From a technical point of view, the sinks drain outside the cabin and not into the waste tank. They vent to the atmosphere by drain masts which are in an enironment which are typically about -40 degrees centigrade in the cruise. They have to be heated otherwise the water flowing through them will freeze. If the heating system (which in some aircraft extends to the pipes leading to the drain masts as well) has failed or isn't performing up to spec, then the system will block with ice. In my experience, this can often be the cause of a sink blockage.
Also, anything you pour down the sinks often ends up on the bottom of the aircraft. My airline recently had tech log entries for some aircraft which stated there was a red/brown stain along the underside of the aircraft, source unknown. If the engineers had run their finger along the stain and then licked it, I would suspect that they would discover that it tasted of a mix of red wine and coffee! As hydraullic fluid looks a bit like red wine, there had been some wasted engineering time discovering the source of the stains.
In addition, the radio altimeter aerials are on the bottom of the aircraft. These have to be kept clean for the very sensitive system to work. I have seen a radio altimeter fail, the cause was found to be an accumulation of coffee residue on it's aerial. In an Airbus, failure of all the radio altimeters causes the fly by wire control system to downgrade two steps when the undecarriage is lowered. This gives the flight deck crew a lot of problems and it is a relatively exciting condition!