Physical Scale Model and Computational Fluid Dynamics Model Study of a CSO Solids/Floatables Screening Structure

The North Hudson Sewerage Authority, which serves the New Jersey communities of Hoboken, Weehawken, Union City, and West New York, is proposing to construct screening structures at various CSO outfalls along the Hudson River to minimize floatables and suspended solids entering the receiving water. The screening chambers were developed to capture ½-inch solids and floatables under permit conditions established by the New Jersey Department of Environmental Protection. Facilities must be in-line, and related cleanout and maintenance operations must be below ground to minimize nuisance impacts on nearby waterfront developments. These parameters resulted in a new facility design, not previously tested or traditionally implemented as part of a CSO abatement program; therefore, a hydraulic study was undertaken. The primary goals for such a screening structure are to minimize head loss, maximize the capture of solids/floatables, and facilitate maintenance of the chamber. Both a physical model and computational fluid dynamics (CFD) model studies were conducted to test a “typical” screening chamber and optimize the baffle and bar rack positions.Flow patterns, hydraulic losses, and debris movement through the facility were evaluated using a Froude scale model, constructed at a 1 to 2.2 geometric scale. Provisions were made to measure water levels upstream and downstream of both the baffle and the bar rack/screen using differential pressure cells. Test results included both tabular summaries of water levels and video recordings made as the test program was conducted.The screen chamber baffle was evaluated at six locations and the optimum position along the length of the chamber was approximately 1.4 times the influent pipe diameter into the chamber. The optimum area of the opening under the baffle was approximately 1.9 times the area of the influent conduit. This position resulted in good distribution of flow to the bar racks at an acceptable head loss.Debris testing was conducted with the bar racks at 45 degrees to the horizontal and 36.6 degrees to the horizontal. The model bar racks corresponded to prototype bar racks with ½ inch clear spacing of the bars. For each test condition, the head loss through the 36.6 degree rack was less than through the 45 degree rack. The minimum head loss occurred when the smallest amount of debris impinged on the bar rack, i.e., more debris remained upstream of the baffle and in suspension upstream of the bar rack. The percentage of impinged debris increases with increasing velocity, i.e., there is a higher percentage of impingement with high flow and low water level.Because physical model tests could not be conducted for the entire range of flows and facility configurations anticipated by the design team, a CFD model was developed to evaluate velocity and flow distributions anticipated in larger solids/floatables screening facilities and higher flow conditions.This paper presents the facility design configurations tested, the physical scale modeling setup and results, and the CFD modeling setup and results. Lessons learned from the hydraulic testing will be presented.