Numerical modelling of two types of MBR
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The document presents a study on reducing membrane fouling in membrane bioreactor (MBR) systems through computational fluid dynamics (CFD) modeling of two configurations: hollow sheet (HS) and rotating cross-flow (RCF). It discusses the methodologies used, results on velocity profiles and wall shear stress, and comparisons between systems, highlighting that RCF systems demonstrate significantly higher shear stress which could improve fouling reduction. Future work includes further investigations on non-Newtonian liquids and energy consumption effects of aeration systems.
Numerical modelling of two types of MBR
- 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. 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. 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. 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. 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. 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. No. 7 of 27 MBR configurations Hollow Sheet (HS) MBR system (Alfa Laval, Denmark) Rotating cross-flow (RCF) MBR system (Grundfos BioBooster, Denmark)
- 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. 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 gl-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. 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. 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. 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. No. 13 of 27 CFD validation (II) • HS MBR system • RCF MBR system
- 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. No. 15 of 27 CFD Results – HS MBR (II) Gas-liquid movement caused by buoyancy Air is homogenously distributed within the module
- 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. 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. No. 18 of 27 Background on rotating systems (I) Impeller Membrane
- 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. 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. 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. 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. 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 gl-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 gl-1) at an angular velocity of 250 rpm.
- 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. 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. 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. 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. No. 28 of 27 Thank you for your attention