To support design upgrades to the existing Primary Settling Tanks (PSTs) of the 26th Ward Waste Water Treatment Plant (WWTP) for the New York City Department of Environmental Protection (DEP), Alden conducted a Computational Fluid Dynamics (CFD) model study to assist in evaluating the hydraulic and sedimentation performance of: 1) A new Flow Distribution Structure (FDS), 2) A new influent channel that services each PST with new ports and target baffles, and 3) New PST effluent weirs and troughs. The study included developing three-dimensional (3D) CFD models and conducting a series of simulations to verify flow and grit distributions favorable for optimal performance. 

A series of FDS designs were modeled for: flow and grit distributions to each operating tank; water surface elevations at inflow chamber of FDS; water surface elevations upstream of the troughs; the relationship between the water surface elevation and flow rates; and weir setting and optimization of the FDS design.  The best design was chosen for grit and flow splits.

The PST design was modeled for flow and grit distributions through ports using: various influent channel configurations; flow and grit distribution within the PSTs and performance in conjunction with the existing baffle boards, target baffles, vertical baffle wall, flow distributors, symmetrical and asymmetrical layouts of inlet piping to influent channel ports, downstream weirs and troughs; prediction of the water surface elevation in the FDS compartment; and estimation of flow and grit distributions of other PSTs.  The results of the model indicated possible design improvements, which were then implemented and tested in the model.  Afterwards, the PST weirs and troughs were modeled in details to ensure favorable hydraulic performance.