By Michael Gardner

Summary: In this article, readers will explore the reasons why spas must be given special consideration to prevent illness in bathers and damage to equipment—high water temperature and small volume.

Customers taking a refreshing dip in their pool or enjoying a relaxing soak in the backyard spa probably aren’t thinking about water treatment. Instead, they rely on water treatment professionals like you to manage their water quality so that pool and spa experience is enjoyable.

Effects of higher temperature
While pools typically run 78°–82°F, spas are usually maintained at 96°–104°F. These increased temperatures affect water chemistry in ways not encountered in pools. For example, high temperature, jetted spa water evaporates faster than calm pool water. When it does, it leaves behind dissolved solids made up of treatment chemicals, minerals, dirt and even sloughed-off skin particles. Tap water can be added to bring the spa back up to the proper level, but it also contains dissolved solids.

Repeated evaporation and replenishment causes the water’s total dissolved solids (TDS) level to build up. If left unchecked, a high TDS level can contribute to corrosion of spa components and possibly cloudy water. TDS cannot be lowered through chemical treatment, so you must partially or completely drain and refill the spa with lower TDS water to reduce the level. High temperatures can also cause calcium hardness to precipitate as scale, particularly on heater elements. This can lead to increased water pressure, reduced circulation, inefficient heating and increased operating costs, if allowed to build up. Once scale has formed, it can be difficult or even impossible to remove.

High temperature water also creates sanitation issues. When bathers get warm, they perspire more, adding organic compounds to the water. In fact, the average person perspires about one pint during a 30-minute soak. Chemical reaction rates also increase with temperature. A commonly accepted guideline is that for every 10°C (18°F) increase in temperature, reaction rates increase two-fold. More organic material in the water and faster reactions between sanitizer and contaminants can quickly deplete a sanitizer residual. For instance, it’s possible for two people in a 400-gallon spa to deplete 2.0 parts per million (ppm) bromine or 1.0 ppm free chlorine in just 15 minutes. This is particularly troubling because potentially harmful yeasts, protozoan parasites, viruses, and bacteria such as Pseudomonas aeruginosa, Legionella pneumophila, and Escherichia coli can flourish at temperatures found in spas. To prevent infection, it’s vitally important to maintain the proper sanitizer residual in spas at all times. In fact, one should always test for sanitizer before entering a spa; test strips make this easy to do, especially when traveling away from home (see Figures 1&2).

Impact of smaller volume
A spa’s relatively small volume also poses several challenges. First, spas have a much higher bather-to-water ratio than pools. Consider this fact: Four people in a 400-gallon spa are the rough equivalent of 250 people in a 25,000-gallon pool—both contain one person for each 100 gallons of water. Spa water must be tested, treated, and replaced regularly to compensate for this increased sanitizer demand. Second, water chemistry must be managed on a much smaller scale in a spa than a pool. Frequently, it’s referred to as “teaspoon chemistry.” For example, an extra pound of dry acid added to a large pool won’t drastically alter its pH, but adding even half this amount of acid to a spa would prove disastrous.

Hot water sanitation
To keep spa water free from pathogens and organic contaminants, it must be properly treated with a sanitizer and oxidizer. Sanitizers kill germs harmful to bathers and also control the growth of algae. Oxidizers eliminate unsavory organic compounds such as urine, sweat, body oil, perfume and other personal care products, and environmental debris so the sanitizer can work unimpeded. (Note: All sanitizers are oxidizers, but not all oxidizers are sanitizers.) “Shocks” made with potassium monopersulfate (MPS), such as Oxone®, are an example of the latter. Because oxidizers reduce the level of organic contaminants in water, they improve the efficiency of the sanitizer. Residential spas should be tested for the required sanitizer level prior to each and every use while heavily used commercial spas should be checked hourly or as regulated by the local health department.

When chlorine is added to water, it becomes the powerful sanitizer and oxidizer called “free chlorine.” The American National Standards Institute (ANSI) and the National Spa & Pool Institute (NSPI) recommend maintaining a free chlorine residual of 3.0–5.0 ppm in all spas. Free chlorine reacts immediately with contaminants in the water until its fighting power is used up. The spent chlorine, called “combined chlorine” or “chloramines,” isn’t an effective sanitizer (or oxidizer for that matter), and it causes eye irritation and a strong chlorine-like odor that signals a need for chlorination.

The goal in both pools and spas is to eliminate all combined chlorine while maintaining a free chlorine residual capable of providing around-the-clock protection for bathers. Chlorine is popular for sanitizing outdoor spas because it can be stabilized against degradation from sunlight by combining it with cyanuric acid. Chlorine for use in recreational waters is available in many different forms, some of which are better suited to spa use than others. For information on specific applications, consult a spa chemical supplier.

When bromine is added to water, it forms hypobromous acid—another powerful sanitizer and oxidizer (see Figure 3). ANSI/NSPI recommendations call for a total bromine concentration of 4.0–6.0 ppm in spas. When bromine reacts with contaminants, bromamines are produced. Unlike chloramines, these compounds remain effective sanitizers and don’t create objectionable odors or irritate eyes and mucous membranes. The reaction also creates a bank of bromide ions that can be regenerated back to active bromine through the addition of another oxidizer such as chlorine or potassium monopersulfate. Bromine, unlike chlorine, is quite stable at elevated temperatures and remains effective over a wide pH range. There’s no known stabilizer, however, to protect bromine from destruction by the sun’s ultraviolet rays. Therefore, bromine is most cost-effectively used to sanitize covered and indoor spas.

Water balance
To prevent scaling or corrosion of spa surfaces and equipment, avoid eye and skin irritation, and ensure efficient use of treatment chemicals, one must keep three key parameters in balance—pH, total alkalinity and calcium hardness. Measured on a scale of 0-14, pH indicates whether water is acidic, neutral or basic; i.e., alkaline. It should be tested daily and maintained in the range of 7.4–7.6 to protect bathers and equipment and maximize the effectiveness of chlorine sanitizers (bromine sanitizers remain effective throughout the pH range recommended for spas).

Total alkalinity (TA), the buffering agent governing water’s ability to resist changes in pH, should be tested weekly. Low TA allows pH to “bounce” or fluctuate rapidly when even small amounts of treatment chemical are added and causes water to tend toward corrosion. High TA makes it difficult to lower pH even when the water is treated with large amounts of adjuster. It can also cause cloudy water, scaling and inefficient sanitizer use. TA should be adjusted (before adjusting pH) to the ideal range of 80–120 ppm.

Calcium hardness, the amount of dissolved calcium in water, should be monitored monthly and maintained at 150–250 ppm. When calcium hardness is too low, water becomes corrosive and may dissolve calcium from plaster, tile and grout. When it’s too high, water may become cloudy and excess hardness can precipitate (as scale on the spa or its equipment) causing rough, unsightly deposits and reducing water flow and heater efficiency.

When to drain, refill a spa
Regardless of how diligently water is sanitized and balanced, spas should be completely drained, cleaned and refilled periodically to prevent problems. To determine the number of days between complete drainage, use the formula developed by Mark McManus, a health inspector for Baltimore County, Maryland:

Spas, like pools, should be tested regularly for sanitizer level, stabilizer (if using cyanuric acid or stabilized chlorine), pH, total alkalinity, TDS, calcium hardness and—less frequently—metals. All test chemistries aren’t equally reliable nor are all kits equally durable or user friendly. Choose a test kit made by a reputable specialty chemical manufacturer. A pool/spa inspector from the local health department can help.

When testing chlorine, be sure to understand the difference between testing with the OT (orthotolidine), DPD (N,N-diethyl-p-phenylenediamine), and FAS-DPD (ferrous ammonium sulfate DPD) methods. OT tests measure only total chlorine; they will not allow you to make that important distinction between free and combined chlorine. DPD tests allow you to determine free, total and combined chlorine. FAS-DPD tests read free and combined chlorine directly. (This issue doesn’t arise when testing bromine since it disinfects in both its free and combined forms. OT, DPD, and FAS-DPD are all acceptable test methods for bromine.) If using a potassium monopersulfate shock as an oxidizer, remember it will cause an artificially high combined chlorine reading in DPD and FAS-DPD sanitizer tests. To prevent such interference, choose a test kit specifically formulated for use with MPS. Finally, always adhere to the ANSI/NSPI levels recommended for spa water shown in Table 1.

Water treatment professionals must keep abreast of research and technological advances to keep their customers’ spas safe for bathing. Moreover, dealers have a responsibility to owners and operators to alert them to the danger of waterborne illnesses that can result from improperly treated spas. Especially with the Internet at our fingertips, resources are plentiful. A good place to start is the website of the Centers for Disease Control and Prevention (

While spas provide a great way to relax or get a therapeutic massage, their water quality must be managed to prevent illness in bathers or damage to equipment. Understanding their water chemistry, regular testing with a quality test kit, diligent treatment, and periodic draining and refilling will keep spas properly sanitized and balanced.


  1. Article text condensed from “Pool & Spa Water Chemistry: A Testing & Treatment Guide” ©2003 and “I Never Liked Chemistry” ©2001, Taylor Technologies Inc., Sparks, Md.
  2. Based on ANSI/NSPI guidelines, issued October 2002, National Spa & Pool Institute, Alexandria, Va.:

About the author
Michael Gardner is communications coordinator at Taylor Technologies Inc., a Sparks, Md.-based manufacturer of water analysis products for pools and spas, potable water and wastewater treatment, boilers and cooling systems, industrial processes, and food and beverage operations. Gardner can be reached at (410) 472-4340 ext. 146, (410) 771-4291 (fax) or email:



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