We've lived in our house for 11 years with zero problems with sewer or septic smells. Until now.

In mid-November our basement flooded. The water came down through a basement closet. Most of our home's pipes run above that closet. The line going to the septic tank had collapsed just outside the house and filled with dirt. It was old and corroded. The septic line was replaced -- about 10 feet of it outside the house leading to the tank. But since then we've had bad smells when bigger volumes of water run from both showers, dishwasher, washing machine and the water conditioner! The smells are more concentrated/stronger after water runs, but there's a general baseline level of smell all the time now. We've had both our plumber and the septic company visit to look, and both said the basement needs to be cleaned because the smells are there. But we've cleaned the basement and the surfaces there don't smell. But after water runs, the air fills with the smell.

We have a 2-story house plus basement. When water is running the smell is stronger in the area of the house where water is used, PLUS the smell is in the basement around that closet where all pipes run above it. When a shower runs, the smell is stronger in the upstairs bathrooms, and in the basement by the closet. When the water conditioner runs, the smell is on the opposite side of the house on the first floor above the water conditioner, and in the basement by the closet with pipes, but NOT on the side of the basement where the water conditioning system actually sits. No smell there. ?? Our basement is cut in half - one half finished where the leaky closet and pipes are, the other half unfinished with laundry, boiler, water heater, sump pump, septic line leading out of the house, etc. separated by a wall and door. The unfinished side with all the utilities does not stink. The closet side stinks, and that's also where the leak happened when the septic line collapsed.

Our plumber and septic company have checked the traps - there are no dry traps.

This may sound dumb but I had no idea about air vents until I started researching these past few weeks. Could it be a vent or vent pipe issue?

This might be a factor ... we did not often use the second floor master bathroom toilet (don't ask me why, old habits die hard and we usually used the guest bathroom) and there is some leaking around the base when the master bath toilet gets used. The toilet was used so infrequently that the water would evaporate and get really low. Now that I've learned about seals, I wonder if there's a problem there. Our plumber and septic company looked at the toilet and said it's not leaking because they didn't see a leak when they were here. They said if it were going to leak, it would leak all the time. It is leaking now that I've flushed it numerous times and I've taken pictures. There's water along the edge of the toilet base where it meets the tile.

But even if the toilet is leaking, would that cause a problem with smells from both showers, water conditioner, washing machine and dishwasher?

I'm looking for ideas for things we should explore & focus on, and questions to ask the plumber, etc. I'm thinking the thing these all have in common is a vent and vent pipes?? Is that on the right track? We cannot see all the pipes running above the basement closet - we'd have to rip out a ground floor living room wall and I'm totally ready to do that now. But I don't want to do it unnecessarily.

Also - don't know if this is a factor but it was odd - when the septic line collapsed and our basement flooded, our ground/first floor toilet was emptied! And the first floor powder room was flooded. It's almost like the toilet threw its water out and emptied. Our plumber said the toilet was loose and he tightened it. But could there be a seal issue there?

Thank you for any help - I'm ready to back my bags and my pets and move in to a nice-smelling hotel! The smells are getting stronger, like the air is thick when water runs, and not tolerable to live with. We gotta find a solution.