If the backup is actually caused by an overflowing septic tank, call a septic system contractor. But the problem may be a clog in the drain pipe running to the septic tank, in which case you need a plumber. To determine which it is, locate the cleanout and open it. The cleanout is a short PVC pipe located between the house and the septic tank that sticks out of the ground or is flush with it, and has a threaded, removable cap.
Look down into the cleanout. If there is no standing water in the bottom, then the problem is a clog between the house and the cleanout. You should call a plumber. But standing water in the main drain pipe at the bottom of the cleanout riser indicates that you have either an overflowing septic tank or a clog between the cleanout and the tank.
