Concerns over leaving septic tank for winter in French house

Reader question: Can a septic tank freeze and if so, how can I avoid this happening, given that our property is empty all winter?

In mainland France it is very rare for winters to be cold enough for a septic tank to freeze.

The fact that most of the tank is buried underground, the volume of the liquid inside (usually at least 2,000 litres) and the warmth generated by the biological reactions which break down solids, means only a prolonged spell of temperatures below -10C is likely to cause freezing in the tank.

Read more: what is cost of septic tank diagnostic when selling a French second home?

If you are really worried, you could spread out some straw or dead leaves over any exposed surfaces of the tank for further insulation.

Problems can occur, however, if the ground into which the tank drains the treated effluent freezes for a long time and deeply.

Frozen ground does not absorb liquid easily and this could lead to a build up of liquid, eventually resulting in partially treated sewage from the tank flooding.

This is one of the reasons why drains from a tank have to be buried in relatively deep trenches when septic tanks are installed.

Similarly, if the drainage area is saturated by heavy rainfall, the capacity of the ground to absorb the treated effluent can be reduced and, in extreme cases, can cause septic tank problems.

A more common problem in houses with septic tanks that are left empty for a long time comes when the U-bends in toilets, sink and shower drains dry out.

This can allow smells from the tank to escape into the house.

Opening windows and doors, and running water through all the drains to fill the U-bends usually solves the problem quickly.

Some people swear by “restart packs” of biological activators for their septic tanks after the house has been empty for a while.

Other people never use them and have reported no problems.