Since I put the Sweet Air vents on my two stacks, I haven't smelled the septic tank gases at all. They are well worth buying.
They just slipped right on the 4 inch pipes and fit tight without using glue. Got a 51mph wind the day after I put them on and they didn't even move. I will have to take them off in the winter for the moisture along with the 40 below weather so gluing is not an option.