The pool water can become cloudy, for example, due to clouding substances (pollen from flowers) transported, suspended limestone, and green algae. These clouding substances are filtered according to the type of filter system present. Some of these opacifying substances may be so small that they cannot be retained by the filter and go back to the pool through recirculation. To make it filterable, flocculation is performed.
For public swimming pools, continuous flocculation is provided, which is an essential component of water preparation, in accordance with DIN 19643.
The most used aluminum salts are aluminum sulphate, aluminum oxychloride and a concentrated solution of the latter. It is also referred to as polyalumincloride (PAC).
In the case of flocculation with aluminum salts, the value of the pool water must be between 7.0 and 7.4. At higher pH values, the resulting flake becomes smaller and smaller and can be filtered more difficultly.
Furthermore, the suspended aluminum hydroxide can partially dissolve in the form of aluminate. Subsequently, if the pH value is corrected again, the aluminate transforms again into aluminum hydroxide, causing clouding again.
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