In the event of an Accidental Fecal Release (AFR), the CDC has published a recommended limit to CYA at 15ppm for practical reasons. The limit is fine by us, as Minimal CYA is our Fourth Pillar of Proactive Pool Care anyway.
The CDC has set a limit to CYA in commercial swimming pools
Cyanuric acid (CYA) serves as a protection shield against sunlight for chlorine. Why protect chlorine from sunlight? Because the UV rays from the sun can degrade chlorine very quickly. Obviously, that can be a big problem for outdoor pools. Studies show that sunlight can be wipe out chlorine by 75-90% in a matter of two hours. The half-life of chlorine—when exposed to direct sunlight—is about 45 minutes. That’s astounding.
So a stabilizer is helpful, otherwise you will be using (and losing) chlorine all day, every day.
The chemistry of cyanuric acid (CYA)
The cyanuric acid molecule is a hexagon with alternating Nitrogen and Carbon atoms. It allows for three molecules of chlorine to attach to the nitrogen atoms, forming a nitrogen-chlorine bond (N-Cl). Without getting too scientific, just know that this N-Cl bond is weak, which allows for chlorine to let go of CYA when it has something to oxidize or kill. When attached to CYA, however, chlorine is protected from sunlight.
Chlorines like dichlor and trichlor have CYA in their formulas. Because of this, they are known as stabilized chlorine, and used primarily in outdoor pools. Stabilized chlorines should not be used in indoor pools.
CYA lowers ORP
Obviously, the protection from sunlight that CYA provides is a huge benefit to chlorine. Just small amounts of CYA can have a major impact on sunlight protection. For outdoor pools, that’s a game changer. But like anything great, there is a limit. At some point CYA gets in the way of chlorine, and slows down its killing speed (measured as oxidation reduction potential, or ORP). We calls this overstabilization. Generally speaking, the more CYA you have, the lower the ORP.
Conventional wisdom in the pool business suggests an ideal range of CYA to be 30-50ppm, with a minimum of 10ppm and a maximum of 100ppm. That being said, it becomes very impractical to disinfect a pool with more than 15ppm CYA in the event of an emergency AFR. Why? Because the contact time (CT) required for chlorine to kill spreadable diseases like cryptosporidium will be close to impossible to reach.
15ppm maximum for CYA
From the CDC: In case of a fecal incident, close the pool, and CYA levels can no longer exceed 15ppm.
Here’s why: chlorine stabilizers (like CYA) slow the rate that free chlorine kills pathogens. Because of the slowed rate of sanitation, pools must have below 15ppm CYA when treating a fecal incident. That way, chlorine can sanitize effectively in a reasonable amount of time. Keeping CYA under 15ppm is really hard to do, especially since many pools have more than recommended anyway (like 50-100ppm). So what can we, as industry professionals, do to comply with this new regulation?
So what do you do if your pool already exceeds 15 ppm CYA? The most economical and reliable way to reduce CYA is to drain and refill to dilute. Hopefully draining completely is unnecessary, just a partial drain and refill should suffice.
It’s our opinion at Orenda that the CDC’s 15ppm limit—while it is a big change—offers an opportunity for new thinking. Pools have been operated the same way for decades; changing the way we think about water can be a good thing. IPSSA meetings are a great place to discuss these new ideas.