Troubleshooting spa/hot tub water chemistry

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Since 1990, HydroTher Hot Tubs have been the #1 choice of architects, consultants, designers and facility operators for commercial aquatic applications.

While water quality is vital to the safety and enjoyment of both spas and swimming pools, there are also major differences when it comes to handling hot and cool water chemistry. Understanding these differences, the impact they have on water chemistry and how to manage them will make hot tub care much easier for hot tub professionals and their clients.

Spa management challenges

There are several spa management factors hot tub users and operators must contend with, including:

Problems with pH

There are three primary factors working to push pH up and total alkalinity (TA) down in hot tub water.


The bubbles introduced into the spa by the blower make the water feel comfortable and help relax bathers’ muscles. However, this action is one of several factors that strongly impact water balance for spas.

Buffering for pools and spas is based on a carbonate system. Carbon dioxide (CO2), the carbonate ion (CO32−), bicarbonate ion (HCO3) and carbonic acid (H2CO3) work together to buffer the water against excessive swings in pH. However, the aeration (and other factors to be discussed later) causes the carbon dioxide to leave the water (see Figure 1).

The result is an increase in the hydroxide ions (OH), which causes pH to increase. There is a loss of the carbon dioxide into the air, so the carbonate ion cannot reform, which causes a decrease in total alkalinity.

Aeration also causes more compounds within the water to sublimate into gas, which allows them to escape from the water. This affects the quantity of sanitizer present in the water, as both chlorinated and brominated compounds can be volatilized and leave the water as a gas. The air quality around the spa can also be affected thanks to the production of chloramines, which are irritating to the eyes and nasal passages.

Aeration also removes more skin cells and oil from bathers’ skin, resulting in the introduction of more contaminants than in a swimming pool environment.

Speed of circulation through piping

Spas have much faster turnover rates compared to pools. Water moves rapidly through the piping, which also has more turns than pool piping. As water flows through the elbows of the piping, the more rapidly moving water, particularly at the outside of the elbow, creates a difference in pressure that causes carbon dioxide to leave the water. Once again, the effect is an increase in pH and a decrease in total alkalinity.

Water temperature

The higher operating temperatures for spas have several ways that they impact the water.  Again, the tendency for carbon dioxide to leave the water is increased as the temperature of the water increases (see Figure 2).

As pH is increased, there is an affect on the amount of sanitizer available to actively kill bacteria. The impact is more dramatic for chlorine than for bromine in the typical pH range for spa operation (see Figure 3).

Hotter water, more bacteria

The higher temperature of the spa also creates an environment that speeds the reproduction rate of bacteria called mesophiles (organisms), which grow most rapidly in temperatures ranging from 20 to 45 C (68 to 113 F). This range matches up very well with both the typical operating temperature of spas and normal human body temperature. In spa water measuring 37 C (98.6 F), some mesophilic bacteria, which are often those that cause disease in humans, can reproduce in as little as 14 minutes (see Figure 4).

The higher temperature of the spa also creates an environment that speeds the reproduction rate of bacteria called mesophiles (organisms), which grow most rapidly in temperatures ranging from 20 to 45 C (68 to 113 F).

Another impact of a hot tub’s higher operating temperature is the rapidity of chemical reactions. Put simply, reactions that tend to occur in water, such as oxidation and pH changes, will happen more quickly in a spa environment. This is why weekly service visits may work well for swimming pools, but can quickly cause issues for spas.

Higher temperature also causes more compounds, including carbon dioxide and various chlorinated and brominated compounds, to become volatile and leave the water. This results in more rapid depletion of sanitizer compared to a swimming pool. That said, brominated compounds are heavier than their chlorinated counterparts, and are less likely to volatilize and leave the water.

Hotter water also increases bathers’ production of sweat, a major source of contaminants that affect water quality. By mass, urea ([NH2]2CO) is the predominant source of organic-N in human sweat and urine. The organic-N compounds react with chlorine to form chloramines, thereby consuming chlorine that could have been used to oxidize a compound or kill bacteria. Chloramines are poor sanitizers compared to hypochlorous acid (HClO), so the introduction of more organic-N compounds can quickly deplete a chlorine sanitizer. Brominated spas will not be affected as adversely, because bromamines are almost as effective as sanitizers as hypobromous acid (HOBr).

Higher temperatures also affect the solubility of calcium compounds, especially calcium carbonate. This means there is more tendency for calcium scale to develop more often and more rapidly in spas, particularly within the heat exchanger.

Less volume, more bathers

Consider this comparison: A typical swimming pool with about 40,000 L (10,566 gal) of water with 40 swimmers results in the same volume of water per person as one bather in a 1,000-L (264-gal) spa. As such, typical spa use creates a much higher level of bather contaminants in the water than occurs with typical swimming pool use. Remember, aeration and higher water temperatures cause increased sweating and the deposition of skin flakes and oil into the water. Obviously, the sheer quantity of contaminants and bacteria in each litre of water cause a strain on the sanitizer.

Dealing with diseases

Unfortunately, the volatile nature of spa water can lead to several bather health issues, which spa operators should aim to prevent.

Pseudomonas folliculitis

This is the most common disease transmitted via spas. It is an infection of the hair follicle caused by bacteria called Pseudomonas aeruginosa. While this bacteria is naturally present on human skin, it is only in low quantities (i.e. not high enough to develop an infection). However, in the warm water of the spa, pores open and allow the bacteria to enter the area around hair follicles. If spa sanitizer levels are inadequate, the number of bacteria entering the follicle can overcome the body’s natural defences and cause an infection. For some individuals, the body’s natural defense mechanisms will overcome the folliculitis infection within a few days. However, those with compromised immune systems may need oral antibiotics to recover.


Pseudomonas folliculitis is often confused with dermatitis, an irritation of the skin that can be triggered by an individual’s sensitivity to a particular compound present in the water. It can also be caused when the water balance is incorrect or sanitizer levels are excessively high.

Determining whether an individual has folliculitis or dermatitis is best left to a physician, but there are some clues of which spa operators should be aware. Typically, dermatitis reactions occur during spa use or within a few hours of using the spa. Also, dermatitis does not usually exhibit the presence of pus if there are raised areas on the skin. In contrast, folliculitis symptoms are typically seen 24 to 48 hours after spa use and pus is present in raised areas.

Legionnaires’ disease

This disease, the milder version of which is known as Pontiac fever, is actually transmitted through breathing of the aerosolized spa water droplets. A person can become infected with Legionella pneumophila without ever entering the spa; simply being around the spa when aeration is occurring can lead to exposure.

To prevent disease transmission, constant maintenance of adequate sanitizer residual is required. With all the factors working to deplete and reduce the effectiveness of the spa sanitizer, this is a fundamental requirement.

Meeting spa maintenance challenges

With all the factors working against the operator of a spa, what can the operator do to ensure an optimal user experience?

Maintaining sanitizer residual

First and foremost, sanitizer residual must be maintained within the recommended range for the chosen sanitizer, whether it is bromine or chlorine. Overall, bromine is less affected by increased pH, brominated compounds are less likely to leave the water due to volatization and bromamines are comparable to hypobromous acid as sanitizers. In order to provide a continuous bromine feed, install a properly sized brominator. Then, test the bromine residual and adjust the flow through the brominator to be sure the feed rate is well suited to the spa’s use pattern. Automatic controllers are helpful, so long as one understands their nuances, particularly when it comes to feeders.

Supplemental oxidation

Supplemental ozone oxidation is also strongly recommended, whether chlorine or bromine is the primary sanitizer. That said, some contend that with bromine-based systems, ozone is consumed, thereby regenerating the bromide ion back to hypobromous acid and negating the oxidizing support. This contention, however, is debatable.

Careful monitoring

In order to maintain proper water balance, spas require close attention. Knowing that the tendency of the spa water is for alkalinity (AT) to decrease while pH increases, operators must rebalance the water frequently to the upper range for alkalinity and lower range for pH.

Buffering systems

Another potential spa chemistry remedy is the use of a phosphate (PO43-)-based buffering system, in addition to a carbonate system. The phosphate system is not subject to the same loss of a component due to volatilization as the carbonate system. However, be sure to check local regulations relative to governing the amount of phosphate permissible for discharge.

Adjusting operations

Operational activities can also improve the ability to maintain proper water balance. These include not operating the aeration feature unless bathers are present and covering the spa when it is not in use. Also, when applying water balance products, operate the circulation pump at the lowest speed and do not use aeration.

Scale inhibitors

Use of scale inhibitors is also recommended to help control calcium carbonate deposits.

Follow draining and refilling requirements

In commercial spas, if the water volume is greater than 4,000 L (1,056 gal), Ontario regulations require make-up water to be added daily based on the number of users (30 L [8 gal] per bather use, to a maximum of 20 per cent of the total spa volume). For spas less than 4,000 L, the following equation below is used to determine how many days may elapse between draining and refilling:

WRI = V/(10 x U)

While these guidelines are for commercial spas, they can also have a positive impact on residential units.

Finally, a spa filter is an environment that encourages bacterial growth. As such, they should be cleaned each time the spa is drained and refilled, using a specially designed filter-cleaning product. Larger spas that require daily water additions should use the equation for refill frequency on smaller spas as a guideline for frequency of filter cleaning.

By Mary Costanzo - Pool and Spa Marketing 

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