Swimming in spreadsheets: Why real-world experiences are integral to managing pool water quality

by habiba_abudu | December 19, 2019 11:15 am

By Roy Vore, PhD, microbiology and Jeff Gaulding, PhD, chemistry

[1]

The great cyanuric acid (CYA) debate is one that has raged within the pool and spa industry for decades. Until recently, the discussion centred on laboratory studies and water quality data from real pools to help guide the technical standard for proper water management. However, a new mathematical model has been created and has entered into this debate.

Aquatic professionals should take heed as models are only as good as their assumptions. They are created to help better understand complex systems, and those who have managed a pool for any length of time know how complex they can be. Although some mathematical models can become quite intricate in structure, it is important not to confuse a complicated model with a good one. A good model uses sound assumptions to make predictions that can be tested and proven through scientific studies or real-world data. Most importantly, predictions need to be tested before being accepted as fact.

The new model recently published by the Council for the Model Aquatic Health Code (CMAHC) Cyanurate Ad Hoc Committee uses a very complex approach, including a multitude of spreadsheets, to attempt to predict outcomes in pool water. It is because of this complexity and these many spreadsheets this article was written. After careful evaluation of this new model, it is clear the assumptions are flawed, and the resulting predicted outcomes do not align with years of collected data in real-world, pool water data.

Flawed assumptions

[2]

Recently, a great deal of attention has been drawn to an idea that pool operators would realize better outcomes if one managed CYA stabilizer and free chlorine (FC) in relationship to each other via an artificial ratio devised through this new model. Several years ago, it was suggested this ratio be 7.5:1 CYA to FC. Now, the CMAHC Cyanurate Ad Hoc Committee suggests it should actually be 20:1.

In reviewing the published study that focuses on this CYA stabilizer to free chlorine relationship, there are at least seven assumptions outlined in the model that do not reflect real-world pool situations. These flawed assumptions are used in an attempt to simplify a complex real system, like pools, into a mathematically solvable format—a spreadsheet. Those flawed assumptions then generate invalid recommendations. Prior to the creation of this new model, the CYA recommendations to be used by pool operators were based on data collected from actual pools.

To emphasize the point, it is important for pool operators to review the puzzling assumptions that provide the foundation of support to the recent 20:1 CYA to FC model. The assumptions in question are listed below and the statements in quotations come directly from the publication and its supplementary material:

1. Swimmer movement

“Velocity of 0 mm/min in the x direction (Ux) was chosen to keep the model as simple as possible.”

To rationalize the model, this means the study assumes bathers in the pool are not swimming, but are rather standing completely still for the entire time they are in the water.

2. Pool depth

“Depth of pool at 0.91 m (3 ft) is a typical shallow pool depth” and “Distance between bathers 1.18 m (3.87 ft) is equivalent to 1.39 m² (15 sf) of surface area per bather.”

So now all of the swimmers are standing in 0.91 m (3 ft) of water, in a grid 1.18 m (3.87 ft) apart. And again, not moving.

3. Swimmers

The model assumes all of the swimmers are children (based on the estimate of how much pool water they drink to get the model’s exposure). Each model child spends approximately two hours a day, 73 days per year in the pool. As a reminder, this time spent in
the pool requires them to arrange themselves in a ~1.48-m² (~16-sf) grid and stand around in 0.91 m (3 ft) of water not moving for those two hours a day, 73 days per year. Does this sound like the behaviour of a typical swimmer, especially that of a child?

4. Pool closure

The model conservatively assumes continual sloughing 24 hours per day, seven days per week at a constant bather load, thus not accounting for partial diurnal recovery from overnight disinfection. So the pool never closes and is always at the maximum capacity. Keep in mind, while each child only spends two hours in the pool, when that time is up they are instantly replaced by another child in the same spot. This continues in the pool for every child in every spot in the ~1.48-m² (~16-sf) grid for 24 hours per day, seven days per week.

5. Circulation

“For Giardia and Cryptosporidium, the maximum probability of infection occurs at very low diffusivity values (<100cm²/min)” and “turbulent diffusion is typically in the hundreds to thousands of cm²/min.”

In other words, the risk is highest when the water is not moving much—so the model assumes the swimmers are not moving and the pump is not running. Further, the ratio described in the model does not factor in circulation and filtration—which is critical to maintaining a clean and sanitary pool—and would be cause for immediate closure of a commercial pool facility.

6. Bather waste

[3]

The model also predicts how much feces each bather is putting into the pool. However, the math behind the model is extremely sensitive to this assumption— if even 18 mg (0.000635 oz) less waste (that is 35/10,000ths of a teaspoon) is added to the water by each bather per hour they are in the pool, then a ratio of CYA to FC of 450:1 would be recommended by the model. Remember, this spreadsheet concludes that facility operators should change decades of pool operating practices based on a guess as to how much bather waste there is. There is a significant difference in conclusions based on an extremely tiny amount of bather waste.

7. Water replacement

To make the math work, the water is never replaced—no splash-out, no replacement water from backwash, no rain, no evaporation… nothing.

Flawed assumptions can lead to inaccurate water treatment recommendations

The study understands the modelling of Giardia does not reflect real-world situations: “It was not possible to develop a model that provides accurate estimates of absolute risk.”

It goes on to suggest: “The high rate of Giardia infection is not realistic but results from the assumptions used in this model: concentration of Giardia in feces, per cent of population infected with Giardia, every visit to a pool with no chlorine and constant 24 hour/day seven days/week high bather load, and no filtration of cysts.”

How can one confidently believe in the increased relative risk of Giardia infection if the study shows that rate of infection is not realistic?

The answer is they cannot, which is what actual data from real pools reveal. There are dozens of recreational water illness (RWI) outbreak reports in numerous scientific journals. The Centers for Disease Control and Prevention (CDC) has been periodically publishing RWI summaries since 1978 with the latest one covering 2000 to 2014 (CDC 2018). These are pools operating under the current guidance on cyanuric acid, so certainly a vast majority of those using CYA are operating far above a 20:1 ratio, and during this period of time there were no reported cases of E coli, Shigella, Pseudomonas, Legionella, Norovirus, and more importantly for this discussion, Giardia, when there was proven to be 1 ppm of free chlorine in the water. This is the real-world data published by CDC, not the output from a spreadsheet.

[4]To give this some perspective, CDC estimates that pools in the U.S. have 301 million swimmer visits per year. Given this figure, the model would predict the following (compared to what is currently being seen above).

Using the assumptions within the model that determine the proposed 20:1 ratio, and assuming there was no trace of chlorine during an outbreak, the incidence of Giardia is overestimated by more than 15 million cases per year. Similarly, the model overestimates the incidence of E. coli by more than 100,000 cases.

When looking at the model’s prediction for pools that were run at the highest ratio of CYA and FC that fits with the current recommendations of 1 ppm FC and 100 ppm of CYA, there should be more than 750,000 cases of Giardia coming from those pools each year. How many have actually been observed from pools that were confirmed to have 1 ppm of FC? None! That said, how confident can the industry be in changing decades of water treatment practices—methods that are demonstrated to effectively prevent illnesses due to pathogens susceptible to chlorine—based on a model that so severely overestimates the scope of the problem?

Conclusion

The key takeaway from this—where both model and real-world experiences agree—is the most important factor in keeping a clean, healthy swimming environment is proper pool maintenance, and keeping a chlorine residual of 1-4 ppm. Cyanuric acid helps prevent sunlight from destroying that residual, which makes it easier and less expensive for pool operators to maintain the appropriate chlorine level. That said, it needs to be asked, who is behind the continuous push to further regulate cyanuric acid in the industry? If the industry is truly concerned about public health, the focus needs to be on maintaining recommended levels of free chlorine to prevent disease from the readily controlled pathogens and on using secondary disinfection systems and proper maintenance practices to combat Crypto outbreaks. Rather than swimming in spreadsheets, it is the aquatic industry’s responsibility to ensure bathers are swimming in clean and clear pool water.

[5]Roy Vore is a technology manager at BioLab Inc. His work focuses on the control of microbial growth in recreational water and household surfaces. He is a Certified Pool & Spa Operator (CPO), a member of the National Swimming Pool Foundation’s (NSPF’s) education committee, an active contributor to the Pool & Hot Tub Alliance’s (PHTA’s) Recreational Water Quality Committee, was a major contributor to the Disinfection Water Quality module of the Model Aquatic Health Code (MAHC), and the lead author of the National Swimming Pool Foundation’s (NSPF’s) Recreational Water Illness handbook. Vore holds a PhD in bacterial physiology, and masters and bachelor’s degrees in microbiology. He has written more than 80 scholarly papers and presentations on the selection and use of industrial biocides, biocide testing methodology, the microbiological of pools and spas, and the governmental regulation of biocides. He can be reached via e-mail at roy.vore@biolabinc.com.

[6]Jeffrey Gaulding received his PhD in chemistry from Georgia Tech and his bachelor of science from Emory University. During his doctoral work he was awarded several fellowships, including being appointed to a National Institutes of Health training grant. Gaulding began his career in the pharmaceutical industry and has been working in the pool and spa industry for several years. He is also a Certified Pool & Spa Operator (CPO) and recently joined the Pool & Hot Tub Alliance’s (PHTA’s) Recreational Water Quality Committee. Gaulding can be reached via e-mail at jeff.gaulding@biolabinc.com.

References

Centers for Disease Control and Prevention (CDC). 2018. “Outbreaks Associated with Treated Recreational Water”—United States, 2000–2014. Morbidity and Mortality Weekly Report (MMWR), 67, 547-551.

Source URL: https://www.poolspamarketing.com/trade/features/swimming-in-spreadsheets-why-real-world-experiences-are-integral-to-managing-pool-water-quality/

---