Septic-tank systems seldom freeze when in constant use. Warm water
and the decomposition of the sewage usually maintain above-freezing
temperatures. In cold regions there is trouble from freezing if various
parts of the system are not covered adequately. If the system is to be
out of service for a period of time or if exposure is severe, it may be
advisable to mound over the poorly protected parts of the system with
earth, hay, straw, brush, leaves, manure, snow, or the like.
In cold regions it is not advisable to install the entire system below
frost depth, as this will remove the effluent from the action of the
aerobic bacteria in the upper layers of the soil and make the system
generally less accessible.
New systems put into operation during very cold weather may freeze
unless large quantities of hot water are discharged during the first
few weeks.