We wish to thank Terry Arko, Product Training Content Manager for Hasa Inc. for his help in preparing this article.

Chlorine has played a vital role in water treatment for centuries. Today, using chlorine-based sanitizers is the main treatment for clean, clear, and safe water in swimming pools. But for chlorine to be useful, it must be present, and in an outdoor pool, without cyanuric acid, chlorine is rapidly burned away to nothing.

Cyanuric acid plays multiple roles in a swimming pool, but its key job is to prevent chlorine from decomposing in sunlight.

To understand how this, and its other effects, are accomplished, it is necessary to have a basic understanding of chlorination chemistry.

When chlorine is added to water, it immediately disassociates into hypochlorous acid (HOCl) and hypochlorite ions, OCl-.

Of the two, hypochlorous acid is the primary killing agent of the chlorine. Hypochlorite is much weaker, and kills bacteria, viruses, and algae at a much slower rate.

In basic chemistry, the amount of hypochlorous acid that is produced in pool water is based on water balance. The pH controls how much of either HOCl or OCl- is produced. At lower pH values, more of the killing agent HOCl is formed, while at a higher pH, more of the weak OCl- is produced.

HOCl is the “Superman” of water treatment, but UV sunlight is the “kryptonite” that can quickly rob it of its power.

In the mid-1950s, Monsanto began to apply for patents for the use of cyanuric acid (CYA) as a stabilizer to prevent the rapid degradation of HOCl in water. When unstabilized chlorine is added to pool water, 90-95% can be lost within two hours from direct exposure to sunlight. Within four hours, all chlorine can be depleted in a pool without any CYA. However, with 30 ppm of CYA in a pool, the chlorine will last roughly up to eight times longer.

Cyanuric acid is available in pure granular form. Adding 2.5 lbs. of pure granular CYA in 10,000 gallons will result in 30 ppm CYA.

The PHTA standard for CYA in swimming pools is 30-50 ppm. When CYA is added to chlorinated water, it combines with the chlorine to form a set of chemical species called chlorinated isocyanurates.

The other way to add cyanuric acid to a swimming pool is through the direct use of chlorinated isocyanurates. These are trichlor tablets, granular trichlor, and granular sodium dichlor.

The advantage of using chlorinated isocyanurates over chlorine alone is that these compounds are stable in sunlight, preventing a photodecomposition reaction of chlorine that would otherwise occur.

It does this because chlorine attaches to cyanuric acid in a loose bond that holds most of the residual chlorine in reserve. Specifically, at a pH of 7.5, with 3.5 ppm of free chlorine (FC) and 30 ppm of CYA, 97% of the chlorine is bound to CYA. Only 3% of chlorine is available at any time for disinfection or to prevent algae.

As stated earlier, chlorine dissociates into HOCl and the OCl- ion. At a pH of 7.5, 50% of the unbound chlorine exists as HOCl and 50% is OCl-. Because 3% of chlorine is not bound to CYA, that means that 1.5% is HOCl and 1.5% is OCl-.

The minimum HOCl concentration in pool water to prevent algae is, by consensus, 0.05 ppm. This amount of HOCl results at a pH of 7.5, with a free chlorine concentration of 3.5 and 30 ppm cyanuric acid – just enough to kill algae.

As this chlorine is used up, the chlorine bound to cyanuric acid is continually released such that there is always 3% unbound and 97% bound.

One way to look at this is to picture an army of 10,000 soldiers facing a battle. The strategy of the army is to send 300 soldiers at a time into battle. When the 300 sent to battle are gone, then 300 more are sent into battle from the reserve army. This is how CYAbound chlorine and the 3% work. As the 3% is used up, it is replaced from the reserve until all the chlorine is used up.

Advances in Understanding CYA

Recently, the pool and spa industry has learned a lot about the relationship between CYA and FC. One of the

biggest revelations of late has been the importance of the CYA-to-FC ratio.

This understanding began when a service professional named Ben Powell developed a chart for preventing algae that showed the free chlorine needed for different concentrations of CYA. Richard Falk, known as Chem Geek at the online forum Trouble Free Pool, was able to derive a simple formula for the minimum free chlorine needed to prevent algae based on the amount of cyanuric acid, expressing it as a ratio. He then spent years educating the pool industry on the CYA-to-FC ratio. Through conversations with Falk, the late Bob Lowry of Lowry Consulting and the Pool Chemistry Training Institute began to promote this ratio.

Simply stated, in order to keep algae from growing and bacteria from forming in a pool that uses CYA, the free chlorine should be 7.5% of the CYA level.

For example, 30 ppm CYA x 7.5% = 2.25 ppm FC. So at 30 ppm CYA, you should maintain the FC at least at 2.25 ppm.

If the CYA goes up to 60 ppm, then 4.5 ppm free chlorine is required. (CYA × 7.5% = 4.5 ppm FC.) At 100 ppm CYA, 7.5 ppm FC is needed.

You can see that as the cyanuric acid goes up, more free chlorine is needed to keep algae and bacteria from growing in the water.

In 2017, the Cyanuric Ad Hoc Committee of the Model Aquatic Health Code (CMAHC) proposed scaling that ratio down so that the free chlorine should be 5% of the cyanuric acid, reformulating the CYA/ FC relationship to be expressed as a 20:1 ratio. In other words, for every 20 ppm of CYA, there must be 1 ppm FC. This ratio was shown to reduce the risk of E. coli by five-fold and the risk of Giardia by two-fold.

Every 20 ppm of CYA means an increase of 1 ppm FC:

•20 ppm CYA/1 ppm FC.

•40 ppm CYA/2 ppm FC.

•60 ppm CYA/3 ppm FC.

•100 ppm CYA/5 ppm FC.

What else does CYA do?

There are several other ways that CYA reacts and performs in pool water. In addition to acting as a shield for chlorine from UV sunlight, CYA is also a buffer that, along with total alkalinity, prevents the pH from drifting.

Other effects of CYA are not as beneficial. Too much CYA reduces the effectiveness of HOCl, slowing its reaction rates against pathogens.

High CYA also lowers the Langelier Saturation Index. In water testing, CYA affects the test results of the carbonate alkalinity test, and pool technicians must account for this. CYA interferes with the total alkalinity test making it appear higher than it really is.

Depending on the pH, CYA will account for a percentage of the total alkalinity test – about 33%.

To ensure that you are reporting the true total alkalinity, there is a simple calculation: Take the CYA reading in ppm and divide by three. Subtract that number from the total alkalinity test result to get the correct total alkalinity value. For example, consider that the total alkalinity is measured to be 80 ppm and the CYA is measured at 60 ppm. Divide the CYA by 3 and subtract that from 80:

60/3 = 20 ppm

80 – 20 = 60 ppm is the true total alkalinity.

Low TA can lead to corrosion of pool equipment and plaster and swimmer discomfort. It is very important to understand this CYA/ TA correction factor.

High levels of CYA combined with levels of copper above 1.0 ppm can lead to the formation of copper cyanurate. This will appear as purple, dust-type material that clings to the waterline, steps, rails, and around light niches. The only real way to get rid of copper cyanurate is to lower both CYA and copper levels. The most practical way to do that is to drain some or all of the pool.

Lowering CYA

The most practical and efficient way to lower CYA is by draining the water. Those using trichlor tablets or shocking with dichlor should be aware that the CYA level increases with every usage and will eventually lead to reduced chlorine effectiveness and the need for more chlorine to keep the water sanitary and safe for your customers.

Users of trichlor must incorporate a practice of draining and dilution regularly to keep the CYA level between 30 and 50 ppm. This can become costly from the standpoint of water usage and water balance chemicals that may be needed.

One of the other ways to keep CYA levels managed is to reduce the use of trichlor tablets and use liquid chlorine to shock or switch to either liquid or cal-hypo as the primary source of sanitizer. Neither liquid chlorine nor cal-hypo contain CYA, so you can set your CYA at 30-50 ppm and forget it. Keep in mind that cal-hypo contributes calcium as a byproduct, and every pound of calhypo raises the hardness by 8 ppm. Liquid chlorine leaves behind sodium chloride as a byproduct, and while this does contribute to the total dissolved solids (TDS), compared to high CYA or calcium levels, it has the least detrimental effect.

There are other ways to lower CYA. There are commercially available products that use a microorganism to break down the CYA. These can be effective, but they are costly, and they must be used precisely according to instructions or they will not work. For example, the water must be warm — not much less than 70 degrees. Also, because the product uses beneficial bacteria, the chlorine level must be low — usually no more than 1 ppm. It may not be recommended to lower the chlorine in the summer when it is hot and sunny. It may be better to try these types of removers in the off-season, but keep in mind that the water needs to be near 70 degrees.

One recent anecdotal method for the removal of CYA has been the use of alum. This is basically a coagulation method where the aluminum sulfate grabs the CYA into a floc, which settles to the bottom of the pool. Alum has been used in water treatment for centuries, and it is used in lake treatment for the removal of phosphates in the same way. Rudy Stankowitz from Florida was among the first service techs to promote the use of alum to other service pros. His podcast and publications are probably the best source for techniques on using alum to remove CYA. Like the biomethods discussed above, preparation is important, and the water again must be around 70 degrees. The pH should be lowered to 7.0 if possible because alum flocs faster at a low pH. Total alkalinity and calcium hardness must be in recommended target ranges. The filters must be bypassed, so set sand filters at “recirculate,” and remove cartridges from cartridge filters. The process takes 8.33 lbs. of alum per 10,000 gallons of pool water, which is circulated for 2 hours. Next, shut off the pumps for 12 hours to allow the floc to settle. The floc is then vacuumed slowly to waste.

Some claim a 20-to-30% drop in CYA after dosing with alum. Keep in mind that if you have very high levels of CYA, this method may be tedious and time consuming. For example, if you had 200 ppm of CYA, a 20% reduction would result in 160 ppm. Unless you really overloaded on alum or did the treatment several times to get to 50 ppm, this would not be helpful in managing your free chlorine. If your goal is to achieve a free chlorine concentration that is 7.5% of the CYA, bringing the CYA down to 160 ppm would still require 12 ppm free chlorine to keep the pool clear. The alum method could be useful in times when draining is restricted. However, proactive drain and dilution is still the most effective and economical way to lower CYA.

Testing CYA

If you use trichlor tablets or dichlor at all, you should be doing regular CYA testing at least once a month. The dilution turbidity test method is one of the best and most accurate methods. There are test strips as well, but they are not as accurate as a turbidity test. When you test for CYA, the pH should be 7.4-7.6, ideally 7.5. Water temperature will affect the accuracy of the test. High temperature (90 degrees or more) will produce a false low reading of 15 ppm or more. Cold water of 60 degrees or less will give a false high reading by 15 ppm or more. If the test measures CYA at 100 ppm, a dilution should be done for accuracy. A simple two-part dilution is 1 part distilled water added to 1 part pool water. Multiply the test result by two. Remember, CYA is very effective at protecting chlorine from the UV rays of the sun, but CYA also affects several other aspects of water chemistry.

Terry Arko has more than 40 years’ experience in pool service, commercial, retail and technical sales. He is a CPO Instructor, member of the Recreational Water Quality Committee of PHTA, board member of California Pool and Spa Association CPSA, and the head instructor of the Pool Chemistry Certified Residential course of Pool Chemistry Training Institute PCTI.