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 the added chlorine is consumed by chemical reactions with the contaminants, resulting in no free available chlorine FAC in the water. As chlorine is added, it reacts with ammonia and nitrogen in the water, oxidizing them to create disinfecting byproducts known as chloramines. Here the added chlorine is being "used up" or combined with contaminants to sanitize the water.
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In May , we published a two-part article about pool sanitizers. This article dives deeper into the science of chlorination. We explore the difference between breakpoint chlorination and hyperchlorination shocking. What is breakpoint chlorination? Breakpoint chlorination is the point where chlorine levels exceed the oxidant demand , and the water begins to build a residual of free available chlorine FAC. Chlorine vs.
When you first add chlorine to water, it immediately begins to oxidize metals like iron and manganese, which reduce chlorine. This initial reaction wipes out a certain portion of chlorine, which is why nothing shows up on the graph until point A.
As more chlorine is added to water, it reacts on contact with other contaminants—not just germs, but non-living organics and nitrogen compounds too—which create byproducts. Organics are carbon-based, and get oxidized by chlorine, further reducing it. But nitrogen? Nitrogen is not oxidized so easily. This will be explained more in depth in a moment Well, chlorine starts replacing hydrogens. Chloramines are [weak] disinfectants As noted before, chloramines are disinfectants--which is why they are referred to as disinfectant byproducts DBPs.
In fact, many water treatment plants add chloramines to their water as a secondary disinfectant. Albeit weak and slow, chloramines first contribute to the total chlorine levels because they help with disinfection. This, however, reaches a threshold where chlorine turns on chloramines, indicated at point B. In other words, chlorine oxidizes all contaminants, which includes chloramines after point B on the graph.
The downward trend on the graph shows chlorine starting to "win the fight" against contaminants until it oxidizes all but the combined chlorine residual. This would look like a more prolonged downward trend toward breakpoint, because breakpoint would be at a much higher dose of chlorine. FAC residual after breakpoint chlorination Only after the oxidant demand has been addressed can disinfection occur.
Therefore, only after breakpoint chlorination has been exceeded can a residual of free chlorine build. Up until that point, chlorine has its hands full trying to oxidize its way to breakpoint. Combined chlorine CC is the chlorine that combined with ammonia and other nitrogen compounds including chloramines and other DBPs.
Combined chlorine is the most accurate measurement of disinfectant byproducts we can test for. We measure all types of chlorine in parts-per-million ppm. Just remember, test kits cannot tell the difference between hypochlorous acid HOCl , and its dissociated, weak form, hypochlorite ion OCl-.
So even though you may read a good amount of free available chlorine FAC , if your pH is high or you have high phosphates , you may still have weak chlorine in your water. If so, your ORP will reflect that. You can calculate any of the three with addition and subtraction. The conventional wisdom in the pool business is a shock of 10x your combined chlorine level in additional free chlorine.
But according to renown chemist Richard Falk, the 10x figure is not accurate. Here is a direct quote from Richard on the PoolGenius Network forum: "The molar ratio of chlorine to ammonia is 1. Because forming dichloramine requires 2 moles of chlorine for 1 mole of ammonia and because of side reactions that can occur, the actual chlorine to ammonia ppm ratio is around x which is where the 10x rule came from.
However, this is wrong since CC is in ppm Cl2 units so no factor of 5. To oxidize monochloramine, it takes from 0. Even if the CC were urea, it takes times the CC level, not 10x.
Of course, the higher the FC level the faster reactions occur, but there is no magic 10x amount. The oxidant demand is greater than the chlorine available to handle it. The oxidant demand in these cases can be chloramines, non-living organics, or any combination of both. Clearly the normal chlorine levels in your pool are not enough to meet the demand.
So think about how the demand itself got there. High combined chlorine is generally because of ammonia being introduced to the pool. Find out what chemicals are being used in and around the pool. Think of pool deck cleaners many of them are ammonia-based , and algaecides many of them are also ammonia-based. We are in favor of a minimalist approach. Why throw more chlorine at the problem, without making an effort to discover the root cause of the problem?
Chlorine is not designed to be a primary oxidizer! It is designed to be a sanitizer and disinfectant. If you are routinely hyperchlorinating your pool, we hope you will reconsider your practices. Applying the right chemistry for the right situations can minimize costs, maximize efficiency and improve the overall swimmer experience.
If we do it right, breakpoint chlorination will be easy to reach, and you can have a safe residual of free available chlorine to keep the water safe. Identify the sources of ammonia, and prevent them from getting in the water. We also recommend supplementing chlorine with enzymes to remove non-living organics and oils.
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Breakpoint Chlorination in Water Treatment Process
Oxidation-reduction potential Continual breakpoint chlorination Continual breakpoint chlorination uses the continual addition of chlorine after bathers have left the pool to oxidise combined chlorine so that all of the total chlorine exists as free chlorine. Last updated: 09 April Content 1 Graphical representation of breakpoint chlorination The above graph demonstrates the theory of continuous breakpoint chlorination. On the right vertical axis is the ammonia-nitrogen i. The bottom horizontal axis represents the ratio of chlorine Cl2 to ammonia NH2 by weight which is zero on the left and increases to the right. The bottom horizontal axis also represents time and increases from left to right.
Understanding breakpoint chlorination
It is these pollutants which can cause stinging eyes and mal odours. These are primarily measured as ammoniacal nitrogen The effectiveness of the disinfection process is related to the pH of the water and the lower the pH the greater the efficiency of disinfection. The process of water treatment used in the UK is known as breakpoint chlorination. What does breakpoint chlorination mean? Breakpoint chlorination is the point where the demand for chlorine has been fully satisfied in terms of chlorine addition to the water. In other words, the chlorine has completely reacted with the bather pollutants leaving a zero chlorine residual.