Phosphates and Algae

- you can’t have one without the other

Phosphates in your pool water are bad news because ortho-phosphate is the major algae food in pools

Algae can get by, and even proliferate, on just a few basics - food, light and water

When you’re an alga spore, the light and water are pretty easy to come by in a swimming pool but food is a bit more complicated because the pool-owner can control how much food is available

Cut off the food supply and algae cannot grow - it's that simple. Nitrogen and carbon are also important factors for the growth of algae but phosphate is the key ingredient  

Phosphates get into your pool: -

  • As a component of perspiration (an important reason to take a shower before swimming)
  • As ortho-phosphate added to the mains supply,  to control scaling and corrosion of metal pipes, by the water companies
  • In 'airborne fertilizer' (bird-poo!)
  • In plant material/debris that blows into your pool, including algae spores, leaves, grass clippings, etc.
  • Some wells, rivers and streams in intensively farmed areas can also have high 'complex phosphate' levels caused by run-off from fertilizers

Complex phosphate breaks down in water, through oxidation and hydrolysis, to form ortho-phosphate

If you have phosphates in the water you will certainly get algae if, at some point, the Chlorine level drops below optimum 1-3 ppm (for example if a child urinates in the pool, when all the available Chlorine gets used up in neutralizing ammonia in the urine)

If you have algae already there is definitely phosphate in the water

As algae needs phosphate to exist, algae itself contains high concentrations of phosphate compounds, organically bound

Chlorine, or an alternative sanitizer, kills the algae but the phosphate is released back into the water and is readily available as food for the next opportunistic algae spores that arrive in your pool

Phosphate levels can be measured quite accurately with the correct Phosphate Test Kit but, once a pool has turned green from an algae attack, the results of testing can be misleading. This is because there could not have been an attack of algae without high levels of phosphate in the water but by the time the water is tested the phosphates have been eaten by the proliferating algae and are now organically bound and therefore cannot be measured by the test. 

As mentioned above, if the algae is then killed by Superchlorination or the addition of Algaecide the phosphate is released back into the pool 

Unless the phosphate is removed the pool is vulnerable to another algae attack. But we KNOW now, don't we? - so we can do something about it

Removal of the phosphate is the best answer

Phosphate can be removed from a pool in 2 ways - by replacement of some or all of the water or by chemically stripping out the undesirable compounds

By replacing half the pool-water, something that ought to be done every few years anyway, and topping up from a phosphate-free source you will halve the phosphates (and the built-up Cyanuric acid) in your pool

This may be sufficient if levels of phosphate are low but it is better to remove all the phosphate if possible, by using chemicals to strip it out and lock it up

Chemical removal of phosphate is achieved by application of aluminium or lanthanum compounds

Aluminium compounds can strip phosphates down to about 500 parts-per-billion (ppb) but to achieve the ideal of under 100 ppb lanthanum carbonate is more effective

Applied via the skimmer, such compounds rest on the sand as crystals and slowly dissolve to coat the sand in a layer of phosphate-hungry lanthanum carbonate and lanthanum phosphate

This effectively removes the phosphates from the pool water and locks them away, Read the manufacturers instructions on the container for dosage rates

Remember - carry out a filter backwash BEFORE applying lanthanum compounds, or you may flush them to waste before they have fully dissolved

Jolly Gel

The simplest and most economic way to remove phosphates is to use Jolly Gel.  This amazing product cleans the water perfectly by removing the tiniest particles of suspended matter, and also strips phosphates from the pool.

Phosphate Starver

An alternative treatment, Lo-Chlor Phosphate Starver, is applied as a liquid (instead of crystals) directly into the skimmer and dilutes into the water very quickly

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Ken Walker - MyPoolGuru©