An aspect of chlorination chemistry that is often overstated is the importance of pH on chlorine’s efficacy. Most pool industry veterans are familiar with the chart or graph that shows how hypochlorous acid is diminished by rising pH. See accompanying graph, Figure 1.

Shown in pool chemistry texts and industry seminars, it has often been repeated that the percent of active chlorine drops by half when moving from a pH of 7.5 to a pH of 8. And this is roughly true at pH 7, there is about 48% active chlorine while at pH 8 there is about 23% active chlorine. But it is only true when there is no cyanuric acid.

Note how things change when a typical 30 ppm cyanuric acid is added. Now, at pH 7.5, there is 1.3% active chlorine, and pH 8 there is 1.1% active chlorine, a drop of only 15%.

What this means is that while pH is an important parameter when it comes to managing water balance, and the pH should be kept within a certain range to prevent corrosion or scale or metal stains, it is really not very relevant when it comes to managing disinfection.

Scrutiny of the graph should make this point even more convincing when it is noted that the percent of active chlorine at pH 8 is significantly higher without cyanuric acid than it is at pH 7.5 with 30 ppm cyanuric acid. It simply doesn’t take that much chlorine to disinfect.

How much chlorine is needed?

How much chlorine is needed? It should be clear by now that it depends. On what? The cyanuric acid, of course!

Unt i l ve ry r e c ent ly, expe r t recommendations about the amount of chlorine to use, relative to the cyanuric acid were based (mostly) on the prevention of algae.

The industry’s understanding about chlorine’s relationship to cyanuric acid is relatively new, dating back to the 1990’s.

In 1997, Ben Powell, a service professional and the creator of Pool Solutions, an information site, and The PoolForum, had the idea that swimming pool chlorine levels should be adjusted according to stabilizer levels to control algae. This was based on years of his own experience servicing commercial pools, information from people who used his forum, a technical paper by John Wojtowicz and some math.

Powell had noticed that ideal chlorine levels, generally between 2 to 4 ppm, were not always effective against algae if the cyanuric acid levels were high.

So, he set about trying to determine how much chlorine should be present at different cyanuric acid levels. The result, printed in the late 90’s, is “Ben’s Best Guess Guide to Swimming Pool Chlorination”.

In the years that followed, Richard Falk, a pool owner with a talent for math and chemistry, was able to refine Powell’s work, working out a more precise relationship between chlorine’s efficacy as a function of cyanuric acid levels.

Falk, known as Chem Geek at the online forum TroubleFreePool, was able to derive a simple formula for the minimum free chlorine needed to prevent algae based on the amount of cyanuric acid. That number corresponds to a hypochlorous acid concentration of about 0.03 ppm.

Falk began with Powell’s best guesses on free chlorine values that are effective for a given cyanuric acid concentration. He then determined that one should have a minimum free chlorine that is 7.5 percent of the cyanuric acid to prevent green and black algae in traditionally chlorinated pools, regardless of phosphate levels.

So if the cyanuric acid is 50 ppm, 3.75 ppm free chlorine should prevent algae.

Yellow, mustard algae needs more hypochlorous acid, with free chlorine that is 15% of the cyanuric acid.

Falk’s Ratio states simply that hypochlorous acid concentrations are proportional to free chlorine divided by cyanuric acid: HOCl FC/CYA Algae prevention is a useful starting point for developing chlorination standards, but Falk recognized that the pool industry needed a more sound scientific basis for recommending a limit on the concentration of cyanuric acid in pools.

It seems certain that that basis should be bather health and safety. So basically, the industry should decide on a ratio that will help prevent diseases.

It has been shown that increasing cyanuric acid while keeping the free chlorine constant slows the kill rate of diseases.

But to put an extra fine point on it, the accompanying graph (Figure 2)comes from a study conducted on the kill rate of Streptococcus Faecalis in the presence of various concentrations of chlorine and cyanuric acid. It clearly shows that increasing the cyanuric acid concentration also increases the amount of time it takes to kill the bacteria.

For example, it can be seen that for the bacteria exposed to 1 ppm chlorine, the average time required for 99 % kill is about 25 minutes in 25 ppm cyanuric acid but goes up to about 55 minutes when the cyanuric acid is raised to 50 ppm.

Since we now know that the active killing form of chlorine (hypochlorous acid) goes down as cyanuric acid goes up, the study shouldn’t be all that surprising.

So now the question is: how much hypochlorous acid is needed to kill diseases in an acceptable amount of time?

This is where things get a little subjective.

Because it depends on what you consider an acceptable risk of infection. And that depends on which pathogens are responsible for more illnesses contracted in treated recreational water. It also depends on which pathogens are responsible for more severe illnesses including death. And it also depends on which pathogens are more chlorine resistant.

In May, of 2019, Richard Falk and other members of the Council for the Model Aquatic Health Code Ad-Hoc Committee set about to answer these questions.

Ultimately, they landed on E. coli, Giardia and Cryptosporidium as pathogens that meet one or more of these criteria.

Then, they calculated the risk of infection from these pathogens as a function of the cyanuric acid to free chlorine ratio. Their model showed that the risk of infection went down by reducing the allowed cyanuric acid to free chlorine ratio, as would be expected. But they recognized that this risk needed to be balanced with practical operations of public pools. For example, if you set the ratio too low, it could eliminate the use of trichlor and dichlor.

As a starting point for acceptable risk, they chose a cyanuric acid to free chlorine of 45:1, because that number corresponds to the maximum cyanuric acid (90 ppm) and lowest free chlorine (2 ppm) allowed by the current MAHC. Another ratio to consider was the maximum cyanuric acid allowed by the MAHC (90 ppm) with the maximum free chlorine allowed by the U.S. EPA (4 ppm), which results in a ratio of 22.5 CYA:FC.

They compared the risks of infection at different ratios to allowed risk of infection set by the EPA for both drinking water as well as untreated water like lakes. They chose the risk of infection from Giardia to be the most relevant of the organisms to consider since the probability of infection from Giardia is higher than that from E. coli or crypto. It was noted that the EPA’s accepted risk of infection for untreated recreational water crosses the curve for the risk of infection from Giardia where the cyanuric acid to free chlorine ratio is 22.

Therefore, they decided that a cyanuric acid to free chlorine ratio of 20:1 should be the recommended guideline, because its a nice round number and would be easy for pool operators to implement.

It is this ratio, 20 parts cyanuric acid to 1 part chlorine, (or where the free chlorine is 5% of the cyanuric acid) that the Ad Hoc committee has recommended for the 2021 edition of the Model Aquatic Health code.