Disinfection of Swimming Pool Water with Chlorine

For chlorine-based disinfection, commonly used chemicals are: sodium hypochlorite, chlorine gas and calcium hypochlorite.

The operators of public swimming pools can make their choice among various disinfection methods. 
These are as follows:

  • Chlorine gas
  • Calcium hypochlorite as pellets or tablets
  • Sodium hypochlorite typically 12 % solution
  • Sodium hypochlorite solution produced on site by electrolysis

For private swimming pools there are also alternative methods:

  • Bromine
  • Hydrogen peroxide
  • Iodine 
  • Silver
  • Trichlorisocyanuric acid (stabilized chlorine)

Chlorine gas is by far the most commonly used treatment globally. For very large volumes of water, chlorine gas is undoubtedly the most economical treatment.However, because of the extremely its hazardous nature and handling requirements, use of chlorine gas is being phased out for safer methods.

Calcium hypochlorite powder or pellets are dissolved in water, typically at 1 % concentration. Due to insoluble materials (up to 7 % of the solution) the liquid i svery cloudy.

Sodium hypochlorite is often used for smaller pools where container handling and associated chemical costs are not as much of a concern. On-site generation (electrolysis) is sometimes used and is showing some success. It is becoming more popular because on-demand generation has certain distinct advantages: no delivery costs, less waste, and reduced denegation.

For On-Site Generation chlorine, hydrogen and caustic soda are produced from salt water using an electric current. There are two basic generator types: pipe cell and diaphragm
Pipe cell electrolysis combines the reaction products to form a sodium hypochlorite solution with a high salt content. This method  can be used in salt-water pools but is unacceptable for typical fresh water swimming pools. Diaphragm electrolysis works with separated electrode chambers, which eliminates salt ci\ontamination of the swimming pool water. The generator output can be fed directly into the pool water as hypochloric acid or it can be temporarily stored (as sodium hypochlorite) in atank, after mixing with the generated caustic soda.

Automatic Control

The chlorine-based disinfection, a free chlorine residual is typically maintaned.

The chlorine concentration can be measured by an amperometric (electrical) method. Swimming pool water contains a high number of containmants influencing the chlorine concentration, the pH and the Redox values. Thus, the actual values can only be reliably observed by automatic measurement with intergrated control.

In order to achieve acceptable, representative values, it is necessary that the swimming pool water be channeled to the sensor in a relatively short time frame. Otherwise, the measurement will not be in accordance with the value of the pool water. 

An accurate measurement can only be guaranteed with the chlorine sensor being calibrated after an initial run-in-time. Typically DPD method are used for the calibration. If the DPD value differs from the instrument reading, the chlorine sensor must be re-calibrated.

By means of a built-in P or PID algorithm a controlled output signal is calculated based on the difference between a setpoint and the  measured value. Thus, the chlorine gas valve, metering pump, or other device is controlled to keep the desired level of chemical in the water. A closed loop process control system is typically used.The actual control variables may also be taken into account in the calculation of the controlled output. Our in-house experts will be happy to assist you with the adjustment of the P-I-D settings (loop tuning). 

The sample line is typically returned to the pool to keep water losses low. Thanks to membrane covered chlorine sensors by ProMinent, the measurements are not as sensitive to water containments as systems using open-type chlorine sensors.