How to chlorinate water correctly in industry

Chlorine is a very effective disinfectant that acts on the cell wall of microorganisms and destroys them. The sodium hypochlorite is the most frequently used chlorine derivative for water disinfection.

The richness or concentration of a hypochlorite solution is usually expressed as a function of the active chlorine content either in ppm (milligrams per litre) or in %.

Sodium hypochlorite is unstableThe solution loses chlorine at a monthly rate of 2 to 4%, with the loss accelerating if the temperature is above 30°C. Figure 1 shows the degradation as a function of time and temperature.

Degradation of hypochlorite as a function of time and temperature

Hypochlorite, on the other hand, is a non-selective oxidantThis means that if the water to be treated contains organic matter or other substances (either dissolved or in the form of suspended material), a large part of the hypochlorite dosed will be consumed in the oxidation of this organic matter, which has several effects:

• Generation of THMs (trihalomethanes) and other toxic by-products.
• Increase in the dose of hypochlorite required to maintain a certain concentration of free chlorine residual (i.e. the amount of chlorine remaining in the water after it has reacted with micro-organisms and impurities).

These are the advantages and disadvantages of the use of sodium hypochlorite for water disinfection:

• It is by far the most economic of all.
• A residual chlorine concentration remains, which serves as a bacteriostatic long-standing and permitted by law.
• It is the most widely used disinfection system in the industry.
• Very compact, space-saving dosing systems.

• The decomposition or reaction of hypochlorite generates undesirable by-productschlorates, chloramines, trihalomethanes (THMs), etc. whose presence in food is becoming more and more common. restricted by new regulations.
• Equally, many of these metabolites are either toxic or carcinogens.
• Securityrequires the storage and handling of a highly toxic and corrosive product.
• It is a non-selective oxidantif the water contains organic matter, the hypochlorite will be consumed in the oxidation of the organic matter, which will require an increase in dosage.
• Strong influence of the pH water in the chlorination efficiency.
• You can leave taste and smell in the water affecting the organoleptic characteristics of the final product.

Illustration 2. Installation diagram of a chlorination system proportional to the water flow rate.

Illustration 3. Example of water meters with pulse generators

For more precise control, it is also possible to install a continuous chlorine meter. This would make it possible to accurately measure the concentration of free chlorine reaching the production and, in addition, to adjust or stop the chlorine dosage in the event that the mains water arrives with sufficient concentration, so as to avoid the hyperchlorination.

If you have accumulation tankThere are two possibilities:

• Without water recirculation, the scheme would be:
  • Dosing proportional to the flow rate at the tank inlet.
  • Optional: Continuous chlorine meter at the tank outlet to the factory.
• With water recirculation system: either the system proposed above can be installed, or:
  • Measuring and chlorination panel (see figure 4): a sensor continuously measures the concentration of free chlorine in the tank and activates or stops the hypochlorite dosing depending on the current and desired concentration. This system makes it possible to continuously maintain the desired concentration of free chlorine in the tank. It is essential that the recirculation system is properly designed for correct homogenisation.

In all cases, it is recommended to do periodically free chlorine concentration monitoring at the furthest points of the site of the water inlet to the establishment and adjust the hypochlorite dosage according to the value obtained.

Illustration 4. Chlorine metering and dosing panel

• It may have a high content of pathogenic micro-organisms, so it is essential to disinfect it.
• May have high levels of dissolved organic matter content or in the form of colloids or suspended matter.
• It may therefore have high levels of turbidity.
• May have high levels of contaminants such as nitrates, sulphates, etc.

1. On the one hand, it is desirable that the well water should be filtered (with a sand filter or similar) to reduce the organic matter content as far as possible. There are two reasons for this:
   1. Minimise the amount of hypochlorite needed to disinfect water.
   1. Minimise the production of undesirable chlorine by-products.

• In cases of high dissolved organic matter content, it is advisable to install an active carbon filter at the filter outlet.

The most common types of equipment for measuring the concentration of free chlorine in water are listed below, and the advantages and disadvantages of each are specified.

Redox potential electrode

A redox electrode is a sensor that measures the oxidation-reduction (redox) potential of a solution. The redox potential is an indirect measure for estimating the concentration of free chlorine in water.

Disadvantages:

• Low measurement accuracy.
• Strong influence of pH on the measurement.

Advantages:

• The sensors have a low price tag.

Galvanic sensor

The galvanic sensor is a type of amperometric sensor consisting of two electrodes: a working electrode and a reference electrode. The working electrode is coated with a metal layer (copper or silver in general) which reacts with the free chlorine present in the solution to produce an electric current. The magnitude of the electric current is proportional to the concentration of free chlorine in the solution.

The reference electrode (gold or platinum coated), on the other hand, is used as a reference point to measure the electrical potential of the working electrode. The electrical potential of the working electrode is measured relative to the reference electrode, which allows the concentration of free chlorine in the solution to be measured.

Disadvantages:

• Influence of pH and temperature on the measurement.
• Not recommended for well water.
• Measurement problems if the water renewal is higher than 20% of the total volume.

Advantages:

• Economic system.
• Self-cleaning system.
• It can work under pressure up to 6 bar.

Amperometric or potentiostatic sensor

Free chlorine gas diffuses through the sensor membrane between the cathode and the electrolyte solution. At the applied potential value, it is electrochemically reduced at the gold cathode. At the same time, the silver anode is oxidised to form silver chloride. The release of electrons at the cathode and the corresponding anode creates a current flow that is proportional to the concentration of free chlorine present in the water. The resulting mA output is conditioned to a 4-20mA current from the probe's on-board electronics. 

The measurement of free chlorine is temperature compensated by the internal sensor.

Disadvantages:

• It is not possible to work at a pressure of more than 1 bar (except for special models).
• The electrolyte should be changed every 6 months.
• The membrane must be changed annually.

Advantages:

• Low influence on flow and pH variations.
• High measurement accuracy

Photometer

DPD (Diethyl-Paraphenylene Diamine) forms a pink colour with the available chlorine almost instantaneously, so that the determination can be carried out by titration or colorimetrically. By reacting only and exclusively with the chlorine present in the water, DPD produces almost instantaneously a pink colouring which means that all factors that may interfere with the measurement (pH, µS, °C, organic matter, etc.) have no influence on the analytical methodology. This colouring is read by the photometer through a sample of reagent (DPD) taken, stabilised and automatically dosed.

Disadvantages:

• Reagent consumption.
• Works with free water outlet (except special models).
• High cost of equipment.

Advantages:

• The highest precision in measurement.
• Automatic calibration of the instrument at the end of each measurement.
• No interference from other chemicals.
• No influence of pH values, conductivity or temperature.

1. No accumulation tank

• With storage tank without recirculation

• With storage tank with recirculation

1. No accumulation tank

• With storage tank without recirculation

• With storage tank with recirculation