Abstract
The secondary settling tank is an essential unit for the biochemical treatment of domestic sewage, and its operational effect influences the quality of effluent. Under the influence of the confluence of rainwater and sewage, wastewater use habits, etc., the inflow of the secondary sedimentation tank changes over time. In this paper, OpenFOAM, an open-source computational fluid dynamics package, was used to study the dynamic behaviors of solid–liquid two-phase flow in the tank under influent flow rate variations. A coupled method including a mixture model, drift equation and a dynamic boundary method is proposed. Numerical investigations were carried out for a 2D axisymmetric sedimentation tank using 12 cases. With increasing influent flow rate, sludge accumulates continuously in the bottom left side of the tank, sludge hopper, and inlet; the sludge blanket thickness near the right end of the tank increases continuously; and the sludge concentration in the tank approximately linearly increases with time, with a low slope. The developed framework is generic and is, therefore, expected to be applicable for modelling sludge sedimentation processes.
Highlights
Secondary settling is referred to in the literature as “the most sensitive and complicated process in activated sludge plants” [1]
The results showed that the effects of the secondary settling tank (SST) baffling structures on the buoyancy-induced turbulence can be divided into three zones: Zone A, where the baffling structures had negligible damping effects, and only the buoyancy-coupled turbulence model provided accurate predictions; Zone B, where the baffling structures partially dissipated the effect of buoyancy on turbulence, and the buoyancy-coupled model continued to provide accurate predictions but the buoyancy-decoupled turbulence model only provided qualitative but similar predictions; and Zone C, where the baffling structures fully dissipated the effect of buoyancy on turbulence, and both models provided similar, accurate predictions
The results showed that the hydrodynamics can only be correctly predicted by a buoyancy-coupled turbulent kinetic energy (TKE) equation
Summary
Secondary settling is referred to in the literature as “the most sensitive and complicated process in activated sludge plants” [1]. Medina and Laurent [6] focused on the development of a computational fluid dynamics (CFD) tool including the hydrodynamics of the settling tank and recent advances in both hindered and compression settling modeling They studied the effects on the sludge blanket height and sludge concentration using the compression function. It is very difficult to reproduce and predict the spatial and temporal evolution of the sludge distribution in the tank It will not need more computational time to simulate the sludge settling process in the tank under dynamic influent flow, the boundary conditions need to consider the influence of time. Different from the previous numerical simulation studies on solid–liquid two-phase flow in a secondary settling tank under steady flow, this study reveals the sludge settling mechanism with respect to influent flow rate variations. The modeling result is expected to aid in the optimization of operating parameters of the tank
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
