In the fourth part, we will look at ground filters, root systems and biological ponds. What about them so they don't suffer in winter?
To begin with, I have one good and one bad news at the same time – extensive wastewater treatment methods are very difficult to control or influence. It means that you (almost) don't have to do anything, but you can't either.
The basis is a thorough autumn check that everything that is supposed to work is working. Pipes and vents are not blocked, shafts are accessible and clean, fittings are functional.
As for ground filters, they have a life of their own until the end of their useful life. Ground filters are covered with soil to protect them from freezing, so they should not freeze except in really extreme conditions.
U root treatment plants, the situation is a little worse, the water level is mostly around 10 cm below the surface. There are two anti-freeze measures: plants and a layer of ice. In other words, the plants function as an insulating material to a certain extent, so they do not cut the rhizome for the winter. Easy. The second measure can only be carried out for "models" of rhizomes that have the ability to change the height of the drain level. When you expect the first hard frost, adjust the height of the drain so that the water level in the root field rises a few centimeters above the surface. A puddle will form. You let the surface freeze (usually overnight) and once a continuous layer of ice has formed, you lower the drain level back below ground level. The ice stays on top as an insulator. In case of frosting and other frosts, it must be repeated. Although this will worsen the transfer of oxygen into the wastewater, it is miserable even without ice. If the root field is large enough and the upstream septic tank is spacious and exported, the treatment plant can handle it.
Do you have a biological pond behind the septic tank? If you have and use various air vents and fountains in the summer, take them outside for the winter so that the ice does not destroy them. If you keep fish, don't forget to cut holes in the ice for them. And in the case of extreme cold, hope your pond is deep enough not to freeze to the bottom, the wastewater should have nowhere to go and your septic tank could overflow.