When a pool flips green, and is loaded with algae, usually there is a tried-and-true clean up procedure pool pros fall back on. Green pools, after all, are not uncommon. Algae has taken over for who knows how long, and now the pool looks like a swamp. But if you have ever tested for phosphates in a green pool, chances are the levels were very low, if not zero.
Where are the phosphates?
Phosphates are essential micronutrients for algae to grow and reproduce...so there must be phosphates in the water of a green pool. Right? Then why are they not showing up on the phosphate test? The answer is simpler than you might imagine.
Phosphates are consumed by algae, and held within its cell walls.
When algae is present in water, phosphate levels seem lower, because so much of them are already consumed. That being said, when algae is killed by chlorine (or an EPA-Registered algaecide), the phosphates and other micronutrients are released back into the water. That's why simply shocking a green pool is not enough to conquer it sometimes. You could come back to water with a low (or no) chlorine reading the next day, because chlorine has been used up killing generation after generation of algae.
Related: Phosphate Removal (Pillar 3)
Test kits usually measure orthophosphates
There are many different types of phosphates that can be present in pools. Test kits usually only measure orthophosphates, which can also contribute to why readings can be so low. Monophosphorus and Polyphosphorus, for example, are not reactive to common tests in the aquatic industry, so they’re invisible to operators. That being said, in the presence of an oxidizer and/or direct sunlight, mono- and polyphosphorus can break down into orthophosphate over time.
We should repeat that. Your phosphorus levels may be higher than what your test kit says, because only orthophosphates show up. But the other types of phosphates still matter! As evidenced in the photo below, this green pool is being dosed with PR-10,000 phosphate remover, and the reaction is obvious.
Within seconds, PR-10,000 reacts strongly with phosphates in this swamp-green pool.
Just look at how quickly PR-10,000 clouds up in that green pool. If there were really no phosphates present, there would be no cloudiness. So not only does algae hold phosphates within its walls, but test kits themselves may also be misleading.
Test kits have their limits
Another thing about phosphate tests is their limitations. Many phosphate tests go up to 1000 parts-per-billion (ppb) for orthophosphate. The water turns blue…and the bluer it gets, the more orthophosphates you have. Well what if your test vial's water is darker blue than the highest 1000 ppb color? How blue is it? Believe it or not, this happens often, and when it does, you have no way of knowing how high your phosphate levels actually are. What if it’s 5000 ppb, or 10,000? Regardless of the severity of your phosphate level, it remains unknown. You just know there's at least the amount shown.
Note: there is a way to test for higher phosphates, but it involves math and pure bottled water. Expand the water sample by diluting the pool water, in an attempt to get your sample color within the color scale on the test kit. Take pure bottled water, and mix with the pool water at a 10:1 ratio (10 parts bottled water, 1 part pool water). Test that sample, and whatever it reads, multiply the result by 10.
But honestly, we don't even bother with the hassle anymore. We used to promote regular phosphate testing, just like we encourage accurate water testing of all water chemistry, both in the pool and from the source water. With phosphates however, because the tests are notoriously difficult and can be flawed, there's any easy way to know if you have enough phosphates to treat.
Pour a cap full of PR-10,000 into your water, and count to 30 seconds. If there is a cloud that continues to grow, there are enough phosphates in your water to justify reducing them.
This method takes the guess work out of test kits. You either have phosphates or you don't. PR-10,000 reacts with all types of phosphates, not just orthophosphates. And keeping phosphates to a minimum is very beneficial and proactive. That's why phosphate removal is our Third Pillar of Proactive Pool Care.
Green pools have very high pH
Another thing that exceeds most test kits is pH. Green pools that are eutrophied and taken over by algae have an extraordinarily high pH, because algae has consumed so much carbon dioxide. Because so much less CO2 is in solution, the pH goes up. This is no different than off-gassing CO2 from aeration, except that algae can take the pH above 10, whereas aeration has its limits thanks to physics.
Remember that phosphates get into our water primarily three ways:
- Chemicals (phosphate-based sequestering agents)
- Naturally (soils, nature, people, wind and rain runoff)
- Tap Water (city water and well water often have phosphates in them)
If you are using phosphate-based pool chemicals, those products contribute to the phosphate level itself. Our philosophy at Orenda is one of minimalism. Proactive pool care, with no chemical conflicts or long-term byproducts. In other words, our chemicals contain no phosphates at all, and can be used in harmony with one another. Avoid the chemical conflicts, and handle the root cause of problems.
To prevent algae, keep phosphates to a minimum, and optimize your sanitizer by supplementing chlorine to handle its oxidant demand. Furthermore, you need approximately 7.5% of your CYA level in free available chlorine to stay ahead of algae. If you do all those things, you should never get algae in the first place. But if you do, you can try our Green Pool Cleanup Procedure to regain control of your pool.
The next day...