In our region, septage does not freeze in the ground, it freezes on the truck trying to get it to the treatment plant. It becomes very difficult for us to get trucks unloaded with frozen valves and frozen septage. If you ever had to deal with thawing out a frozen septic truck, it would become very clear why we will not pump in weather unconducive to hauling liquid waste. Septic Pumping is also made difficult, if tank doesn't have a riser at ground level and the ground is frozen which doesn't permit digging.