computational fluid dynamics analysis of vortex grit removal system- chien- weftec 2010.pdf

Computational Fluid Dynamics Analysis of a Vortex Grit Removal System

Mark H. Chien1, Alexander Borys1, and Joseph L. Wong1* 1 East Bay Municipal Utility District, 375 11th Street, Oakland, California 94607 *To whom correspondence should be addressed. Email: [email protected] ABSTRACT A vortex grit removal system (two tanks rated at 70 MGD each) was installed at East Bay Municipal Utility District (EBMUD) in its Main Wastewater Treatment Plant in 2006 to improve grit removal from influent wastewater. Although use of the system has made some reductions in energy use and odors at the plant, independent performance testing concluded that the specific grit removal efficiencies were not being met. EBMUD retained AECOM to develop a computational fluid dynamics (CFD) model to analyze the vortex grit system, and to evaluate design modifications that would potentially improve grit removal. The CFD model included a particle tracking component to assess grit travel through the system. Numerous design and operational modifications were tested and evaluated based on in-tank velocity and turbulence, relative grit removal, and short-circuiting potential. Based on model results, AECOM recommended removal of several vortex grit tank components, including an internal influent baffle, the cover of the grit hopper and an in-tank propeller. AECOM also recommended that both vortex grit units be operated during dry weather, resulting in average flow rates of approximately 35 MGD per unit. After the modifications were implemented, a repeat performance test was completed. Preliminary test results indicate that removal efficiency has increased for 50-mesh, 70-mesh and 100-mesh grit. KEYWORDS: grit, vortex grit tank, computation fluid dynamics, CFD, Flow-3D INTRODUCTION

Grit in the influent stream of municipal wastewater treatment plants, if not removed at the headworks, causes numerous impacts downstream that can significantly affect overall plant operation and maintenance costs. These impacts include (but are not limited to) accelerated wear on downstream equipment (e.g., dewatering centrifuges, pumps, heat exchangers), increased downtime and cleaning costs for digesters and other facilities, and reduced capacity of digesters. EBMUD contracted for the construction of a vortex grit removal system (two tanks rated at 70 MGD each) in its Main Wastewater Treatment Plant in 2006 to improve grit removal. Additional benefits from the project were to decrease energy use and odors compared to the existing aerated grit removal system. Although use of the vortex grit system has made some reductions to energy use and odors, an independent grit removal performance test (Black & Veatch, 2009) concluded that the specified removal efficiencies were not being met. Test results indicated that the removal efficiencies for 50-mesh, 70-mesh and 100-mesh grit were 77%, 65% and 60%, respectively, compared to performance specifications of 95%, 85% and

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Copyright ©2010 Water Environment Federation. All Rights Reserved.6020

65%. As a result, EBMUD retained AECOM to develop a computational fluid dynamics (CFD) model to analyze the vortex grit system and evaluate design modifications that would potentially improve grit removal by the vortex grit system. METHODOLOGY AECOM developed a model of one of the vortex grit tanks in Flow-3D, a CFD modeling software package. Base Hydraulic Model AECOM’s base hydraulic model representing existing conditions included definition of the system geometry, modeling of the fluid volume as a system of discrete cells, application of boundary conditions (flow and water surface elevation), dynamic simulation of fluid flows, and field verification. The modeled system geometry included the influent and effluent channels, the vortex grit tank, the internal influent and effluent baffles, the grit collection hopper at the bottom of the tank, the hopper cover plate, and the rotating propeller above the hopper (Figure 1). An orthogonal mesh of cells was applied to the entire fluid volume. The fluid cell sizes varied depending on the complexity of the geometry and ranged from 1 in. x 1 in. x 1 in. cells near the propeller to 4 in. x 4 in. x 4 in. cells in the influent and effluent channels. The resulting mesh included approximately 800,000 active (fluid) cells. The downstream (effluent channel) boundary condition was specified as a known water surface elevation, which is controlled by the downstream primary sedimentation tanks. The upstream boundary condition was specified as a flow rate. The upstream water surface elevation was calculated during the dynamic simulation of the base hydraulic model.

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Figure 1. Vortex Grit System Components AECOM ran the base hydraulic model at flow rates of 35 and 70 MGD to achieve steady-state hydraulic conditions. The model was validated with field hydraulic measurements of water surface elevations and channel velocities. Model results indicated an overall total head loss through the entire system (from the influent channel to the effluent channel) of 19.5 inches at 70 MGD. The location of a hydraulic jump in the effluent channel was also correctly predicted by the model (Figure 2).

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Figure 2. Base Model Velocity Magnitude - Isometric View Propeller Analysis AECOM used the impeller sub-model feature available within Flow-3D to dynamically simulate the effect of the rotating propeller above the hopper cover plate. This sub-model defines a cylindrical object representing the area swept by the propeller blades, and describes the effect of the rotating blades as a steady-state net momentum change imparted to the fluid by the object. The effect of the blades is set using calibration constants which describe the magnitude of momentum transfer and the ratio of axial to radial forces. The calibration constants were developed using the General Moving Object (GMO) sub-model, another feature used to model moving objects in Flow-3D. Unlike the impeller sub-model, AECOM used the GMO sub-model to define the actual geometry of the individual propeller blades and, based on the known rotational velocity, calculated the momentum effect of the propeller on the surrounding fluid. Since detailed propeller analysis using the GMO sub-model was too computationally-intensive in conjunction with the dynamic simulation of the base model (a 24 ft. diameter tank, and influent and effluent flows), the propeller shaft and blade were analyzed in a small, closed “laboratory vessel” of 6 ft. x 6 ft. x 5 ft. in order to determine its effects on the fluid. Calibration constants derived from the GMO sub-model were then applied to the impeller sub-model described above and used as part of the dynamic simulation of the entire system model for each of the applicable model runs. This approach allowed for detailed analysis of the propeller in isolation, without unnecessarily expending computational resources on repeatedly simulating the constant-speed propeller motion once the effects on the fluid were determined. Grit Particle Model Grit behavior was modeled using a particle tracking module. Two particle sizes were chosen as representative of coarse and medium grit (Table 1). Based on previous laboratory test results, AECOM used a specific gravity of 1.4 for “wet” grit particles (e.g., including grease and other surface active agents, moisture, etc.). For each model run, these particles were introduced into the influent channel after hydraulic steady-state was reached. Because the model had limited

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ability to describe grit travel along the tank bottom and into the hopper, the grit removal efficiency was determined for each model run based on grit retention in the vortex grit tank. In other words, it was assumed that the grit would eventually enter the hopper after being retained in the tank. Development of a fully-calibrated grit model proved to be challenging and would have required extensive additional resources. Instead, AECOM interpreted the grit removal results as a relative indicator of grit removal efficiency, rather than as a prediction of actual removal efficiency in the field. Relative grit removal efficiency results were compared among the various model runs, and significant differences in the results between two runs were considered to be a good indicator of potential changes in actual grit removal efficiency. Simulation of Design Modification Potential design modifications were tested by AECOM both individually and in combination, including removal of the propeller blades, removal of the hopper cover plate, removal of the internal influent and effluent baffles (Figure 1), installation of an effluent channel baffle of varying heights (Figure 3), removal of the internal effluent baffle guide vane, and adjusting the height of the internal effluent baffle guide vane (Figure 1). Numerous other modifications were considered but were not evaluated. The potential design modifications were evaluated by AECOM based on reduction of velocity and turbulence inside the tank, relative grit removal efficiency using the grit particle model, and short-circuiting potential based on minimum fluid travel time through the system. Changes in tank velocity and turbulence were evaluated by comparing isometric, plan, and elevation view color plots of velocity magnitude and turbulent kinetic energy for different model runs.

Figure 3. Effluent Channel Baffle Location

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RESULTS Base System Hydraulic Model Evaluation of the base model at the design flow rate of 70 MGD confirmed the primary vortex flow pattern in the tank. The maximum fluid velocity within the tank was determined to be 6 feet per second (Figure 4). The propeller-assisted secondary (toroidal, or donut-shaped) flow pattern described by the manufacturer was not observed by AECOM in the model of the tank. The flow within the tank is described with plan and vertical section view velocity magnitude plots as shown in Figure 4, 5 and 6.

Bottom Cut (2.5 ft. Above Tank Floor)

Top Cut (8 ft. Above Tank Floor)

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Bottom Cut (2.5 ft. Above Tank Floor)

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Figure 4. Base Model Velocity Magnitude - Plan View

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Figure 5. Base Model Velocity Magnitude - Vertical Section A-A

Figure 6. Base Model Velocity Magnitude - Vertical Section B-B Isolated Propeller Analysis Based on analysis in the isolated laboratory vessel, AECOM found that the propeller had a significant radial component and that fluid was propelled radially-outward at the tank bottom. Also, AECOM found that the close proximity of the propeller off the floor (three inches) hindered the development of an upward axial motion in the center of the tank (i.e., fluid could not easily re-circulate back to the center of the vessel under the propeller). The result is that the

SECTION A-A

SECTION B-B

Influent

Effluent Channel

Effluent Channel

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actual flow pattern that was developed in the isolated laboratory vessel is outward at the bottom, up along the vessel walls, and back down in the center toward the propeller (Figure 7). Note that this pattern was not observed in the base model (full-size tank, Figure 8) because, as discussed below, the overall effect of the propeller was not significant compared to flows into, out of, and around the tank.

Figure 7. Propeller-Induced Flow in Isolated Laboratory Vessel Simulation of Design Modification A summary of the design modification simulations performed by AECOM and the modeling results is presented as model runs A through N in Table 1.

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Table 1. Summary of Model Runs and Results

Model Run Prop

eller

(1)

Hopp

er C

over

Int.

Influ

ent B

affle

Int.

Efflu

ent B

affle

Efflu

ent

Guid

e Van

e

Efflu

ent C

hann

elBa

ffle

Relative Coarse(2)

Grit Removal

Relative Medium(3)

Grit Removal

Min. Fluid Travel

Time (4)

Max. Tank

VelocityMax.TKE

%(5,6) %(5,6) sec ft/s ft2/s2

70 MGD (Design Flow Rate)A. Base Model √ √ √ √ Std None 74% 2% 56 6 1.5B. Remove Propeller X √ √ √ Std None 76% 2% -- 6 1.5C. Remove Hopper Cover √ X √ √ Std None 77% 2% -- 6 1.5D. Remove Internal Influent Baffle √ √ X √ Std None 90% 2% 53 5 1.3E. Remove Internal Influent Baffle

and Effluent Guide Vane√ √ X √ None None 82% <1% 50 GR GR

F. Remove Internal Effluent Baffle (incl. Effluent Guide Vane)

√ √ √ X None None 83% 2% 44 6 0.9

G. Remove All Internal Baffles √ √ X X None None 62% 2% 45 4 0.7H. Add 6" Effluent Channel Baffle √ √ √ √ Std 6 in. 83% 1% -- 6 1.5I. Add 12" Effluent Channel Baffle √ √ √ √ Std 12 in. 83% 2% -- 6 1.5J. Combination (FINAL) X X X √ Std None 88% 12% -- GR GRK. Combination X X X √ Std 19.5 in. 89% 7% -- 5 0.9L. Combination X X X √ Raised None 87% 9% 67 GR GR

35 MGDM. Base Model √ √ √ √ Std None -- -- -- 2 0.6N. Combination X X X √ Std 19.5 in. 100% 72% -- 2 0.2

NOTES:√ Component present.X Component removed.-- Not evaluated.GR AECOM reported results velocity and turbulence results graphically. Maximum values were not specifically determined.TKE Turbulent Kinetic EnergyStd Standard guide vane provided by manufacturer.1. Propeller removed means removal of the blades, but retention of the drive shaft and fluidizer impeller inside the grit hopper.2.

3.

4. Used as an indicator of short-circuiting potential.5.6. Modeled grit removal efficiencies are not intended to predict actual grit removal efficiencies in the field. Grit particles size and

specific gravity were chosen as representative values that would exhibit observable differences in removal efficiency. The significance of the modeled removal efficiencies is the relative differences in values between simulation runs.

Removal efficiencies calculated based on capture of particles within the tank, as opposed to capture within the grit hopper.

Coarse grit particle (including attached organic matter) defined as a diameter of 649 µm and a specific gravity of 1.4, nominally representating a particle size range between 18 and 50 mesh size.Medium grit particle (including attached organic matter) defined as a diameter of 254 µm and a specific gravity of 1.4, nominally representating a particle size range between 50 and 70 mesh size.

DISCUSSION A narrative discussion of the key model runs and findings is provided below.

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Propeller Removal (Model Run B) AECOM determined that the propeller was a weak momentum source relative to the fluid flows into, around, and out of the tank. The propeller has a diameter of 3 feet in a 24 foot diameter tank, with a rotational velocity of approximately 21 rpm. The propeller only affected fluid flow in the very near-field portion of the tank, with no observable effect on the larger flow patterns throughout the tank (Figure 8). The propeller-assisted secondary flow pattern was not observed. Because the imparted momentum is radially-outward adjacent to the propeller, AECOM concluded that the propeller may actually be preventing grit from reaching the center of the tank and the entrance to the grit hopper. Based on this analysis, AECOM recommended removal of the propeller.

Figure 8. Propeller Effect in Base Model Tank – Propeller Blades Not Shown Hopper Cover Removal (Model Run C) AECOM’s model results did not show any conclusive differences when the grit hopper cover plate was removed. The grit particle model was not able to accurately model grit travel along the tank bottom; therefore, the removal of the hopper cover could not be fully evaluated by the model. However, AECOM obtained additional information from two other wastewater treatment plants in Boston and Milwaukee suggesting that removal of the hopper cover plate appeared to increase grit removal. Internal Influent Baffle Removal (Model Run D) AECOM found that removal of the internal influent baffle reduced velocity and turbulence inside the tank and increased relative grit removal, albeit with a small increase in short-circuiting potential (Figure 9). Based on these model results, AECOM recommended removal of the internal influent baffle.

Run A Run B

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Figure 9. Velocity Magnitude Near Tank Bottom - Model Runs A and D Internal Effluent Baffle Removal and Related Modifications (Model Run E, F, L) Removal of the internal effluent baffle and associated guide vane decreased tank turbulence and increased relative grit removal, but also caused an increase in short-circuiting potential (model run F, compared to model run A). Removal of only the guide vane was also evaluated, while the effluent baffle itself was retained (model run E, compared to model run D), but AECOM found that this increased short-circuiting potential and decreased relative grit removal. Finally, raising the effluent baffle guide vane (i.e., raising the outlet point) was evaluated (model run L, compared to model run J), but results suggested a performance decrease. AECOM concluded that removal or modification of the effluent baffle and guide vane was not advisable. Effluent Channel Baffle Addition (Various Model Runs) Baffle heights from 6 inches to 19.5 inches were added at the tank outlet as a possible means of reducing flow velocities. Each baffle had a 6 inch gap at the bottom to allow grit passage. Preliminary results indicated that an effluent channel baffle improved relative grit removal (model runs H, I, compared to run A). However, the results were inconclusive when an effluent channel baffle was evaluated in combination with other design modifications (model run K, compared to run J). Because the performance benefit was not conclusive, and the addition of an effluent channel would have required additional operational effort for cleaning and possibly for baffle height control, AECOM did not recommend addition of this baffle.

Flow Rate Reduction (Model Runs M, N) Evaluation of the model at a reduced flow rate of 35 MGD showed a significant reduction in maximum tank velocity from 6 ft/sec at 70 MGD to 2 ft/sec at 35 MGD (model run M, compared to model run A, Figure 10) as well as a significant increase in relative grit removal (model run N, compared to model run K). Reductions in tank velocity and relative grit removal were significantly greater under reduced flow conditions than for any of the modifications tested at the design flow rate. While reducing flow rate is a viable solution to increase grit removal efficiency, it is less desirable because it limits process capacity and may cause unintended settling in the influent channel.

Run A – Base Model Run D- No Influent Baffleft/s

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Figure 10. Velocity Magnitude Near Tank Bottom - Model Runs A and M Combination Run (Model Run J) Three potentially beneficial design modifications (internal influent baffle removal, propeller removal, and hopper cover removal) were evaluated simultaneously. The results were similar to removal of the internal influent baffle alone and confirmed that the modifications could be applied in combination. CONCLUSIONS Based on the model results, AECOM recommended the following design modifications:

Removal of the internal influent baffle; Removal of the propeller blades (while retaining the drive shaft and a grit fluidizer in the

grit hopper); and Removal of the grit hopper cover plate.

AECOM also recommended that both vortex grit tanks be operated during dry weather, resulting in reduced flow rates in each vortex grit tank. The three design modification recommendations (removal of the internal influent baffle, the propeller blades and the grit hopper cover) were implemented in April 2010, and a repeat of the grit removal performance test was performed by Black & Veatch in July 2010. Preliminary test results indicate that removal efficiency has increased (Table 2).

Run A – 70 MGD Run M – 35 MGDft/s

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Table 2. Grit Removal Performance Test Results Average Grit Removal Efficiency

Grit Mesh Size Plant Flow 50-mesh 70-mesh 100-meshParticle size range (1) MGD > 300 µm 212 to 300 µm 150 to 212 µm

Before Modifications (2) 58 77% 65% 60%

After AECOM-Recommended Modifications (Preliminary Data) 61 90% 75% 61%Performance Specification -- 95% 85% 65%

NOTES:1. Size based on sieve analysis after drying and volatilization of organic matter.2. Grit Removal Performance Test - VGT2 EBMUD Main Plant, Black & Veatch, February 2009. ACKNOWLEDGEMENTS The authors would like to express their appreciation to the AECOM project team (Ryan Edison, David Wood, Megha Bansal and James Kern) and the Black & Veatch project team (Jim Clark, Sanjay Reddy, and Gary Hunter) for their dedicated efforts throughout this project.

REFERENCES

AECOM. 2009. Computational Fluid Dynamics Analysis for Vortex Grit Removal System – Technical Memorandum. AECOM. 2010. Computational Fluid Dynamics Analysis for Vortex Grit Removal System – Report Addendum. Black & Veatch, Inc. 2009. Grit Removal Performance Test – Vortex Grit Tank No. 2 EBMUD Main Plant.

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computational fluid dynamics analysis of vortex grit removal system- chien- weftec 2010.pdf

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