UK tap water can contain enough phosphate to feed algae and chew through sanitiser. Here is when it matters and how to remove it.
Many UK water companies dose orthophosphate into the mains supply to inhibit lead pipe corrosion. The phosphate that protects your kitchen taps is the same phosphate that feeds algae and bacteria in your hot tub. In most cases the level is too low to cause problems, but persistent green algae or unexplained sanitiser demand sometimes traces back to high phosphates.
Phosphate is a key nutrient for algae and bacteria. The same chemistry that lets pondweed thrive in farm runoff lets algae get a toehold in a hot tub if phosphate is high enough.
In the presence of high phosphate, even adequate sanitiser levels struggle to keep ahead of biological growth. The result is foggy water that does not respond well to shocking, or recurring algae episodes.
Phosphate test strips are inexpensive (around 8 pounds for a bottle of 50). Test at every refill.
UK tap water phosphate is typically 1 to 5 mg/L PO4. Above 200 micrograms (0.2 mg/L) in your hot tub is enough to feed algae actively.
Note that some water-treatment chemicals (especially certain off-brand stain and scale removers) themselves contain phosphate. If your hot tub phosphate has crept up despite stable refills, check the back of every bottle in your chemical cupboard.
Phosphate removers are lanthanum chloride or aluminium-based products. They bind phosphate into an insoluble precipitate that the filter catches.
Dose per the bottle, run the jets for 4 hours, then rinse the filter. Often two doses 24 hours apart are needed to bring high phosphate down.
For recurring problems in a phosphate-heavy supply, use a hose-end pre-filter that includes a phosphate-removal cartridge. They cost around 25 pounds and last a few refills.
Yes, in their annual water quality report. Many also publish it on their website by postcode.
Yes. It is dosed precisely for safety and is well below any health threshold. The hot tub issue is unrelated to drinking water safety.