or body oils, urine, dirt, and all kinds of other debris. The more cyanuric acid there is in a pool for the same amount of chlorine, the worse chlorine performs. And if we want the industry-recommended 1-3 ppm chlorine to be effective against algae, we’re going to have to keep the cyanuric acid at a commensurate concentration.
The idea that cyanuric acid mitigates chlorine’s efficacy is beginning to really take hold in the pool and spa industry. For decades, the industry was taught a misleading notion about the dramatic effect that pH has on the killing power of chlorine, represented in the accompanying chart.
One can see from the chart that as the pH goes up, the active form of chlorine, represented in red, becomes smaller. Meanwhile, the less active form of chlorine, represented in grey, becomes larger.
This dramatic effect that the pH has on the active (killing) form of chlorine continues to inform industry experts who argue that elevated pH values in swimming pools is bad — even dangerous — for bather health and safety.
It seems evident from that chart that one must maintain the pH of the pool as low as possible to provide the maximum disinfection and oxidation strength of chlorine.
But this notion is misleading for two reasons: First, even at high pH levels, the substantial loss of active chlorine still provides more than enough disinfection power to handle nearly everything in the pool. Second, it neglects the contribution of another incredibly relevant chemical that is also in most outdoor swimming pools: Cyanuric acid.
And the fact of the matter is that the chemistry depicted in the chart is valid only in swimming pools that do not contain cyanuric acid (stabilizer, or conditioner).
And how many outdoor, chlorinated pools do not contain cyanuric acid these days?
When moderate levels of cyanuric acid are in the water, the situation changes dramatically. See accompanying chart on page 16.
From this chart, it can now be seen that while the pH does play a role in the different species of chlorine present, that role is now minor. In a pool with just 30 ppm cyanuric acid, it can be seen that the major determining factor of how much active, killing chlorine is present is the cyanuric acid itself.
If such is the case, why do industry experts continue to emphasize the impact the pH has on chlorine’s ability to do its job?
One aspect that is interesting about the effect that cyanuric acid has on the percentages of active chlorine is that it is evident that swimming pools need far less active chlorine than the industry seems to believe.
Let’s look at some realistic, realworld examples At a pH of 7.5, with 3 ppm chlorine, and no cyanuric acid, the percent of active chlorine is close to 50 percent. That’s 1.5 ppm active chlorine (HOCl).
How about with cyanuric acid? At a pH of 7.5, with 3 ppm chlorine and 30 ppm cyanuric acid, the percentage of active chlorine drops to about 1.4 percent. That’s 0.042 ppm active chlorine.
Using 30 ppm cyanuric acid, 3 ppm chlorine, and a pH of 7.5 falls well within industry standards, and most pools maintaining such levels should be in good shape.
Meanwhile, what happens if we bring the pH to 8.5? This is considered much too high by most industry experts for bather health and safety.
At a pH of 8.5, with 3 ppm chlorine, and no cyanuric acid, the percent of active chlorine drops to about 8 percent. That’s 0.24 ppm active chlorine (HOCl), which is a lot more active chlorine (nearly 6 times higher) than is in the pool that was in accordance with the above industry standards.
This shows two main things: 1. pH isn’t nearly so important to sanitation for bather health and safety as many believe.
2. Swimming pools need a lot less active chlorine than we used to think.
But these pools will only need less chlorine because the cyanuric acid is held in check.
The best rule of thumb is: To prevent algae, keep the chlorine concentration at 7.5 percent of the cyanuric acid concentration. At 30 ppm, this corresponds to 2.5 ppm chlorine. If you let the cyanuric acid creep up higher, you will find that you will need a correspondingly higher concentration of chlorine to get the job done, which, these days, can really increase costs.
Better yet, test your phosphates. As algae food, phosphates really do make a difference in how much chlorine is needed to prevent algae. When phosphates are low — less than 500 ppb — you can reduce your percentage of chlorine needed to 5 percent of the cyanuric acid level. At 30 ppm cyanuric acid, that’s just 1.5 ppm chlorine.
While phosphate removal does not kill or prevent algae, it slows the reproduction (growth) rate, allowing free chlorine to stay ahead of it. So while phosphate removal does have an upfront cost, over time, it saves money.
Advanced Oxidation Process
The newest method of chemical treatment and procedure for removing organic material from swimming pool water is the advanced oxidation process (AOP).
Although it is a relatively new technology for the pool industry, in water treatment applications, organic compounds have been treated with AOP for years.
Advanced Oxidation Processes are systems that produce hydroxyl radicals for chemically treating the water. Hydroxyl radicals are shortlived, extremely powerful oxidizing agents capable of reacting with a broad spectrum of contaminants. In fact, hydroxyl radicals (OH·) are the second most potent oxidizers known to man.
There are several ways to produce hydroxyl radicals, but the most common method for swimming pool applications involves combining ozone and UV technologies. The first step is to produce ozone, and the second step is to irradiate the ozone to form hydroxyl radicals.
The reaction is a two-step process as follows: O3 +H2O + UVLight →H2O2 +O2 H2O2 + UVLight → 2OHThe resulting hydroxyl radicals that are formed are superior to both UV and Ozone in terms of oxidative capacity as well as disinfection efficiency.
A 2006 study entitled “Combined ozone and Ultraviolet Inactivation of Escherichia Coli” by Magbanua et al. examined what some have called the synergistic effect of using the two technologies simultaneously. It was found that when UV and Ozone are reacted together with E. coli, they achieve a 3-log reduction or 99.9 percent inactivation — far more than the sum of the contributions of each disinfectant.
A 2008 study by Cho et al. looked at the effect of hydroxyl radicals against cryptosporidium. It was found that the hydroxyl radical is 104 to 107 more effective against crypto than other popular disinfectants such as ozone, chlorine dioxide, or free chlorine.
Advanced Oxidation Processes are not only superior when it comes to disinfection and sanitation but also in terms of oxidation. Not only do hydroxyl radicals react quickly with natural organic matter, they have also been shown to reduce chlorine’s disinfection byproducts such as trihalomethane and haloacetic acid.
A 2005 study called “Removal of Disinfection By-products Precursors with Ozone-UV Advanced Oxidation Process” reported that the process was able to remove 50 percent of the total organic carbon at typical ozone/UV doses. The same doses were shown to reduce trihalomethane formation by 80 percent and haloacetic acid formation by 70 percent.
These effects are still being studied. The end products of complete oxidation of AOP are carbon dioxide and water.
But oxidation does not always go to completion, and the process can itself lead to disinfection byproducts, depending on the contaminant in the water.
AOP are referred to in the plural form because there are several ways to achieve hydroxyl radicals. The most common ways are to mix UV with ozone, UV with hydrogen peroxide, or hydrogen peroxide with ozone.
Many of the manufacturers for pool and spa units use the ozone/ UV process. However, the way that each manufacturer produces the ozone also varies from company to company.