HYDRODYNAMIC MODELING HELPS IMPROVE SECONDARY CLARIFIER PERFORMANCE

Hydrodynamic modeling offers several benefits in the evaluation of secondary sedimentation tank (SST) performance, and can often provide valuable insights into the factors impacting performance. This paper discusses the successful application of hydrodynamic modeling to evaluate SST performance at the 40-mgd Truckee Meadows Water Reclamation Facility (TMWRF) in Reno, Nevada. The facility has six square SSTs (92 feet × 92 feet × 11.5 feet) that have often operated for sustained periods at effluent suspended solids (ESS) concentrations exceeding 20 mg/L. Because of the adverse impact of these ESS loads on the downstream tertiary filters, improving SST performance has been a high priority for the plant. Hydrodynamic modeling of the SSTs was initiated in an attempt to better understand the factors limiting performance and to recommend potential modifications for improvement. A calibrated hydrodynamic model of the existing SSTs was developed, and used to simulate the impact of different modifications and operational changes. These simulated modifications and operational changes have the cumulative potential to reduce ESS concentrations by more than 50%. The implementation of just one operational change has resulted in a reduction of more than 35% in the ESS concentrations, without any structural modifications to the SSTs.