Phosphates, Algae, and Chlorine Demand

Pool operators should know about phosphates, and have probably heard of nitrates…but little has been understood about what these elements actually do or why do they matter. High phosphates seem to weaken chlorine, as evidenced by low chlorine readings, lowered ORP, and the most visual of all evidence: algae. What’s really going on?


Chlorine demand

Chlorine’s vital role in our swimming pools is disinfection, and its secondary role is oxidation. According to the IPSSA Basic Training Manual (emphasis added):

The primary purpose of disinfection is to kill pathogenic (disease-causing) organisms such as bacteria, parasites, viruses and other organisms such as algae, mold, mildew and spores. The secondary purpose [of chlorine] is oxidation of swimmer waste such as sweat, urine, saliva, mucous and other bodily fluids. In addition, we need to oxidize or destroy all the man-made and natural creams, sunblock, soap, oils, deodorants, makeup, lipstick, lip gloss, hair spray, hair conditioner, styling gel, perfume, cologne, foot powder and baby powder to name a few.” — Chapter 5, pg. 55.

The important takeaway here is that chlorine has a lot to deal with. These obstacles can slow down the “killing speed” of chlorine. What really matters is the killing speed of chlorine compared to the growth and reproductive rate of organisms like algae. As an aside, chlorine also oxidizes nitrites into nitrates, which are a micronutrient--like orthophosphate--for organisms like algae. Nitrates are the dissolved form of nitrogen found in swimming pools, much like orthophosphates are the dissolved form of phosphorus. The IPSSA Intermediate Training Manual states:

"Nutrients are present in several forms in pools and spas including dissolved inorganic, dissolved organic, particulate organic and biotic forms. Only dissolved forms are directly available for algal growth: for nitrogen and phosphorus these include Ammonia (NH4), Nitrate (NO3-), Nitrite (NO2-), Orthophosphate (PO4-3), as well as dissolved Carbon Dioxide (CO2) and dissolved Silica (SiO2)"

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You think your pool has an algae problem? Behold, Lake Erie.

Growth rate vs. Kill rate

kill v grow boxing-1

When the growth rate of a microorganism exceeds the killing rate of chlorine, you are likely to have an outbreak. Like an algae problem. Living things need both nitrogen and phosphorus as micronutrients. In swimming pools and spas, nitrates provide the nitrogen, and orthophosphates provide the phosphorus. When ample micronutrients are present in the water, bacteria and algae can grow more quickly, and potentially grow faster than chlorine can kill them. We are fortunate to have spoken directly with renown pool chemistry expert, Richard Falk. He told us how phosphates affect water chemistry:

"The rate of chlorine kill is the same [with high phosphates], but the rate of growth can be slower with low phosphates. It's a race of reproductive growth that is faster with more phosphates (up to a limit determined by sunlight and temperature) vs. killing by chlorine.

This explains why chlorine demand tends to be higher with high phosphate levels in a swimming pool. It’s not that phosphates directly affect chlorine’s strength—like we originally were taught, and published—but rather they help fuel contaminant growth. Those contaminants are constantly being attacked by chlorine, which uses it up.

That needs to be repeated: phosphates have no direct relationship with chlorine strength. Rather, the difference phosphates cause is in the growth rate of contaminants like algae. That growth rate is what raises the chlorine demand, not the phosphates themselves.

Related: Phosphate Removal (Pillar 3)

Multiple generations of algae

pillar 3 phosphate removal, green pool, algae pool, PR-10,000, orenda phosphate removerVisualize a green pool, full of algae. It’s nasty. Do you see it in your mind? A pool service tech knows to shock a pool to kill the algae, and usually it works. But have you ever shocked the pool, and come back the next day to see it’s still murky and green, but you have no free chlorine? It can happen, and it’s actually not uncommon to come back to a zero chlorine reading shortly after shocking an algae pool. But how?

Algae holds its micronutrients (like orthophosphate and nitrate) within its cell walls. So when chlorine kills it, those nutrients are released back into the water. And in the right conditions, a new generation of algae can grow…only to be killed by residual chlorine.  And again, and again, and again. The IPSSA Basic Training Manual states: 

“Algae can double in population in about 3 to 8 hours. To prevent uncontrolled growth, the kill rate [of chlorine] must exceed the growth rate for bacteria or algae. Specifically, this means killing more than half of the bacteria or algae in the time that it takes to double in population.” — Chapter 5, pg. 65

So if algae can double in 3 to 8 hours, it can multiply several times each day. That’s quite a burden on chlorine…on top of everything else it has to do for the water. Let’s go back to Richard Falk:

"Phosphate removers should not be used to clear an existing algae bloom since the oxidation of algae will release additional orthophosphate. Use them after clearing algae from a pool."

In other words, a solid one-two punch to clear an existing algae bloom is to shock with chlorine, and follow it up with a phosphate remover like PR-10,000. Chlorine kills the algae, and phosphate remover wipes out the orthophosphate so the next generation does not have that critical nutrient to grow.

Orenda Procedure: Green Pool Cleanup

Chlorine is the best algaecide

When microorganisms reproduce faster than chlorine can kill them, an outbreak can occur. Nutrients like phosphorus (orthophosphate) and nitrogen (nitrate) allow microorganisms to grow. Depriving the water of those nutrients can slow growth, though it does not directly kill the organisms. Chlorine is a great algaecide, with the right free chlorine to cyanuric acid ratio. Back to Richard Falk:

"As with algaecides, [phosphate removers] are a preventative measure, but are not necessary if the appropriate minimum FC/CYA ratio is maintained as chlorine alone can kill algae faster than it can reproduce.”

This ratio, as discussed in several of our other articles here, here and here, is a 7.5% reduction factor. So if you have 100ppm cyanuric acid stabilizer, 7.5% of that is 7.5 ppm. That means you need free chlorine levels at a minimum of 7.5 ppm to keep algae at bay. Think about that. It’s no coincidence that most pools we see with algae have cyanuric acid over 50 ppm.


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If you experience higher chlorine demand when fighting algae and high phosphates, we hope this article simplified the science behind it. Phosphates are just one piece of the puzzle, and a critical piece indeed. Reducing them can be beneficial, especially when you're dealing with elevated levels of CYA.

If you are continually struggling with a high chlorine demand, contact us. It may or may not be algae or microorganisms causing it. It could be non-living organics and oils (especially in busy commercial pools). In any case, we are happy to try and help you figure out what's really going on. It's what we do.  

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WATCH: Orenda PR-10,000 concentrated phosphate remover in action!

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