513 - A CFD Analysis to Minimize Sedimentation in Primary Effluent Channel by Structural Modifications

A new primary effluent channel (PEC) was to be designed at a wastewater treatment plant. Up to 286 million liters per day (ML/d) of water with a solids concentration of 2,680 milligrams per liter (mg/L) in the 2.4-m wide PEC is expected to be distributed between four weirs upstream of corresponding bioreactors. Return activated sludge (RAS) enters the PEC through two parallel pipes at the bottom of the PEC, and primary effluent can exit the PEC through a pipe at the bottom of the PEC upstream of the RAS pipes. The main objective of the analysis was to minimize the sediment deposition upstream of the weirs in the PEC without compromising the equal distribution of flow and solids between the weirs by structural modifications. In this analysis, ANSYS CFD, a commercial software package for computational fluid dynamics (CFD), with user defined functions for secondary clarifier modeling was used to simulate the flow structure of water-solids mixture and distribution of solids concentration in 3D. The original design with a constant channel width with a flat floor was first evaluated. After that, channel geometry was modified to reduce the sediment deposition in the channel, and the final geometry consisted of a channel tapering in lateral and vertical directions. Deviation of water-solids mixture flow and solids flow from the ideal flows per weir was smaller than 5 percent. Mass of solids deposited in the channel decreased by 79 percent by narrowing the width and raising the floor of the channel.