You can blow down your primary condensate drain line every year and/or install a safety float switch to prevent this from happening again.

Very costly if you ignore a septic tank and don’t have preventive measures in place with your PM to clean them out 3 to 5 years. 

Check with your local laws as what you are describing may be considered a rooming house and there may be laws to prevent this. 

I have installed new laminate floors and a new complete central air unit.

Always look for preventative measures that avoid catastrophe.

You can put it in your lease that it's required and you can verify it when they move in by making them show you a copy of the policy, but what prevents them from dropping it the day after they move in? 

Air BnB is a short term rental/hotel operation, it just is, there's no debate.People interested in doing this legally are going to need to get a conditional use permit or the residential zoning code will have to be amended to allow short term rental/hotel uses by right.

It  caused by moisture vapor in the air.

Just be careful that no humans come into the unit while that ozone machine is running as it can kill you as there is no oxygen on the air.

It is very cold here, so don't really want to leave the windows open to air it out.