Our basement was lowered by the previous owners and since moving into this home two years ago we inherited a sewage ejection pit and a separate sump pump pit. The fixtures that feed into the SEP are the toilet, sink, shower, washer, laundry sink and furnace humidifier. All the p-traps are operational at all times and the floor drain for the furnace is always full of water – this floor drain runs directly to the SEP.
When the SEP ejects sewage smells run into the home. We believe that water is drawn from the floor drain at some point in the ejection process allowing the sewer gases to emit from the drain – though we are not sure. We only get smells when the SEP ejects.
We have replaced the check-valve twice, and have followed the advice of others and have drilled a 3/16 hole below the check-valve to prevent air locking.
We haven't ruled out the possibility of a venting issue and/or crack in the pipes though we did have two smoke bombs preformed and are going to administer a peppermint test ourselves tomorrow.
We are wondering if we should redirect the furnace floor drain to the sump-pump? Also, if the floor drain has a p-trap then how are the gasses getting past it?
We're considering this only because we're assuming that the only way the sewage smell could be coming into the home is through the floor drain.
Any advice whatsoever would be much appreciated.
We would be happy to post a photo of the SEP plumbing

Have you done any testing to confirm the floor drain goes to the SEP or the sump pump? How is your SEP vented, is there a sealed separate vent stack?

Agree with Ballengerb1: There is a chance that SEP is not vented properly. Trace the vent and make sure it is a dedicated vent that goes all the way to the roof. It should NOT connect or combine with any other vent in the house. Also, is your SEP lid air-tight sealed, including all the cables penetrating it ? Check it out, let us know what you have found out. If you can, post a photo of the top of the SEP for us to have some visual to work with. Back to you. Milo