If you’ve got a pool with a persistent algae problem that doesn’t seem to respond to a reasonable amount of chlorine, it’s a good idea to test for both cyanuric acid and phosphates.
That’s because cyanuric acid reduces the kill rate of chlorine, while phosphates amplify the growth rate of chlorine.
If the phosphates are high (beyond about 500 ppb can become problematic) a phosphate remover can go a long way in slowing algae growth so that chlorine can work more efficiently.
The APSP’s 2009 National Standards offers the following justification for phosphate removal: Phosphorous is a non-metallic element and an essential nutrient for all living organisms, including bacteria and algae. Phosphate does not create a chlorine demand since the phosphate ion does not react with free chlorine. However, inadequate maintenance and sanitation can allow algae and bacteria to reproduce rapidly and utilize phosphorous as a nutrient. Under these circumstances, the multiplying algae or bacterial populations will cause a chlorine demand. In other words, phosphates are bad for pools and spas because they are a major nutrient for algae. Some pools that have high phosphate concentrations have difficulty controlling algae despite the chlorine.
Phosphates are typically removed from swimming pools with lanthanumbased chemicals. Some examples include lanthanum chloride, lanthanum sulfate, and lanthanum carboxylate. Added to water, an ion exchange reaction takes place. The lanthanide drops its counter ion in preference for the phosphate. Thus, lanthanum phosphate forms, a compound that is insoluble in water, and precipitates out of solution.
Service professionals caring for pools with high phosphate levels should note that when a phosphate remover is used, the water usually appears worse before it gets better. The water becomes cloudy because the product formed, lanthanum phosphate, is insoluble in water. But that is what the chemical is designed to do, and techs can tell homeowners to have patience with the filtration system before things get better.
