Mechanical backflow prevention for fecal matter

You can get a mechanical backwater valve at any hardware store or building materials dealer! You have to weigh the risk for yourself.

Thank you. You are right. For me, it’s only about being able to assess the risk as accurately as possible. So far, I have gotten the impression that apart from references to DIN and insurance companies, there is probably no verifiable technical reason why a proper mechanical valve shouldn’t also work safely. Provided it is regularly checked.

...there is no verifiable technical reason why a proper mechanical flap should not also function reliably. Provided regular checks are carried out.

and in case of doubt, that is where it will fail. If something hangs under the flap, the flap is useless in case of backflow. Manufacturers in Germany simply do not want to take that risk.