Comprehensive Evaluation of Secondary Sedimentation Performance

The paper describes a project in which data analysis, field-testing, and multi-dimensional modeling were used to evaluate and recommend construction of an innovative inlet dissipator to the high purity oxygen activated sludge sedimentation tanks at the North San Mateo County Sanitation District (NSMCSD) wastewater treatment plant located in Daly City, California. The modeling work was calibrated to solids and velocity profile measurements taken under conditions of elevated loading and then used to evaluate the impact of a series of enhancements to the square, radial-flow sedimentation tanks at NSMCSD. It was concluded that a combination of inboard launders and a new inlet could increase the performance and capacity of the sedimentation tanks by 25% and 40%, respectively, by permitting return activated sludge rates to rise without detrimental effects on tank performance. This will prevent thickening failure without causing overflow of solids due to excessive inlet turbulence.