Here are a few links that relate directly and indirectly to your problem. After reading them, if you have more ?'s, by all means post up. But please add what you've done so we don't 'duplicate' the advice.
Sewer Gas I, Sewer Gas II, Sewer Gas III, Sewer Gas IV, and Sewer Gas V.
My best to ya and hope this helps.
Jay J -Moderator
© 2016 Renovate Your World LLC