Calcium Phosphate Scale

For over two years, pool chemistry expert Eric Knight has been researching one of the most misunderstood topics in pool maintenance: calcium phosphate scale. What he discovered challenges nearly everything the industry thinks it knows about those mysterious white flakes in salt cells and stubborn deposits in pool heaters.

If you've ever been told that phosphate remover will solve your scaling problems, or that those flakes in your saltwater pool are calcium phosphate, prepare to have your assumptions turned upside down. The reality is far more complex—and far less common—than most pool professionals realize.

The Calcium Phosphate Confusion: What We Think We Know vs. Reality

The pool industry has long perpetuated a myth that's costing pool owners time, money, and unnecessary chemical treatments. As Eric Knight explains in this revealing episode:

There is a prevailing belief in this industry, and it's still on one of our competitors' websites, that if you have flakes in your saltwater pool, meaning your salt system is scaling up, that it's calcium phosphate. I have great news for you. There is a very, very, very, very, very, very low probability that that's the case.

— Eric Knight, Rule Your Pool Podcast

This misconception has led to widespread misdiagnosis of scaling problems. Pool professionals routinely recommend phosphate removers to address what they believe is calcium phosphate scale, when in reality, they're dealing with calcium carbonate—a completely different beast that requires different treatment approaches.

The confusion stems from a fundamental misunderstanding of what calcium phosphate actually is and under what conditions it forms. Unlike calcium carbonate scale, which forms based on the Langelier Saturation Index (LSI), calcium phosphate operates under entirely different rules.

The Science Behind Calcium Phosphate: It's Not Just One Thing

One of the biggest revelations from Knight's research is that "calcium phosphate" isn't a single substance—it's a broad category encompassing dozens of different mineral combinations. The research revealed an overwhelming variety of forms:

Calcium phosphate is a broad term to talk about a whole bunch of mineral ion pairs that contain both calcium and some form of phosphate. I'm going to rattle them off... There's a group called apatite, hydroxyl apatite, also called hydroxyapatite, chlorapatite, fluorapatite, tetracalcium phosphate, monocalcium phosphate... You get the point. It goes on and on and on and on.

— Eric Knight, Rule Your Pool Podcast

The most common form found in swimming pool applications is hydroxyl apatite, discovered through actual samples extracted from pool heaters. This finding came from collaboration with industry experts Q Hales from On Balance and chemist Richard Falk, who provided crucial real-world data that made this research possible.

What makes calcium phosphate particularly problematic is its incredible hardness. Unlike calcium carbonate flakes that you can literally crush between your fingers, calcium phosphate formations can be harder than concrete. This structural difference explains why some scaling problems seem impossible to resolve with traditional methods.

The Extreme Conditions Required for Calcium Phosphate Formation

Perhaps the most eye-opening aspect of Knight's research is revealing just how extreme the conditions must be for calcium phosphate to actually form in pool water. The chemistry isn't just demanding—it's almost prohibitively so for typical residential pools.

The formation depends on a complex formula involving calcium hardness, temperature, pH, and most critically, phosphate levels. Knight breaks down a real-world example:

Let's take a pool, Jared, that's 82 degrees in the summertime... 350 calcium hardness... And an 8.0 pH, which is very common... You would have to have 13.3 parts per million of phosphates. In other words, 13,300 parts per billion of phosphates.

— Eric Knight, Rule Your Pool Podcast

To put this in perspective, most pool professionals consider 1,000 parts per billion of phosphates to be high. The levels required for calcium phosphate formation are more than ten times higher than what's typically considered problematic for algae growth.

The situation becomes slightly more favorable for calcium phosphate formation under commercial pool conditions—higher temperatures, higher calcium levels, and the use of phosphate-based chemicals. Even then, we're talking about phosphate levels in the 6,000-11,000 parts per billion range, which are extraordinarily high by any standard.

Where Calcium Phosphate Actually Occurs (Hint: Not in Your Salt Cell)

Despite widespread belief to the contrary, calcium phosphate scale is virtually never the culprit behind salt cell problems. Knight's research, backed by years of field experience, reveals the real locations where this type of scaling occurs:

It is very unlikely to deposit in a salt cell. We have never seen it happen, ever. And we get calls about flakes on the hotline... People keep posting on Facebook. What are these flakes? Somebody very credible in the industry just posted it within a month, too, saying, hey, that's calcium phosphate. Again, common misperception.

— Eric Knight, Rule Your Pool Podcast

The actual locations where calcium phosphate forms tell the story of why temperature is so critical:

• Pool heaters: The most common location, where water temperatures can reach 150°F for brief periods
• Sand filters: In extreme cases, deposits can become so hard that the filter tank must be cut open to replace the media
• High-temperature commercial pools: Therapy pools and swim schools that maintain water temperatures in the 88-90°F range
• Industrial applications: Cooling towers and boilers where temperatures reach 212°F

The temperature factor cannot be overstated. The higher the temperature, the lower the phosphate threshold for precipitation. This is why calcium phosphate problems are virtually unknown in typical residential pools but can occur in specialized high-temperature applications.

The Real Culprit: Why Your Salt Cell Flakes Are Probably Calcium Carbonate

If it's not calcium phosphate causing those frustrating flakes in your salt cell, what is it? The answer is much simpler and more manageable: calcium carbonate. This familiar form of scale operates according to well-understood LSI principles and responds to traditional treatment methods.

Calcium carbonate flakes in salt cells typically result from:

• High LSI readings indicating scale-forming water conditions
• Localized high pH conditions near the electrode plates
• Temperature fluctuations that affect water chemistry balance
• Inadequate water circulation through the salt cell

The good news is that calcium carbonate responds well to acid cleaning, can be prevented through proper LSI management, and doesn't require the extreme chemical intervention that calcium phosphate would demand.

Practical Prevention and Treatment Strategies

Understanding the true nature of calcium phosphate leads to much more effective prevention and treatment strategies. Here's what pool professionals and owners need to know:

For Calcium Carbonate (the likely culprit):

• Monitor and maintain proper LSI balance
• Regular acid cleaning of salt cells
• Ensure adequate water circulation
• Address pH drift promptly

For Calcium Phosphate (the rare occurrence):

• Monitor phosphate levels in high-risk situations (heated commercial pools using phosphate-based chemicals)
• Maintain lower calcium hardness levels in high-temperature applications
• Consider non-phosphate alternatives for scale inhibition
• Understand that once formed, calcium phosphate may require mechanical removal

The key takeaway is that prevention is infinitely easier than treatment when it comes to true calcium phosphate scale. Once formed, it may require grinding, jackhammering, or extended acid treatment to remove.

The Industry Impact: Rethinking Common Practices

This research has significant implications for how the pool industry approaches scaling problems. The widespread misdiagnosis of calcium carbonate as calcium phosphate has led to:

• Unnecessary phosphate remover treatments
• Frustrated customers whose problems persist despite "correct" treatment
• Missed opportunities to address actual LSI-related scaling issues
• Confusion about the effectiveness of various treatment approaches

Pool professionals who understand these distinctions can provide more effective service, save customers money on unnecessary treatments, and build trust through accurate problem diagnosis.

The research also highlights the importance of industry collaboration and scientific rigor. Knight's work involved reaching out to material scientists, medical doctors, and chemistry researchers—professionals who normally have nothing to do with swimming pools but whose expertise proved invaluable in understanding the underlying science.

Conclusion: Knowledge is Power in Pool Chemistry

The calcium phosphate story is ultimately about the power of proper diagnosis in pool chemistry. By understanding what calcium phosphate actually is, under what extreme conditions it forms, and where it's likely to occur, pool professionals can make better decisions and provide better service.

For the vast majority of pool owners dealing with scaling issues, the solution lies in traditional calcium carbonate management through LSI control and regular maintenance. The rare cases of true calcium phosphate scale require recognition of the extreme conditions involved and appropriate mechanical or chemical intervention.

Most importantly, this research demonstrates that even well-established industry "knowledge" deserves periodic examination. Sometimes the most valuable discoveries come from questioning what everyone assumes to be true and diving deep into the actual science behind our daily practices.

The next time you encounter scaling problems in a pool, remember: it's probably not calcium phosphate, and that's actually good news for everyone involved.

Episode Chapters

• 00:00 Introduction to Calcium Phosphate Scale
• 05:00 The Many Forms of Calcium Phosphate
• 10:00 Common Industry Misconceptions
• 15:00 The Math Behind Calcium Phosphate Formation
• 25:00 Where Calcium Phosphate Actually Occurs
• 35:00 Prevention and Treatment Strategies