By The Zagosa Team. Published .
Reviewed or updated .
Plenty of UK water companies add orthophosphate to the mains to hold back corrosion in lead pipes. The phosphate protecting your kitchen taps is the very same phosphate that feeds algae and bacteria in your hot tub. Most of the time the level is too low to matter, but stubborn green algae or unexplained sanitiser demand can sometimes be traced back to high phosphates.
Why phosphates are a problem
Phosphate is a prime nutrient for algae and bacteria. The chemistry that lets pondweed run riot in farm runoff lets algae get a grip in a hot tub once phosphate climbs high enough.
With phosphate running high, even a sensible sanitiser level can struggle to stay ahead of biological growth. The result is hazy water that shrugs off shocking, or algae that keeps coming back.
Testing your level
Phosphate test strips are cheap, around 8 pounds for a bottle of 50. Test at every refill.
UK tap water phosphate typically runs 1 to 5 mg/L as PO4. Anything above 0.2 mg/L (200 micrograms) in your tub is enough to actively feed algae.
Watch out too for water-treatment products, especially some off-brand stain and scale removers, that contain phosphate themselves. If your tub's phosphate keeps creeping up despite steady refills, read the back of every bottle in your chemical cupboard.
Getting rid of it
Phosphate removers are lanthanum chloride or aluminium-based. They lock phosphate into an insoluble precipitate that the filter then catches.
Dose per the bottle, run the jets for four hours, then rinse the filter. High phosphate often needs two doses 24 hours apart to bring it down.
For a phosphate-heavy supply that keeps causing trouble, fit a hose-end pre-filter with a phosphate-removal cartridge. They cost about 25 pounds and last a few refills.
FAQ
Will my water company tell me the phosphate level?
Yes, in their annual water quality report, and many publish it on their website by postcode too.
Is high phosphate water safe to drink?
Yes. It is dosed precisely for safety and sits well below any health limit. The hot tub issue has nothing to do with drinking water safety.