Phosphates in Pools

Phosphates in pool water have become a common problem, yet the topic is largely misunderstood or misrepresented. Do you know what phosphates are, where they come from, and why they cause issues for pool chemistry? If not, this article will simplify information from many outside sources to explain it to you. Phosphates are a significant enough problem that we made Phosphate Removal our Third Pillar of Proactive Pool Care.

What are phosphates?

Phosphates are types of phosphorus commonly found in water. There are many different types of phosphates present in swimming pools, yet our test kits usually only test for one type: orthophosphates. Phosphates are a misunderstood issue in pool chemistry because 1) they are difficult to test for, and 2) they cannot be removed by chlorine alone. In fact, chlorine does not even interact with phosphates. To remove phosphates, you will need a phosphate remover

Where do phosphates come from?


Nature and the environment surrounding the pool

When compared to other topics in pools, there is surprisingly little scientific detail about phosphates available online. But there are some reliable sources. According to the Water Research Center:

fertilizer by pool, phosphate in fertilizer

“Phosphates exist in three forms: orthophosphate, metaphosphate (or polyphosphate) and organically bound phosphate. Each compound contains phosphorous in a different chemical arrangement. These forms of phosphate occur in living and decaying plant and animal remains, as free ions or weakly chemically bounded in aqueous systems, chemically bonded to sediments and soils, or as mineralized compounds in soil, rocks, and sediments.”

Translation: phosphates are found in soil, which is a combination of broken down rocks (with phosphorus in them), and decaying plant and animal remains. They are prevalent in fertilizers too. When wind and rain runoff introduces these soils into our water, phosphates are included in that. Another source can be leaves from trees, grass clippings, and other natural debris.

Pool chemicals

Sequestration vs. ChelationPool chemicals like metal sequestering agents and scale inhibitors tend to be phosphate-based (but not ours). The products work well for preventing stains and carbonate scale, and are common in the swimming pool business. Adding these sequestering agents to our pools is basically adding liquid phosphates. And if you also use a phosphate remover, those products conflict, as the phosphate remover will be used up removing the sequestering agent. You pay for both products, and they cancel each other out. And when you wipe out the sequestering agent, it releases its once-bound metals and minerals, and stains and scale can occur afterward. Nevertheless, phosphate-based pool chemicals are a very common source of phosphates in pool water. Which brings us to the most common source of phosphates in pools...

Tap Water

Many localities add phosphates to protect their water pipes from internal corrosion and/or scale formation. This is the same concept as using phosphate-based sequestering agents in pools...except it is done at a much larger scale at the water treatment plant. As a result, if your pool is filled by public water, there’s a good chance you will have some phosphates in your pool from the start (and every refill). We have seen phosphate levels way over 1000 ppb out of city water sources before. Some treatment plants use sequestering agents, and others just add orthophosphates directly. It depends on what the drinking water facility is trying to overcome.

Wells also are notorious for high phosphates. Think about it...the water table means water percolates through the soil and down into the groundwater, which is where the well draws from. 

Related: Pillar 3: Phosphate Removal 

Do phosphates really matter in pools?

The short answer is yes, phosphates matter. In all pools, but especially for outdoor pools, phosphates are inevitable. Obviously some pools have more than others based on the factors listed above, but keeping a pool phosphate free is not practical. We simply want to keep the levels low enough to not become an issue.

There is a common myth about phosphates that even we at Orenda were taught to believe (and we published it too...sorry...but don't worry, we corrected it and re-filmed the video too). The myth was that phosphates directly impact chlorine efficiency by dissociating Hydrogen from HOCl and making it the weaker Hypochlorite Ion (OCl-). We learned the hard way, several weeks later, that is not true. Chlorine and phosphates do not interact directly with each other at all, and phosphates do not reduce chlorine.

Despite being a nuisance, phosphates are rarely discussed in CPO courses, and to many experts, they are not even regarded as a problem. Perhaps this is because chlorine does not oxidize phosphates, and it takes a specialty chemical to remove them. The symptoms–or consequences–of high phosphates are what people tend to focus on. Primarily, algae. According to renown chemistry expert Richard Falk, algae growth is directly correlated to phosphates, and can be mitigated by regularly removing phosphates to keep their level to a minimum (below 500 ppb).

To be clear, simply removing phosphates does not kill algae. In fact, Orenda does not make any product that kills algae or anything else. Such sanitizers and algaecides must be EPA registered. 

Symptoms and consequences of high phosphates in a pool

Phosphates over 500 ppb, again, according to Falk, can fuel optimal growth and reproduction of microorganisms like algae. It is the reproducing organisms that chlorine gets used up fighting against. So while it may appear that high levels of phosphates coincide with high chlorine demand, it is actually an indirect relationship. This video explains more:


Chlorine demand and algae

kill vs. grow, chlorine kill algae, growth rate, sanitization, phosphates, orendaSay you're struggling to keep up with high chlorine demand. Normally, the largest contributor to chlorine demand is the oxidant demand, and the second highest is the sanitizer demand. In other words, most contaminants are non-living organics and nitrogen compounds, followed by reproducing living contaminants (sanitizer demand) like algae. Both reduce chlorine, but the difference is oxidants do not reproduce. Algae do.

This is why most attempts to clean up a green pool wind up with the pool consuming a huge amount of chlorine and taking the free chlorine down to zero overnight, and the pool might not even be cleared yet. The reason for that is when the nutrients are available in the pool, algae can be killed and new generations can reproduce every few hours. So you didn't just kill one generation of algae when you dumped in that chlorine and algaecide. You killed multiple generations until the chlorine was depleted. Our Green Pool Cleanup procedure is gaining popularity because we don't just shock the pool with chlorine, we remove the phosphates too.

Again, to reiterate: removing phosphates does not kill algae, nor does it completely prevent algae. Phosphate removal just helps minimize the growth rate. Sanitization is essentially a battle between the kill rate (of the sanitizer) and the growth/reproductive rate of the contaminant. The Orenda Four Pillars of Proactive Pool Care try to address both sides of this equation.

When the kill rate of the sanitizer is faster than the growth and reproductive rate of the microorganism, the sanitizer can stay ahead. When the growth rate exceeds the kill rate, you can get an outbreak.

Calcium phosphate

calcium phosphate blocksCalcium phosphate (Ca3(PO4)2) is a type of scale that can sometimes form in pool systems (usually heaters and filters). It can occur with very high levels of phosphates (over 5000 ppb), high calcium hardness, and high pH. In other words, it's kind of like a phosphate-enriched form of carbonate scale, though it forms in a different way. We have another article about it here. Calcium phosphate scale hardens more dense than carbonate scale, and can turn sand filters into what feels like concrete. On numerous occasions, we have been called about it, because it can take a jackhammer to break apart the sand. See the photo of the calcium phosphate boulders of sand from a commercial pool filter.

To give you an idea of how high your phopshates need to be for calcium phosphate scale to precipitate, this formula from Richard Falk helps us solve for the phosphate level needed:

PO4 = 10^[11.755 - log(CaH) - 2log(t) - (0.65 * pH)]

In his example, the pool has 375 ppm calcium hardness, 7.5 pH, and 80ºF (26.67ºC). In that scenario:

10^[11.755 - log(375) - 2log(26.67) - (0.65 * 7.5)] = 28.44 ppm = 28,440 ppb phosphate

As you can see, in fairly normal pool chemistry, you would need a LOT of phosphate in your water. Of course, you could have much higher calcium hardness, or higher temperature (in your heater, for instance), and a higher pH (like in a salt cell, though calcium phosphate in salt cells is rare). We're not chemists, but in our experience, we have seen this quite a bit, but it's almost always in commercial pool filters and heaters. Perhaps pressure and or extra time help precipitate it, but we don't know for sure. 


Phosphates are an essential nutrient that fuels the growth of living contaminants like algae. They are virtually unavoidable because phosphates are found in certain pool chemicals, tap water and natural sources. While many people think phosphates are no big deal, we disagree, because we can dramatically simplify pool care by removing them from water, and keeping the levels below 500 ppb. Consider phosphate removal as part of your pool maintenance routine.

BONUS: Eutrophication

Have you ever seen a pond or lake consumed with algae? This occurred because there was an overabundance of phosphorus and other micronutrients in the water, allowing for optimal algae growth. Eventually the algae takes over, and the condition is called eutrophication. Eutrophication, as defined by the Dictionary is:

"The process by which a body of water becomes enriched in dissolved nutrients (such as phosphates) that stimulate the growth of aquatic plant life usually resulting in the depletion of dissolved oxygen" - Merriam-Webster Dictionary


Yes, that's Lake Erie. And yes, the green is algae.

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