De or het waterhardheid?

The correct Dutch article is:

de waterhardheid

Rule Words ending in -heid: almost always de. See the rule

How to use waterhardheid

the waterhardheidde waterhardheid
this waterhardheiddeze waterhardheid
that waterhardheiddie waterhardheid
our waterhardheidonze waterhardheid
your waterhardheidjouw waterhardheid
every waterhardheidelke waterhardheid
which waterhardheid?welke waterhardheid?
a big waterhardheid / the big waterhardheideen grote waterhardheid, de grote waterhardheid