Once chlorine has combined with nitrogen compounds, there are a few reliable ways to address it. Combined chlorine can be reduced chemically (with chlorine and chlorine supplements like enzymes) or physically (with secondary sanitizer/oxidation systems or special filtration).
But the best way to address combined chlorine is to identify and prevent nitrogen from getting in the water in the first place.
This article expands on our previous article about it. We want to show you both the chemical and mechanical options available to help you lower combined chlorine in a swimming pool.
As a refresher from our other article about combined chlorine:
Combined chlorine (also called chloramines) is various forms of chlorine that have combined with nitrogen compounds in water. Without nitrogen, there would be no combined chlorine.
CC = TAC - FC
Combined chlorine = total available chlorine - free chlorine
If your total chlorine level is higher than your free chlorine, you have combined chlorine, and therefore nitrogen in your water.
Ideally, combined chlorine stays at zero, meaning all the chlorine in your water is available to kill, and has not been tied up dealing with nitrogen compounds. Germs are the priority for chlorine, as our primary goal with chlorination is to keep water safe, clean and clear.
Forcing chlorine to handle organics and nitrogen (and usually organic nitrogen, which is even more complex) is asking too much of chlorine. Chlorine is an exceptional disinfectant, but only a mediocre oxidizer. At the levels used in swimming pools, chlorine alone is not strong enough to outright destroy compounds like urea.
And yet, most pools struggle with combined chlorine. So this article focuses on seven proven ways to reduce combined chlorine.
Before getting into the reactionary ways to reduce combined chlorine, the best way to handle this is to be proactive. Instead of focusing on the symptom (combined chlorine), focus on the cause: identify the source(s) of nitrogen getting in your water. Then think about ways to prevent that nitrogen from getting into the water in the first place.
If you already have high combined chlorine in your pool, you have some options. Here we have seven proven ways to reduce combined chlorine, both indirectly and directly. They are broken into two categories of methods: chemical and physical.
Superchlorination is by far and away the most widely used and accepted practice for addressing combined chlorine. Superchlorination is dumping an excess of free chlorine (the industry standard is 10x the level of combined chlorine, though that's more than necessary) into the pool. This surge of oxidizing power should destroy the oxidant demand. It also kills just about everything that might be lurking in the water too, which is a plus.
The downside is that the pool must be closed for superchlorination, and when it's done, the FAC level needs to be brought back down, or wait long enough for it to drift down on its own.
With respect to the breakpoint chlorination process, superchlorination is the rocket boost that finally pushes past the demand, achieving and exceeding the breakpoint, and building a free chlorine residual. In the chart above, when chlorine struggles to get over the hump at point (B), superchlorination is a reliable option.
Non-chlorine shock like potassium monopersulfate (KMPS) can be added to the water. This can help destroy the nitrogen precursors to chloramines. KMPS does not disinfect, nor does it destroy break apart chlorine that has already combined to nitrogen.
To be clear, non-chlorine shock does not directly reduce combined chlorine. Instead, it attacks the nitrogen compounds themselves, so chlorine has less of them to combine with. KMPS is a powerful oxidizer, so use only as directed when bathers are not in the pool.
First, Enzymes metabolize carbon-based body products like body oils, deodorants, cosmetics, lotions and sunscreen. That way, chlorine has less to oxidize. Less oxidant demand means more chlorine in the war against nitrogen compounds.
Second, enzymes break down carbon bonds, which means they can break apart organic nitrogen compounds into simpler, easier-to-manage substances. With carbon gone, a complex organic nitrogen (like urea) becomes a much less complex inorganic nitrogen (like ammonia).
Removing carbon with enzymes radically simplifies the molecules, lowering the energy required for chlorine to address the inorganic nitrogen. Commercial-grade enzymes like CV-600 can bring an enormous improvement in chlorine efficiency.1
So while enzymes do not directly remove nitrogen or chloramines, the net effect to a pool operator appears that way. We regularly recommend using enzymes to not only help reduce combined chlorine, but also to improve indoor air quality.
Related: Pool Water Chemistry Resources
There are some other chemicals that are being introduced into the market in the United States, but at this time, we don't have enough data on them. One of them is chlorine dioxide, which is a very powerful form of chlorine, capable of destroying chloramines with relative ease. It has been used in Europe for some time, but relatively new to the US.
These products are often used in tandem with a flocculant to help filter out microparticles in water, further improving water clarity and chlorine efficiency. Less particulates in the water usually improves water quality. As we learn more about these methods (and vet them ourselves), we will update this, and probably write new articles about them if they show promise.
In our experience of visiting nearly 400 indoor pools, well over 90% of them already had UV systems installed. UV helps reduce combined chlorine by separating the nitrogen compound from the chlorine, but does not destroy the nitrogen compound itself, which can combine with more chlorine later on.
Secondary disinfection systems like UV and Ozone are quite popular in commercial pools. Ultraviolet (UV) systems can destroy formed chloramines, but not the nitrogen compounds themselves.
UV is not an oxidizer. It is a secondary sanitizer that disrupts cells and prevents them from reproducing. It also happens to break apart combined chlorine molecules (monochloramine and dichloramine) without destroying the nitrogen itself. UV splits the bond between nitrogen and chlorine.
Once trichloramine is formed and off-gassed, UV can no longer touch it...for obvious reasons. This means UV has a limited impact on indoor air quality. But if trichloramine does stay waterborne long enough to pass through a UV chamber, it can be destroyed, depending on the water clarity.
UV, for this reason, is our recommendation for indoor pools. It is less effective in outdoor pools because people tend to wear sunscreen, and sunscreen blocks UV. UV makes a noticeable impact on water and air quality in indoor pools, but it is only a piece of the puzzle.
Related: Comparison of the Best Secondary Systems for Swimming Pools
Ozone is also a point-of-contact system, like UV, so ozone also has a limited impact on indoor air quality. One advantage ozone has over UV, however, is that ozone will oxidize and destroy just about anything. Yes, that includes nitrogen compounds, organic bather waste, and waterborne chloramines. So ozone can fully destroy chloramines and the nitrogen they rode in on.
Ozone is also a powerful sanitizer that can kill germs and diseases. Ozone is the clear favorite for outdoor pools. When used indoors, it needs a degassing chamber to prevent undissolved ozone from getting into the pool, as it can irritate bathers indoors.
Advanced Oxidation Process (AOP) is similar to Ozone, in that it is a point-of-contact oxidizer. It has the ability to destroy pathogens and oxidize nitrogen compounds too.
Hyper-Dissolved Oxygen (HDO) is a relatively recent technology in the pool industry, though it has been around for many years in other industries like food processing and agriculture. It injects hundreds of millions of nanobubbles of purified oxygen into solution; bubbles so small they can stay in solution for days.
Image: Hyper-dissolved Oxygen (AquaFuzion®) installed on a high-end residential pool. Photo courtesy of Pure Vision Technologies. Used with permission.
This means HDO is not just a point-of-contact system. Oxygen helps boost chlorine and enzyme activity. So while the oxygen itself does not reduce combined chlorine, it boosts chlorine, which does.
Air quality, however, is where HDO stands out. There is a noticeable improvement to indoor air quality using HDO. This is largely because there is so much oxygen in the water, that when it off-gasses, it introduces pure oxygen above the surface of the water (right in the breathing zone). Fresh air dilutes chloramine gases. We know from personal experience that it dramatically improves indoor air quality...despite not reducing chloramines.
Four large activated carbon filters for a waterpark in Denmark. Photo: Eric Knight
Activated carbon (or charcoal) filters are widely used in Europe to remove chloramines. They are very effective, and are used on whole-house drinking water systems too. For this reason, we had one installed on our drinking water, because I tested 0.25 ppm of combined chlorine from our kitchen sink.2
After the activated charcoal filter was installed, that number (along with free chlorine, which was 1.25 ppm) are both zero.
The benefit is highly effective elimination of combined chlorine. The downside is activated carbon filters also neutralize free chlorine. And the media doesn't last forever, so it needs to be replaced over time. Pools using this will consume more chlorine, so this might not be feasible for most pools in the United States. It works well for pools following the European DIN standard, which has less than 1 ppm (or 1 mg/L) of free chlorine.
Dilution is a viable option for pools that have no chloramines in their fill water. Simply draining or backwashing water to waste makes room for tap water to replace it.
This is also viable even if you have some chloramines in the tap water, assuming your fill line feeds water into the surge tank or pre-filter on a pool with a UV, ozone, or AOP secondary system. The secondary system will destroy the chloramines before they make it into the pool. You can also run the fill water through an activated carbon (charcoal) filter prior to circulating in the pool.
If you have the option for Reverse Osmosis (RO) filtration, that will also remove combined chlorine from water. This is not always practical, especially for larger pools. But RO filtration is widely used in areas with drought-related water restrictions, where dilution is not always an option.
Combined chlorine is a measurement of chlorine that has combined with nitrogen compounds. We generally refer to all of these molecules as chloramines, even though chloramines are just one specific group. There are more disinfection byproducts (DBPs) that we can list on here. The variations happen, in part, because of incomplete oxidation. When chlorine runs out of steam after halfway destroying something, you're left with a half-destroyed byproduct.
You can reduce combined chlorine physically using dilution or secondary systems (UV, ozone, AOP). You can chemically reduce combined chlorine utilizing superchlorination to exceed the breakpoint, and you can supplement that effort using non-chlorine shock and/or enzymes.
It's easy to think that indoor air quality can be "perfect" if the water quality is perfect. But that's simply not true. If the air quality happens to be great, it's because the air system is also well-designed. Water and air quality must be in harmony. Too often, we see indoor pools with HVAC systems that do not match the needs of the pool they serve.
Does your facility have challenges with combined chlorine and indoor air quality? If so, you're not alone. Let us know if you want help properly diagnosing the issues so you can solve them at their source.
1 Residential-grade pool enzymes tend to be more surfactant than enzyme, and they often fall short of meeting the bather demands of commercial pools. In our experience, Orenda CV-600 and CV-700 perform the best.
2 According to the US Environmental Protection Agency (EPA), there are a few reasons why water utilities use monochloramine intentionally. Part of it is to extend the reach of the water and still have some disinfection. The main reason they cite, however, is monochloramine is less reactive and therefore leaves less other byproducts in the water compared to chlorine. Counter-intuitive, right? Monochloramine itself IS a chlorine byproduct. In fairness, it's less harmful than many of the other chlorine disinfection byproducts like cyanogen chloride, trichloromethane (chloroform), and dozens of other variations.