Using Computational Fluid Dynamics (CFD), Flow Science was contracted by CDM to perform a flow analysis of the North Basin of the Sunset Reservoir in San Francisco, CA. The 89.4 million-gallon (MG) North Basin is nearly rectangular and features horizontal dimensions of approximately 994 x 625 ft, with a depth of 31.3 ft. The inflow and outflow rates were constant at 7 million gallons per day (MGD).The computations determined the effects on reservoir water quality upon conversion to chloramine. We focused on determining the water age distribution and limiting free-ammonia formation associated with prolonged residence times. The CFD model was verified against small-scale model studies. The CFD model reproduced all of the main flow features observed in the tests.

Initial results showed that the water age was approximately 24 days in the recirculation zone, an unacceptable scenario. A final configuration was chosen that featured a high-momentum inlet that enhanced the mixing in the reservoir and significantly reduced the maximum water age. San Francisco PUC is in the process of implementing these modifications.