My parents do have an issue with their septic tank and leach field where their lowest toilet doesn't always flush right and it is venting related. In that case though you can still pour buckets of water in and the toilet doesn't overflow. It just doesn't empty with the required whoosh to drag everything out.
Sanitary sewer systems are designed way oversized so that the pipes are normally never full. This leaves airspace in the top half of the pipes that effectively vents the system. In my parents case I installed that lowest toilet and I extended the main horizontal waste line further into the basement to make the connection. This line goes straight to the septic tank after leaving the basement and, when it rains, it fills solid with water just enough that when the fancy pressure assisted low flow toilet wants to suddenly blast a limited amount of water into the system, there is no room for that to happen without trying to accelerate the entire volume of 50' of 4" horizontal sewer line. I did install a proper vent line in the proper place where the 3" line from the toilet comes through the wall into the basement but did not connect it to the vent system that goes to the roof. Instead I used an air admittance valve ((AAV) that will let air into the drain system to prevent a vacuum forming but won't let air or sewer gasses out. The short section of 3" drain between the toilet and the main sewer line has sufficient volume to accept a flush but it doesn't accept it quickly enough when the air is trapped due to high water in that lowest section of drain line. This only happens when it rains enough for the level in the septic tank and leach field to rise. The entire system otherwise looks normal and the toilet doesn't overflow, it just doesn't work "right".
You can get the toilet to flush right by going into the basement and unscrewing the AAV. This lets the trapped air burp into the basement as the toilet flushes. There is probably no great danger from sewer gas as the full pipe blocks off gasses from the septic tank but I always put the AAV back on after each flush.
Sanitary sewer systems are designed way oversized so that the pipes are normally never full. This leaves airspace in the top half of the pipes that effectively vents the system. In my parents case I installed that lowest toilet and I extended the main horizontal waste line further into the basement to make the connection. This line goes straight to the septic tank after leaving the basement and, when it rains, it fills solid with water just enough that when the fancy pressure assisted low flow toilet wants to suddenly blast a limited amount of water into the system, there is no room for that to happen without trying to accelerate the entire volume of 50' of 4" horizontal sewer line. I did install a proper vent line in the proper place where the 3" line from the toilet comes through the wall into the basement but did not connect it to the vent system that goes to the roof. Instead I used an air admittance valve ((AAV) that will let air into the drain system to prevent a vacuum forming but won't let air or sewer gasses out. The short section of 3" drain between the toilet and the main sewer line has sufficient volume to accept a flush but it doesn't accept it quickly enough when the air is trapped due to high water in that lowest section of drain line. This only happens when it rains enough for the level in the septic tank and leach field to rise. The entire system otherwise looks normal and the toilet doesn't overflow, it just doesn't work "right".
You can get the toilet to flush right by going into the basement and unscrewing the AAV. This lets the trapped air burp into the basement as the toilet flushes. There is probably no great danger from sewer gas as the full pipe blocks off gasses from the septic tank but I always put the AAV back on after each flush.