I would be comfortable with 'Gwastraff Dŵr' - it works, though both are acceptable.
It is common, when discussing/labelling various bins and recycling receptacles to name them in such a way, for example...
Gwastraff gardd for the brown bin - garden waste
Gwastraff cyffredinol for the green bin - general waste
The same as in English, you could use both 'Water Waste' and 'Waste Water'.
Hope this helps.