Swimming pool disinfection
Disinfection of swimming pool water
Micro-organisms polluted swimming pools. Every swimmer adds 1.000.000 to 1.000.000.000 microorganisms to the water. The water itself contains microorganisms, as well. After oxidation a disinfectant must be added to the water to kill pathogenic microorganisms.
Demands on disinfectants
Disinfectants used for swimming pool water disinfection must meet certain demands. They should be harmless and non-irritating to swimmers and attendants. They must be active in small concentrations and remain their activity for a long time.
Contrary to drinking water disinfectants, disinfectants for swimming pool treatment must be active in the pool itself, because pollutions and pathogenic micro-organisms are constantly added to the water. Therefore the water has to maintain a residual disinfectant concentration. The disinfectant must be easily traced and measured and should be safe to use.
Disinfection methods for swimming pool water
In some countries, sodium hypochlorite is used for both oxidation and disinfection of swimming pools. When it is added to water, sodium hypochlorite increases the pH value. It is better to use chlorine as a disinfectant and an oxidizer at a pH value of 6,5. Often, acid is added to lower the pH value.
Demands on swimming pool conditions
Chlorine-based disinfectants are among the most frequently applied disinfectants and oxidizers for swimming pool treatment. Chlorine is usually added as hypochlorous acid (HOCl) or hypochlorite (OCl-).
Chlorine kills pathogenic microorganisms that are present in the water. When too much chlorine is present, it can cause eye and mucous membrane irritation, as a result of chloramine formation.
Threshold and maximum levels are set for chlorine concentration. For available chlorine the minimum concentration in swimming pools is set to 0,5 milligram per litre. The maximum level is set to 1,5 mg/l. When using cyanic acid (stabilizer) minimum and maximum values are set to respectively 2,0 and 5,0 mg/L. For outdoor swimming pools and indoor pools smaller than 20 m2, the maximum level is set to 5,0 mg/l.
Lowering the chlorine concentration is undesirable, because this increases the risk of waterborne diseases.
Alternative disinfectants can be used as well, these decrease the required amount of chlorine or cause chlorine addition to be irrelevant.
The pH value is measured daily. It should be between 6,8 and 7,8. At a pH of 7,0, the amount of free chlorine present is 70%, while this concentration decreases to 20% at pH of 8,0.
The water and air temperature in swimming pools is usually high. Furthermore the humidity is high. This influences the activity of disinfectants and the behaviour of substances that are formed in the swimming pool during disinfection. When sodium hypochlorite is used, chlorine gas is formed due to reactions with the acid that is added to lower the pH of the water. Chlorine gas must be removed, because it can be harmful to human health and corrosive on materials. Chloramines, formed through reactions of ureum and chlorinated disinfectants, are corrosive as well.
More information on water disinfection?:
What is water disinfection? Necessity of drinking water disinfection History of water disinfection Waterborne diseases Factors that influence disinfection Conditions of water disinfection Regulation drinking water disinfection EU USA