Settling and separation


Computational Fluid Dynamics (CFD) has become an indispensable tool for the analysis and optimization of settling and separation processes in various industries. This technique provides valuable insights into fluid flow behavior, allowing engineers to improve the efficiency and effectiveness of equipment used for these processes. In particular, CFD has been successfully applied to the design and optimization of clarifiers in the wastewater industry and in separation processes within the chemical and process industries.

In the wastewater industry, clarifiers are crucial for the treatment of water, as they facilitate the settling and removal of suspended solids, thereby reducing the pollutant load. CFD simulation enables engineers to analyze the flow patterns, residence time, and solid-liquid separation efficiency within clarifiers. By simulating different operating conditions and design parameters, engineers can optimize clarifier performance, minimize the footprint, and reduce operational costs. This approach has led to improved designs, such as the incorporation of baffles and optimized inlet and outlet configurations, resulting in more efficient sedimentation processes.

In the chemical and process industries, CFD has also been applied to the design of process equipment such as cyclones, filters, and membranes, where it can provide insights into the behavior of multi-phase flows and the impact of various design parameters on performance. For instance, CFD simulations have been used to optimize the design of cyclones for improved particle separation, by altering the geometry, inlet velocity, and gas-solid interactions.

By providing a deeper understanding of the fluid flow and transport phenomena involved, CFD has enabled engineers to design more efficient and cost-effective equipment, ultimately enhancing the sustainability of these industries

Sludge modeling

Nowadays, the settling of sludge in biological reactors for water and wastewater treatment is commonly analyzed using the classic laws of sedimentation, established by Newton and Stokes, for discrete, non-flocculating spherical particles.

The forces acting on the discrete settling of particles include:

  • The force of gravity
  • The drag force

The gravitational force acting on the particles primarily depends on the volume of the particles and the difference in densities between water and solid matter. The drag force, which is generated due to viscous friction once motion has been initiated, depends on several factors, such as the shape of the particle, its projected surface in the direction of flow, Reynolds number, and the viscosity of the liquid medium. For most applications, particles are assumed to be spherical.

Furthermore, it is important to consider the fact that the concentration of dispersed solid matter can alter the rheology of the mixture in a suspension. The local concentration of dispersed particles affects the viscosity of the mixture, and in some cases, the mixture may exhibit non-Newtonian behavior when the concentration reaches a certain level.

Taking these factors into account, the process parameters considered to model the settlement of sludge particles include the density of the medium, particle diameter, viscosity of the liquid medium, and the impact of local particle concentration on the water-particle mixture.

Since different types of sludge exhibit distinct settling characteristics, it is necessary to numerically model each sludge individually. Sludge Volume Index (SVI) data is used for each reactor to numerically calibrate the interaction between water and sludge. The SVI is employed to describe the settling characteristics of sludge in biological reactors. Utilizing available experimental data, our engineers can accurately reproduce the settling behavior of any sludge for any reactor through numerical simulations. This can be easily demonstrated by conducting a virtual SVI test, which is a numerical reproduction of the same test typically performed in laboratories.

Circular clarifiers

Circular clarifiers are widely used in wastewater treatment plants for the separation of suspended solids from treated water through sedimentation. These clarifiers are characterized by their circular shape and radial flow pattern, which facilitates efficient solids settling and minimizes short-circuiting. Computational Fluid Dynamics (CFD) simulations have become a valuable tool for understanding and optimizing the performance of circular clarifiers.

CFD analysis provides insights into the hydrodynamics and flow patterns within circular clarifiers, including the effects of inlet and outlet configurations, weir placement, and sludge collection systems. By simulating the flow and identifying areas of poor circulation, engineers can optimize clarifier design to minimize dead zones and short-circuiting, ensuring the efficient removal of suspended solids.

Additionally, CFD simulations can incorporate particle tracking and settling models, which help in predicting the behavior of suspended solids within the clarifier. By accounting for factors such as particle size distribution, density, and settling velocity, engineers can evaluate the efficiency of solids removal and estimate the required detention time for achieving desired effluent quality.

CFD analysis can also be used to study the influence of various design and operating parameters on clarifier performance, such as the clarifier diameter, side water depth, and flow rate. By exploring the effects of these parameters, engineers can make informed decisions that enhance the overall efficiency and capacity of the clarifier.

Square clarifiers

Square clarifiers play a significant role in wastewater treatment plants for the separation of suspended solids from treated water through sedimentation. These clarifiers typically consist of a flat-bottomed square or rectangular tank, along with a mechanical scraper system to remove the settled sludge. Computational Fluid Dynamics (CFD) simulations have become a valuable tool for understanding and optimizing the performance of square clarifiers, with particular emphasis on the complexities of modeling the motion of scrapers in a transient simulation approach.

In CFD analysis of square clarifiers, understanding the hydrodynamics, flow patterns, and the influence of inlet and outlet configurations is crucial for efficient solids settling and minimizing short-circuiting. However, incorporating the motion of scrapers into CFD simulations adds an additional layer of complexity due to their transient behavior and interaction with the flow and settled sludge.

Modeling the scraper motion requires a transient simulation approach, which captures the time-varying effects of the scrapers on flow patterns, solids settling, and sludge removal. This can be achieved by employing dynamic mesh techniques or sliding mesh approaches, which allow the scrapers to move within the CFD model while updating the computational mesh accordingly.

The transient simulation approach enables engineers to evaluate the impact of scraper design, motion, and speed on the overall clarifier performance. By analyzing the interaction between scrapers, flow patterns, and settled sludge, engineers can optimize the scraper system design and operation to enhance sludge removal efficiency and reduce the risk of solids re-suspension.

Furthermore, CFD simulations can be combined with experimental data to calibrate and validate models, improving their accuracy and predictive capabilities. This synergy between computational and experimental approaches contributes to the development of more efficient and robust wastewater treatment processes.

In conclusion, CFD simulations of square clarifiers in wastewater treatment, with an emphasis on modeling the motion of scrapers using a transient simulation approach, provide valuable insights into the complex interplay of hydrodynamics, solids settling, and sludge removal. By capturing the transient behavior of scrapers and their interactions with the flow and settled sludge, engineers can make informed decisions that improve the overall performance and efficiency of square clarifiers in wastewater treatment plants.

In our videos we introduce CFD solutions for settling and separation

CFD for wastewater treatment: simulation of secondary clarifier

Secondary clarifiers are settling tanks in wastewater treatment plant, used for continuous removal of the mixed liquor TSS from the treated effluent and for the thickening of the settled activated sludge.

This animation shows a transient multiphase simulation of a rectangular clarifier. Clear water is collected with a system of perforated piping system at 0.5 m of submergence. The sludge is recollected at the bottom-left part of the basin with two suction pipes.

Multiphase Simulation of sludge settling, as it is conventionally carried out in SVI-tests (Sludge Volume Index).

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