When washing machines, toilets or showers are installed in a basement below the backflow level (i.e. street level), and their water flows through natural gradients into the public sewage system, under certain abnormal circumstances this can lead to flooding. When the public sewage system becomes overloaded, if there is no backflow protection, a mixture of rainwater, domestic wastewater and industrial wastewater may flow back through the drainage system into your building thus flooding the surrounding rooms.
If the backed up water flows into rooms without a backflow stop, material damage to furnishings or damage to construction elements due to damp penetrating the walls will occur which'll also add up to the substantial cost involved in the cleaning up. Neither the local authorities nor insurance companies will accept the costs of water damage caused by backflow.
The risk of backflow exists at any time, as the public sewerage systems are designed, for purely economic reasons, to cope only with average rainfall and not to cope with pipes cracking, blockages or other extreme events, such as torrential rainfall. Several German supreme court rulings have confirmed that the responsibility for protecting a building from backflow lies entirely with the owner/s of the building.
By using backflow flaps from ACO Haustechnik drainage points with gradients to the sewer, e.g. floor gullies or continuous pipes, can be protected long-term and in a way which is easy to retrofit. If drainage points have no gradient to the sewers, then the wastewater is disposed of by automatically functioning wastewater lift plants, which prevent backflow.
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