STA POOL PLANT – CONTROLLING PH
Controlling pH Robbie Phillips and Richard Lamburn, STA’s Technical Pool Plant Team look at the control of pH – the key to safe, effective disinfection and comfortable, stable pool conditions
hat exactly is pH? ‘pH is the log of the reciprocal of the concentration of hydrogen ions in a volume of water’, or simply a measure of the acidity or alkalinity of the pool water. As the number of hydrogen ions in the solution increases, the pH decreases and the water becomes more acidic. Conversely, as the number of hydrogen ions in the solution decreases, the pH increases and the water becomes more basic. pH is measured on a logarithmic scale which ranges from 0 (acids) to 14 (base), with 7.0 being neutral. Keeping pool water within ideal pH ranges: • Ensures effective micro-organism destruction • Increases bather comfort • Prevents damage to the pool, its components and equipment. Ideal pH levels depend on the other four components of water balance (total alkalinity, calcium hardness, water temperature and total dissolved solids), but typically range from 7.2 to 7.8, with an ideal of 7.2 to 7.6 – the lower part of the range the better while still maintaining correct water balance. Low pH levels can cause chlorine to dissipate rapidly. Equipment corrodes, and pool surface materials etch or crack. There is debate over lower pH levels as at the lower levels the disinfectant effect becomes greater. However, we will reach a point where bathing comes uncomfortable and the protection of water balance is compromised (corrosive). At high pH levels, less hypochlorous acid (HOCl) (disinfectant) forms and chlorine becomes less effective. Algae growth might increase, the water may cloud, scaling can occur and circulation pipes can calcify, and filter runs (the time between backwashing filters) often shorten. At high levels skin can be affected.
Sodium Carbonate (pH plus)
Lemon juice Carbonic acid (dissolved CO2)
Dry acid (pH minus) Harpic toilet cleaner, Chlorine gas
Conc. Hydrochloric/Sulphuric acid
As you’ve probably noticed, pH levels fluctuate constantly. This problem is magnified if total alkalinity levels are permitted to drop. Pool chemicals, rain, air pollution, the fresh make up water added to the pool, plaster and other pool surface materials and equipment the water comes in contact with, and waste products introduced into the pool by swimmers, all cause the pH to change. Some products like hydrochloric and sulphuric acid, cyanuric acid, sodium bisulphate, also trichlor tablets are acidic and all cause the pH to drop. Others, like dichlor or carbon dioxide are slightly acidic. While other chemicals like sodium and calcium hypochlorite, also sodium carbonate are alkaline. To keep pH in the proper range, the pool operator must test frequently using a photometric method and pH meter, and adjust as necessary.
COMMON PH ADJUSTMENT CHEMICALS Raise pH
Sodium carbonate (soda ash)
As the number of hydrogen ions in the solution increases, the pH decreases and the water becomes more acidic. Conversely, as the number of hydrogen ions in the solution decreases, the pH increases and the water becomes more basic” www.swimmingpoolnews.co.uk 43_SPN_Oct_15_STA.indd 43
Tiling grout and screed can be destroyed by water lacking in calcium for example, often due to low pH levels. Corrosion of metal parts can cause leaks with potential undermining of pool structure
Hydrochloric acid Sulphuric acid Sodium bisulphate (dry acid) Carbon dioxide (CO2)
The control of pH together with water balance can save substantial costs due to mainly corrosive water conditions. Tiling grout and screed can be destroyed by water lacking in calcium for example, often due to low pH levels. Corrosion of metal parts can cause leaks with potential undermining of pool structure. They can all potentially cause significant repair bills, together with expensive facility closures and loss of revenue. STA 01922 645097 www.sta.co.uk
SPN October 2015 43 23/09/2015 18:26
Published on Oct 1, 2015
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