This is where all your rags, flushable wipes, dental floss, Q-Tips, condoms, and kitchen grease end up.
  They form these greasy mats on the diffuser banks and restrict oxygen to the wastewater. 
  Less oxygen means a lower bacteria population, and that leads to odor, because the waste components don't get fully broken down.

So, if you want to help keep the sewer odor down, please stop flushing trash down the toilet.