New Thinking: Chlorine/Cyanuric Acid in Balance

New Thinking: Chlorine/Cyanuric Acid in Balance Image

The swimming pool industry has long embraced the recommended standard for chlorine concentrations of 1 to 3 ppm or 2 to 4 ppm, depending on who's doing the recommending. The industry has also observed a recommended cyanuric acid range of 10 to 100 ppm, a range that is viewed as necessary to adequately prevent degradation of chlorine residuals by sunlight.

There are, however, those who now challenge the basic presumptions that chlorine, when used as the sole sanitizer, should be applied within a set concentration range. In a nutshell, the idea is that the proper chlorine concentration should be based on the level of cyanuric acid present in the water - the higher the CYA concentration, the higher the concentration of free available chlorine.

It's an idea that has received little exposure or discussion. Yet according to chemical consultant and author Robert Lowry, one the industry's most prominent authorities on water chemistry, the time has come to rethink the way the industry manages water chemistry.

Specifically, Lowry is recommending that FAC should be maintained at 7.5 per­cent of the CYA concentration, which ideally should not exceed 50 ppm.


There's no doubt this idea is a radical departure from pool professionals' basic understanding of water chemistry, but it comes from a qualified source.

Since joining the industry back in 1973, Lowry has started two chemical companies (Robarb and Leisure Time Chemicals), authored 12 books, co-founded Service Industry News and developed 111 products. He has worked as an independent consultant since 1995.

Lowry's seminars on water chemistry in pools and spas are widely considered the gold standard for
water treatment education within the industry.

"I've always tried to share in­formation with the industry that I believe to be true," he says. "Sometimes that means challenging our basic assumptions. The relationship between CYA and chlorine is one of those assumptions."

Lowry's re-examination of the CYA/chlorine relationship began as a result of his authorship of the Independent Pool and Spa Service Association's two chemical training manuals, which he wrote in 2006 and 2009 respectively.

"I received a correspondence from theoretical chemist Richard Falk," he recalls. "He told me that some of the information in the man­ual was incorrect, especially where CYA and chlorine are concerned."

Falk shared with Lowry a number of citations to support his critique. One of those came from an out-of-print book ti­tled "The Chemistry of Water Supply, Treat­ment and Distribution" by Jay O'Brien
(Ann Arbor Publishing, 1973).


Falk pointed Lowry to a chapter about the equilibria of isocyanurates. "According to O'Brien, there was more chlorine tied up by CYA than I had previously believed," Lowry recalls. "I subsequently spoke with some of the chemists at Monsanto, which makes cyanuric acid, and also some of the companies that manufacture trichlor. These conversations confirmed the view that there is indeed an equilibrium relationship between chlorine and CYA."

(Lowry also cites chemist Ben Powell for first developing the 7.5 percent concept back in the '70s.)

"Based on existing information," Lowry explains, "I came to realize that at any given time, you only have 3 to 4 percent chlorine available to do anything. That led me to look at the level of chlorine we need to kill algae and bacteria."

Given that there are potentially hun­dreds of organism species in a swimming pool, Lowry began exploring which of those should be used as a standard for the necessary levels of FAC at a given concen­tration of CYA.

"Should we use as a standard E. Coli, Pseudon1onas, Giardia, Cryptosporidium or some type of algae? When you look at the required chlorine concentration for various organisms, the differences are huge," he explains. "The basic idea is to determine how much chlorine we need to prevent growth. We don’t need to kill ev­erything's that in there, but instead prevent growth to the point that these organisms have no negative effect on bathers and water quality. Simply put, we need to kill algae faster than it can reproduce."

With that fundamental question in mind, Lowry lit on the sin1ple answer: Because algae is tougher to kill than most forms of bacteria, maintaining enough chlorine
available to prevent algae growth will en­sure there is plenty to take care of bacteria in a residential pool. Public or commercial pools require enough chlorine to oxidize organics and accommodate larger numbers or people.

"I discovered that in order to stop the growth of algae, you need only .05 ppm of HOCl, which certainly doesn't sound like much," he says. "The problem is that in the presence of cyanuric acid at a concentra­tion of 30 ppm, as an example, only 3 per­cent of chlorine is available to do anything. Under those conditions, a concentration of 3 ppm chlorine means that you only have 1 ppm available, but we also have to con­sider that at a pH of 7.5, only 50 percent of that concentration is available as HOCl, which brings us to .05."

(The amount of HOCl and OCl is de­termined by the pH of the water and at a pH of 7.5 there is 53 percent in the HOCl form.)


The problem, he explains, is that when CYA levels increase, the percentage of available chlorine decreases; at 60 ppm CYA, 98 percent of chlorine is bound.
[The bound chlorine never gets above 98.2 percent.] For pools treated with trichlor, the level of CYA increases rapidly and con­centrations over 100 ppm are typical.

"For every 10 ppm of trichlor that you add, your increasing CYA by 6 ppm, so obviously the level is going to rise very quickly, which in turns binds larger percentages of chlorine," he explains. "If your chlorine demand is 1 ppm chlorine that means every 10 days, your CYA concen­tration is going up by 6 ppm, and that's if the chlorine demand is only 1 ppm. If the demand is 2 ppm, which is very common, CYA levels increase at twice that rate."

In effect, he says, a homeowner or ser­vice technician may keep chlorine residu­als at recommended concentrations but unbeknownst to them have inadequate FAC to be effective. All of which explains why water quality will decline even though the chlorine levels are kept in what we've been taught is the proper range.

(This phenomenon has often been loose­ly referred to as "chlorine lock" a largely inadequate tern, that betrays the fact that chlorine and CYA exist in a state of equi­librium.)

"At first, when the concentration of CYA is 20, 30 or zero, chlorine at a normal concentration will get the job done,” Low­ry explains. "But once you get in ranges above 50 ppm, which can happen within a few weeks, or less depending on the chlo­rine demand, you come to the point where you start to see algae and you’re left won­dering what's going wrong.

While algae itself doesn't present a health hazard per se, common sense tells us that if you're not killing the algae, you also might not be killing the bacteria."

While at first blush, this concept of the
CYA/chlorine equilibrium, might seem to demonize CYA and chlorine products that contain it, Lowry is quick to point out that without CYA in the mix, it's nigh onto impossible to maintain chlorine residuals be­cause of how quickly chlorine is destroyed by UV light.

"There's no question that in a chlori­nated body of water exposed to sunlight, you need cyanuric acid," he says. "What the equilibrium equation tells us is simply that we need to think in terms of maintaining lower levels of CYA."

To that point, Lowry recommends rejig­gering the standard to limit CYA concen­trations to 50 ppm, meaning that when that level is reached it's helpful to switch to chlorine that doesn't contain CYA, sodium hypochlorite or calcium hypochlorite being the most commonly used.

And, at whatever the CYA concentra­tion, FAC should be maintained at 7.5 per­cent of that concentration, i.e. at 100 ppm CYA, you need 7.5 ppm FAC.

"It turns out that's the only way we can be sure we have enough FAC to do the job," he concludes.

For a detailed discussion of this issue, check out Lowry's paper at CYAControlsYourPool.


The implications of the adjoining discussion are far-reaching. To help make sense of the issue, Lowry offers the following takeaways:

The free chlorine level should be 7.5 percent of CYA. The recommended level of 2 to 4 ppm doesn't always work.

This is not hard to maintain or calculate. If you add CYA separately and then use liquid chlorine as the chlorinating source, the CYA will not change and you will always know what the free chlorine level should be (target). As an example, you add 30 ppm of CYA and you will always need about 2 ppm of free chlorine (30 ppm x .075 = 2.25 ppm). Or if you have 50 ppm of CYA, you will need about 4 ppm of free chlorine (so x .075= 3.75 ppm).

Unless you add more CYA, the free chlorine required will not change. This gives you a target for free chlorine and makes it easy to know if you have enough free chlorine to provide safe water and prevent algae from growing.

CYA maximum is 50 ppm. At that level, switch from using trichlor as main chlorinating source.

As stated in the article, the buildup of CYA from trichlor use is huge. For each 10 ppm of chlorine added to the water from trichlor, 6 ppm of CYA is added. In a typical residential pool, the chlorine loss to sunlight even with CYA and low swimmer loads is about 1.5 ppm per day or about 10 ppm of chlorine per week. This means that if you started with a low CYA of only 30 ppm, in about three weeks you would be at 50 ppm. This is the maximum CYA that we recommend. Even using the APSP guideline of 100 ppm maximum, in 11 weeks you would have more than 100 ppm. Of course using the CL/CYA percentage of 7.5 percent, this would mean that you would need 7.5 ppm of free chlorine.
It's better to use trichlor until CYA reaches 50 ppm and then switch to liquid chlorine, liquid bleach or calcium hypochlorite.

Using liquid chlorine or bleach will not raise the pH of the water.

Most people believe that using liquid chlorine or bleach will raise the pH of the pool water. This is not so. It is true that the pH of liquid chlorine is about 13 and bleach is about 11.
So adding these will initially raise the pH. However, when chlorine is degraded by sunlight or used in disinfection, the result is that HOCI that becomes HCI and this amount of acid is almost equal to the amount of base (sodium hydroxide) in the liquid chlorine. So the result is almost a net zero change in pH. I have said that because in some liquid chlorine there is a slight excess of sodium hydroxide and this small amount may have a minor effect on pH.

A benefit of using CYA is less exposure to high levels of chlorine.

Using the free chlorine level of 7.5 percent of CYA can result in using a FAC level that is higher than EPA recommends - currently 4.0 ppm. However, EPA's maximum is based on free chlorine in water without CYA. Based on the O'Brien work,which was converted into a formula by Richard Falk, we know that the amount of chlorine bound to CYA is between 96 and 98 percent.

This of course means that at 4 ppm of free chlorine with 30 ppm of CYA at a pH of 7.5 that swimmers are exposed only to 0.112 free chlorine (4.0 ppm x 0.028 = 0.112 ppm). In contrast, at even 1.0 ppm free chlorine without CYA results in an exposure of 1 ppm of free chlorine. So having CYA in the water results in an exposure to chlorine 10 times less than without it.

The roughly 3 percent of chlorine that is not bound to CYA and is free to kill organisms and destroy organics is in equilibrium. As some chlorine not bound to CYA is used in oxidation or destroyed by UV, more chlorine is released from the CYA to maintain the 3 percent. So the 97 percent of the chlorine bound to CYA is a reservoir that replenishes the 3 percent as needed.

Realize that CYA is a double-edged sword.

CYA is needed to protect chlorine from sunlight degradation. It prevents exposure to high levels of chlorine and acts as a buffer to prevent pH from drifting lower but too much in the water can slow down chlorine disinfection. Using CYA keeps the chlorine in the water eight times longer than without it. With no CYA the loss of chlorine to sunlight is 75 percent in two hours, so in four hours you have essentially no chlorine.

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