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Chlorine as a Biocide

Good morning. Welcome to the new topic on " Chlorine as a Biocide". Many of us have learnt in school that chlorine acts as bleach and is used to kill germs in the water. Chlorine is a strong oxidiser and can readily react with most organic matter. Microorganisms are made up of material that is most vulnerable to chlorine present in water.

Chlorine is generally added to the water stream either as chlorine gas or as hypochlorite solution of the desired concentration using controlled volume pumps.

Chlorine added to water reacts in the following order:

a. reaction with inorganic metals/compounds

b. amines

c. microorganisms

The effectiveness of chlorine added to water as biocide depends upon the presence of interfering inorganic substance and amines. The term chlorine demand includes the total chlorine required to react with inorganic compounds, amines and the microorganisms which are the actual target.

Chlorine added to water as a biocide has two main objectives:

  1. Kill the microorganism and make it suitable for drinking ( drinking water treatment)

  2. Kill/prevent the growth of algae and bacteria in the water stream ( scale and corrosion prevention)

In industry, chlorine dosing is adapted for controlling the growth of algae and bacteria to protect the pipelines from scaling and corrosion. This method is largely used in cooling water circuits ( cooling water basin and circulating pipes) where the tendency of algae growth is high.

Chlorine dosing is controlled manually by looking into periodic samples analysed in the laboratory or automatically using ORP or free chlorine analysers in the water circuit where chlorine is dosed.

Excessive chlorine dosing results in higher free chlorine concentration in the water stream and the free chlorine can attack the base metal of piping creating chemically induced corrosion problems.

Hence it is important to monitor and control the chlorine dosing in any water stream. If you have any specific queries related to the design, sizing of chlorine dosing systems please write to engineers at

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