No. 1 of 27
Numerical modelling of two types of
MBR
T. R. Bentzen, N. Ratkovich, & M.R. Rasmussen
Department of Civil Engi...
No. 2 of 27
Outline
Introduction & Objectives
• Reduction of fouling (methods)
• MBR configurations
Methodology
• Experime...
No. 3 of 27
Motivation
Membrane Fouling
• Scouring effect (shear)
• Particle removal
Energy consumption
• Aeration (air sp...
No. 4 of 27
Introduction (I)
Membrane fouling
• Fouling is the main bottleneck of the widespread of MBR systems.
• Decreas...
No. 5 of 27
Introduction (II)
Air sparging
• Gas-liquid (two-phase) flow
• Coarse bubbly flow
• Airlift (buoyancy)
Liquid ...
No. 6 of 27
Objectives
• To develop tools for design, analysis and optimization of the
hydrodynamic current conditions in ...
No. 7 of 27
MBR configurations
Hollow Sheet (HS) MBR system
(Alfa Laval, Denmark)
Rotating cross-flow (RCF) MBR system
(Gr...
No. 8 of 27
HS MBR system (= HF + FS)
 TMP across the entire membrane surface
• HF:
• Blackflusing + high packing density...
No. 9 of 27
Rotational cross-flow (RCF) MBR (Grundfos
BioBooster®)
• It operates…
• between 20 – 40 lmh
• pressurize syste...
No. 10 of 27
CFD model for HS MBR system
Single filtration module
• 86 HS membranes
• 7 perforated pipes (air sparging)
Tw...
No. 11 of 27
CFD model for RCF MBR system
Single filtration module
• 2 membranes
• 1 impeller
Single-phase flow
Moving mes...
No. 12 of 27
CFD validation
• Experiments & CFD simulations were carried out with air
and water.
• Validations were made f...
No. 13 of 27
CFD validation (II)
• HS MBR system
• RCF MBR system
No. 14 of 27
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
*Min...
No. 15 of 27
CFD Results – HS MBR (II)
Gas-liquid movement caused
by buoyancy
Air is homogenously
distributed within the m...
No. 16 of 27
CFD Results – HS MBR (III)
Animations for an air flow rate of 37 m3 h-1 with Star
CCM+
Volume fraction Veloci...
No. 17 of 27
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-...
No. 18 of 27
Background on rotating systems (I)
Impeller Membrane
No. 19 of 27
Background on rotating systems (II)
Wall shear stress in rotating systems
• Impellers generate scouring effec...
No. 20 of 27
CFD Results – RCF MBR (I)
Tangential velocity measurements (for water)
• A good agreement between the experim...
No. 21 of 27
CFD Results – RCF MBR (II)
Wall shear stress (for water)
• Shear stress depend on impeller velocity
Rotationa...
No. 22 of 27
CFD Results – RCF MBR (III)
Wall shear stress (for AS)
• It was inferred from CFD simulation that values of t...
No. 23 of 27
CFD Results – RCF MBR (IV)
Wall shear stress (cont.)
• Velocity factor (𝑘) was found to
be 0.795 ± 0.002 (R2 ...
No. 24 of 27
CFD Results – RCF MBR (V)
Area-weighted average shear stress
• An empirical relationship, to determine the ar...
No. 25 of 27
Conclusions
Wall shear stress…
• is homogenously distributed for both systems
• in rotational system is highe...
No. 26 of 27
Future work
• Build a setup to study HS and RCF MBR using CMC (non-
Newtonian liquid)
• To calculate how aera...
No. 27 of 27
Consultancy services on CFD for MBR
• CFD has become a frontline tool for virtual simulations (conceptual des...
No. 28 of 27
Thank you for your attention
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Numerical modelling of two types of MBR

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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

  1. 1. No. 1 of 27 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
  2. 2. No. 2 of 27 Outline Introduction & Objectives • Reduction of fouling (methods) • MBR configurations Methodology • Experimental measurements • CFD modelling Results & discussion • Velocity profiles • Wall shear stress Conclusions & future work
  3. 3. No. 3 of 27 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
  4. 4. No. 4 of 27 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)
  5. 5. No. 5 of 27 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
  6. 6. No. 6 of 27 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
  7. 7. No. 7 of 27 MBR configurations Hollow Sheet (HS) MBR system (Alfa Laval, Denmark) Rotating cross-flow (RCF) MBR system (Grundfos BioBooster, Denmark)
  8. 8. No. 8 of 27 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.
  9. 9. No. 9 of 27 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.
  10. 10. No. 10 of 27 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
  11. 11. No. 11 of 27 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
  12. 12. No. 12 of 27 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)
  13. 13. No. 13 of 27 CFD validation (II) • HS MBR system • RCF MBR system
  14. 14. No. 14 of 27 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
  15. 15. No. 15 of 27 CFD Results – HS MBR (II) Gas-liquid movement caused by buoyancy Air is homogenously distributed within the module
  16. 16. No. 16 of 27 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
  17. 17. No. 17 of 27 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
  18. 18. No. 18 of 27 Background on rotating systems (I) Impeller Membrane
  19. 19. No. 19 of 27 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.
  20. 20. No. 20 of 27 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 %.
  21. 21. No. 21 of 27 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
  22. 22. No. 22 of 27 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
  23. 23. No. 23 of 27 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.
  24. 24. No. 24 of 27 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
  25. 25. No. 25 of 27 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.
  26. 26. No. 26 of 27 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
  27. 27. No. 27 of 27 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
  28. 28. No. 28 of 27 Thank you for your attention

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