phosphates

Causes, Sources & Solutions Phosphates can increase the likelihood growth of algae in swimming pool water and can enter the water from such sources as:  leaves, grass and dirt,decaying plant matter, fertilizers, mineral treatment chemicals, contaminated well water, acid rain, contaminated soil, ground water runoff, bird droppings, bather wastes, urine and sweat.  Phosphate is a vital plant nutrient and the presence in swimming pool water, even at low concentrations, can cause accelerated algae growth.  Higher levels of phosphates can make algae control more difficult and increase the amount of sanitizer required to maintain satisfactory control of algae.  It is possible to remove modest levels of phosphates, by treating the pool water with a phosphate precipitating lanthanum product.   Very high levels may require so much of the lanthanum compound, as to render this approach impractical, unless there is no option of water replacement.   In order for phosphate reducing products to work the concentration of phosphates must be reduced to extremely low levels:  parts per billion.  The benefit of such product use is by adding another layer of protection against algae growth.  Proper pool water maintenance is always the first line of defense.   Testing for phosphates is not universal, but in those cases where algae control is proving difficult, despite apparently ideal pool water conditions, testing for phosphates and nitrates might be advantageous.  
Phosphate eliminators are based on a lanthanum (a rare earth element) compound.  Upon addition to swimming pool water, the lanthanum reacts with the phosphates in the water and precipitates them from solution.  The lanthanum reduces the phosphate concentration from parts per million to parts per billion.  Phosphates are a necessary nutrient for algae growth and their nearly total removal from the water can impede their growth.  The phosphate eliminators can be used with chlorine, algaecides and most pool chemicals.   Used properly and in conjunction with sanitizers, phosphate eliminators can add another layer of algae control.  They should be added periodically to keep the phosphate levels depressed, because swimmers and their associated wastes are a source of new phosphates.  A phosphate test can be performed occasionally to determine the need to add additional product. It is recommended twice a year unless other known problems are present.