 2,953 cases with a confirmed cause. All 13 deaths occurred in people affected by a Legionnaires’ Disease outbreak. Among the 208 outbreaks, 34 percent were associated with a hotel (i.e., hotel, motel, lodge, or inn) or a resort, and 51 percent started during June-August.

It is clear that those pools and spas could be better maintained.

Because 100 percent of the deaths were caused by Legionnaire’s Disease, it might be instructive to think about what chlorine levels are necessary to prevent the growth of Legionella, the bacteria responsible.

Warning — this is about to get really technical.

According to the Environmental Protection Agency’s Office of Water, the EPA regulates Legionella under the Surface Water Treatment Rule (SWTR). The SWTR has treatment technique requirements to control for the Giardia parasite and viruses. The SWTR’s treatment technique requirements presume that if sufficient treatment is provided to control Giardia and viruses (i.e., 3-log inactivation of Giardia and 4-log inactivation of viruses), then Legionella risks will also be controlled.

“Log inactivation” is a way to express the percent of microorganisms that have been killed or rendered unable to replicate through the disinfection process. So a 1-log inactivation value means that 90 percent of the microorganisms of interest have been inactivated, a 2-log inactivation corresponds to 99 percent inactivated, and a 3-log is 99.9 percent inactivated.

Basically, it’s a measure of how effective a disinfection process is at killing microorganisms under specific conditions.

The EPA’s Surface Water Treatment Rule has established “CT” values for systems using chlorine and other disinfectants. “C” stands for the concentration of the disinfectant, and “T” stands for the time needed to obtain a certain level of inactivation. These CT values are specific to a set temperature and pH. See accompanying table.

The EPA requires that public pools be maintained between 1 and 4 ppm chlorine. One can calculate how fast those levels can inactivate Giardia by using CT Values at specific temperatures and pH values. Going by the EPA’s limits for chlorine of between 1 and 4 ppm, at a temperature of chlorine that is used. *Note: 14 minutes was calculated from a best fit line corresponding to 4 ppm chlorine. See graph on page 6. It’s important to note, however, that one can only achieve these inactivation times in the absence of cyanuric acid.

In the presence of virtually any cyanuric acid, it has been shown that disinfection rates are slowed. Numerous studies have shown the effect that cyanuric acid has in slowing chlorine’s efficacy. See accompanying graphic on page 6.

Unfortunately, no such data could be found that studies the effect of cyanuric acid on the specific kill rate of Giardia with chlorine.

Nonetheless, it is pretty well understood that pathogen kill rates are proportional to the concentration of hypochlorous acid present in the water.

Many of us are familiar with the idea that when chlorine is added to water, it forms hypochlorous acid and the hypochlorite ion. Hypochlorous acid is responsible for the majority of chlorine’s power. Further, many of us know that as the pH rises, there is more hypochlorite and less hypochlorous acid, which is a big reason

of 77 degrees and a pH of 7.5, Giardia can be inactivated in roughly 14 and 45 minutes, depending on the amount why industry standards prescribe maintaining the pH between 7.2 and 7.6.

But when cyanuric acid is added to the water, a lot of the hypochlorous acid becomes loosely bound to it, meaning that there is less active chlorine around to perform disinfection.

That means that kill rates are slowed when cyanuric acid is present, and that the kill rates will be proportional to the hypochlorous acid present, and NOT the total free chlorine concentration.

Fortunately, it doesn’t take all that much hypochlorous acid to kill many of the microorganisms that get introduced to swimming pools.

The World Health Organization suggests the minimum Oxidation Reduction Potential (ORP) for disinfection is 650 mV, which corresponds to about .01 ppm hypochlorous acid. This is a level that has been shown to deactivate many pathogens.

It is possible to calculate the amount of hypochlorous acid present for specified concentrations of cyanuric acid and at specified pH values, thanks to the hard work of scientists who used published equilibrium constants to determine how much of each species is present under those conditions.

Thus, for a pool with no cyanuric acid and a measured Free Available Chlorine (FAC) of 1 ppm and a pH of 7.5, the hypochlorous acid concentration will be about 0.5 ppm.

Meanwhile, given the same conditions and the addition of 50 ppm cyanuric acid, the hypochlorous acid will be about .01 ppm — just adequate by WHO standards.

So, 1 ppm FAC may be good enough, but it probably wouldn’t be a bad idea to bump it up a notch (2-3 ppm) particularly in highly used pools and spas, especially considering the incidence of waterborne illness.

That said, while the latest APSP “American National Standard for Water Quality in Public Pools and Spas” acknowledges that cyanuric acid reduces kill rates, they state, “We do not have any empirical evidence that a disease outbreak has been linked to any particular cyanuric acid level in a properly sanitized pool (i.e when at least 1 ppm free available chlorine was present).”