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Ways to Reduce Chlorine Demand

Chlorine demand is a common concern for commercial pools. Is your swimming pool consuming more chlorine than you would like, or more than it should be? There are many reasons why this can happen, so in this article we’ll explain a few of them, and offer a remedy for each.

What reduces chlorine?

Chlorine has two primary functions in water chemistry: sanitzation/disinfection, and oxidation.

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Reduction by Oxidation

The term “reduce”, or “reduction” of chlorine is the antonym to “oxidize”. The process of oxidation is the exact opposite of the process of reduction, which is why ORP (oxidation-reduction potential) is so commonly misunderstood.  Oxidation is the loss of electrons by an oxidant and given to an oxidizer. Reduction is the gain of electrons by the oxidizer (chlorine or perhaps ozone or a hydroxyl radical). Because electrons (e-) are negatively charged, they reduce the valence of the oxidizer.

Chlorine in its powerful killing and oxidizer form, Hypochlorous Acid (HOCl) is reduced by electrons into weak chlorides (Cl-) that can no longer oxidize (steal more electrons). Therefore, that chlorine has been reduced, or used up. It’s not free chlorine anymore.

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Metals are the first things to be oxidized when chlorine is added to water.

Compared to its strength as a sanitizer, chlorine is a relatively weak and inefficient oxidizer. Depending on what the oxidant is, its complexity determines how much chlorine will be consumed. Iron is easy to oxidize and occurs quickly. Sunscreen, on the other hand, is quite complex, and takes more chlorine to remove. And that’s nothing compared to a nitrogen compound like urea, which requires chlorine combining with ammonia and creating all sorts of disinfection byproducts (DBPs) like chloramines. These byproducts are measured as combined chlorine. See the breakpoint chlorination curve above.

Related: Water Chemistry Lesson: Combined Chlorine

Oxidation is chlorine’s method of eliminating non-living contaminants from water, like organic waste (bather waste, oils, sunscreen, cosmetics, lotions, etc.) and nitrogen compounds (ammonia, urea, etc.).

Reduction Consumption by Sanitization

Unlike oxidation, chlorine and other sanitizers do not steal electrons in the process of sanitization/disinfection, because the target contaminant is a living thing, not an oxidant. But like oxidation, chlorine also gets consumed in the killing process, it’s just not technically “reduced” because of electrons.

Sanitization is chlorine’s way of killing all living contaminants, and chlorine is really good at it. Killing germs and algae is what chlorine was made for. It is incredibly efficient at killing, whereas it is not so efficient at oxidizing complex non-living organics and nitrogen compounds.

Chlorine Demand

Let’s break down these demands into the two categories of sanitization and oxidation.

Sanitizer demand

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As long as chlorine’s kill rate is greater than the growth (reproduction rate) of the contaminant, the water will not be at risk of outbreak.

Living organisms that reproduce will consume chlorine. Naturally, these will be more prevalent in warmer water, because things like algae grow better in warm water. Cold water is not conducive to algae or bacterial growth…or at least, not nearly as fast. The name of the sanitization game is making sure chlorine’s killing speed is faster than the contaminant’s reproduction rate.

Algae are a huge part of chlorine demand in outdoor pools. Algae not only consume chlorine, they also consume carbon dioxide, which raises the pool’s pH, causing increased acid demand too.

Algae growth is almost always a consequence of overwhelmed chlorine, either by overstabilization, or by an abundance of nutrients like phosphates.

Germs and parasites are far more of a concern for the health and safety of a swimming pool, but fortunately, there are very few of them in the water, relatively speaking. Most germs and viruses are killed very quickly by chlorine (again, provided the pool is not over stabilized with too much CYA). Parasites like Cryptosporidium, however, are notoriously hard to kill, even with very high levels of chlorine. The contact time (CT) required to kill parasites is almost not even comparable to simple germs and other bacteria.  We could go on and on about recreational water illnesses (RWIs), but the CDC has already published the best RWI-prevention practices, and we are focusing on the consumption of chlorine for this article.

How to minimize sanitizer demand

To reduce the amount of chlorine consumed on germs, there are basically two ways you can attack the issue. First, and perhaps most obviously, is to optimize chlorine efficiency and strength.  One way is to supplement chlorine with a secondary disinfection system like UV, Ozone or AOP. This will take some of the burden off chlorine by killing just about anything that passes through the pump room. But therein lies the weakness of secondary systems: they are point-of-contact only. They do not offer a residual that can circulate throughout the pool itself–a fact that reinforces the need for a primary residual sanitizer like chlorine.

Not all secondary disinfection systems are ideal for any pool. UV, for instance, is not ideal for an outdoor pool because of a particular organic waste that is common in the water: sunscreen. Sunscreen blocks UV light, and therefore limits the efficacy of UV on outdoor pools. They still help, but they are not nearly as beneficial as something like Ozone or AOP.  Indoor pools, however, can benefit greatly from UV, though UV itself will gradually break down chlorine just like direct sunlight would.

Another option to supplement chlorine is with enzymes, though enzymes are not sanitizers. Enzymes handle the non-living organic oxidant demand–which happens to be the single-biggest category of contaminants in a pool–so there is more free chlorine available to kill germs. More on enzymes in a moment.

Optimizing chlorine isn’t just about strengthening it, it’s also about minimizing things that hold it back. Namely, cyanuric acid. CYA is beneficial for sunlight protection, but too much CYA dramatically weakens chlorine (%HOCl) and slows it down. Slow, weak chlorine means higher contact times (CT) when killing living pathogens.

phosphate removal, phosphate remover, concentrated phosphate remover, reduce phosphates, phosphate cloudThe second way to minimize sanitizer demand is to limit the growth and reproduction rate of algae. At any given time, an outdoor pool will be faced with algae spores trying to produce a colony. There is almost certainly more algae than there are germs, so we want to slow their growth and reproduction rates so chlorine can stay ahead. This can be accomplished by using Concentrated Phosphate Remover (CPR) as a proactive measure.

Oxidant Demand

Oxidants are the non-living contaminants in your water. The most common, by far, are non-living organics like sunscreen, cosmetics, lotions, deodorants, body oils, sweat, etc. Then there are the nitrogen compounds like nitrites, urea and ammonia, which eventually get reduced down into nitrates and other various disinfection byproducts. And depending on your tap water, metals can also be a major source of oxidant demand. Iron in particular can be a real nuisance.

How to minimize oxidant demand

Managing the oxidant demand dramatically reduces the overall chlorine demand.

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AAD enzyme purge

Depending on the oxidant type, there are several things that can help. Just like sanitizer demand, supplementing chlorine with secondary oxidation really helps. This does not include UV, because UV is not an oxidizer. We are talking about Ozone and AOP systems here. These systems help destroy non-living organics, nitrogen compounds, and pretty much anything else that can be oxidized. As mentioned earlier, we strongly recommend supplementing chlorine with enzymes like Amino Acid Digester (AAD). AAD breaks down and removes oils, sunscreen, cosmetics, mucous and most of the other things that come off our bodies. And because enzymes circulate alongside chlorine throughout the entire system, their benefit is 24/7 in every corner of the pool.

As for metals like iron, copper and manganese, they should ideally be pre-filtered out from the tap water fill line. If that’s not an option and metals are already in your pool, you can bet high iron levels will consume chlorine rapidly, and leave behind brown-tinted water or ugly stains. If the metals are in your pool, you can either sequester them or chelate them with Metal and Scale Inhibitor (MSI). But be aware, the first time you use MSI, its purge dose can wipe out your chlorine levels entirely for a few days (talk about a temporary chlorine demand!). That’s because it takes time for a chelating agent like MSI to find all the metals and minerals (like calcium) to bind to. Once it does its job, MSI coexists with chlorine just fine.


The vast majority of chlorine demand is from non-living organics, aka bather waste. The second biggest contributor is nitrogen compounds like urea and ammonia, which require a lot of chlorine to oxidize out of the pool. Then metals. Then there’s the sanitizer demand, which, unless you’re facing an algae outbreak, is proportionally much less prevalent in the water than the oxidant demand. Most of the sanitizer demand is small (but critically important for health and safety!), and chlorine is made for killing germs.

To reduce chlorine demand, consider supplementing chlorine with secondary oxidizers/disinfection systems, and enzymes. For metals in particular, MSI is a good option for chelation to prevent oxidation.

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Combined Chlorine, Chloramines and Ammonia

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.

1. Naturally

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 (NO2) and nitrate (NO3). 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.

3. Bathers

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.