De or het drainering?

The correct Dutch article is:

de drainering

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

How to use drainering

the draineringde drainering
this draineringdeze drainering
that draineringdie drainering
our draineringonze drainering
your draineringjouw drainering
every draineringelke drainering
which drainering?welke drainering?
a big drainering / the big draineringeen grote drainering, de grote drainering