a) What are advantages and the disadvantages of vacuum waste system?
A vacuum waste system gets rid of the dead otters pretty quick, even the big ones. But two loos often mean flapper valves, computers and other such like. These and vacuum pumps add an extra level of complexity and resultant weakness are causes of unreliability.
b) What are advantages and the disadvantages of recirculating waste system?
They are cheaper but I (and most passengers) don't like seeing bits of mashed up !!!! and bog paper "flushing" the loo. Also, if I want to smell other people's !!!! I'll join one of those kinky clubs that politicians and High Court judges belong to.
c) How often the toilet is used in short/medium aircraft? How full the waste tank after flight?
It depends on who the passengers are and the timing of the flight. I've been on one hour flights where nobody had used the loo and others where the traps have been in continuous use. But a one hour flight generally (with say 33 users) only yields about 30-40 litres and I believe our waste tank is 200 litres or so and has one warning light when 75% full and another at 95%.
d) Do you think it is worth to use recirculating waste system in modern short/medium aircraft? If yes/no, why?
NO! See b) above. Plus a vacuum system also gets rid of most of the things bloody women throw down the traps.
PM