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.