“Sedimentation in rechteckigen Durchlauf-” does not sound like a tourist attraction does it? The Regenklärbecken Inselallee Nord is more than just a basin. It is a testament to Dresden’s commitment to managing its water resources.
Regenklärbecken Inselallee Nord also called RKB, plays a vital role in the city’s drainage system. According to DWA-A 166, a Regenklärbecken is a rain basin in the rainwater channel of a separate system. It separates sedimentable substances like sludge and floating materials like fats and oils from the rainwater.
These basins can be designed with or without a permanent accumulation of water. The permanent accumulation helps to retain light liquids easily. This is advantageous for basins far from settlements because it does not require a connection to a sewage canal. These basins must be desludged periodically. Any collected light liquids must be disposed of professionally.
Regenklärbecken Inselallee Nord without permanent accumulation are emptied after each rain. Optionally, it happens after a few hours of settling time. This is also called decanter or batch operation. The emptying process directs everything to a sewage canal. This ensures that the entire contents of the RKB are treated in the sewage treatment plant.
Alternatively, the clear water zone can be drained into the water body after settling. Only the sludge zone goes into the sewage canal. The state of North Rhine-Westphalia requires the entire basin contents to go to a sewage treatment plant. It can also go to a retention soil filter.
Under certain conditions, heavy metals and other pollutants may be released from the sludge. This is due to biological and chemical processes. The new DWA-A 102 worksheet recommends that Regenklärbecken only be built without permanent accumulation.
If the rainwater channel continuously carries unpolluted foreign water, a RKBoD can still be emptied after rain. This removes the sludge for disposal at the sewage treatment plant. The foreign water refills the emptied basin without adding new sludge. Until the next rain, the structure has a permanent accumulation. DWA-A 166 explicitly allows this operation.
The design of an RKB with or without permanent accumulation was done in two steps. This was according to the DWA-M 153 leaflet. First, the M 153 evaluation point system checks if a RKBmD or RKBoD provides sufficient water protection. Then, the basin must be large enough. Its surface load must not exceed a specific value. This value is often 10 m/h at the critical discharge Qkrit. The previous design method did not consider storage effects. These effects are significant in RKB without permanent accumulation. The new DWA-A 102-2 worksheet assesses the total efficiency of the substance retention. It considers the target parameter AFS63. This is the annual specific load emission in kg/(ha*a). Special operating conditions can be considered in a verification process.
According to A 102-2, the sedimentation effect in simple Regenklärbecken is quite low. This often requires relatively large structures. Instead, inclined clarifier systems with the same operation are recommended. These can also be retrofitted into existing RKB.
The basin’s constructive design follows DWA-A 166 guidelines. RKB are sedimentation plants. They are usually designed like a continuous flow basin in a mixed system. Key aspects include the inflow distribution and the arrangement of the clarification and basin overflows. According to DWA-A 166, RKBmD require a minimum depth of 2 meters. This ensures a sufficiently large sludge storage space. A 166 also requires additional verification. This relates to the geometry and proportions of the sedimentation chamber. It also relates to the weir load in l/(s*m) of the KÜ and BÜ weir thresholds. A specific inflow rate must also be verified for circular basins.
Hydraulic coordination of the basin and clarification overflow is essential for proper function. The BÜ must only start when the critical discharge flows over the KÜ. Large inflows during heavy rain can cause excessive flow velocities in the sedimentation chamber. This would stir up already deposited sludge. The clarification overflow can be designed as a self-regulating UFT-FluidClari. It is available in slotted and tubular versions. The UFT-FluidBend or UFT-FluidFlap can also be used. They are used at the basin overflow which is usually designed as a fixed weir.
The emptying of RKBoD usually requires water level measurement. It also requires suitable EMSR technology to control the emptying pump or slide after the rain ends. This happens after the specified settling time.
The Regenklärbecken Inselallee Nord exemplifies Dresden’s dedication to environmental protection. It showcases innovative engineering in urban water management.