The water in a septic tanks rarely
freezes. There is usually enough biological activity happening in the
tank, bacteria breaking down the incoming wastes, to keep the water
But, the line from the house to the septic tank can freeze in severe climate areas if it is shallow. The best prevention for this is to insulate the ground above the line. Piles of leaves raked from the lawn, or bales of hay, can be used as natural insulation. Snow itself is a good insulator. In some cases the homeowner may want to dig down a few inches over the drain line and install a sheet of Styrofoam insulation, which will provide good thermal protection. You can buy sheets of Styrofoam at Lowes and Home Depot.
A leaking toilet or faucet may make the situation worse, by providing a trickle of water that can freeze into an ice dam in the pipe. High-efficiency gas furnaces, some of which create a tiny flow of water, can have the same effect.
If the line from house to tank freezes, it can be opened by jetting with water to melt the ice. This has the strong potential to be a messy job, and is best left up to a professional septic-tank company.
Some homeowners have tried using salt to thaw a frozen sewer line. It is difficult to get the salt to the ice, and long-term the added sodium will tend to clog the leach field.
Another possible freeze point is the sump, for those systems which use a pump. Ice in it can disrupt the operation of the float valve and, potentially, the pump itself. Insulation of the area over the sump, and especially the removable cover, is the best solution. Styrofoam can be installed on the bottom side of the cover.
The final possible location for freezing is the drop box and leach field (drainfield). Insulation can be installed over the drop box. If the drainfield is wet in the summer time, it may well freeze solid in winter. A wet drain field is a clear sign of problems with the septic tank system.
In extreme situations, where insulation id not able to prevent freezing, electric sewer heaters are available. So long as there is electric power, these will keep the piping around them warm.
Caveat: Check with your local authority before using any of these ideas.