We live in a country property on septic. We are in Ontario and having an extreme cold snap (-20). Our drains have begun to back up, some water is getting away but very little. The pipe to the septic tank is not very far underground. I believe it must be freezing between the tank & the house. Any suggestions on how to dethaw the pipe. Not sure what kind of pipe it is out to the tank but inside it is cast material. Any suggestions as to how to dethaw would be very helpful. As well as how to ensure it doesn't happen again if possible. Thank you for any advice.

Fill a sink or tub on your lowest floor of the house and then let it drain. Line cold be frozen but that's rare, tank and field fill is also a possibility.