Actually, in anything 1960 or later, it flushes to a tank.
Prior to that, it vented overboard, and generally flash-froze and the liquids evaporated. Solids froze and fell. IN fact, prior to the development of the Nuclear-Tipped Missile, this practice in passenger aircraft made it possible for you to be hit by an. . . .
Wait for it. . . .
Icey BM
Grinning, ducking for cover, and running like all get-out. . . .