The biggest odor complaints ...

The biggest odor complaints and health hazards come from the buildup of hydrogen sulfide.  Hydrogen sulfide builds when sewage becomes septic due to anaerobic conditions which can occur from stoppages or slow flowing lines. A properly designed collections system will keep flow regular and moving. Regular preventive maintenance such as treating for tree roots and jetting loose grease deposits will keep the flow moving. The public may complain of odors if the manhole covers do not seal properly. 

If all of the good practices are being followed, and there is still a problem, there are chemical and biological products that can be fed in to alleviate the problem. We have used ferric chloride to neutralize H2S in a lift station. I recently watched a presentation on an enzymatic odor reducer that worked by breaking down the grease deposits within the system.