Draining potable water is normal procedure on our planes here in germany in winter. Kinda dangerous during the walkaround next morning on the huge ice patch below the tail of the plane...
Years and years ago, maybe 20, that practice was banned at Manchester and we had to drain into a tank.
Towards the end of my career, in Maintrol, there was a succession of phone calls from the outstations,every night when a frost was forecast "do you want it draining tonight ?" Buggered if you did, buggered if you didn't