Heated baths are provided in most fitness centres and resorts; whirlpool baths are found in hotels, homes, and healthcare centres. The discussion continues from BioTalk 19 with the evaluation of disinfection methods.

Chlorine
Chlorine based disinfectants are effective when levels are maintained at least to 2 to 4 ppm (mg/L) as free residual chlorine. The two major physical factors that cause the reduction in chlorine levels are surface area and temperature. Spa baths with gas jets provide a higher water surface area and the high temperature settings both aid the loss of chlorine levels. For this reason it is essential that regular, if not continuous, chlorine level monitoring is essential for safe bathing. Chlorine in water forms hypochlorous acid. If the water pH is above 7.8 the disinfection capability is impaired due to near complete conversion of hypochlorous acid to hypochlorite ion. The unpleasant odour that can be associated with chlorine disinfection is due to the formation of chloramines. The presence of urine and perspiration can aid the formation of chloramines. High chloramine levels can not only be irritating to the skin and eyes but can also compromise the disinfection capability of chlorine. Adequate ventilation and periodic high level slug dosing (to 10 ppm of free chlorine) as well as regular emptying of spa water should resolve such problems. The failure to achieve a free chlorine residual level of 0.5 ppm or more after high level slug dosing is an indication that the spa water needs to be discarded. Although it must be noted that some spas may collapse if emptied, this needs to be checked with the manufacturer and if necessary partially empty the spa and refill with fresh water regularly to achieve the same water replacement.

The pH and associated alkalinity levels require adjustment occasionally for most forms of chlorine disinfection. The optimal pH is 7.2 to 7.8 and the maximum total alkalinity should be 60 ppm.

Bromine
Bromine is available commercially either as sodium or potassium bromide used in conjunction with sodium hypochlorite or potassium persulfate, or as 1-bromo-3-chloro-4,4-dimethylhydantoin supplied in tablet form. The recommended residual free bromine levels should be maintained between 4 and 8 ppm. The required pH and total alkalinity levels required for bromine disinfection are the same as for chlorine, i.e.  pH 7.2--7.4 and maximum total alkalinity 60 ppm. Continuous dosing of bromine is readily achievable in practice, but the response to rapid changes in demand may be slow. High level slug dosing with chlorine may still be required under heavy load usage. There tends to be less odour problems associated with the use of bromine than chlorine. Liquid bromine is too dangerous to handle and should not be used.

Ozone
Ozone is a very powerful oxidant but it can only be generated in situ and commercial units appear generally to be safe to use. Ozone dozing is only practical in off-pool circulating water and some form of removal process is required before water is returned to the spa. A free disinfectant residual (e.g. chlorine at 2 to 4 ppm) is recommended.

Ultraviolet light-hydrogen peroxide
Ultraviolet light appears to have excellent potential for off-pool water disinfection so long as the water turbidity remains low (less than 1 NTU). If the water contains material that will cast a shadow as it passes under the light source then any micro-organisms in that shadow will not be exposed. Dosage levels need to be defined assuming a minimum circulation rate of two cycles per hour. In swimming pools using these systems the residual in-pool biocide levels of hydrogen peroxide from 30 to 50 ppm are used.

Reference: Australian Guidelines for Heated Spa Pools -- NHMRC.