Conceptually, it's easy to think of a spa as simply a warm swimming pool of manageable size. And after years of experience with swimming pools, some may consider spa water care to be a scaled down version of the same process.
But there are important differences that make spa water care very much its own field. Perhaps most obvious of these differences is temperature — roughly 20 degrees separates an average pool (approximately 80 degrees) from an average spa (around 100).
That 20 degrees makes a big difference. It boosts the production of body wastes, speeds chemical reactions that alter water balance and consume sanitizer, and it provides a comfortable environment for bacteria to reproduce.
Not only is the water hotter, it is more frothy and fizzy. It surges through piping and is squirted through nozzles, which has a significant effect on its chemistry — most pointedly, it drives carbon dioxide (CO2) out of the water.
This is a crucial point. A lot of the differences you find in spa chemistry as opposed to pool chemistry are based on that lack of CO2, lost through turbulent flow and helped on its way by high temperature. This loss of CO2 ends up affecting pH balance and total alkalinity, which can play havoc on a homeowner's attempt at water balance.
A key chemical equation which governs the presence of CO2 in spa water is HCO3-OH- + CO2. In this reaction, carbonate (HCO3-) breaks apart to form CO2 and hydroxide (OH-).
But if you lower the level of CO2 in the water, more carbonate reacts to make up the difference. The immediate result is that pH tends to go up because of a loss of carbonic acid (which normally helps keep pH down) and an overproduction of hydroxide (which drives pH up).
At the same time, total alkalinity has gone down due to a loss of bicarbonate.
These two effects — the tendency to raise pH and decrease alkalinity — are signature differences to keep in mind when caring for a spa as opposed to a pool, whose water balance tends to stay relatively steady.
In addition to these considerations, the loss of CO2 leads to the increased formation of calcium carbonate, which, in hot water, is less soluble and thus more likely to be deposited as scale or seen suspended in cloudy water. (Unlike most reasonable elements or compounds you find in the natural world, calcium solubility decreases with higher temperature.)
Of course, with its extremely high local temperatures, the spa heater is the first component affected by scale, but the ugly film can be deposited anywhere in the spa, showing up on surfaces in a thin white layer perfect for making gleaming spa surfaces look old and unattractive.
Higher temperatures and vigorous water movement not only affect pH, alkalinity and calcium, they also have a strong effect on sanitizer stability.
As a general statement, most reactions take place more quickly at higher temperatures, and those involving halogen sanitizers (chlorine and bromine) are no exception. These are consumed faster at higher temperatures and therefore produce the by-products of sanitization and oxidation faster as well.
When comparing the two, chlorine has an atomic mass that is less than half that of bromine, and the higher temperatures and aeration of spas encourages this lighter, more volatile element to exit the water more rapidly — whether as free chlorine or in a chlorinated compound. This makes it more difficult to maintain a residual in a chlorinated spa.
Also, as chlorine and its compounds leave the spa and float up into the ambient air, they are likely to enter the lungs of bathers, which research has determined is unhealthy at best and may put bathers at risk of illness. Plus, it smells bad.
Finally, it's worth noting that bacteria also grow faster at higher spa temperatures, a fact that comes into play when sanitizers are used up.
Volume Vs Bathers
The difference in water volume is another consideration for spas in that the bather load per unit volume of water is much greater, a problem exacerbated by the effect on bathers of higher spa temperatures. Looking at the ratio of bathers-to-gallons, two people in a 300-gallon spa is the same as 200 people in a 30,000-gallon pool.
But now imagine an enormous party of 200 people splashing and sweating in an ordinary backyard pool; that gives a clearer picture of the typical bather load in a spa. At the same time, the spa's hot water causes those 200-equivalent bathers to sweat at a higher rate, and the release of their natural human debris — the flaking of skin, release of oils — occurs at a faster rate, too, adding to the pronounced bather-load effect.
The spa's small body of water and these exacerbating factors make it more difficult to keep water clean and clear, whereas in a residential pool (which is more likely to see five or 10 bathers in an average day, tops) there's a lot more volume, and the bather load is diluted, reducing its impact.
A Pound Of Cure
Many of the standard commonsense approaches to water treatment remain unaltered as one moves from pool to spa. That is, maintain a proper sanitizer residual, shock to aid the oxidation process, follow proper water balance procedures, test and adjust water balance frequently and especially make sure to keep the filter clean.
But what happens when it all goes south?
If water balance is becoming incorrigible, and you are reading a high pH and low total alkalinity situation described above, adjust alkalinity first, and consider using a phosphate based pH buffer which is unaffected by the loss of CO2.
As with pools, spas can develop sanitizer demand due to the build up of contaminants from heavy bather loads. In this situation, chlorine or bromine is being added to the water, but continues to produce no measurable residual. The reason is that you have so much contamination in the water that every bit of chlorine you put in is immediately being put to work.
Here we come to what is maybe the most important distinction between pool water care and spa water care — the ability to dump without significant loss when things get hairy or when enough contamination has built up in the spa. There's no need to struggle with the black hole of sanitizer demand or out-of-control water situations. The thing to do at that point is to simply drain and refill.
Education Is The Answer
The difference between pool and spa water care is really just another good way of categorizing the large amount of training and understanding necessary to hold a competent discussion on the subject.
And that's no simple task.
When it's all said and done, many people find recreational water chemistry intimidating, but it need not be. Industry training and education capabilities have grown in scope and sophistication, and now instruction in water care is available from a variety of sources in a variety of modes.
Manufacturers offer excellent training opportunities around the country and online versions, which can be accessed at any time of day or night.
Now more than ever, the industry needs people who can explain water issues to spa customers with ease and confidence — that is the most important function of the pool and spa store.
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