Optimizing Mixing Requirements for Moving- Bed … for Study 4 Ideal Aeration Grid Design Create...

Optimizing Mixing Requirements for Moving-Bed Biofilm Processes

HEADWORKS | BIOLOGICAL | SEPARATIONS | MEMBRANES | OXIDATION-DISINFECTION | BIOSOLIDS | INDUSTRIAL SYSTEMS WWW.DEGREMONT-TECHNOLOGIES.COM

Optimizing Mixing Requirements

Discussion Outline

Process Description

Justification/Need for Study

Objectives

Experimental Approach (Pilot-scale)

CFD Model Creation and Verification of pilot-scale results

Example of Model Application (Full-scale)

09/09/2011 > DEGREMONT TECHNOLOGIES - INFILCO DEGREMONT INC. 2



Process Description


Moving-Bed Biofilm Processes

IFAS (Integrated Fixed-film Activated Sludge) or

MBBR (Moving-Bed Biofilm Reactor)



Justification/Need for Study

4

Ideal Aeration Grid Design

Create upwelling and downwelling zones for good mixing of media

Minimize energy demand without sacrificing oxygenation or mixing

Problem

Aeration grids do not create ideal mixing• Media floats on surface: “rafting”

• Media bunches in corners and deadzones

Interaction between grid configuration and mixing is complex• Difficult to improve upon grid design due to complex physics involved in mixing

Need

Develop a CFD model that can:• Simulate a three phase system (solid, liquid, and air) over a large computational domain

• Represent the macroscale mixing tendencies IFAS/MBBR systems at various configs.

Goal

Improve understanding of mixing

Guide the selection and arrangement of aeration grids



Study Objectives


1. Create a model capable of simulating the movement of media

2. Verify the simulated results with empirical observations from a pilot-scale system

3. Apply model to full-scale application



Experimental Studies

Test Tank

Volume = 22,500 gal

L x W x D = 25’ x 8’ x 16’

Storage TankTest Tank

6” Pump




Aeration Grid

Made of 9” Fine Bubble Diffusers

Magnum Tube Mini Panel

Max Air 9” Disc

http://www.wastewater.com/aeration-products/tseries.htm



Section Number


Aeration Grid


Grid segregated into 9 sections

25’

23’15.5”

8’

6’

9”

13”

1 2 3 4 5 6 7 8 9




Aeration Grid


Grid segregated into 9 sections

Sections were manually turned on/off



CFD Model Development

Models Applied

Eulerian Multiphase model

• Water: Continuous phase

• Air bubble: Secondary Phase

• Solid Phase: Secondary Phase (Granular Model)

Discrete Phase Model (DPM)

Model Closures

Standard k-ε turbulence model was used

Phase Interactions are handled through the “drag” term

Air Assumptions

Size is conserved throughout simulation

Bubble diameter = 2 mm

Airflow for mixing determined using empirical correlation: Course Bubble – MaxAir: Q (SCFM/KCF) = 0.2771 *X + 9.6386

Fine Bubble - 9" Disc: Q (SCFM/KCF) = 0.0782*X + 8.917

Fine Bubble - Magnum Tube: Q (SCFM/KCF) = 0.047* X + 9.6121

Fine Bubble - Mini Panel: Q (SCFM/KCF) = 0.0947*X + 7.0615

Where X = Media Fill Fraction and Q = Air flow (scfm/kcf)

Course Bubble – MaxAir: Q (SCFM/KCF) = 0.2771 *X + 9.6386

Fine Bubble - 9" Disc: Q (SCFM/KCF) = 0.0782*X + 8.917

Fine Bubble - Magnum Tube: Q (SCFM/KCF) = 0.047* X + 9.6121

Fine Bubble - Mini Panel: Q (SCFM/KCF) = 0.0947*X + 7.0615

Where X = Media Fill Fraction and Q = Air flow (scfm/kcf)



Model Development

Media Assumptions

Size is conserved throughout simulation

Media density = 964 kg/m3

Media weight = 132 kg/m3

Media porosity (ε) ≈ 88% (1*ε+(1- ε)*964=132)

Porous media was modeled as a solid spherical particle:

• Real diameter = 22 mm; specific surface area= 450 m2/m3

• Preserving surface area, equiv. diameter = 1.44 mm

• Preserving drag forces, equiv. diameter = 13 mm

• Pellet diameter = 5 mm in simulation



Model Development

Geometry and Meshing Assumptions

For pilot-scale study, tank was symmetrical

• Simulated ¼ of domain

For full-scale study, tank was not symmetrical

Hybrid mesh was used in both pilot- and full-scale simulations



Model Results

Simulated Conditions

From experimental study, well mixed and poorly mixed conditions were correlated with different aeration grid configurations

large portion unmixed10001000113.3


Small portion unmixed01010101013.3




Notice a lot of left to

right movement11111111111.7

mixed well and

Uniform roll01100011011.7

987654321

ObservatrionAir LateralsAir Flow

scfm/1000 cf







Notice a lot of left to

right movement11111111111.7

mixed well and

Uniform roll01100011011.7

987654321

ObservatrionAir LateralsAir Flow

scfm/1000 cf

Well Mixed

Poorly Mixed



Model Results

1. Multiphase Model Results

2. DPM Results



Multiphase Model Results

Water Velocity: Streamlines

Well Mixed case shows consistent velocities with four distinct rolling patterns

Well Mixed Poorly Mixed




Water Velocity: Centerline Contour

Low velocity and high velocity zones are critical to create upwelling and downwelling zones





Air Volume Fraction

Air is better distributed in the Well-Mixed case





Media Volume Fraction

Well Mixed: f > 0.1 = 6% f > 0.08 = 16%

Poorly Mixed: f > 0.1 = 11% f > 0.08 = 22%









Model Results

1. Multiphase Model Results

2. DPM Results



DPM Model ResultsCFD Simulation

Rationale/Justification for DPM approach from 2008 Pilot Study

Example of “good” mixing

21

Multiphase Model


Multiphase Model

Velocity Contours

DPM Simulation

Velocity Contours



DPM Model ResultsCFD Simulation

Rationale/Justification for DPM approach from 2008 Pilot Study

Example of “poor” mixing

22

Multiphase Model


Multiphase Model

Velocity Contours

DPM Simulation

Velocity Contours



DPM Model Results

DPM Justification

f_mediamultiphase 1/ν_watermultiphase ≈ 1/ν_waterdpm

Characteristics of Well Mixed case • well-defined zones of high and low velocity

• Velocity field oriented in the vertical direction

23



Model Verification


Well Mixed case



Model Verification


Poorly Mixed case



Full-Scale Application

Optimize Air Grid to Minimize “Rafting”

Large Municipal Wastewater Treatment Plant• 35 MGD

• Impending nutrient regulations

Used DPM Model for Evaluating Grid Configurations • Need for rapid evaluation

• Evaluated 30+ configurations in <2 months

26




27

Possible Solutions

Adjust airflow to improve mixing

Rearrange grid

Add components to grid

Change diffuser-type




28

Water Velocity: Centerline Vector




29





30





31





32





Applied Solution

Using the CFD model simulations as guides, an optimal solution was found that minimized “rafting”

Solution did not add significant cost to project

Solution did not delay project

33



Conclusions


1. CFD model mimicked mixing tendencies of pilot-scale tank

2. DPM simulations provided a time-saving computational method that correlated well with multiphase simulations

3. For full-scale application, multiple scenarios were investigated in a short period

4. Optimal solution applied for full-scale application



Questions?

Acknowledgements

ANSYS: Jaydeep, Narayana, Genong

IDI: Mudit Gangal, Vishal Pandey, Amit Kaldate, Paul Lacey


Optimizing Mixing RequirementsPellet Size Estimation

Pellet has buoyancy and drag forces on it.

Buoyancy force:

Drag force:

If we want to preserve the total surface area:

If we want to preserve the total drag force:

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