Phosphates in Pools

May 23, 2016

Phosphates in pool water have become a big 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? There is a surprising shortage of research on the topic available online, so we hope this article helps explain what's really going on. This article is just a summary of information from multiple outside sources, simplified for 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

So here's why they matter: phosphates are an essential micronutrient for microorganisms like algae.

Where do phosphates come from?



Pool chemicals like metal sequestering agents and scale inhibitors tend to be phosphate-based (but not ours). Adding these products 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. If you wipe out the sequestering agent, it will release its metals (or calcium), and stains and scale can occur afterward. Nevertheless, phosphate-based pool chemicals are a very common source of phosphates in pool water.


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:

“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.”

fertilizer by pool, phosphate in fertilizer

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.

Tap Water

Many localities add phosphates to protect their water pipes from internal corrosion. 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.

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 

Why do phosphates even matter?

Especially for outdoor pools, phosphates are virtually unavoidable. Despite being a nuisance, phosphates are rarely discussed in CPO courses, and to many experts, they are not even regarded as a problem. Algae is the problem everyone talks about...but the growth of algae is dependent upon micronutrients like phosphates (and nitrogen, carbon, etc.). 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).

Symptoms of High Phosphates

There is a common misconception about phosphates that even we at Orenda were taught to believe. The myth is that phosphates directly impact chlorine efficiency, or use up chlorine. That is not true. Chlorine and phosphates do not interact directly with each other. That being said, high phosphates (over 500 ppb) 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:


You can read more about this in our article about phosphates and chlorine demand. 

Algae and Sanitization

kill vs. grow, kill rate, growth rate, sanitization, phosphates, orendaThe most obvious symptom of high phosphates is algae. And this may seem weird, because if you have ever tested a pool with algae for phosphates, chances are the levels were low, if not zero. That's because the algae is containing the phosphates within their cell walls. Rest assured, there are phosphates in the water if you see algae.

Removing phosphates does not kill algae (a sanitizer like chlorine or an EPA-Registered algaecide does), but it 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.

On the killing side, how can we optimize our sanitizer? The idea of having enough free available chlorine (FAC) sounds great, provided it is truly free, and available. What is your CYA level? Do you have at least 7.5% of your CYA level in FAC? Do you have combined chlorine? How about your bather load? Metals? Other reducing compounds? Chlorine gets reduced rapidly when oxidizing things in swimming pools, but its primary function is sanitizationSanitization is all about the kill rate vs. the growth rate. So the action step is simple: supplement chlorine so you can optimize the killing rate.

On the other hand, we have phosphates fueling the growth rate. If we remove phosphates and keep them to a minimum, that growth rate can be minimized too. So optimizing sanitizer and removing phosphates is a viable one-two punch against outbreaks.


Phosphates are a micronutrient that fuels the growth of contaminants like algae. They are virtually unavoidable because phosphates are found in certain pool chemicals, tap water and natural sources. Removing phosphates can slow the growth and reproduction rate of algae, but will not kill or prevent algae from surviving. 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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