Swimming Pool Blog

Phosphates and Nitrates in Swimming Pools

by Rob Cox, July 23, 2012
Phosphates and Nitrates in Swimming Pools

Phosphates and Nitrates are naturally occurring combinations of Phosphorous and Nitrogen with oxygen atoms. They are also produced synthetically for various industrial and agricultural products.

In lakes and ponds, a measurement of good health would be a proper level of Nitrates and Phosphates to assist in the growth of beneficial underwater plants and algae. Nitrates and Phosphates are one of aquatic plants main food sources. Add a little sunlight and Carbon Dioxide and you have a perfect recipe for a healthy lake or pond.

In swimming pools however, we don't like anything green growing. Swimming pools will always have a residual of Phosphates and Nitrates in the water, and roughly speaking, ppb (parts per billion) is OK, but ppm (parts per million) will give you trouble with algae, if you let your chlorine regimen slip now and then.

Phosphates and Nitrates need oxygen to form. Where do they get this oxygen? Many times, these sneaky petes are stealing the "O" from HOCl, which renders our killing form of chlorine inert. And, while they're doing this, they are filling the cabinets with tasty food for algae to enjoy. 


How do Phosphates enter the Pool? 

  • Windblown fertilizer
  • Soaps, lotions, creams
  • Mulch or soil run-off
  • Bird droppings
  • Ducks and Dogs
  • Sweat, Cosmetics, Urine
  • Contaminated fill water 
  • Leaf debris
  • Acid rain

How do I know if I have Phosphates in the Pool?

  • Difficulty maintaining a chlorine residual
  • Repetitive algae blooms
  • Test it ~ with a phosphate test kit.

How to Remove Phosphates from a Pool

  • Add a Phosphate remover containing Lanthanum
  • Remove and replace the water from a low-phosphate supply

Phosphates and Nitrates are in your pool, all the time, you can't see them or taste them, but they are there. To keep them at bay, keep your pool as clean as possible, and keep your water balanced, including a constant chlorine residual. Be careful with fertilizers as you apply them, and if your pool is prone to flooding from surrounding planter beds, do what you can to mitigate the possibility of overflowing into the pool.

I took care of a pool years ago that had a huge phosphate problem, primarily due to the regular mud-slide that would come into the pool nearly everytime it rained. At the time, I didn't know that it was a Phosphate problem, I thought it was an "Algae problem". When the idea of phosphates was introduced to the pool industry, we added a new product, called Phos-Free, and our algae problems were over. The mud-slide problem continued, unfortunately.