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How to remove and prevent scum lines

Scum lines are very common in pools

In our business (commercial swimming pools), almost every pool has a scum line. Yes, even gutter pools can have them. For the sake of this article, let’s define a scum line as a visible layer of deposits at the water level that adhere to surfaces: walls, ladders, rails, tile lines or gutters. These deposits are not always the same. In fact, we find they are usually a mixture of non-living organics and anything that adheres to them. Because non-living organics in pools are often sticky and/or slimy, any number of suspended contaminants can join in on the scum line party.

We have seen hair, lint, grease, dirt, leaves, and other fun stuff adhered to pool walls at the water level. We have also seen deposits of calcium carbonate scale, calcium phosphate scale, and dried salt.  This article will explain how and why scum lines form; and more importantly, how to prevent and remove existing scum lines.

Chlorine vs. Bather Waste

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Figure 1. 100 parts-per-million chlorine in a bowl of water, versus a small amount of WD-40. It congealed into a hardened mess. Is chlorine really cut out for oxidizing oils and non-living organics? Surely 100 ppm should be enough to handle a small amount of oil. Right?

Non-living organic waste, or bather waste, comes in many forms. Body oils, sweat, mucous, urine…but also what we put on our bodies: lotions, hair products, sunscreen, etc. All of these contaminants contribute to the oxidant demand that chlorine must overcome to do its primary job: sanitation. Have you ever seen what chlorine does to a high concentration of bather waste? We experimented with 100 ppm chlorine in a bowl of water with a squirt of WD-40 (a type of oil). See the Figure 1.

The reality is this: chlorine is designed to be a sanitizer. It has to overcome the oxidant demand in order to exceed breakpoint chlorination, but chlorine is inefficient at it. Rather than oxidizing the oil out of the water in the bowl, it actually turned into a sort of hard gel. Very odd, indeed.

This gel is very sticky and all types of dirt and debris can attach to it.  But why only at the water line?

Non-living organics and oils float

Who remembers the elementary school experiment where you pour vegetable oil into a glass of water? It floats and stratifies, because oil is less dense than water. This same principle applies to non-living organics. They act like greasy oils and float to the surface. This explains why scum lines are never found deep below the surface; they adhere at the water level only. Below is a video to demonstrate more on that.

The same ‘scum’ fouls filters and leads to costly sand changes.  That is, of course, unless you chemically refurbish your sand. Nobody denies that scum lines are a nuisance in a public pool. Lifeguards spend countless hours scrubbing them with brushes and tile cleaners, and sometimes have to use pumice stones to remove hardened scum lines. Okay, to be fair, the latter is when you have hardened calcium carbonate scale on the tile line…and we will address that in a moment. The point is, these scum lines never show up three feet underwater, or near the main drain, right?

They always show up at the water line because oils float. So let’s treat the oils accordingly.

Scale and scum lines are different, but can coexist

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Salt generators give off heat during electrolysis. They also release a byproduct of sodium hydroxide (caustic lye) which raises the pH and the saturation index of the water. High pH and heat tend to encourage calcium carbonate to fall out of solution and harden.

Because scale is often in the same location as a scum line, the two problems are associated with one another. The truth is, they are completely different problems caused by completely different factors. While scum is at the water line because non-living organics float, scale forms near the water line because of temperature. To be more precise, scale forms when the water is over-saturated with calcium carbonate (according to either the Langelier Saturation Index (LSI) or Ryznar Index). One of the factors in both of those indices is water temperature.

The warmer the water, the higher the LSI or Ryznar, and the more likely it is that calcium carbonate will fall out of solution and harden. That’s what we call scale.

Have you ever noticed that carbonate scale forms first in heat exchangers and/or salt cells? Or how about on your shower head or faucet at home? Hot tubs scale way more often than swimming pools. For visible scale in the swimming pool, calcium will always harden in the warmest areas first. Think of the sunniest, darkest tile that gets the hottest. Or a spillway. You can probably picture it in your head. If not, we have included some photos.

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Before and after treating the pool with MSI and balancing the water to LSI standards. The scale was gone in two weeks.

How to prevent and remove scum lines


AAD enzyme, pool enzyme, tile scum preventionThe first thing is to address bather waste with something other than chlorine. We recommend a commercial-grade enzyme like our Amino Acid Digester (AAD). It circulates throughout the entire pool along with chlorine to break down and digest oils and other bather waste. It bubbles off carbon dioxide and effectively removes the oily waste from the water. Following our dosing recommendations for your pool should prevent scum lines from forming.

If carbonate scale is involved, circulating Metal & Scale Inhibitor (MSI) can prevent scale from forming in the first place.


Already have existing scum lines? If they are just grease related, using AAD in circulation and raising your water level can help remove them without labor. That said, normally they form at or right above the water line, due to evaporation loss and the wet-dry effect. So you may still need to clean the tiles. No problem, just use a good tile cleaner. If there is carbonate scale, you may want an acid-based tile cleaner for quick removal. Just like AAD, however, MSI can gently remove scale on your water line…but does so gently over time. It also needs to be soaking the scale, so scale above the water line will need manual cleaning.

Thanks for your time, and if you have any questions or need help, contact us.

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MSI: One very dynamic pool chemical

Meaningful pool care in more ways than one

Think of the most dynamic pool chemical you know of. What does it do? When we think of the word “dynamic”, it suggests multiple uses and a range of positive results. Metal & Scale Inhibitor (MSI), to us, is one very dynamic pool chemical. It serves many functions, including the ability to both prevent and undo common problems in swimming pools and other water treatment systems. Let’s share some of them.

1. Preventative water chemistry

Water chemistry affects everything in a water system. If your water is corrosive, it can cause damage to surfaces and equipment. Conversely, if it is scale-forming, the scale itself can cause damage too; pipes with scale in them can increase water pressure and reduce flow rates.

The most important factor in preventing corrosion and scale is LSI balance. After that, it helps to have a chelant like MSI to hold minerals and metals in solution. Take calcium carbonate scale, for example. If calcium is held in solution by MSI, it takes a much more for scale to form. Think of MSI as allowing more grace to your water chemistry.

It’s not that MSI prevents these problems from happening…it’s that it gives you, as an operator, an advantage in handling minerals and metals.

2. Protecting pool plaster

Pool plaster is most vulnerable while it is curing. Plaster curing can take 30 days more more while the surface hydrates underwater and hardens. Given that calcium chloride is often used as an accelerant in plaster mix, it provides available calcium to water that is probably calcium-deficient. Water will stop at nothing to reach equilibrium. As the universal solvent, water dissolves whatever calcium it can find and bring it into solution for its own benefit; the most readily available source is plaster or tile grout.

But what about vinyl liner pools, or fiberglass? The LSI still applies, but the difference is water has no source of calcium. Therefore, low LSI water corrodes its way through everything trying to find it. The damage can be severe over time. For vinyl liners, this can mean fading, wrinkling or even wearing through the liner itself, allowing water underneath it. It’s no good.

MSI provides more grace to the LSI. In effect, it seems to buffer the LSI acceptable range, making it more difficult to form scale, and more difficult to corrode. Of course, this buffering benefit has its limits. At some point, the LSI can be so far out of range that no amount of MSI can stop the inevitable consequences. Use MSI immediately upon filling (and refilling) a swimming pool. It gives you more time to adjust the chemistry to get in proper LSI range.

3. Dechlorination

With any sequestering agent or chelant, one of two things happens: it gets oxidized by chlorine, or it wipes out chlorine. MSI’s initial “purge” dose wipes out chlorine. In most cases, this is an annoyance; chlorine must be hand-fed for two or three days to catch up, because otherwise the pool is unsafe. Fortunately, the weekly maintenance dose of MSI is small enough that it hardly has any impact on chlorine levels. More importantly, the good news about MSI is it will not be oxidized out of the water by chlorine. That means it stays in the water and works to your benefit over a longer period of time. It’s just the initial concentration of the purge dose of MSI that overpowers chlorine…and when dosed properly and evenly circulated throughout the pool, chlorine levels can maintain just fine.

Dechlorination is sometimes needed, however. For example, if you need to do certain types of maintenance, or bring chlorine levels down from recent hyperchlorination, MSI can help. Rather than using sodium thiosulfate, MSI can accomplish more than just lower chlorine. After all, you get the benefits of this dynamic pool chemical while lowering the chlorine level to where you want it.

Realistically, dechlorination is a rare need for swimming pools…but sometimes it’s urgent. MSI can drop chlorine levels down in just minutes.

4. Chelating Metals, Preventing Stains

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

Metals are the first substances to be oxidized when chlorine enters the water. Oxidation can change the color of certain metals. Eventually, when the water is oversaturated with dissolved metals, or when they are oxidized, they can fall out of solution and stain. Just like with calcium saturation, water can only hold so much.

Stains come in many different colors, and there are entire businesses devoted to stain removal. We are not one of them…but again, MSI is a dynamic pool chemical.  Just like with calcium, MSI gives more grace to the water’s ability to hold metals in solution. With MSI in your water, metals have a far more difficult time coming out of solution and staining, because the metals are chelated.

Chelating metals means MSI isolates each metal ion and binds with them. Binding with metal ions prevents them from being oxidized.

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When stains originate from behind the plaster, the stains will continue to occur. MSI cannot prevent these types of stains.

Can MSI remove existing metal stains?

Over time, MSI has shown it can gently dissolve stained metals back into solution and lift them from the pool surface. We told you MSI is a dynamic pool chemical. Unlike acid-based products, however, this process is slow. You may want to combine its efforts with ascorbic acid (vitamin C) to speed up the process. That said, be sure that the stain in question is, in fact, caused by metals in the water. Some stains come from behind the pool surface itself, like rebar being too close to the surface. In those cases, MSI cannot prevent or permanently remove that problem.

5. Scale removal

Yes, just like removing stains, MSI can dissolve calcium back into solution as well. Of course, the long-term solution is LSI balanced water…but even then, hardened scale won’t move easily. MSI is the chemical to soften and remove scale deposits around your pool. It can even remove scale on spillways and water falls, as long as the MSI-treated water has time to affect the scale area. Raise your water level to remove scale on a tile line, for example.

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In a matter of weeks, MSI gently removed carbonate scale from this pool’s spillway. In addition, this pool used AAD enzymes. Look at the difference in water clarity!

Why use one dynamic pool chemical vs. multiple single-use chemicals?

Why not?