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What makes spa chemical demand unique?

By Marcelle Dibrell

The month of January is not known as a time a time for swimming, but rather for soaking, and while many outdoor pools have been closed for the winter, the spa is still open for business.

One of the key elements that determines the spa’s allure is the water’s temperature, and it’s instructive to examine if there might be any difference in how a body of water should be treated when the water’s temperature is raised. 

In terms of water chemistry, some may wonder how much difference there could be between a pool and spa. But there are a few factors that change when we move from a pool to a spa.

In general, spas are relatively small bodies of water, usually no larger than about 500 gallons, that contain hot water which is rigorously circulated, typically incorporating vigorous air-blowing jets into the stream. 

This hot, turbulent water is intensely relaxing to most bathers, but it also gives rise to some fundamental differences in how it should be treated. Spas are not miniature swimming pools, and treating them as such will ultimately lead to problems.

There are two major factors that distinguish spas from swimming pools.

The most obvious is the water’s temperature. Hot water changes things a lot. For one thing, human bodies respond to hot water differently than cold water. 

Skin pores open, and oils and sweat are released. The net result is an increased amount of bather waste into a relatively small volume of water. 

But in addition to changing the bather’s load on the water, thus changing the way that sanitizers respond, other water chemistry reactions are altered. Beyond increased demand, chlorine behaves differently at elevated temperatures, and the concentration of disinfection by-products changes. 

Water balance parameters are also affected by heat. Many of the chemical reactions that take place in a pool or spa are equilibrium reactions, and equilibrium is affected by temperature. 

In addition, the heat places stress on some of the equipment and piping, and changes the erosion rate. 

Also, hot water increases the rate of evaporation, leading to an increased concentration of materials dissolved within. 

Finally, the heat changes the biological response of algae as well as many types of bacteria.

The second factor that distinguishes spas from pools is aeration. Turbulence has a profound effect on several parameters.

One of the most obvious effects of turbulence is to increase bather loads. The jet action helps slough away dead skin cells into the water, which increases sanitizer depletion.  

The turbulence can also cause foaming. 

Like heat, aeration increases evaporation rates. 

A final crucial point is that the vigorous aeration of spa water changes the rate at which gases both enter and exit the water.

The effects of these differences are magnified by a spa’s size. Due to the fact that spas are so much smaller than the average pool, bather load is dramatically increased.

According to one estimate, two people in a spa is equivalent to 80 people in a typical swimming pool.

It is for all of these reasons that spas cannot be managed the same as swimming pools, and the following articles will explore some of these effects in greater detail.                                         

Paid subscribers have full access to the complete coverage on spas. Contact our circulation department at 949-916 0292 or email for details.

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