Planes returned to base after dumping fuel due toilet issues 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?
An aircraft with full lavatory waste tanks can not continue on it's flight as it is a requirement to have a specified number of serviceable toilets on every flight.  It is usually the responsibility of the Purser or In Charge Flight Attendant to check the waste tank volume when they prepare for each flight. On most aircraft there is a control panel with a display that shows the actual waste volume. 

Here are some possible reasons why the waste tanks became full:


They were not checked before the fight
They were not emptied on the ground
They malfunctioned and ran continuously 
The display on the waste control panel malfunctioned


In any case these tanks can not be emptied while airborne, and the only course of action is to land and have the tanks emptied or serviced. 

On a long haul flight like PVG to AMS the crew would not have been legal to continue their duty day after returning to PVG and the flight would have to be cancelled or delayed until a new rested crew could be found to operate the flight.