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Prominent Controller

The operators of public swimming pools can make their choice among various disinfection methods for swimming pool water treatment. The German Standard DIN 19643 lists all Chemicals admissible for disinfection.

These are as follows:

  • Chlorine gas
  • Calcium hypochlorite according to E DIN EN 900 as pellets or tablets
  • Sodium hypochlorite according to E DIN EN 901 as ready-made 150 g/l solution
  • Sodium hypochlorite solution produced on the place of usage by electrolysis

 

For disinfection of swimming pool water, Chlorine gas is most commonly used on a global scale. Especially for large amounts of water, chlorine gas constitutes an economical solution. Due to its exposure, however, use of chlorine gas has been reduced substantially in Germany and especially in Switzerland.

Calcium hypochlorite is solubilized in water as powder or pellets. In most cases, a 1% disinfection solution is made up. Due to insoluble particles comprising 7 % of the particle solution, the product becomes slightly lacteally cloudy.

Sodium hypochlorite is very often used for smaller swimming pools where handling with the containers and the respective chemical costs are not of primary importance. Chlorine electrolysis is an interesting method of disinfection of swimming pool water with sodium hypochlorite.

disinfection of swimming pool water with chlorineFor chlorine electrolysis chlorine, hydrogen and caustic soda are produced from salt water and electric current. 
We differentiate between pipe cell electrolysis and diaphragm electrolysis. Pipe cell electrolysis combines the reaction products to a sodium hypochlorite solution with a high content of residual brine (salt not transferred during reaction). This is of use in salt-water pools but is a disturbance factor in normal swimming pools. Diaphragm electrolysis works with separated electrode chambers, so that no residual brine will get into the swimming pool water. The chlorine generated can be directly channeled into the water as hypochloric acid or it can be temporarily stored as sodium hypochlorite with the generated caustic soda.

Automatic Control

The chlorine-based disinfection methods are in most cases adjusted to a chlorine concentration of 0.4 to 0.6 mg/l in the water. In detail, the German norm DIN 19643 requires the following values depending on type of pool and type of water treatment:

  • General pools: 0,3 - 0,6 mg/l
  • General pools with Ozone stage: 0,2 - 0,5 mg/l
  • Warm whirlpool (with or without Ozone stage):  0,7 - 1,0 mg/l - under defined microbiological conditions up to a maximum of 1.2 mg/l

The chlorine concentration must be registered exactly by an amperometric measuring method. During bathing, there are a high number of disturbance variables, influencing the chlorine concentration, the pH value and the Redox value. Thus, the set value can only be observed constantly by automatic measurement with integrated control.

In order to achieve acceptable, representative values, it is necessary that the swimming pool water be channeled to the probe in a relatively short time frame. Otherwise, the measurement will not be in accordance with the value of the pool water. The German standard DIN 19643 requires that the measuring water is to be taken 20 cm below the pool water level and to be channeled to the measuring electrode by the shortest possible way. This requirement is equally valid for pH Redox and temperature measuring.

A clear measurement can only be guaranteed with the chlorine probe being calibrated after the required starting time. For this purpose the standardized DPD method is used. The DPD method allows reassigning the manually taken value to the transmitter. Control measuring according to DPD indicates directly,  whether these two values correspond or not. If the DPD value differs from the continuous measuring, the chlorine probe must be calibrated.

By means of an integrated P or PID control algorithm a control variable is calculated from the differentiation value between set value and actual value. Thus, the chlorine gas valve or the metering pump or other actuating elements are controlled. A control circuit closed in itself is created, where control variables may also be taken into account. For the adjustment of the control parameters, our specialists will be happy to assist you. 

Back leading the measuring water to the swimming pool water circuit is recommended to keep losses of swimming pool water as low as possible.

Thanks to closed chlorine probes by ProMinent, the measurements are not as sensitive to measuring water changes as with systems using open chlorine probes.