White sediment issue

[forgetaboudit]

So I'm getting my new to me keggle ready for my first biab.
I have boiled water in it to test, but it keeps leaving a white sludge/sediment at the bottom. I've scrubbed it down with PBW with no changes.
I'm hoping its not something going on with the keg that'll cause me to not be able to use it.

Any ideas to what the cause is, or what the "stuff" is?
Ill post photos when I get home.

 

[bucketnative]

Does this sound like what you're seeing: https://www.homebrewersassociation.org/forum/index.php?topic=5792.0

Decarbonation followed by CaCo3 precipitation? Is your water particularly hard?