De or het waterontharding?

The correct Dutch article is:

de waterontharding

  • Plural: de wateronthardingen
  • Diminutive: het wateronthardinkje
Rule Words ending in -ing: almost always de. See the rule
Meaning (Dutch) het proces waarbij calcium- en magnesiumionen (kalk) uit leidingwater worden verwijderd, Waterontharder - Wikipedia.

How to use waterontharding

the wateronthardingde waterontharding
this wateronthardingdeze waterontharding
that wateronthardingdie waterontharding
our wateronthardingonze waterontharding
your wateronthardingjouw waterontharding
every wateronthardingelke waterontharding
which waterontharding?welke waterontharding?
a big waterontharding / the big wateronthardingeen grote waterontharding, de grote waterontharding
the waterontharding (plural)de wateronthardingen, deze wateronthardingen

Meaning, plural, diminutive, pronunciation and translations: WikiWoordenboek (Dutch Wiktionary) (CC BY-SA 4.0). Example sentences: Tatoeba (CC BY 2.0 FR).