Presentation given at the 4th worshop on CFD modeling for MBR applications - 7th october 2011 - Aachen, Germany

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Numerical modelling of two types of
MBR
T. R. Bentzen, N. Ratkovich, & M.R. Rasmussen
Department of Civil Engineering
Aalborg University - Denmark
4th workshop on CFD modeling for MBR applications
October 7th, 2011, Aachen - Germany

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Outline
Introduction & Objectives
• Reduction of fouling (methods)
• MBR configurations
Methodology
• Experimental measurements
• CFD modelling
Results & discussion
• Velocity profiles
• Wall shear stress
Conclusions & future work

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Motivation
Membrane Fouling
• Scouring effect (shear)
• Particle removal
Energy consumption
• Aeration (air sparging)
• Pumping
• Viscosity
Biology
Filtration
Hydrodynamics
Particle size
distribution
Influent
TMP - Flux
Effluent

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Introduction (I)
Membrane fouling
• Fouling is the main bottleneck of the widespread of MBR systems.
• Decreases permeate flux
• Increases trans-membrane pressure (TMP)
Control/reduction of fouling
• Process hydrodynamics can decrease and/or control fouling…
• By air sparging (two-phase flow)
• By high liquid cross-flow velocity
• Increase permeate flux
• Surface shear stress → scouring effect
• Increase mass transfer (cake layer → bulk region)

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Introduction (II)
Air sparging
• Gas-liquid (two-phase) flow
• Coarse bubbly flow
• Airlift (buoyancy)
Liquid cross-flow velocity
• Single-phase flow
• High liquid velocity
• Rotational system

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Objectives
• To develop tools for design, analysis and optimization of the
hydrodynamic current conditions in MBR systems in relation to
energy efficiency using CFD.
• Optimization of the design of MBR in relation to:
• Air sparging
• Liquid cross-flow velocity
• To study shear conditions close to the membrane in multiple
phase currents

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MBR configurations
Hollow Sheet (HS) MBR system
(Alfa Laval, Denmark)
Rotating cross-flow (RCF) MBR system
(Grundfos BioBooster, Denmark)

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HS MBR system (= HF + FS)
 TMP across the entire membrane surface
• HF:
• Blackflusing + high packing density
• HS:
• Less fouling due to gravity-based operation (no
pumps).
• Activated sludge (AS) does NOT accumulate and/or
stick to membrane surface.
• AS flows upwards between the membrane while
permeate passes through membrane .
To ensure that AS circulates properly…
• Air bubbles are used to create a two-phase cross-
flow velocity.
• Generates scouring effect to remove particles that
are attached to the membrane surface.
• Aerator is located at the bottom.

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Rotational cross-flow (RCF) MBR (Grundfos
BioBooster®)
• It operates…
• between 20 – 40 lmh
• pressurize system (~5 bar)
• up to 5 times higher sludge
concentration than in conventional
MBR systems (TSS up to 50 gl-1).
• Rotating impellers between
filtration membrane discs
prevent fouling.
• Impellers ensures low viscosity
in the reactor biomass due to
the non-Newtonian behaviour
of activated sludge (AS).
•  energy consumption and 
flux.

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CFD model for HS MBR system
Single filtration module
• 86 HS membranes
• 7 perforated pipes (air sparging)
Two-phase flow
• Mixture model
k- turbulence model
Enhanced wall treatment
CFD software
• CFX v13
• Star CCM+ v6.04

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CFD model for RCF MBR system
Single filtration module
• 2 membranes
• 1 impeller
Single-phase flow
Moving mesh
• Rigid body motion
Laminar, k- & k- turbulence model
Enhanced wall treatment
CFD software
• CFX v13
• Star CCM+ v6.04

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CFD validation
• Experiments & CFD simulations were carried out with air
and water.
• Validations were made for both cases using experimental
velocity measurements
• Micro-propellers (MP)  HS
• Measure liquid velocity
• Laser Doppler Anemometry (LDA)  RCF
• Optical technique to measure velocity field in
transparent media
• Cannot be used with AS (non-transparent substance)

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CFD validation (II)
• HS MBR system
• RCF MBR system

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CFD Results – HS MBR (I)
Validation for an air flow rate of 55 m3/h
Experimental ~0.23 m/s CFD ~0.30 m/s
*Mind the different scales

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CFD Results – HS MBR (II)
Gas-liquid movement caused
by buoyancy
Air is homogenously
distributed within the module

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CFD Results – HS MBR (III)
Animations for an air flow rate of 37 m3 h-1 with Star
CCM+
Volume fraction Velocity Wall shear stress

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CFD Results – HS MBR (IV)
Wall shear stress:
Air flow rate
(m3 h-1)
CFD
(Pa)
37 0.71
55 0.88
83 1.17
37 m3 h-1 55 m3 h-1
83 m3 h-1

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Background on rotating systems (I)
Impeller Membrane

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Background on rotating systems (II)
Wall shear stress in rotating systems
• Impellers generate scouring effect.
•  in shear stress prevent particles to attach to membrane surface
due to larger tangential velocities.
• Where 𝜔 is angular velocity, 𝜈 is kinematic viscosity (𝜈 = 𝜇 𝜌), 𝑘 is
velocity factor and 𝐺′(0) and 𝛼 are dimensionless velocities in the
tangential direction.

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CFD Results – RCF MBR (I)
Tangential velocity measurements (for water)
• A good agreement between the experimental measurements and the
CFD simulation results, with an error up to 8 %.

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CFD Results – RCF MBR (II)
Wall shear stress (for water)
• Shear stress depend on impeller velocity
Rotational speed
(rpm)
CFD
(Pa)
50 1.3
150 6.2
250 12.6
350 24.2
50 rpm 150 rpm 250 rpm 350 rpm

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CFD Results – RCF MBR (III)
Wall shear stress (for AS)
• It was inferred from CFD simulation that values of the shear stress
were accurate (250 rpm).
40 g/l30 g/l 50 g/l

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CFD Results – RCF MBR (IV)
Wall shear stress (cont.)
• Velocity factor (𝑘) was found to
be 0.795 ± 0.002 (R2 = 0.957)
• 𝑘 for impeller with vanes is
between 0.35 and 0.85.
• CFD model was modified to
account for NN behaviour for 3
different TSS concentrations
(30, 40 and 50 gl-1) and 4
rotational speeds (50, 150, 250
and 350 rpm).
• 𝛼 was found to be 0.525 ±
0.008 (R2 = 0.946), that can be
used for the different angular
velocities and TSS
concentrations.
Shear stress vs. radius for three different TSS concentrations (30,
40 and 50 gl-1) at an angular velocity of 250 rpm.

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CFD Results – RCF MBR (V)
Area-weighted average shear stress
• An empirical relationship, to determine the area-weighted average
shear stress in function of angular velocity (in rpm) and TSS was
developed:
𝜏 = 0.369
𝛺
𝑙𝑛 𝛺
+ 0.013 𝑇𝑆𝑆2 − 2.873

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Conclusions
Wall shear stress…
• is homogenously distributed for both systems
• in rotational system is higher than airlift system
• is up to 10 times higher in rotational system
But...
• Is there a shear stress threshold to remove particles???
• A proper validation of CFD models was made in terms of velocity
measurements (i.e. MP and LDA systems) with water.
• MBR operates with AS and measurements cannot be made.
• Local shear stress at any place of the membrane surface and area-weighted
average shear stress was determined.

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Future work
• Build a setup to study HS and RCF MBR using CMC (non-
Newtonian liquid)
• To calculate how aeration systems influences oxygen
distribution in the reactor (process aeration) and the self
cleaning of the membrane (scouring aeration).
• To study energy consumption of both systems

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Consultancy services on CFD for MBR
• CFD has become a frontline tool for virtual simulations (conceptual design
& performance prediction)
• CFD not only gives qualitative data very exact quantitative predictions.
• CFD tools and techniques are extensively validated with experimental and
analytical results allowing more robust models for R&D.
• Software expertise:
• CAD: Rhino
• Meshing: GAMBIT, ICEM, Star CCM+
• Solver: Star CCM+, CFX, Fluent
• Domain expertise
• CAD repair for meshing
• Moving mesh
• Turbulence modelling
• Multiphase modelling
• Thomas R. Bentzen: trb@civil.aau.dk
• Nicolas Ratkovich: nr@civil.aau.dk

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Thank you for your attention