How is the pressure difference in passenger plane toilets generated?
How is the pressure difference in passenger plane toilets generated?


The pressure in flight is generated by using the vacuum outside. This dosen't work on the ground or at low altitude, so there's a pump as well to draw out the air to create an artificial vacuum.


  Considering that the toilets should also work on the ground, I presume
  pumps are used - on the other hand, maybe the low pressure is stored
  inside the tanks from the last flight?
  Also, pumps would mean the toilets don't work when the aircraft is not
  powered up - is this true? 


Not sure about this one, but since the aircraft is seldom not powered when a person is onboard (maintenance can't do much without power either), I can't think of this being a significant problem. There might be a little bit of leftover pressure. 

Airbus states that the vacuum pump is powered off the AC power bus, so it may be powered from the ground or the APU when the engines aren't on.


  Also, how was the engineering problem
  solved of keeping the pumps or the tubes connecting to the outside
  free from waste?


As is seen in the picture below, the tank is rinsed out through with the help of a second port on the access cover to clean it out. The rest of the system I believe is designed to last until the next maintenance, when (I think) it's connected up to a high pressure hose to thoroughly clean it out.



A320 Toliet System:



A320 System Details.