achieving greater efficiency for 360° rotational grit removal technology using empirical data and...

Achieving Greater Efficiency for 360° Rotational Grit Removal Technology Using Empirical Data and CFD Analysis M. Couture1, A. Steele2, M. Bruneau3, A. Gadbois4, B. Hohman5 Martin Couture1, Equipment Department Director, John Meunier Inc., a subsidiary of VWS [email protected] Alan Steele2, US Sales Manager, John Meunier Inc., a subsidiary of VWS [email protected] Michel Bruneau3, Headworks Product Specialist, John Meunier Inc., a subsidiary of VWS [email protected] Alain Gadbois4, Technology Vice President, John Meunier Inc., a subsidiary of VWS [email protected] Brittany Hohman5, CFD Development Manager, VWS [email protected] ABSTRACT The current article covers the results of two years development aimed at identifying the best possible configuration for a new type of Vortex Grit Chamber using CFD modeling. The model was based on actual field data generated by a project using already existing technology. This data provided a true to life, full scale starting point for our development efforts. Once the CFD model proved and validated, various development ideas were used to establish the best possible Vortex Grit Chamber configuration on a 360 degrees configuration. KEYWORDS: Vortex, Grit, Grit Chamber, CFD, 360º, MECTAN, Fluent® INTRODUCTION The headwork of a wastewater treatment plant is not only the first step of the wastewater treatment, but also a paramount treatment step, as it sets the tone for the overall pollution abatement performance of the whole WWTP. Generally speaking, the headwork is composed of mechanical screens, screening washer compactors, and grit chambers. Modern grit chambers remove grit by inducing a vortex pattern in the circular chamber. A drive paddle in the vortex unit maintains circulation under all flow conditions. Grit slurry pumps periodically remove the accumulated grit from the hopper at the bottom of the grit chambers. The efficiency of the grit chambers is of importance for the remainder of the wastewater treatment process: removal of solids improves treatment efficiency, improves downstream hydraulics and

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Copyright ©2009 Water Environment Federation. All Rights Reserved.773

protection against excessive wear and tear in pumps. Traditionally, for design purposes, grit particle sizes have included particles larger than 65 mesh (0.008") with a specific gravity of 2.65. Removal of at least 95% of these particles always has been the target of grit removal design. Empirical studies in the literature validating these performances have been few and far between. The objective of this study is to: validate the hydraulic dynamics of a vortex grit removal system, using empirical data, and to use three-dimensional computational fluid-dynamics (CFD) to achieve enhanced grit removal performance. THE RIDGE CREST, CA CONVENTIONAL MECTAN GRIT REMOVAL SYSTEM The CFD models were implemented trough the Fluent® software (ANSYS). The fluid-dynamics models were established using the modeling of the exact geometry of the conventional MECTAN vortex grit removal package plant unit installed in Ridge Crest, CA, as shown on Figure 1. Figures 2 and 3 are presenting parameters and rendition of the same tank in Fluent® format. CFD simulations were validated with regards to experimental data from onsite trials operated with the following methodology. The sand dosage method was used during these performance tests. During these tests, the quantity of injected sand was sufficient and the velocity in the channel was high enough to avoid the settlement of sand before the grit removal system. While sand was injected, two samples with equal flow rates were taken simultaneously, upstream and downstream of the grit chamber using two submersible pumps installed at the inlet and outlet of the grit chamber. The samples were sent to an external laboratory (KREBS) where analyses of sand granularity and density were done. Grit samples where sieved through three different mesh sizes (50, 70 and 100) corresponding to 300μm, 250μm and 150μm. Hence four classes of grit were obtained corresponding to particle sizes (<150, 150-250, 250-300, >300). From a modeling point of view the simulation was performed using the standard K-ε model and the sand particles are taken into account through the discrete phase model (DPM). The Fluent® system proved highly helpful in understanding the dynamics of the vortex behaviour in the vortex grit chamber. Figures 4 and 5 are presenting velocity tracking inside the vortex system, showing the variation in velocities in various areas and their potential impact on particle removal or recirculation. By the same token, Figure 6 shows the tracking of velocity vectors in the reference configuration. This figure is particularly interesting in presenting the impact of the paddle mixer configuration and location on the flow circulation in a vortex based flow pattern. Figure 7 shows particles tracing in the reference model configuration. Each rendition is in 3D and for a particular particle size category. This is the rendition that provided the core material to establish the capture rate of the particles in the grit removal efficiency review. Figure 8 presents the comparative results from the field testing described above to the simulation results of the vortex grit chamber. All grit particles are assumed to be of 2.65 specific gravity, the basic value allocated for grit chamber design. The maximum difference between the trial results and simulation results is less than 3% across the 150-300 �m range of particle sizes. The modeling method is hence validated and assumed to be coherent and consistent with the real

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physical behaviour of the system. This validated model and methodology will thus be used for further simulation with a different geometry configuration. DEVELOPIN THE 360º DESIGN The project’s central idea is to develop a new and more efficient Vortex Grit Chamber configuration, while adapting the design to the current modern approaches. The conventional MECTAN Grit Chamber design, like the one used for Ridge Crest, CA, essentially derives from the original Swiss design created in the late 1950’s. The configuration is typically referred to as 270º, named after the rotation angle of the water from the inlet channel to the outlet channel in the design. This implies that the inlet and outlet of the unit are on the same side of the Vortex Tank. While very practical for bypass installation, the 270º design required the outlet channel to pass around the Vortex Grit Chamber’s tank to be connected to downstream treatment systems. In the late 1980’s, configurations using in-line inlet and outlet configurations appeared. This approach addressed successfully the flow direction issue, while not offering any grit removal performance increase. The present research effort by John Meunier Inc. and Veolia to define a MECTAN 360º design aims at the following:

• Create a tank configuration that can provide in-line possibility and, potentially, positioning the outlet channel in any desired direction to facilitate the plant design without affecting the unit performance.

• Establish a tank configuration that would provide enhanced Grit Removal efficiency compared to the existing 270º results and general market requirements.

• Establish by the use of computer modeling assistance, all the hydraulic parameters and capacity of each component used in the 360º Vortex Grit Chamber Design.

RESULTS AND OPTIMIZATION OF GRIT REMOVAL THROUGH CFD MODELING Once the fluid dynamics using the Fluent software were obtained, various geometry modifications were tested in order to identify which changes would benefit the most the performances of the grit removal chamber. The global approach is to maintain the calibrated solids and hydraulic behaviour identical to those we have used for the Ridge Crest model, while building modified Vortex Grit Chamber configurations. Multiple configurations were developed and most were modelled and tested with the Fluent® software. These five configurations are shown on Figure 9. Here is a recapitulation of the versions evaluated and the lessons learned:

• Reference Mectan 270º configuration based on Ridge Crest’s exact hydraulic conditions and results.

• Mectan 360º configuration with turbine-type arrangement replacing the false floor. The fins of the turbine are redirecting the flow downward to the lower portion of the vortex

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tank, thus enhancing the typical motion “downwards on the sides and upwards in core” of the Vortex System.

o This configuration was originally expected to be very promising. o Modeling proved it to be detrimental compared to the reference configuration. o Apparently, the level of turbulence induced by the presence of the turbine-type fins

was detrimental to the grit removal process. • Mectan 360º configuration with a partial concrete slab simply separating the inlet opening,

located in the lower portion of the vortex tank, and the outlet channel, located in the upper portion of the vortex tank. This option included normal MECTAN type paddle mixing.

o This configuration presented a global increase of grit removal efficiency compared to the reference design.

o Most significant performance increase was with the 100 micron grit size category, where the model presented a 20% increase in capture.

• Mectan 360º configuration with a complete conical false floor with a no central opening and a minimum fringe peripheral opening of 2” all around the tank. This option included normal MECTAN type paddle mixing.

o This configuration presents highly detrimental removal capacity, far worst than the Reference configuration.

o This configuration was used to establish that the central circulation in the tank is capital to reach conclusive results with the design.

• Mectan 360º configuration with a complete Conical Ring false floor with a central opening ½ the diameter of the Vortex tank. This option included normal MECTAN type paddle mixing.

o This configuration presents essentially the best results of the complete test, slightly lower than the flat ring configuration.

o Efficiency increase compared to the Reference system is substantial in finer grit sizes.

o Difference between Conical Ring and Flat Ring configurations is of 3 to 5% in places.

o Both solutions present the same advantage and almost identical grit removal curves.

• Mectan 360º configuration with a complete flat false floor, or Flat Ring configuration, with a central opening ½ the diameter of the Vortex tank. This option included normal MECTAN type paddle mixing.

o This configuration presented a substantial increase of efficiency compared to the reference design in all categories.

o It has proven to be the highest ranking configuration modelled, topping the Conical Ring configuration by some percents.

o Performance increase was of 10% in the 200 micron grit size and of 26% in the very difficult 100 micron grit size.

• Mectan 360º configuration with a complete Flat Ring false floor with a central opening ½ the diameter of the Vortex tank and no mixer at all.

o This is not a configuration per se, but a test using the best configuration so far. o This configuration without the paddle mixer proved to be more efficient than all

other configurations except the Flat and Conical Ring configurations with mixers.

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o This test serves as a validation of the fact that those configurations are effectively highly desirable and attractive as a basis of design.

o It also indicate that designs using these configuration could operate in a very efficiency degraded mode, should the paddle mixing system present a fault for a period of time.

o Finally, it brings in the fact that the paddle mixing system effectively acts as a grit removal system performance enhancer.

Figure 10 shows the particle track on the most efficient system, the Mectan 360 with the ring floor configuration. Short circuiting is minimized, leading to improved abatement of grit. Figure 11 presents the particle removal efficiency for the 100-300 µm particle size range for the most efficient configuration, as it is compared to the reference configuration (270O grit removal system). This figure clearly shows that higher removal efficiencies were obtained on the smallest, and most difficult, particle sizes to remove, reaching almost 50% increase in grit removal efficiency at 100 μm. As the particle sizes increase, both the reference configuration and the improved configuration achieve increasingly similar abatement performance, as would be expected since larger particles are easier to remove. CONCLUSIONS The importance of grit removal in modern wastewater treatment processes is of growing concern since as a primary treatment it impacts the down-stream operations in the wastewater treatment process. Computational Fluid Dynamics (CFD) is a powerful and precise tool that allows the study of a very wide variety of applications in the water industry. During this study, CFD has proven to be a valuable tool that mimicked the exact behaviour of a full size grit removal unit (validation) tested at Ridge Crest, CA and also helped identify a new optimum geometrical configuration for a new generation of 360º Vortex Grit Chambers. This new geometrical configuration has lead to higher separation efficiencies than the previous 270º configuration, especially for smaller grit particles in the 100-150 micron range, which often prove the hardest to remove. Other findings also provided information indicating that the design’s paddle mixing system actually enhances the overall performance of the design, while the same design without this paddle mixing provides a substantial efficiency increase when compared to the reference design. More information will be generated by full size tests and validation of the selected configurations. Also, the power and ease of operation of CFD assisted validation provides a powerful tool to further optimize the Vortex Grit Chamber design, particularly considering the enhanced efficiency required for very fine grit sizes. These will be the topic of further development efforts on the technology.

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FIGURES

Figure 1: Ridge Crest, CA Conventional Mectan Package Plant Unit

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Figure 2: Geometry of the Ridge Crest MECTAN unit

Figure 3: 3D model rendition of the Ridge Crest MECTAN in 270 degrees configuration

300”

60”

35”

24”

48”

62.5”

4”

6”

30” 84”

10”

18” 16”

18”

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Figure 4: Particle velocity tracking in Vortex Grit Chamber vertical model sections

Figure 5: Particle velocity tracking in Vortex Grit Chamber horizontal model sections

Flow Area: 62.1% Mean Velocity: 0.0635 m/s

Flow Area: 51.9%Mean Velocity: 0.0373 m/s




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Figure 6: Particle velocity vector tracking in Vortex Grit Chamber model sections

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Figure 7: Particle tracing in 3 D model based on particle size categories

Removal Efficiency ~95%

300μm 250μm 150μm

Removal Efficiency ~87% Removal Efficiency ~60%

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Figure 8: Comparison of the on-site testing and CFD simulation trials for the Ridge Crest grit removal project.

Figure 9: MECTAN 360º configurations tested with Fluent® CFD program

Fix Turbine Slab

Fringe Opened Full Conic Apron

Closed Fringe Ring Conic Apron

Closed Fringe Ring Flat Apron

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Figure 10: Particle tracking on the most efficient MECTAN 360º configuration

Removal efficiency: Effect of geometry

0,0%10,0%20,0%30,0%40,0%50,0%60,0%70,0%80,0%90,0%

100,0%

100 150 200 250 300Particle diameter (microns)

Effic

ienc

y

Reference configuration

Most efficient Configuration

Figure 11: Removal efficiency of the most efficient configuration over the 150-300 �m range of particle diameter

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achieving greater efficiency for 360° rotational grit removal technology using empirical data and...

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