Oh, you call your air admittance valve, a studer valve in Canada? I was wondering what the hell that was.
OP,
why do you have two shut-off valves on your waste pipes?
As zinfamous mentioned, flush the pipe work with bleech, run the hot/cold water taps on the kitchen sink and open the foul water manhole or inspection chamber outside to check that the foul water is flowing happily from A to B (i.e. that there are no partial blockages from the kitchen sink to the foul water drainage outside).
Edit: I'm guessing that they are to stop sewage backing up the system if there is a blockage further down the line; you can deliberately shut off the pipes. It looks as if they are fully open, but double check. Plus if you check that what you are putting in is coming out the other end (i.e. by looking in the manhole / inspection chamber outside) then they are both open.
Edit 2: If you find that there seems to be a blockage, when you check the manhole / inspection chamber(s) outside, then you will have to rod the drains from that chamber back to house using
these drain rods (or you can pay someone to do it for you - its an unpleasant job). You need to rod each straight / slightly curved section of drainage, so you will need to find every rodding point. The whole idea is that every change of direction in the drainage should have a rodding point. That is how drainage systems are designed in the UK. I'm not sure if the same applies in Canada - I'm assuming that it is.