Improving the performance of industrial clarifiers using three-dimensional computational fluid dynamics

Abstract

Sedimentation is one of the most popular wastewater treatment processes, and is used to separate solid particles from carrier fluid in settling tanks known as clarifiers. The clarifier, as the last major facility in wastewater treatment plants (WWTPs), can limit or define the performance of the overall WWTP. This paper presents a novel three-dimensional unsteady computational fluid dynamics (CFD) model to improve the efficiency of an industrial clarifier that had been experiencing underperformance and reduction in wastewater handling capacity. We propose a numerical technique to address the transient process of removing sludge from the floor of clarifiers by using rotating rakes. The CFD model was first applied to analyzing the ramifications of the current clarifier geometry on performance. The results show that the root causes for underperformance are related to the unconventional top side feed design of the clarifier, which leads to significant asymmetry in the flow distribution. The CFD model was next used to investigate various design modifications with the goal of improving the clarifier performance. A few geometry modification ideas such as an inward baffle, dissipating inlets, and a submerged skirt were found to create a more uniform flow distribution in the clarifier, significantly reducing the backflow into the feedwell and the velocity of the flow exiting the feedwell, which helps the solid particles to settle in the clarifier. These three designs were found to reduce the effluent total suspended solids (TSS) by more than 80% and thus significantly improve clarifier performance. It is believed that the CFD model developed in this study can become a computationally efficient tool for investigating the performance of industrial clarifiers with complex geometries and rotating rakes. ARTICLE HISTORY

The influence of wind in secondary settling tanks for wastewater treatment—A computational fluid dynamics study. Part I: Circular secondary settling tanks

“…Governing equation. The equations describing 3-D, transient, turbulent, and density stratified flow in a circular SST are as follows (Das et al, 2016):…”

“…In the last three decades, many researchers and engineers have developed and applied computational fluid dynamics (CFD) models to optimize SST internal geometry to improve process efficiency and to understand the effects of influential factors on the efficiency of SSTs (Das et al, ; De Clercq, ; Ekama et al, ; Patziger, ; Ramalingam et al, ; Samstag, Dittmar, Vitasovic, & McCorquodale, ; Vitasovic, Zhou, McCorquodale, & Lingren, ; Weiss, Plósz, Essemiani, & Meinhold, ; Zhou & McCorquodale, , , ). Most of the previous researches focused on the influential factors inside the SST, such as tank geometry, energy dissipating inlet structures, other baffle configurations, and flow conditions.…”

Development and applications in computational fluid dynamics modeling for secondary settling tanks over the last three decades: A review

“…Table summarizes the description of turbulence models commonly used for Reynold Averaged Navier‐Stokes (RANS) closure (Bridgeman, Jefferson, & Parsons, ). Although the SKE model has been widely applied in the CFD SST modeling over the last 30 years, Das et al () noted that it performs poorly for complex flows with strong streamline curvature. Therefore, two modified versions of the SKE model (Realizable k‐ε and RNG k‐ε models) have been developed and applied in recent sedimentation studies.…”

“…However, to the best of the authors knowledge, the RNG k‐ε model was only applied in the studies of settling tanks either in drinking water treatment or PSTs in wastewater treatment (Das et al, ; Laine, Phan, Pellarin, & Robert, ; Patziger & Kiss, ; Tamayol et al, ; Tarpagkou & Pantokratoras, ), where the particle concentration is low, and its buoyancy effect can be ignored and no study applied shear stress transport and Reynolds stress models in the SST modeling. Only Das et al () applied the Realizable k‐ε model to optimize the complex inlet structure of an SST. However, the study did not show the model calibration with the field data to show the superiority of the Realizable k‐ε model.…”

“…In the last two decades, CFD has been applied to simulate multiphase flow in wastewater treatment facilities, such as primary settling tanks (PST) (Liu & Garcia, ; Patziger, Günthert, Jardin, Kainz, & Londong, ), aeration tanks (Karpinska & Bridgeman, , , ; Rehman, Maere, Vesvikar, Amerlinck, & Nopens, ), SSTs (Das et al, ; Gao & Stenstrom, ; McCorquodale et al, ; Patziger, Kainz, Hunze, & Józsa, ; Ramalingam et al, ; Weiss, Plósz, Essemiani, & Meinhold, ; Wicklein & Samstag, ; Xanthos, Ramalingam, Lipke, McKenna, & Fillos, ), and sludge digestions (Craig, Nieuwoudt, & Niemand, ; Kariyama, Zhai, & Wu, ). Among these applications, SSTs have been most widely studied.…”