Breakpoint Chlorination: What is it? Why is it so important?

How many operators truly understand breakpoint chlorination? Both new and experienced operators continue to face challenges in comprehending its significance and determining whether their system has achieved an adequate residual level. Breakpoint chlorination is not simply about attaining an arbitrary free chlorine concentration. It is, in fact, defined as “the point at which enough chlorine has been added to a quantity of water to satisfy its disinfecting demand” (Breakpoint Chlorination). 

While newer operators are still familiarizing themselves with the concept of breakpoint chlorination, even seasoned professionals often find it difficult to fully grasp its implications. Furthermore, how can operators be certain that their system has reached an adequate residual level? This question warrants further consideration. To clarify, breakpoint chlorination is not about reaching a random free chlorine value but about ensuring the system is properly disinfected and the required demand has been met. 

Stage 1:  Chlorine Demand 

In this stage, chlorine is first introduced to water where it will react with reducing agents like hydrogen sulfide, iron, and manganese.  During this stage, all the chlorine is consumed by the reducing agents which will result in no free chlorine residual.  This is known as “Zone 1” of the breakpoint chlorination curve. 

Stage 2:  Formation of Chlororganics and Chloramines 

It is well documented that ammonia is present in water.  Because of that, as chlorine is increased, it will begin to react with ammonia to produce monochloramine, dichloramine, and trichloramine. Many communities disinfect with chloramines because chloramines maintain a residual longer in a long distribution system. The major drawback with chloramines includes taste and odor issues as well as they are a less effective disinfectant. This is known as “Zone 2” of the breakpoint chlorination curve. 

Stage 3:  Destruction of Chlororganics and Chloramines to Breakpoint 

Here is where all the chlororganic and chloramine compounds that have been formed begin to break down with the addition of more chlorine.  It is here where customers will complain that there is too much chlorine in the water.  It is also here that when these complaints come in, the chlorine feed does not get decreased but rather increased instead. That is right, that just doesn’t seem to make any sense whatsoever.  Once the compounds are destroyed, the breakpoint curve will drop. Any residual at this point will be total chlorine. This is known as “Zone 3” of the breakpoint chlorination curve. 

Stage 4:  Free Chlorine Residual 

Here in “Zone 4”, the addition of chlorine will be strictly free, available chlorine that is highly effective for disinfection. In this zone, the residual chlorine is present to ensure ongoing protection against recontamination as the water travels throughout the distribution system.  

At the beginning of this article, we stated that many operators are unsure whether their system has reached an adequate residual level. In order to determine whether said system has reached the adequate residual level, collect your instantaneous sample and divide it in half. One of the samples will be tested for total chlorine, while the other needs to be tested for free chlorine.  Once you obtain your residual results, take the free chlorine and divide it by total chlorine then multiply it by 100. If your result is greater than 75% then you have an acceptable chlorine residual level. Although 75% is an acceptable result, your best result will be 80% or greater. 

Free Chlorine ÷ Total Chlorine × 100 = Percent 

This article was funded under RCAP’s EPA NPA 1 23 – 25 grant.