Here is an aerobic “all in one tank” system that has the pump tank compartment caved in. Looking into the lid of the pump tank chamber you can see that the concrete wall has been pushed in so that it is visible through the opening. The normal position of the sidewall is out of view when looking into the lid opening.
This view is of the outside of the tank with some of the dirt from the side area removed. You can see directly into the pump tank chamber.
I am not exactly sure what forces were at play when the side caved in but it may not have happened if there was steel reinforcement embedded in the concrete.
This is what it looked like after the new 500 gallon concrete pump tank was installed. The 4″ pipe coming out of the aerobic chamber was plumbed to the new pump tank with 1/8″ per foot slope. The electric for the pump and the 1″ PVC effluent line were connected to the existing system and it was inspected with a PASS.