Recently, at my place, a sort of filter for hard water was installed. They told me that it contains sodium triphosphate which is dissolved into all the water that enter home, i.e. both cold and hot water to prevent calcium carbonate to accumulate into the plumbing system, dishwasher, etc.... From what I can understand, there is no way this device is considering the flow rate in order to decide how much sodium goes into the water. It just happens.

According to what I googled, sodium triphosphate is used as an additive in many foods and, even though is not considered harmful, high levels may lead to health risks. Also, I have found some links (not in english) stating that such devices should be installed in the hot water plumbings since sodium triphosphate and calcium carbonate interact only at certain temperatures. According to these sources, drinking cold water is not harmful since no chemical reaction took place (but still sodium is present in the water, I believe) and, also, once I use the same cold water for cooking it gets hot and a reaction may take place.

I am quite puzzled and I don't know what to do with this filter. I can move it to hot water only, or I can keep it where it is (both cold and hot) if it's not harmful but I couldn't find any clear information. Is there something that may help?

EDIT 1: this is the exact product I am using.

EDIT 2: its chemical formula is Na5 P3 O10

EDIT 3: this seems to be the device that was installed and this is one of the components that is taking care of the dosing (that is equal to 4-5ppm of sodium triphosphate, apparently)

  • $\begingroup$ How is this filter installed ? Is it on a bypass of the main pipe ? How is the flow controlled ? Trisodium phosphate is a bit soluble in water. So, without control, it should be quickly dissolved and eliminated. $\endgroup$
    – Maurice
    Jan 13, 2021 at 12:28
  • $\begingroup$ it is installed at the very entrance of the water inside the house and, as far as I know, the flow is not controlled in any way. Also, If it is quickly dissolved...what is the purpose of having it in the first place? $\endgroup$
    – gabt
    Jan 13, 2021 at 12:45
  • $\begingroup$ If the sodium triphosphate is in big chunks, it will not be quickly dissolved. But I am afraid it is not sodium triphosphate. By reading your reference, it is sodium polyphosphate. There are several sodium polyphosphates. For example, the sodium tripolyphosphate is poorly soluble and is known to be useful for removing calcium ions from tap water. It is not a dangerous substance. You can maintain it in your filter. It will make a complex ion with calcium, so there will not be any calcareous deposits in your tubing and other equipment. It seems to be a good idea. $\endgroup$
    – Maurice
    Jan 13, 2021 at 14:49
  • $\begingroup$ @Maurice I added the exact chemical formula! $\endgroup$
    – gabt
    Jan 13, 2021 at 14:55
  • 1
    $\begingroup$ @Maurice Sodium triphosphate: Sodium triphosphate (STP), also sodium tripolyphosphate (STPP), or tripolyphosphate (TPP) ..... Not to be confused with Trisodium phosphate Na3PO4. Note that sodium diphosphate is not Na2HPO4 either, but Na4P2O7. $\endgroup$
    – Poutnik
    Jan 13, 2021 at 15:17

1 Answer 1


It appears to be safe, provided that you are putting it into one of the delivery systems that it’s made for which I found through the link you provided.

Here is one of the delivery systems:


Per the specs, it delivers 3ppm, which means that you’re only introducing 1ml per 333L of water, which is approximately what the average person uses per day.

Contributing to phosphate load shouldn’t be a concern as someone (with “normal” diet &renal function) has a concentration of 10-300ppm of phosphorus in their urine!

This company also sells water filters for regionally specific water issues like high arsenic, and water softeners with different ion exchange resins based on the ions present in the water. They appear to have a solid understanding of drinking water chemistry, but it’s your domestic water, not mine, so caveat emptor

  • 1
    $\begingroup$ Hm, but 3 ppm can hardly make any difference for hard water. E.g. if we have a hard water 25 dGH ( old unit, old school ), it means 250 mg of CaO equivalent. And as 1 CaO is equivalent to 1 Na5P3O10, forming complex, it is approx 250*(5*23+3*31+10*16)/(40+16)=1640 mg of Na5P3O10, so 1640 ppm. It would be less, as there is no need to cover all calcium. $\endgroup$
    – Poutnik
    Jan 14, 2021 at 7:11
  • $\begingroup$ ok! so yes, I checked which device is the one I have installed and edited the question accordingly. I haven't thought about checking on it. $\endgroup$
    – gabt
    Jan 14, 2021 at 8:28
  • $\begingroup$ also, I wanted to add that you considered 333l of water per day but we only use roughly 7/8l for actual drinking and cooking hence the amount of phosphorus should be even lower, right? $\endgroup$
    – gabt
    Jan 14, 2021 at 8:38
  • $\begingroup$ @Poutnik yes, agreed this won’t do much as a primary water softener. The website mentions that it can be installed after the water softener to reduce scaling in pipes as water softeners aren’t 100% effective. $\endgroup$ Jan 14, 2021 at 19:21
  • $\begingroup$ @gabt that 333L is average household use for ALL water, including washing & showering to understand how much phosphate was being introduced to the waste water as far as environmental impact is concerned. You’re only consuming a trace amount from the water you drink added by the system as I imagine most of it is reacting with the water before it leaves the tap. $\endgroup$ Jan 14, 2021 at 19:33

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