Glen, a description of the specific design of your septic system will be necessary before I can even hazard a guess for a solution to the problem. Also, provide the geographic location of your system.
What is the seasonal low temperature?
What is the maximum depth of frost in your area?
Where specifically is the sewage freezing?
Is it freezing in the raw sewage pipeline between the house and the septic tank?
Is the sewage in the septic tank freezing?
Is it freezing in the septic tank effluent pipeline?
Is the effluent pipeline a gravity flow system, or is it a pressure effluent pipe?
What is the volume, in gallons, of the average daily sewage flow?
Provide the information requested, and answer the questions posed, and I will attempt to provide a solution to the problem, site unseen.
John Aldrich (Septic Tank Yank)
Septic System Consultant
Timnath, Colorado USA