Definition - What does Breakpoint Chlorination mean?
Breakpoint chlorination is defined as the point where enough chlorine has been added to a quantity of water to satisfy its disinfecting demand. In other words, it is the point where all undesirable contaminants have been removed from the water. At breakpoint chlorination, all chlorine added to the solution is consumed by chemical reactions with the contaminants, resulting in no free available chlorine (FAC) in the water
For the purposes of wastewater treatment, breakpoint chlorination is a means of removing ammonia from a solution, which changes to an oxidized volatile form. Increased chlorine residual is produced by adding chlorine to water that contains ammonia or organic matter that contains nitrogen.
Breakpoint chlorination is achieved by the continual addition of chlorine to water until the point where the chlorine successfully suppresses a decrease in the corrosion rate, which makes the diffusion process difficult to occur due to a thickening of rust layers.
The negative effects of chlorine on metal are primarily due to its corrosive properties. Excessive chlorine content can result in pitting corrosion. Excessive chlorine is also detrimental to human tissues. The strong oxidizing effect of chlorine causes hydrogen to separate from water in moist tissue. This causes the release of hydrogen chloride and nascent oxygen, which then causes corrosive tissue damage.
In regard to swimming pools, breakpoint chlorination has a certain amount of free chlorine that is used to fully remove combined chlorine (CC) from the pool water. It is necessary to avoid the unsuccessful termination of combined chlorine content in the pool, although if chlorine is overused it will be necessary to use oxidizers to reduce the level.
Corrosionpedia explains Breakpoint Chlorination
Chlorine is added to water so it will react with and break down contaminating compounds. As chlorine is added, it reacts with any ammonia and nitrogen in the water, oxidizing them to create disinfecting byproducts known as chloramines. At this point any chlorine added to the water is immediately "used up" and combines with contaminants to sanitize the water. During this process, the overall chlorine residual increases.
However, as additional chlorine is added, it begins to react with the chloramines present in the water. The chlorine and chloramines cancel each other out, reducing the chlorine residual concentration. When the excess chlorine can undergo no further reactions (or when the chlorine demand is satisfied) breakpoint chlorination is said to be achieved. The addition of chlorine beyond the breakpoint creates a presence of free available chlorine (FAC), i.e., uncombined chlorine that can act as a disinfecting agent.
The following equation is used to determine the amount of combined chlorine present:
Combined Chlorine = Total Chlorine - Free Available Chlorine
Breakpoint chlorination removes combined chlorine by adding available chlorine. Sufficient chlorine is put into a pool to raise the free available chlorine level to 10 times the amount of combined chlorine in order to reach the breakpoint.
Combined chlorine is available chlorine that has combined with wastes containing nitrogen or ammonia. Combined chlorine causes an obvious "chlorine odor" and can cause skin and eye irritation.