CYA Effects on Disinfection
The Effect of CYA on Disinfection
- Ability for diarrheal fecal accident remediation
- The efficacy of 1 ppm free available chlorine (FAC) to be at least as effective as monochloramine for inactivation of Giardia
A. Use the CDC diarrheal fecal accident requirements for CYA during normal operation
B. Set the CYA limit so that free chlorine will be at least as effective at killing Giardia as combined chlorine.
No matter which form of chlorine is used (calcium hypochlorite, chlorine gas, trichloroisocyanuric acid, etc.), when it dissolves in water, the active form of chlorine, HOCl, is formed. HOCl is a very strong disinfectant that can oxidize contaminants in the water as well as kill harmful germs (i.e. pathogens) such as bacteria and viruses.
Cyanuric Acid Chemistry
Following is the structure of CYA. It is a dynamic molecule and can be present in two different forms, called the enol and keto tautomers.
Chemistry of Chlorine and Cyanuric Acid Together
In addition to the ability to bind hydrogen ions, CYA can bind up to three chlorine atoms. When it has three chlorine atoms attached, it is called trichloroisocyanuric acid (trichlor). When it has two chlorine atoms attached, it is called dichloroisocyanuric acid (dichlor). Trichlor is a common slow dissolving chlorine tablet often used in residential pools. Dichlor is a fast dissolving chlorine shock that is usually sold in granular form. The following figure shows the addition of chlorine to CYA to make trichlor.
The ability of chlorine to kill pathogens is often described in terms of CT values. A CT value is simply the concentration (C) multiplied by the time (T) needed to kill or inactivate an organism. Typically, these values are reported for 3-log reductions of organisms. A 1-log reduction would inactivate 90%, a 2-log reduction would inactivate 99%, and a 3-log reduction would inactivate 99.9% of the organisms. CT values are usually given in the units of ppm·minutes. So a CT value of 1 ppm minute for chlorine would typically mean that 1 ppm of chlorine was able to kill 99.9% of the organisms in 1 minute. A CT value of 100 ppm minutes would typically mean that:
- 100 ppm was able to kill 99.9% of the organisms in 1 minute
- 1 ppm was able to kill 99.9% of the organisms in 100 minutes.
* pH 7.5, 25 °C, 3 log, 1 ppm FC
Algae is normally not a significant public health concern, so there is little peer reviewed scientific journal data on the effect of CYA on kill rates for algae. One study was conducted by Sommerfeld and Adamson in 1982.xxv The authors saw that chlorine activity was reduced slightly by 25 ppm CYA and that higher stabilizer concentrations up to 200 ppm generally resulted in no further reduction in activity.
Oxidation Reduction Potential
ORP is used in commercial pools to control the feed of chlorine. It is also a measure of the oxidizing power of chlorine in solution. For both these reasons, a discussion of the effect of CYA on ORP is relevant to disinfection.
- CYA has been shown to decrease the inactivation rate of chlorine towards bacteria, viruses, amoeba, protozoa and algae.
- CYA decreases the ORP of chlorine.
- Even at low concentrations (i.e. <10 ppm) of CYA, the concentration of HOCl, the active oxidizer and disinfectant in pools, is drastically reduced by the presence of CYA resulting in slower disinfection rates.
- The CDC recommends that the CYA concentration be reduced to 15 ppm before performing a diarrheal fecal accident remediation as the CDC was unable to achieve 3-log reduction of crypto with >16 ppm of CYA present in the water.
- Chloramine is not considered a sufficient sanitizer for commercial pools because the kill rates are too slow to prevent bather-to-bather disease transmission (1 ppm free chlorine will inactivate Giardia in 45 minutes vs 750 minutes for 1 ppm chloramine (i.e. – 17 times longer))
- To retain the minimum efficacy equivalent to monochloramine, the CYA to FAC ratio should be no greater than 14 to 1.
- Using this 14:1 CYA to FAC ratio, the maximum CYA concentration should be 14 ppm for a minimum chlorine concentration of 1ppm or 28 ppm for a minimum chlorine concentration of 2 ppm.
- Considering the EPA limits of 1-4 ppm free chlorine, we recommend that CYA concentrations be kept below 14 for all chlorine levels to ensure adequate bather safety.
- Pool and Spa Operator Handbook, National Swimming Pool Foundation, Colorado Springs Co, 2017.
- Williams, K.G., Aquatic Facility Operator Manual, 6th revised edition, National Recreation and Park Association, 2012.
- Goldstein, F., Professional Pool and Spa Operator Manual, Association of Pool and Spa Professionals, 2017.