By Marcelle Dibrell

Recreational water chemistry can be a complex topic. The issues become even more complicated when moving from the relatively large volume, cool waters of a swimming pool, to the small volume, hot waters of a spa.

So many variables change in a hot tub.

The most obvious aspect is water cleanliness. In well-used spas, it is more difficult to maintain hygiene because of the water volume, jet action and higher temperature, all of which maximize bather load.

It is often said that four people in a 400-gallon spa is equivalent to 200 people in a 20,000-gallon pool, but even this may be a low-ball estimate when one accounts for extra skin sloughing from the scrubbing jet actions, and excess sweat from the high temperatures.

The average spa user can produce up to three pints of sweat per hour at 102 °F, releasing ammonia, creatine, urea and many other chemicals for chlorine to oxidize, significantly increasing chlorine demand.

Then there is the bacteria introduced from swimmers’ bodies, which thrive in the trillions.

There is also the bacteria and pathogens within the hot tub’s plumbing itself, nestled in the dark warm confines of a biofilm matrix, with only the superficial succumbing to the effects of chlorine.

Pathogens such as Legionella and many others living within the plumbing of treated hot tubs are responsible for thousands of hospital visits and sometimes fatalities every year.

Disinfection by-products are also a bigger concern in a spa, relative to a swimming pool.

The same amount of bather produced contaminants in a smaller volume of water leads to an increase in disinfection by-products, which are also produced in greater quantities with increased temperature.

Some of these by-products are known to be harmful to humans who may inhale them directly into their lungs as they soak with their faces just above the water’s surface.

Evidence has shown that this can be more problematic at indoor pools and spas that lack adequate ventilation. However, chlorine consumption and disinfection issues are just two of the myriad differences between pools and spas.

Another aspect is temperature’s effect on the Langelier Saturation Index (LSI), which predicts the propensity for scale. All other things being equal, temperature alone raises the LSI and increases the possibility of scale. Contributing to this effect is a tendency for upward pH drift, caused by aeration from the jets.

Many of these topics will be covered in greater detail in accompanying articles.

In this special issue of Service Industry News, we’re talking hot tubs, and the complex chemistry caused by a change of degrees.