Combined chlorine is a concern shared by commercial pool operators worldwide. But what is combined chlorine? How does combined chlorine form? Is combined chlorine the same as chloramines? If not, what’s the difference between combined chlorine and chloramines? The science is advanced, but as usual, we are here to distill the information down and simplify it for you.
What is combined chlorine?
Combined chlorine is chlorine that has combined with nitrogen compounds present in water. Such compounds include monochloramine, dichloramine and trichloramine (which actually off-gasses into the air). At least, the three types of chloramines are what most people are familiar with. Combined chlorine includes a myriad of other disinfection byproducts (DBPs) like chloroform and other trihalomethanes. There are too many to name in this article.
Combined chlorine is easy to measure. Simply measure the Total Available Chlorine (TAC) and subtract Free Available Chlorine (FAC) from it. TAC – FAC = Combined Chlorine (CC). It’s essentially a measurement of chlorine present that is no longer free and available. Once chlorine oxidizes and combines with nitrogen compounds, it is no longer free chlorine. That being said, combined chlorine still has some disinfecting power, albeit far weaker than Hypochlorous Acid (HOCl).
Some health departments have limits to combined chlorine levels. Some states set a maximum level at 0.4 ppm. If your pool exceeds that, the health department can get involved. We also know that most people can smell the “pool smell” of chloramines at any level above 0.2 ppm of combined chlorine.
Here is an example of calculating combined chlorine:
2.5 ppm TAC – 1.7 ppm FAC = 0.8 CC
The nitrogen question: how does it get into pools?
To understand combined chlorine, chloramines and other issues in pools, we need to start with a basic understanding of nitrogen. And how nitrogen gets into a pool. There are basically four (4) ways nitrogen can get in the water.
Nitrogen gas itself (N2) makes up something like 80% of our breathable air. It’s very prevalent in our environment, but because Nitrogen gas has a triple covalent bond, living things cannot use it in its natural form. And yet, Nitrogen is needed by all living things, plants and animals alike. It is a micronutrient essential to life. So how is it used?
Certain plants (like legumes) and bacteria carry a special enzyme called nitrogenase that can break the triple covalent bond holding Nitrogen gas together. This allows for usable Nitrogen compounds to be formed, such as ammonia (NH3), ammonium (NH4+), nitrite (NO−2) and nitrate (NO−3). These compounds are found in soils throughout nature. Naturally, these forms of nitrogen can find their way into swimming pools, maybe through soil runoff, or dirt on our skin, etc.
2. Source water
Believe it or not, municipal water treatment plants sometimes deliberately add nitrogen compounds to water. They do this to produce chloramines, which have some disinfection power (but not much). The benefit of chloramines vs. chlorine, however, is staying power. Chloramines last a lot longer in the pipes than normal chlorine.
The downside of this is obvious: chloramines are introduced through the source water.
One of the most common sources of Nitrogen, especially in commercial pools, comes from bathers. And I don’t just mean people, I mean any bathers. This includes ducks and other birds that enjoy your pool, dogs, and other living things. But for now, let’s focus on people.
There is a compound called urea that is found in human sweat and urine. Urea is primarily made up of ammonia (??), which is a nitrogen compound. Urea is the culprit behind campaigns against swimmers peeing in the pool, because urea takes a lot of chlorine to oxidize out of the water.
4. Pool and other cleaning chemicals
Finally, people add chemicals to (and around) their pools. Some chemicals are ammonia-based, such as deck cleaners, common algaecides and disinfection chemicals. We see this all the time. A pool operator has a mystery combined chlorine problem, but a low bather load. Come to find out, they clean their pool deck every night with an ammonia-based cleaner. Or they fight algae with an ammonium sulfate algaecide. Be aware that these products work great in the short term, but leave nitrogen behind, which will become combined chlorine.
The combined chlorine process
In a nutshell, the combined chlorine cycle works something like this: nitrogen compounds in the water are oxidized by chlorine. It takes a lot of hypochlorous acid—the strong, killing form of chlorine—to convert nitrogen to its next form, and the next, and next, and eventually off-gassed out of the water. We say “a lot of hypochlorous acid”, because eventually it takes a 15:1 molar ratio of HOCl to Ammonia to get it out of the water. 5:1 creates monochloramine. 5-10:1 converts monochloramine to dichloramine. And 15:1 creates trichloramine, which eventually off-gasses out of the water (and then becomes a nightmare for air quality).
The nitrogen cycle in a swimming pool gets really complex, with all sorts of chemistry reactions that can occur. What you need to know is chlorine will eventually oxidize nitrogen out, but not easily. Chlorine is a great sanitizer, but comparatively, it’s not so great of an oxidizer. Eventually chlorine will overpower nitrogen and get it out. This is known as reaching breakpoint chlorination. Breakpoint chlorination is when chlorine overcomes nitrogen and organic loading, and begins to build a free chlorine residual.
How to reduce combined chlorine and chloramines
Combined chlorine needs to be destroyed, either through secondary sanitation systems or enough chlorine to complete its oxidation. Chloramines will eventually go airborne as trichloramines, which then become an air problem. We strongly recommend both a proactive and reactive approach to addressing combined chlorine.
Proactive methods to reduce combined chlorine
Minimize chlorine’s burden on things like non-living organic bather waste so it can focus on germs and oxidizing nitrogen. Such an approach can be accomplished by use of NSF-Certified enzymes, like AAD. AAD (Amino Acid Digester) breaks down carbon-based organic waste in the water, which chlorine would otherwise have to oxidize. With AAD, chlorine has much less carbon waste to fight, and therefore it is freed up to go after nitrogen. AAD is highly effective at optimizing chlorine efficiency.
Another proactive approach is to manage water with a slightly lower pH, like 7.2 to 7.4. Why a lower pH? Because the lower the pH, the higher percentage % of HOCl your chlorine will be. This means your chlorine will be stronger.
To manage a pool with lower pH, however, pool operators must maintain water balance according to the LSI and/or Ryznar index. If you use our LSI Calculator App, you will see that a lower pH creates more aggressive water. To compensate for a lower pH, you will need to have an above-average level of calcium hardness or carbonate alkalinity. Calcium Hypochlorite chlorine is a great choice for pools looking to optimize chlorine efficiency with a lower pH.
Another proactive approach is to make sure cleaning agents used in and around the pool are free of ammonia and other nitrogen products. You might be surprised! A lot of pool deck cleaners are basically pure ammonia.
Reactive methods to reduce combined chlorine
Once chlorine has combined with nitrogen compounds, short of shocking the pool with a lot more chlorine (hyperchlorination), there are secondary systems that can help. Medium pressure Ultraviolet systems (UV) can be very effective at destroying monochloramine and dichloramine. Ozone is another secondary system which can both sanitize and oxidize. Both of these systems are proven to help.
Another technology, relatively recent to aquatics, can also help in a major way. It’s called hyper-dissolved oxygen (HDO). Without making sanitation claims, the principle of HDO is simply to dissolve a lot of oxygen into the water, which can accelerate oxidation throughout the body of water. Unlike Ozone or UV, which are contained to the pump room—and therefore point-of-contact systems—HDO sends oxygen out into the pool, where it works alongside chlorine. Just like AAD enzymes, HDO can be out in the field where the people are, working to help chlorine get the job done.
To keep control over combined chlorine, do your best to minimize nitrogen introduction. Check chemicals for anything like “ammonium” in the label…and avoid using them. Optimize chlorine efficiency, either by lowering pH or using AAD enzymes (or both). Use a secondary system like UV, Ozone or Hyper-dissolved oxygen. These strategies can make a huge difference.