I have just remodeled a bathroom for a lady. Old sink was removed without noting what kind of drain was installed. Added new vanity with new sink and trap. After finishing, there is a sewage smell coming from the sink drain that she says was not there before we started. Then we realized another sink drain pipe is running horizontally past this one in the wall and filling up the trap with smelly water. We added another p-trap going up before exiting to make a hump or a Z-trap. That helped a lot, but we are still getting some smell. All I can figure is that drain water from the other sink is surging over the hump and still getting in our trap. Any suggestions on how the previous plumber prevented this smell?