Investigation of Activated Sludge Behavior in Secondary Sedimentation Tanks by Two Models

This article employs mixture model and discrete phase model to explore the flow and activated sludge distribution in a horizontal secondary sedimentation tank (SST). The influences of sludge diameter, density, and entrance velocity are investigated and cross checked with two models. A stagnation region (5 m ≤ x ≤ 9 m) appears at the bottom hopper, where sludge tends to accumulate. Around 4.5 m is the appropriate depth for this tank. Particles with higher density or larger diameter benefit to the separation of sludge. The minimum diameter 50 μm for sludge of 1,050 kg/m3 at an entrance velocity 0.10 m/s is a threshold to guarantee the separation. Lower entrance velocity helps the sedimentation of sludge. The critical diameter for particle with the same density increases with entrance velocity. Different densities cause different behaviors of settled sludge after returning to the tank. Threshold values of diameter, density of sludge particles, and entrance velocity are presented for the first time for SSTs with the similar structure. All these will contribute to the design and optimization of SSTs.