Phosphates in pool water

May 23, 2016


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Has anyone ever told you about phosphates in pool water? Do you know what they are, or what they do to pool chemistry? For most people, the answer is no...including many pool professionals who have not had to fight phosphates before. After all, they did not seem to be an issue 10+ years ago, so what changed? This article is just a summary of information from multiple outside sources, simplified for you. We have another article that goes more in depth about different types of phosphates here.

What are phosphates?

We are talking about compounds that contain phosphorus. 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.”

inorganic phosphates, fertilizer phosphates, pool phosphates, phosphates, phosphate remover, eutrophication Fertilizers contain phosphates. With rain and wind, these phosphates often find their way into swimming pools.

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, as well as other pool chemicals (such as metal sequests, which are actually phosphate based).

Phosphates are not always a bad thing. Remember the 2015 water crisis in Flint, Michigan? Well it turns out that one of the ways the local health department is mitigating the problem is by adding phosphates to the water, so more scale develops to coat the inside of the lead pipes, thus reducing lead in the water.

Flint is not alone, as many other localities add phosphates to protect their water pipes from internal corrosion. As a result, if your pool is filled by public water, there’s a good chance you have phosphonic acid in your pool from the start. According to one source, however, less than 20% of US municipalities have a significant level in their tap water.

Related: Pillar 3: Phosphate Removal 

pool algae, green pool, orenda pool, orenda, phosphate remover, pr10000 Phosphates are food for algae, but removing phosphates doesn't necessarily kill algae.

Since phosphates are natural in the ecosystem, they will inevitably get into your pool water in many ways other than tap water: rain water/ground runoff, leaves in the pool, etc. For outdoor pools, these sources are 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.

We respectfully disagree.

Symptoms of High Phosphates

The little research available seems to suggest that phosphates do not affect water clarity much, if at all. In fact, many experts argue that phosphates are a non-issue if your pool is properly balanced. And yet, we know that high levels of phosphates can be problematic. One symptom is an increase chlorine demand...but it turns out that is an indirect result of phosphates being present, as explained by renown water chemistry expert Richard Falk. The relationship between phosphates and chlorine demand is better explained here.

What’s the big deal? Do phosphates even matter in pool water?

Most of the research suggests inorganic phosphates are not harmful to people; which is good news, considering local governments add phosphonic acid to our drinking water (as a scale and metal sequest). And plenty of online forums suggest they aren't anything to worry about, provided you have enough free chlorine. So why even worry about them?

Well, there are two sides of this. First, phosphates are a micronutrient needed for any microorganism. Removing phosphates does not prevent or kill algae (a sanitizer like chlorine or an EPA-Registered algaecide does). Phosphates do, however, can fuel the growth and reproductive rate, which is not a good thing. Second, 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 on the one hand, we have phosphates fueling the growth rate (bad), and on the other hand, we have a dependency on chlorine for just about everything. Are you sure your sanitizer's killing power is better and faster than the growth rate of algae?

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.



The phosphate topic keeps coming back to algae, because algae in swimming pools is a very prevalent problem. Phosphorous in various forms contribute to the eutrophication of water. It is especially a problem for lakes and ponds...but swimming pools can have eutrophication issues as well. 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

Removing phosphates takes out a key ingredient which can help slow the growth rate (but not prevent it). It's all about speed and chlorine efficiency in the sanitization game, so we want to give chlorine the best chance possible to prevent algae.

Operating costs

More chemicals means more costs, regardless of which chemicals you’re using. A healthy, balanced pool under regular maintenance may not have the need to address phosphates often, if at all. Phosphate removal products can get expensive if you use them regularly, so it all depends on the situation you’re in. If the pool is in a constant struggle, it’s okay to ask for help! Each pool is unique and should be treated as such.

Are phosphates worth testing for?

Yes, knowledge is power. Phosphate test kits readily available in pool supply stores, and they’re inexpensive and easy to use. It can’t hurt to know what you’re up against. Levels under 250ppb are considered low. 250-500ppb should be considered when balancing the rest of the chemicals, and 500ppb+ should be addressed directly. It is good practice to have your pool’s phosphate levels checked at least monthly. There is a reason that phosphate removal is our Third Pillar of Proactive Pool Care.

It may be in a pool owner/operator’s best interest to start the pool season with a treatment of PR-10,000, and end the season with it as well. Depending on the variables of your area, those two treatments alone may be plenty. If not, follow the dosing calculator on our website.

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