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What is breakpoint chlorination according to park

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Here is the full definition and explanation of Breakpoint Chlorination as given in Park's Textbook of Preventive and Social Medicine:

Breakpoint Chlorination (Park's Textbook)

Background - Chlorine Demand and Break Point

When chlorine is added to water, it first reacts with organic matter, ammonia, and other reducing substances. The chlorine demand of water is defined as:
"The difference between the amount of chlorine added to the water, and the amount of residual chlorine remaining at the end of a specific period of contact (usually 60 minutes) at a given temperature and pH of the water."
In simpler terms, it is the amount of chlorine needed to destroy bacteria and oxidize all organic matter and ammoniacal substances in the water.
The point at which the chlorine demand is fully met is called the "break point". Beyond this point, any further chlorine added begins to appear as free residual chlorine (HOCl and OCl-) in the water.

Definition of Breakpoint Chlorination

As chlorine is added to water containing ammonia, it first forms chloramines (combined chlorine), which are less efficient disinfectants than free chlorine.
If the chlorine dose is further increased:
  1. A reduction in residual chlorine occurs, because the added chlorine destroys the chloramines.
  2. The end products of this destruction carry no residual chlorine.
  3. This fall in residual chlorine continues as the chlorine dose increases.
  4. After a certain stage, the residual chlorine begins to rise again in proportion to the added dose.
The point at which:
  • Residual chlorine reappears, and
  • All combined chlorines (chloramines) have been completely destroyed
...is called the breakpoint. The dose of chlorine needed to reach this point is called the breakpoint dosage.

Key Point

"Breakpoint chlorination achieves the same results as superchlorination in a rational manner and can therefore be construed as controlled superchlorination."
This distinguishes it from simple superchlorination (which involves adding very large doses of chlorine to heavily polluted water and then dechlorinating), because breakpoint chlorination is a more precise and controlled process.

Practical Significance

  • A minimum free residual chlorine of 0.5 mg/L for a contact period of at least one hour is required to kill bacteria and viruses.
  • Free residual chlorine provides a safety margin against subsequent microbial contamination during storage and distribution.
  • The correct dose = chlorine demand + 0.5 mg/L free residual chlorine.
  • Park's Textbook of Preventive and Social Medicine, p. 815
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