A computational fluid dynamics (CFD) simulation identified the causes of uneven sludge loading and limited storm-flow capacity at the Brandenburg-Briest Wastewater Treatment Plant in Germany.

The plant, operated by BRAWAG GmbH, has two identical secondary clarifiers. For years, one clarifier received approximately 20% more sludge than the other, contributing to sludge carryover during storm events and effluent limit exceedances. A review using the German DWA-A 131 design standard confirmed the imbalance but did not identify its cause.

THINK Fluid Dynamix developed a time-dependent, multiphase CFD model of the plant's distribution structure and a secondary clarifier. The simulation showed that while the distribution structure divided water flow nearly evenly, it did not mix the incoming sludge. An uneven sludge distribution from the biological treatment stage therefore carried through the structure. A targeted simulation reproduced the approximately 20% difference observed at the plant.

The analysis also identified a storm-flow capacity limitation. Under normal conditions, simulated effluent sludge concentration was 0.8 mg/L, but during heavy rainfall it reached 2,500 mg/L as sludge levels rose near the clarifier's water surface. The study concluded that the clarifier was undersized for storm flows.

The findings will inform planned upgrades at the facility, with the engineering team focusing on the uneven inflow and clarifier storm capacity rather than modifying the distribution structure alone.