Rainwater from areas below the backwater level may only be connected to the public sewerage system separately from domestic wastewater via automatic wastewater lifting units, which must be located outside the building, in a backwater-free manner in accordance with DIN EN 12056-4 (lifting above the backwater level, backwater loop). Areas below the backwater level that contribute to drainage and have a gradient towards the building, such as garage entrances, building entrances or excavated areas leading to basement flats, must be kept as small as possible.
In exceptional cases, e.g. in the case of boundary developments and inner-city courtyards or small areas below the backwater level with a gradient towards building entrances, such as garage ramps, the wastewater lifting system may also be installed inside the building as a dual-lift system, provided the building is adequately protected against flooding by structural measures.
Rainwater from small areas (approx. 5 m²) of basement access points and the like may be allowed to percolate. If this is not possible, such areas may be drained via backwater valves in accordance with EN 13564-1 where a natural gradient exists, provided that suitable measures, e.g. thresholds at cellar entrances, prevent low-lying rooms from being flooded by rainwater whilst the backwater valve is closed. A flood test is required for this purpose.
Wastewater lifting systems that drain areas below the backwater level, which could endanger buildings or other property in the event of flooding, must be designed in such a way that no damage can occur in the event of a once-in-a-century rainfall event r(5,100). These areas include, for example, building entrances, cellar entrances, garage entrances and courtyards. For small areas below the backwater level that have a gradient towards building entrances, such as garage ramps, the wastewater lifting system may also be installed inside the building.
For large areas below the backwater level that do not endanger buildings or property, a flood verification in accordance with DIN EN 752 must be carried out using the 5-minute rainfall event occurring at least once every 30 years (r(5,30)). In such cases, the wastewater lifting system must be designed to cope with at least the five-minute rainfall that may occur once every two years (r(5,2)).
Rainwater lifting stations must comply with DIN EN 12050-1 for wet-installed systems, but without faecal separation, or DIN EN 12050-2, and must be used. The stations must be designed as dual-lift versions.
