Phosphates are a vital nutrient that is used for plant growth. Though while great for nature, they can be extremely troublesome for the pool owner. Phosphates cause chlorine not to activate and sanitize therefore allowing algae to bloom at tremendous rates. If you have ever encountered this problem, then you know what I'm talking about. So you ask, "where do phosphates come from and how do they get in my pool?"
Water treatment facilities may add phosphate to tap water to prevent pipe corrosion and reduce concentrations of heavy metals in drinking water. Tap water is chemically treated with chlorine, soluble silicates, phosphate polymers and many other chemicals. So basically just by filling your pool you're adding phosphates.
Normally phosphate levels are below 125 ppm & are of no concern. Higher levels of phosphates can enter thru decaying plant matter, fertilizers, ground run-off, urine, and even sweat. No matter what route they take to get there, when you have them you will know. You will see your pool start to turn even though the chlorine level is outrageously high.
It is possible to remove moderate amounts of phosphates using a precipitating product. They can be rather expensive, but at times you have no other choice. The dosage level will vary from brand to brand, but from experience once the product is used the pool should start to clear overnight.
Extremely high levels of phosphates, 1000ppm & over, may render using a remover pointless. The cost would outweigh just draining the pool and starting over. Unless of course water replacement is not an option.
Need to test your pool for phosphates? Try this phosphate test kit.