STA POOL PLANT – WATER BALANCE
Water: A Balancing Act Maintaining the chemical equilibrium of swimming pool water, really is a balancing act, and Colin Davis, STA Pool Plant Tutor and Pool Plant Ambassador shares his expert advice here on calculating water balance to ensure safe and hygienic conditions are maintained for bathers
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ater is said to be in balance when it is neither corrosive, erosive nor scale forming. Balanced water is important in our swimming pools because it will prevent erosion, corrosion and scale formation increasing the life and reliability of our pool plant and equipment. Water balance can be calculated in several ways and the various models include Ryzner, Hoover, Hallopleau and Dublin, and Claud Smitt, each featuring a formula by which balanced water can be calculated. In pool water chemistry the most common adopted models to calculate water balance are the Langelier Saturation Index and Palintest Balanced Water; both are simplified considering that swimming pools operate over a relatively restricted range of parameters. The properties of the water are determined by the hydrological cycle, which creates continuous movement of water on, above and below the surface of the earth. The surface water will evaporate, condensation will form cloud cover, eventually resulting in precipitation and in turn, infiltration and surface water run-off. The evaporative phase of the cycle purifies water, which then replenishes the land with freshwater. The flow of liquid water and ice transports minerals across the earth. Natural water is never completely pure; it will contain dissolved chemical substances and gases which impact on pool operators. Gases are absorbed as the water passes through the atmosphere containing dissolved oxygen, nitrogen and carbon dioxide, to form carbonic acid which aids the dissolving of minerals when the rain falls on the earth. As rainwater passes through the earth’s surface the characteristics of the water are determined. With hard impervious rock the rain runs off the surface into streams and rivers; this water has little opportunity to dissolve minerals resulting in soft water with low Total Dissolved Solids – the rocks are often covered with acidic peaty soil and the water may then have a low pH value. In soft porous rock areas, the water will
seep into the ground dissolving mineral salts and calcium; this water is generally hard water and has a high pH value, high TDS and will be scale forming. Experience has shown that the Langelier Saturation Index gives the pool operator a reliable indication as to the water’s condition. The absence of corrosive or scale forming tendency depends on the water being in balance. Both corrosion and scaling arise from a complex equilibrium, which exists in water between dissolved carbon dioxide, carbonates and bicarbonates. Excessive carbon dioxide in the water results in corrosive tendency; if there is an excess of carbonates and bicarbonates in the water then it will be scale forming and these salts will be deposited from the water in the form of calcium, or magnesium carbonate. The situation is further complicated by the fact bicarbonates are inherently unstable, and are readily precipitated as carbonates when the water is heated. The Langelier Saturation Index uses measurements of: • The pH value • Total Alkalinity • Calcium Hardness • Temperature • Total Dissolved Solids. A simple formula is then used to calculate the water balance using some correction factors or alternatively using the Lovibond app PoolM8. Once measured, the operator can then act on the water balance results, taking the appropriate corrective action. It is essential that each of the five parameters are kept within their own range to ensure correct water balance; sometimes you may achieve an overall result within the ideal parameters, but some values outside of their range can offset the overall result. PWTAG recommends water balance value to be maintained between -0.5 to +0.5, ideally 0. Maintaining the water balance in a slightly scale-forming condition (Langelier index +0.1 to +0.4) can prevent erosion, corrosion and
Balanced water is important in our swimming pools because it will prevent erosion, corrosion and scale formation increasing the life and reliability of our pool plant and equipment” 44 April 2022 SPN 44_SPN_April_2022_STA.indd 44
excessive scale formation. A negative result indicates the water is corrosive and a positive result scale forming. The five factors are not equally weighted on the overall outcome, the pH value will have the greatest overall impact on the result followed by – Total Alkalinity, Calcium Hardness, Temperature and TDS. As with any water testing requirements, the person should be appropriately trained and qualified having the right level of understanding to undertake the test. STA’s Level 3 Award in Pool Plant Operations qualification, which is CIMSPA professionally endorsed, covers everything operators need to know to ensure safe, clear and hygienic water practices are adhered to in pools, spas and interactive water features, and once completed it is valid for five years. The training would also deal with such issues as ensuring all testing equipment is clean and free from any contamination, as this will affect readings; photometers must be calibrated to ensure accuracy and of course the correct tablets being used for the equipment. The selection of a standardised testing location in the pool is also very important to give reliable results. There are many options on water testing equipment that will complete the water balance test parameters, but a photometer will always be the recommended choice as it provides a high degree of accuracy on all tests, including all the water balance tests. A quality instrument will last for years providing it is looked after, and should be considered as an investment in your facility. Getting it wrong can lead to serious consequences, which overtime will impact on the facilities ability to achieve optimum water quality and is likely to result in unplanned expenses. STA 01922 645097 www.sta.co.uk
www.swimmingpoolnews.co.uk 07/04/2022 12:14