Pool water balance is a term that describes the scale or corrosion activity of pool water. These aspects must be controlled while remaining within correct pH for disinfection efficacy and bather comfort. Water balance is affected by four factors:
– pH
– Total alkalinity
– Calcium hardness
– Temperature.
These factors are discussed individually below.
pH
pH is a measure of the relative acid/alkali strength of a solution. pH is measured on a scale from one to 14, with 7.0 being neutral. Correct pH is essential for three reasons:
– Equipment protection
– Bather comfort
– Sanitiser (disinfection) efficiency. When pH is too high (relative to the other water balance parameters), water is more likely to have scale-forming properties. When pH is too low, water will become corrosive to pool equipment and surfaces. The pH of the eye fluid is around 7.4, so good quality water within
the prescribed pH range should not cause eye irritation. As pH increases, free chlorine loses
oxidative activity. At a pH of 8.0, only 20 per cent of free chlorine is immediately available as hypochlorous acid to kill germs; whereas, at a pH of 7.5, about 50 per cent is immediately available.
pH change in pools is caused by the addition of disinfectants, which can be strongly acidic or alkaline, and the acids present on pool users’ skin. Aeration in spa pools tends to drive the pH up by removing acidic gases. Dilution water may affect the pH in extreme cases. See Table 8 Summary of the Characteristics of a Range of Pool Water Disinfectant for the pH effect of disinfectants.
Total Alkalinity
Total alkalinity is a measure of the amount of alkaline salts present in the water. Total alkalinity works as a shock absorber to reduce pH fluctuation when alkalinity is above regulatory limits. Conversely, total alkalinity above 200 ppm can make any necessary pH adjustment difficult.
Higher total alkalinity is appropriate when using acidic disinfectants, such as chlorine gas, trichlor or BCDMH.
To increase total alkalinity, the number of dissolved alkaline substances should be increased.
Calcium Hardness
Calcium hardness is a measure of the amount of calcium salts present in the water. Relative to the other water balance parameters, if calcium hardness is too high, scaling of heaters and pool finishes may occur. If calcium hardness is too low, etching of cement and tiles and corrosion of heating and circulation components may occur.
Calcium behaves differently from most chemicals as it becomes less soluble as temperature rises.
In areas of high calcium source water, specialist advice should be sought prior to the establishment of recommended water balance parameters and choice of disinfectant and pH chemicals.
Temperature
The higher the temperature, the more likely scaling is to occur, because calcium solubility is lowered. At a lower temperature the water can absorb more calcium. Concrete, marblesheen or tiled pool surfaces may become etched, particularly at low temperatures.
Total Dissolved Solids (TDS)
TDS is a measure of all solids and salts dissolved in pool water. TDS is increased by the addition of chemicals and salts from pool users and concentrated further by the evaporation of water. Salt in salt chlorinated pools constitutes the bulk of TDS and must be accounted for when measuring TDS.