A colleague of mine experienced this and I was astonished. Flying KLM from PVG to AMS a couple of hours into take off toilets became full. The pilot dumped fuel and returned to PVG. All passengers were re-booked next day. Funnily, she had the same issue fly from JFK to LHR last year. (Probability that she was on both planes - amusing)
Thinking of aviation fuel, environment, cost of re-flying passengers. I wonder why the aircraft makers not allow dumping 'contents' of 'toilet tank' (on the open sea) instead of dumping gallons of fuel.
I just wonder how often does such issues happen? Would toilet tank emptying not be checked before take off?