Unintended Consequences: Resolving Foam Trapping In BNR Basins Through CFD Modeling

The 20 MGD North Durham Water Reclamation Facility operates a 5-stage Bardenpho process to meet total nitrogen and total phosphorus limits. The aeration basins were physically trapping foam, causing propagation of filamentous organisms. Foam was trapped in the aerobic and post-anoxic zones of the BNR process due to poor surface transport across the baffle wall separating the post anoxic zone from the reaeration zone.It was determined that the foam trapping was due to flow patterns in the reactor induced by the anoxic zone jet mixing system. Computational fluid dynamics (CFD) modeling was used to evaluate potential baffle alternatives to alter the mixing flow pattern and encourage surface transport of foam. Several baffle configurations were evaluated over the anticipated range of flow conditions. The CFD modeling aided in determining a baffle configuration which optimized surface transport while minimizing detrimental impacts on process performance.