For some strange reason these are at the perfect height for 9 year old boys, I don't know if that is something to do with our plumbing or our future employment policies!
Once in the vacuum tube everything, and water from the showers, washing machines, sinks etc all goes to the sewage plant where it is biologically treated. Here is the sewage plant.
The output from the sewerage plant is clean water that is piped into a hole in the snow and some sludge which is automatically incinerated (and helps to provide heat for the station).
If you fly away from the station into what we call deep field then the situation is a little more difficult. No nice, warm, clean toilets to use out in the middle of nowhere, you can pee onto the snow but, er, solid waste needs to be brought back in a special container to incinerate on station.
So now you know.