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
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:
- 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.
- 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.
- 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?
- 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.
- 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.
