What is breakpoint chlorination according to park

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Breakpoint Chlorination - According to Park's Textbook

Background: Chlorine Demand and the Break-Point

When chlorine is added to water, it first reacts with organic matter, ammonia, and other reducing substances before any "residual" chlorine is left over. 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."

In other words, it is the amount of chlorine consumed in destroying bacteria and oxidizing all organic matter and ammoniacal substances.

When chlorine is first added to water containing ammonia, it combines with ammonia to form chloramines (combined chlorine), which are less effective disinfectants than free chlorine.
As the chlorine dose is progressively increased beyond this point, the added chlorine actually destroys the chloramines, causing a paradoxical fall in residual chlorine.
This decline continues until all the combined chlorine (chloramines) has been completely destroyed.

The Breakpoint

"This point at which the residual chlorine appears and when all combined chlorines have been completely destroyed is the breakpoint, and the corresponding dosage is the breakpoint dosage."

In graphical terms, the "breakpoint" is the lowest trough on the residual chlorine curve plotted against chlorine dose. Beyond this point, any further addition of chlorine results in free residual chlorine (HOCl and OCl⁻) increasing proportionally with the dose added.

Significance of Breakpoint Chlorination

Breakpoint chlorination achieves the same results as superchlorination but in a rational, controlled manner.
It is therefore described as "controlled superchlorination".
The presence of free residual chlorine for at least one hour is essential to kill bacteria and viruses.
The minimum recommended concentration of free chlorine is 0.5 mg/L for one hour.
Free residual chlorine provides a safety margin against subsequent microbial contamination during storage and distribution.

Correct Chlorine Dose Formula

Correct dose = Chlorine demand of the water + Free residual chlorine of 0.5 mg/L

Limitations

Chlorine (even at breakpoint) has no effect on spores, protozoal cysts, and helminthic ova, except at higher doses.

- Park's Textbook of Preventive and Social Medicine, p. 815 (Section: Break Point Chlorination)

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