(1) Niveau der Ablaufstelle
(2) RSE (Rückstauebene) entspricht Niveau Kanalschacht + 15 cm
Definition of the backwater level
A key step in the design of drainage systems is determining the physically effective backwater level. To this end, the relevant level at the drainage outlets must be established as a reference value. The following are decisive:
- for sewage discharge points, the water level in the trap
- for rainwater, the upper edge of the outlet (cover)
- as well as the highest effective reference point from the connected sewerage system
The requirement for an additional 15 cm above the level of the manhole or inlet grate provides increased safety in the event of a backwater incident. All drainage points below the described backwater level must be protected. In the example (B) shown above of a sloping site (hillside), therefore, not only the basement but also the ground floor would need to be protected against backwater!
Determining the type of drainage
The position of the backflow prevention system in relation to the connected sewer is also a key factor in product selection. Wherever possible in the design phase, a sufficient gradient should always be established in accordance with the principle of gravity drainage. This enables drainage during normal operation without the need for additional pumping, thereby saving on investment, operating and maintenance costs. Both backwater valves and backflow pumps and backwater lifting stations utilise this principle.
Determining the utilisation rate
If it is not possible to do without a drainage point, the downstream backflow prevention system must ensure that drainage is still possible even during a backflow situation. This therefore requires the pump to be in operation. The pump must drain into the outlet pipe via the backwater loop.
In particular, for lifting stations, a dual-pump system with two pumps of equal capacity must be installed in accordance with DIN EN 12050-1 or DIN EN 12050-2, depending on the application, if the wastewater flow must not be interrupted. The second pump improves operational reliability.
Backflow lift and backwater pumping stations offer even greater operational reliability in the event of a pump failure, as drainage continues provided there is sufficient gradient to the sewer. Consequently, no pump energy is consumed. Only when backflow occurs does a flap close, and drainage then takes place via the integrated pump.
It is possible to do without using a drain outlet:
- in domestic settings, if the drain point is not in use and, independently of this, a toilet facility is available above the backwater level
- for wastewater containing mineral oil, provided there is no risk of stored light liquids escaping, and
- in the case of Grease wastewater, provided that kitchen operations can be suspended
