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

6. Bather waste

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.