Chlorination is the process of adding chlorine-based products to water, inactivating pathogenic microorganisms, in addition to oxidizing a large number of organic and inorganic substances - such as hydrogen sulfide gas, Iron, and Manganese - making it suitable for human consumption as chemically and biologically potable water.
The main products used in this process are sodium hypochlorite, calcium hypochlorite, and, to a greater extent and frequency, Chlorine Gas (Cl2) widely used in sanitation companies. The latter, when dissolved in water, forms hypochlorous acid (HOCl) and the hypochlorite ion (OCl-).
The reaction equilibrium between these two substances (HOCl and OCl-) is highly dependent on pH. The bactericidal power of hypochlorous acid is much more efficient (about 80 times more efficient) than hypochlorite ion, so it is recommended that chlorination be carried out at pH values below 7 to ensure a greater presence of this substance, reducing the amount of chlorine-based products needed, making the process more effective and economical.
In the chlorination process, we determine that Chlorine can be RESIDUAL, COMBINED, or TOTAL.
The FREE CHLORINE is determined through the presence of chlorine (HOCl) and (OCl-) in the water that has not reacted with inorganic and organic substances and microorganisms. The excess is known as RESIDUAL CHLORINE.
COMBINATED CHLORINE, on the other hand, occurs in waters that contain ammoniacal compounds that react with chlorine to generate chloramines (mono, di, and tri). The result of this reaction is the combined chlorine. It also has disinfection power, but much lower than the oxidation potential of hypochlorous acid, and even than the hypochlorite ion.
The TOTAL CHLORINE, finally, is the concentration of Chlorine, adding up all the forms mentioned above.
A typical Water Treatment Plant may contain up to three stages of chlorination: Pre-chlorination, Disinfection, and Post-chlorination.
Pre-chlorination
As soon as the raw water arrives at the WTP (Water Treatment Plant), Chlorine is dosed to increase the efficiency of the organic matter, microorganisms, and metals control, oxidizing the materials that result in taste, odor, and Color. When these substances are no longer present, the addition of more chlorine causes it to dissolve in the form of free residual chlorine, ensuring the success of pre-chlorination.
This fundamental phase for the success of the treatment process requires efficient and robust instruments for high performance and 100% effective results. The most indicated instruments for Chlorine analysis are: TW-52I-HP (process) and DM-CL (Field meter).
Disinfection
In this stage, the last chlorine dosage is applied before the potable water leaves the WTP. It ensures that the supplied water reaches the consumer, free of microorganisms, and compliant with regulatory standards that establish water quality parameters for human consumption.
It is essential to have high operational reliability and instruments of high precision and accuracy to meet the standard, such as the TW-52I-HP amperometric chlorine analyzer for the process, the DM-CL field meter for free, total, and chlorine dioxide, and the AI-CL2-HP online colorimetric analyzer for free and total chlorine, compliant with Standard Methods and with automatic calibration.
Post-chlorination
Due to the volatility (evaporation) of Chlorine, water tends to lose its disinfection power. For this reason, it's necessary to continuously monitor and restore the residual Chlorine concentration in very long distribution processes and water tanks.
Here, we continue to follow potability standards, maintaining the necessary amount of chlorine with frequent measurements. The equipment remains the same, both for the process and field use.
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