Do you see that bit where the snow has melted well that's our neighbours septic tank, actually it's the flat rocks that cover the top of it to be more exact. The reason the snow has melted is due to all the heat that is released from the tank. Some of this is due to the hot water that is flushed through it every day but some must also be due to the chemical reactions that are taking place as bacteria break down the stuff inside. Its bad when the water coming to the house freezes in the winter but much worse when the septic tank freezes, fortunately this is more rare which I guess is because of the hot water and heat producing bacteria. Its a pity all this heat goes to waste, it would be much better if the septic tank was inside the house, or maybe not.