How to manage Iron in Swimming Pools

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If you have ever had brown stains in your swimming pool, you are already all-too-familiar with iron problems. In this article, we discuss how iron gets into our water and why it creates stains. Then we will discuss options for managing iron to prevent problems like stains and excessive chlorine demand.

Metal Oxidation creates higher chlorine demand

Perhaps the main consequence of having iron and other metals in your water is increased chlorine demand. Metals are the easiest thing for chlorine to oxidize, and therefore the first things to be oxidized. As oxidants, metals like iron reduce chlorine rapidly. This is why at the beginning of the breakpoint chlorination curve (up to point A on the chart below), there is no noticeable increase of chlorine residuals until these “chlorine reducing compounds” like iron are conquered.

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The first things chlorine attacks are oxidants like metals. Iron is at the top of that list.

Pool operators with iron issues may notice a higher consumption of chlorine, but often it is overlooked. The cost may not be noticeably higher because the pool might be constantly introducing new water with iron in it. In other words, it’s the baseline, and nothing to compare it to. But rest assured, iron absolutely reduces free chlorine in water. Below we will discuss ways to control metals like iron, but first, let’s talk about where iron comes from.

How does iron get into water?

Iron is found in almost all natural water sources. According to a drinking water equipment manufacturer, LennTech, iron is in seawater, rivers, lakes and groundwater too. It up to those of us who manage and treat water to remove it. So iron usually gets into our swimming pools via the tap water. Certain areas of the United States have more iron in their tap water than others. For instance, the upper midwestern states (Ohio, Michigan, Indiana, Illinois, Wisconsin and Minnesota) are challenged with notorious iron problems. Swimming pools in those areas must deal with high levels of iron out of the faucet, or be plagued with iron staining.

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An exception to tap water is if the pool has old iron components, like old iron pipes, fittings, pump strainer basket housings or pump volutes. If these components have been worn down (like having low pH water flowing through them long enough), iron is sure to find its way into the pool.

There are some exceptions for other metals too, like copper. Copper can get into water from products like copper algaecide, mineral disinfection systems, and from corroding heat exchangers or copper pipes. Pools have turned green from copper before, and sometimes it is mistaken for algae. But let’s get back to iron.

Dissolved iron in water is mainly present as ferrous hydroxide (Fe(OH)2+). We could try to explain the chemical formulas and reactions, but you can read these three sources if you are interested in that level of detail. Source 1, Source 2, or you can go down the Wikipedia rabbit hole as we did. But for this article here, we will try and keep this as simple as possible.

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Usually, iron gets in our swimming pools in a dissolved, soluble state. This means it is mostly invisible and not yet oxidized. If drinking water is chlorinated or chloraminated, however, some of the iron may become oxidized and have a tinge of brown color to it. Iron, when oxidized, turns reddish-brown, and chlorine is consumed (reduced) in the process. If the iron was not yet oxidized in the pipes en route to the swimming pool, it sure will be when it is met with chlorine or a secondary oxidizer like ozone.

Oxidation is what creates staining.

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Oxidized iron stains usually present themselves first near return inlets.

Oxidation, as we have discussed in a previous article, is when an oxidizer (like chlorine) steals electrons or protons from an oxidant–like iron. So the key to preventing iron staining is to prevent iron from being oxidized. And to do that, we have a couple of options.

How to prevent iron staining

We can try to remove iron, either from the source water or from the pool itself, and/or we can chemically manage it by using a chelating agent or sequest.

Filter Iron out of the tap water

Iron filters exist and can be installed on pool fill lines. They do need to be replaced periodically; no filter has unlimited capacity. Such iron filters have a wide range in price depending on the removal rate you need, and the flow rate of your water. For instance, a 2″ fill line on a commercial pool will need a larger, more expensive iron filter than a garden hose filling a residential pool.

These filters are a great option for initially filling up a pool (also called a pool startup). The iron–and other metals that may be present in the fill water–will be filtered out prior to adding chlorine to the pool.

Remove Iron from the pool water

If your pool is already full and is challenged with iron problems, there are options to physically remove iron from the pool. Most solutions in the pool business are a two-part process. The first part is to sequester the metals–which basically means to cluster metal ions together into larger particle sizes–and then use a filter that can capture the sequestered metals. Such products do exist on the market already.

If you have a D.E. filter, it could be sufficient in and of itself to capture sequestered metals. Removal would be as easy as doing a media replacement. This method has difficulty removing already-oxidized iron, however. Ferric iron, for example, is not as easy to sequester as ferrous iron. More on that in a moment. In order for this plan to work on existing stains, you would need a way of lifting those stains and getting iron into a state where it can be more easily sequestered or chelated.

Chelate or Sequester iron to prevent oxidation

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Chelants like NaturallyFREE binds to (and isolates) individual ions, whereas sequestering agents cluster multiple ions together.

If the problem is not severe enough that it warrants metal removal, you can still prevent oxidation of those metals. Both sequestering agents and chelating agents can bind to metals and hold onto their protons or electrons. If stains already exist, sequestering and chelation is more difficult because the metals are already oxidized and insoluble. An additional step of lifting the stain back into suspension is necessary. To do this, consider using citric acid (also called ascorbic acid, or vitamin C).

naturally free scale and metal sequest

Our chelating agent NaturallyFREE is proven to chelate ferric and ferrous iron, but it is very slow to remove stains. It also works better in warmer water. If the water is colder than about 60ºF, the product will be very slow or even dormant. Keep this in mind when using NaturallyFREE.

Sequestering and chelation do not remove metals from the water. They just bind metals to prevent oxidation and staining. You would need some sort of filter to capture and remove the metals. As mentioned before, Regenerative DE filters can capture chelated or sequestered iron. We have even seen it happen on sand filters, but only a few times.


Stains occur when metals get oxidized and become insoluble. Otherwise, metals like iron are in solution or suspension, and mostly invisible. Metals are introduced to our pools through the tap water, with few exceptions like copper algaecide or corroded metal equipment in the pump room. The options to solve this problem are either a strategy of removal, or chemically isolating the metals to prevent further oxidation. In either case, if stains are already present, a citric/ascorbic acid might be needed to lift the stains. Contact us if you need help deciding what to do.

Six Factors that weaken chlorine in swimming pools

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Slow Chlorine = Weak Chlorine

Sanitization and disinfection are the two primary responsibilities of chlorine. Chlorine’s secondary responsibility–or, more accurately, it’s secondary obligation–is oxidation. In terms of residual sanitizer out in the pool water, chlorine is the first line of defense against common recreational water diseases like pseudomonas aeruginosa; bacteria like e. coli; other germs like staphylococcus aureus and giardia; and living organisms like algae.

So its speed is critical.

If chlorine is slowed down, its kill times, referred to in biochemistry as contact time (CT) are proportionally increased. And that’s not a good thing when it comes to keeping water safe. We want chlorine to kill germs and viruses fast and efficiently. Slow chlorine is weak chlorine, and there are several factors discussed in this article that drag chlorine down.

To help protect people against those nasty waterborne pathogens, there are secondary disinfection systems like UV, but they are a point-of-contact system.  In other words, secondary disinfection systems can only affect what they touch, and they only touch what passes through them. Chlorine, on the other hand, is flowing with the water throughout the entire pool and system…it’s everywhere. So let’s get into what slows it down.

Factors that slow chlorine

1. Metals

When chlorine is introduced to the water, it reacts first with metals like iron and manganese. Chlorine oxidizes these metals first and foremost, which in turn knocks out available chlorine early on. The reaction looks like this:

Cl+ Fe † FeCl2

You can see this on the chart below. Notice that chlorine residual does not present itself until point A. What happened prior to that? Why did it not start building residual from the start?  The answer is metals between 0 and point A. The chart labels it as “Destruction of chlorine residual by reducing compounds.” For more information on the science of breakpoint chlorination, read this.

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2. Non-living organics (bather waste)

Why are we depending on chlorine alone to remove non-living organic waste from the pool? Chlorine is critical to the safety and wellbeing of everyone in the pool because of disinfection…not removing bather waste. But alas, the bather waste (such as sweat, urine, body oils, mucus, lotions, cosmetics, deodorants and hair gels) must be oxidized to get them out. Right?

Wrong. Oxidation is not the most efficient or effective means of removing bather waste from a swimming pool. Try using SimplyPURE enzymes instead. They are strong enough for wastewater; yet safe enough to meet the NSF/ANSI 50 standard. The enzymes flow throughout the entire pool system–alongside chlorine–and devour organic contamination to reduce the burden on chlorine. By using enzymes, chlorine is freed up to sanitize and disinfect…and that’s what we need it for in a pool.

More specifically than non-living organics, however, are nitrogen compounds. Nitrogen compounds combine with chlorine (to create combined chlorine). The breakpoint chlorination process shows this occurring, then the eventual destruction of combined chlorine compounds like monochloramine and dichloramine. Eventually, with enough hypochlorous acid (HOCl) to break it down and convert it to nitrogen trichloride (aka trichloramine), these compounds will off-gas into the air. They then become an air quality problem.

3. Direct sunlight (UV)

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Direct sunlight breaks down chlorine. Without a stabilizer (cyanuric acid) in the water, as much as 90% of free available chlorine could be destroyed within just two or three hours.

We conducted fairly diligent research for this article. We found plenty of sources indicating that direct sunlight breaks down chlorine and bromine; a fact that is irrefutable. What we were looking for, however, we have not yet found. Does anyone know if UV sanitation systems for pools also break down chlorine? And if so, how much? Is it similar or even more severe than sunlight? We think it must be less severe than sunlight, but we honestly do not know. It would be great if a UV manufacturer could contact us and let us know.

But back to what we know. Direct sunlight breaks down chlorine in a matter of hours. Obviously, broken down chlorine is ineffective at sanitation, so therefore sunlight makes this list. Fortunately for outdoor pools, several decades ago a wonderful discovery was made, called…

4. Cyanuric Acid (chlorine stabilizer) Overstabilization

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The more stabilizer you have in your pool, the slower the chlorine.

Without cyanuric acid (CYA), chlorine would break down in a matter of hours in direct sunlight. Something about the UV rays breaking apart the HOCl and OCl- itself, but the specific chemistry is not important to this conversation. Just know that without cyanuric acid, also known as chlorine stabilizer, outdoor pools need to be constantly replenishing free chlorine. It’s good to have some stabilizer in an outdoor pool.

The key is moderation. Overstabilization is the problem to avoid, and it’s all about the free chlorine to cyanuric acid ratio (FAC:CYA), which we will discuss in a moment. The US Centers for Disease Control (CDC) released a mandate for regulating the use of cyanuric acid in commercial swimming pools. The new regulation stipulates CYA levels cannot exceed just 15 parts per million! That can be a real problem for pools that use trichlor.

CYA’s dramatic impact on chlorine kill rates

Why limit the use of a stabilizer that protects chlorine from sunlight? Because it also weakens chlorine. Pool industry experts disagree and debate whether or not “chlorine lock up” is a real thing. The notion being that CYA prevents a certain amount of chlorine from being used. The numbers we have heard range between 5-10%, but the best article we have found on the topic of chlorine lock is from Service Industry News. Read the article. It’s worth your time. And we quote:

“Richard Falk derived his ratio in part by recognizing that even if the free chlorine is the same, the concentration of hypochlorous acid (effective chlorine) changes when cyanuric acid is introduced at different levels…

…He did this by recognizing hypochlorous acid, HOCl (the killing form of chlorine) is proportional to the ratio of free chlorine to cyanuric acid:


Beginning with Powell’s best guesses on free chlorine values that are effective for a given cyanuric acid concentration, Falk determined that one should have a minimum free chlorine to cyanuric acid ratio of 7.5 percent to prevent algae in traditionally chlorinated pools.

Falk’s ratio has made doing the math to prevent algae incredibly easy.

FC = 7.5% x CYA

For example, if the measured cyanuric acid in a swimming pool is 30, then a pool operator should maintain a minimum free chlorine level of 2.25 ppm.

2.25 ppm FC = 7.5% x 30 ppm CYA

If the cyanuric acid is at 70 ppm, the free chlorine should be maintained at a minimum of 5.25 ppm.

5.25 ppm FC = 7.5% X 70 ppm CYA

So, assuming Falk’s numbers are correct, the factor of 7.5% is an important one to understand. If your pool has 100ppm cyanuric acid, you basically don’t have free chlorine until you exceed 7.5ppm chlorine. That’s your new baseline. Crazy, right?

5. High pH

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In non-stabilized water (meaning no CYA), The higher the pH, the lower the concentration of HOCl in free chlorine readings. The weaker form of chlorine, hypochlorite ion (OCl-) increases and surpasses HOCl around 7.5 pH.

As you can see from the chart above, pH has a direct impact on the dissociation of H+ from hypochlorous acid in the pool. Hypochlorous acid (HOCl) is the strong, killing form of chlorine We need it in the water! The higher the pH, the less of it there is, as it is replaced by the weaker chlorine, the hypochlorite ion (OCl-). So yes, more alkaline pH can weaken chlorine. Conversely, the lower the pH, the stronger the chlorine, because it has a higher percentage of the strong HOCl.

Let’s be clear about something that sounds somewhat contrarian to industry textbooks and the chart above. The pH of your water dictates HOCl concentrations in pools with no cyanuric acid (CYA). With CYA present, the impact pH has on HOCl concentrations is very little, if not negligable. See the chart below that puts the previous chart in context.

cyanuric acid, CYA, HOCl, pH does not control chlorine strength, pH and HOCl, pH chlorine strength

Notice the red line (HOCl concentration) plummets in the presence of CYA. pH now has a negligible impact on HOCl concentration, and chlorine speed/strength is now dictated by CYA. The new blue line at the top of the right graph represents chlorine bound to CYA, also called isocyanurates.

6. Phosphates

pool sanitization, phosphates, pool phosphates, kill rate versus growth rate, preventing algae, green poolYes, phosphates indirectly impact chlorine too. Phosphates are an essential micronutrient for organisms to reproduce and grow. In particular, algae. Reproducing algae consumes more and more chlorine in the sanitization battle between the sanitizer and contaminants.

Phosphates are an invisible problem in swimming pools that brings about many consequences. And since chlorine cannot simply oxidize phosphates out of the water, they need to be removed by a dedicated phosphate remover like Blue PRO. Phosphate levels over 500 ppb fuel the reproduction of algae, provided other conditions are met. And while removing phosphates does not kill algae, it can certainly slow its reproduction rate, allowing the sanitizer (chlorine) to stay ahead in the battle, preventing outbreaks.