Unevenly Distributed Sludge Load: CFD Simulation Identifies the Causes
Two identical secondary clarifiers and a distribution structure that operates flawlessly according to calculations. Yet for years, one clarifier has received about 20% more sludge than the other. A time-dependent 3D CFD simulation identified why.
German wastewater treatment plants are facing a wave of modernization. The amended EU Urban Wastewater Treatment Directive (KARL), in effect since January 2025, will gradually tighten requirements through 2045 and affects approximately 4,700 plants in Germany. The investment requirement is estimated at 20 to 25 billion euros over the next two decades. Anyone undertaking retrofits now needs a robust planning foundation.
That was precisely the situation at the Brandenburg-Briest wastewater treatment plant operated by BRAWAG GmbH. For years, one of the two secondary clarifiers had been receiving about 20% more sludge than the other, with the well-known consequences: sludge carryover during storm events and effluent limit exceedances. A recalculation based on the German design standard DWA-A 131 confirmed the problem but could not explain it. The distribution structure functioned perfectly according to the calculations: the water flow was divided almost exactly in half. So why was there this imbalance in sludge distribution?
A Digital Look Inside the Clarifiers
To answer that question, THINK Fluid Dynamix® built a detailed digital model of the distribution structure and a secondary clarifier, based on the plant’s original drawings. The time-dependent multiphase simulation showed how water and sludge actually behaved under normal operation and during heavy rainfall, replicating real operating conditions.
The result: although the distribution structure divided the water flow almost exactly in half, it did not mix it. If the sludge arriving from the biological stage was already unevenly distributed, which happens regularly in real operation, that imbalance simply carried through. A targeted test simulation of exactly this scenario reproduced the roughly 20% difference measured on site, closely matching real operating data. That pinpointed the cause: not the structure itself, but the uneven inflow feeding it.
The Second Finding: A Clear Capacity Limit
A second simulation uncovered another weak point. Under normal operation, the secondary clarifier performed flawlessly, with an effluent sludge concentration of 0.8 mg/l. In the simulated storm scenario, however, the clarifier failed: the sludge level rose to near the water surface, and the calculated effluent value reached 2,500 mg/l. The simulation confirmed that the clarifier was undersized for storm flow.
Implications for Practice
The insight here reaches well beyond Brandenburg-Briest: distributing water evenly and distributing sludge evenly are two different problems. A distribution structure can work perfectly on the hydraulic side and still let an uneven sludge load pass straight through. Standard design checks do not always catch this. Facilities with a similar layout often carry this risk without realizing it.
For BRAWAG and the engineering firm managing the upgrade, the simulation delivered a solid basis for decision making: investing in changes to the distribution structure alone would not have solved the problem. Planning now targets the real cause, the uneven inflow feeding the structure, as well as the clarifier’s storm capacity. That means avoiding a costly wrong turn before any construction began.
Image 1: Copyright Airbus GeoBasis-DE/BKG (©2009) + Google
