STA POOL PLANT – PH & ACTIVE FREE CHLORINE
pH In The Disinfection Process And An Introduction To Active Free Chlorine In the December issue of SPN, Luke Griffiths, STA’s Pool Plant Qualification Development Manager looked at the importance of pH to swimming pool water in general. In a follow-up to that piece, in this issue he provides an introduction to active free chlorine and focuses on the impact pH has on the effectiveness of it
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n the pH range found in swimming pools, free chlorine is made up of two forms of chlorine which co-exist: • Hypochlorous acid (a very effective disinfectant) • Hypochlorite ion (a very weak disinfectant). The relative proportions of hypochlorous acid and hypochlorite ions are dictated by the pH level. As can be seen in the table below, at pH 7.0 approximately 80% of the free chlorine is in the form of hypochlorous acid, whereas at a pH of 8.0 this drops to around 20%. As you can see from the graph to the right, the nearer the pH is to 7.0 the more chlorine is in the form of hypochlorous acid and more effective the chlorine is as a disinfectant. As the pH rises the less effective hypochlorous acid becomes.
ACTIVE FREE CHLORINE Active free chlorine refers to the hypochlorous acid part of the free chlorine, which, as discussed, is an extremely effective disinfectant. At concentrations above 0.5mg/l active free chlorine is effective as a biocide in killing most microorganisms. The table below, adapted from PWTAG’s TN60 provides the free chlorine levels in yellow, the pH levels in orange and the corresponding active free chlorine figures in white. So, for example – a water sample with a pH result of 7.2 and a free chlorine result of 1.4mg/l indicates an active free chlorine level of 0.95mg/l.
WHAT THIS MEANS FOR SWIMMING POOLS, SPAS AND INTERACTIVE WATER FEATURES For chlorine efficiency PWTAG recommend a pH value in swimming pools of between 7.0 and 7.4 to be the range when using chlorinebased disinfectants and ideally 7.0 to 7.2 for a better disinfectant effect. For swimming pools unable to maintain pH levels between 7.0 to 7.2, the upper range of 7.2 to 7.4 should be supplemented by increasing the level of free chlorine to 2.0mg/l to provide sufficient active free chlorine to neutralise most bacteria. For spas and interactive water features maintaining pH values between 7.0 and 7.6 is acceptable. pH in spas and interactive water features can elevate due to aeration and agitation which can cause an increase in pH as CO2 escapes − hence the wider margin. But we must ensure the free chlorine concentration is sufficient to provide an adequate level of active free chlorine concentration. Particularly as these systems are more susceptible to
microbiological issues such as Legionella, Pseudomonas, Coli forms and Biofilms – partly due to the relatively high pollution from the high ratio of bathers to low water volume. This, combined with the higher temperatures in which spas are operated and the potential of higher temperatures for interactive water features, is a combination which makes it essential there is sufficient active free chlorine. To conclude, whether it be a swimming pool, spa or interactive water feature it is imperative pH and free chlorine levels are maintained within the recommended parameters and considered collectively to ensure there is sufficient active free chlorine to control microorganisms within the water. STA 01922 645097 www.sta.co.uk
STA’S LEVEL 3 AWARD IN POOL PLANT OPERATIONS QUALIFICATION CIMSPA professionally endorsed, STA’s Level 3 Award in Pool Plant Operations qualification covers everything operators need to know to ensure safe, clear and hygienic water practices are adhered to in pools, spas and interactive water features. For more information on the qualification visit the STA website or you can contact Luke direct on 01922 645097 or by email at luke.griffiths@safetytrainingawards.co.uk for further details and advice.
FOUR INDUSTRY EXPERTS TO GIVE KEYNOTE TALKS AT SPATEX 2022
...the nearer the pH is to 7.0 the more chlorine is in the form of hypochlorous acid and more effective the chlorine is as a disinfectant” 36 February 2022 SPN 32_SPN_Feb_22_STA.indd 36
Luke and STA’s expert Pool Plant Ambassadors will also be at SPATEX 2022 from the 1 – 3 February, and on day one they will be hosting a seminar in Arena 2, which starts at 10.15am. A networking opportunity on STA’s Stand B65 with free refreshments will follow the final session. Delegates will also receive ½ CPD Point for attending all the STA sessions.
www.swimmingpoolnews.co.uk 21/01/2022 16:29