PhD defense of Nicolas Ratkovich (03/05/2010)

1. www.mbr-network.eu
Understanding hydrodynamics in membrane
bioreactor systems for wastewater treatment:
two-phase empirical and numerical modelling
and experimental validation
Nicolás Ratkovich
Faculty of Bioscience Engineering
Ghent University
May 3rd 2010, Ghent - Belgium

2. Introduction
Waste water treatment processes
• Goals
- Produce clean effluent
- Recover nutrients and energy from waste stream
• Biological treatment
- Conventional Activated Sludge (CAS) – Gravity-based separation
Influent Effluent
Air
Bioreactor Settler
2

3. Introduction
Waste water treatment processes (cont.)
• Biological treatment (cont.)
- Membrane Bioreactor (MBR) – Filtration-based separation
Influent Effluent
Side-stream
Air
Bioreactor Membrane
Effluent
Influent
Immersed
Air
Bioreactor
3

4. Introduction
Waste water treatment processes (cont.)
• Comparison (pros & cons)
CAS MBR
Sludge production ↑ ↓
Effluent quality ↓ ↑
Disinfection ↓ ↑
Footprint ↑ ↓
Problem Settling Fouling
Energy consumption ↓ ↑
Cost ↓ ↑
4

5. Introduction
MBR economics
5

6. Introduction
MBR economics
• Energy
- MBR > CAS 0.15
O&M costs breakdown ($/m3)
- MBR ≈ CAS + TT
0.10
• Total cost
- MBR > CAS
- MBR ≈ CAS + TT 0.05
• Effluent quality 0.00
- MBR > CAS CAS MBR CAS+TT
- MBR = CAS + TT
Cote et al. (2004)
Energy optimization
TT: Tertiary treatment (polishing) (air sparging)
6

7. Introduction
Membrane fouling (drawback)
• Caused by attachment of…
- suspended solids and
- soluble substances
• Mechanisms of fouling:
Resistances
Operation
Clean membrane
Filtration
Membrane
Relaxation Pore blocking
Backwash
Cake build up
7

8. Introduction Gas
slug
Air sparging
• Used as fouling control
• Gas-liquid (two-phase) flow in
vertical tubes
• Slug flow
• Advantages
- Airlift (buoyancy)
- Scouring effect (shear stress)
Air flow
8

9. Motivation
Influent
• Membrane Fouling
Effluent
- Scouring effect (shear) Biology
- Particle removal
Particle size
Hydrodynamics
distribution
• Energy consumption
- Aeration (air sparging)
- Viscosity
Filtration
TMP - Flux
9

10. Outline
10

11. Objectives
To observe and measure…
• Behaviour of developed gas slug
• Shear stress using Shear Probes (SP)
• Gas slug rising velocity using High Speed Camera (HSC)
To develop…
• CFD and empirical models
To quantify…
• Pressure drop and energy consumption of the system
11

12. Introduction
Slug flow
• 3 Zones
• Large shear stress values Falling film zone
• Dynamic shear stress
Wake zone
Liquid slug zone
*Taha & Cui, 2006
12

13. Experimental set-up
Tube diameter:
• 9.9 mm
Fluids used:
• Water + electrolyte
Flow rates:
• Liquid: 0.1 - 0.5 l⋅min-1
• N2: 0.1 - 0.3 l⋅min-1
13

14. Experimental set-up at UBC (SP)
Electrolyte solution
Fe(CN )6 + e → Fe(CN )6
3− 4−
• Cathode (probes):
Fe(CN )6 → Fe(CN )6 + e
4− 3−
• Anode (pipe fitting):
Shear probe (magnitude) Shear stress
0.70 1
Probe 1 Probe 1
0.60
0.8
0.50
Shear stress (Pa)
Voltage (V)
0.6
0.40
0.30
0.4
0.20
0.2
0.10
0.00 0
0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Time (s) Time (s)
14

15. Experimental set-up at UBC (SP)
2 Shear probes (flow direction)
Two voltage signals Shear stress
Gas slug Gas slug
15

16. Experimental measurements
Experimental measurements
• i.e. 0.11 - 0.06 m s-1 (water-N2)
2
Shear probe
1.5
1
0.5
Shear stress (Pa)
0
-0.5
-1
-1.5
-2
-2.5
-3
4 4.5 5 5.5 6 6.5 7
Time (s)
16

17. Experimental measurements
Shear stress Histogram (SSH)
• i.e. 0.11 - 0.06 m s-1 (water-N2)
0.45
0.4
0.35
Gas
slug
0.3
Frequency (-)
0.25
0.2
0.15
0.1
0.05
Liquid
slug
0
-3 -2 -1 0 1 2 3
Shear stress (Pa)
17

18. Experimental measurements
0.5
Gas 0.1 l/min
Gas 0.2 l/min
Gas 0.3 l/min
0.4
0.3
Frequency
0.2
0.1
0
-3 -2 -1 0 1 2 3
Shear stress (Pa)
Liquid 0.1 l·min-1
18

19. Computational Fluid Dynamics
CFD model
• Numerical methods and algorithms
• Analyze problems that involve fluid flows
• Interaction of liquid-gas
19

20. Computational Fluid Dynamics
Slug flow CFD model
• Fluids flow in a vertical tube
- Superficial velocities (gas + liquid)
- Volume fraction
• Validated against…
- Shear stress measurements (SP)
- Gas slug rising velocity (HSC)
20

21. Validation SP and HSC with CFD
1.5
1
Phase, Velocity (m/s), Shear stress (Pa)
0.5
0
-0.5
-1
-1.5
-2
-2.5
wall
Phase
-3 Velocity
Shear stress
-3.5
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
Time (s)
21

22. Validation SP and HSC with CFD
Liquid slug: well predicted
Gas slug: shifted to the left
0.5
Exp 0.1 l/min
Exp 0.2 l/min
Exp 0.3 l/min
0.4 Sim 0.1 l/min
Sim 0.2 l/min
Sim 0.3 l/min
0.3
Frequency
0.2
0.1
0
-3 -2 -1 0 1 2 3
Liquid 0.1 l·min-1 Shear stress (Pa)
22

23. Validation SP and HSC with CFD
TB rising velocity (9.9 mm) uTB = 1.2 um + 0.345 [g d ]0.5
U TB um
=C +k
(g d ) 0.5
(g d ) 0.5 1.2
HSC
Sim y = 1.00x + 0.41
R2 = 0.99
• Theoretical values 1
- C = 1.2 0.8
- k = 0.35
y = 1.04x + 0.30
0.5 R2 = 0.99
UTB/(gd)
0.6
0.4
0.2
0
0 0.1 0.2 0.3 0.4 0.5 0.6
Um/(gd)0.5
23

24. Shear Stress Histograms
Empirical model
• Correlate shear stress with…
- Magnitude
- Direction
- Gas-liquid flow rates
• Occurrence of both peaks (height + width)
- Better fouling control (Ochoa et al. 2007)
• Bimodal SSH based on Gaussian distribution
0.3 0.25 0.3
0.25 0.25
0.2
0.2 0.2
= +
0.15
Frequency
Frequency
Frequency
0.15 0.15
0.1
0.1 0.1
0.05
0.05 0.05
0 0 0
-3 -2 -1 0 1 2 3 -3 -2 -1 0 1 2 3 -3 -2 -1 0 1 2 3
Shear stress (Pa) Shear stress (Pa) Shear stress (Pa)
Gas slug Liquid slug
24

25. Bimodal distribution
SSH
• liquid-gas flow rates that equilibrates the peaks
0.3
0.1 - 0.43 l/min
0.2 - 0.49 l/min
0.3 - 0.54 l/min
0.25
0.4 - 0.58 l/min
0.5 - 0.63 l/min
Liq - gas
Relative frequency (-)
0.2
0.15
0.1
0.05
0
-3 -2 -1 0 1 2 3
Shear stress (Pa)
25

26. Pressure drop and energy consumption
Pressure drop
22000
Gas flow 0.0 L/min
Gas flow 0.1 L/min
Gas flow 0.2 L/min
21000 Gas flow 0.3 L/min
Total pressure drop (Pa)
20000
7%
19000
18000
17000
16000
0 0.1 0.2 0.3 0.4 0.5 0.6
Liquid flow rate (L/min)
26

27. Pressure drop and energy consumption
Pump power
0.25
Gas flow 0.0 L/min
Gas flow 0.1 L/min
Gas flow 0.2 L/min
Gas flow 0.3 L/min 7%
0.2
0.15
Epump (W)
0.1
0.05
0
0 0.1 0.2 0.3 0.4 0.5 0.6
Liquid flow rate (L/min)
27

28. Pressure drop and energy consumption
Blower power
0.25
Gas flow 0.0 L/min
Gas flow 0.1 L/min
Gas flow 0.2 L/min
0.2 Gas flow 0.3 L/min
0.15
Eblower (W)
0.1
70 %
0.05
0
0 0.1 0.2 0.3 0.4 0.5 0.6
Liquid flow rate (L/min)
28

29. Pressure drop and energy consumption
Total power
0.4
Gas flow 0.0 L/min
Gas flow 0.1 L/min
0.35 Gas flow 0.2 L/min
Gas flow 0.3 L/min
0.3
0.25
25 %
Etotal (W)
0.2
0.15
0.1
0.05
0
0 0.1 0.2 0.3 0.4 0.5 0.6
Liquid flow rate (L/min)
29

30. Pressure drop and energy consumption
Optimal bimodal SSH
• Pressure drop: ↓4% • Blower power: ↑9%
• Pump power: ↑2% • Total power: ↑2%
16000 0.6
Total pressure drop
Epump
Eblower
Etotal
15500 0.5
Energy consumption (W)
Total pressure drop (Pa)
15000 0.4
14500 0.3
14000 0.2
13500 0.1
13000 0
0 0.1 0.2 0.3 0.4 0.5 0.6
Liquid flow rate (L/min)
↑ gas flow ↓ fouling does not result in large increase in energy consumption
30

31. PhD thesis structure
31

32. Objectives
CFD modelling of Norit Airlift system
• Membrane module
- Membrane resistance (1 tube)
- Bundle of tubes (700 tubes)
• Air diffuser
- Ring aerator
- Disk aerator
• Modelling exercise
- In-situ measurement is tough
32

33. Norit airlift system
Membrane module
• Length 3 m
• 700 tubes
• ID 5.2 mm
33

34. Membrane Module (1 tube)
water
Membrane tube
outlet
• 3D single UF tube
- Hydrodynamics
- Filtration
- Single phase flow Permeate
outlet
• Membrane resistance
- Viscous resistance (Darcy’s law)
- Inertial resistance
water
inlet
Membrane Outside
tube volume
34

35. Membrane Module (700 tubes)
Membrane module
• Step-wise extrapolated to 700 tubes
• Two resistances
- Membrane resistance
- Bundle of tubes resistance
CFD model
• Calibrated in single-phase flow
- Mass balance to determine
resistance values (TMP + Flux)
35

36. Air diffusers
Two types
• Ring aerator outlet
Ring
aerator
• Disk aerator
Water inlet
Disk
aerator
36

37. Air diffusers
Inlet of membrane
Ring aerator Red 0.05 – Blue 0
volume fraction of air
module
Module
Air diffuser
Disk aerator
Module
Air diffuser
37

38. Module + air diffuser Red 0.2 – Blue 0
volume fraction of air
Module + air diffuser
• Ring aerator:
- Air near the wall
• Disk aerator:
- Air in the bulk
Membrane
Module
3m
Ring
aerator
Water
inlet
Diffuser
Disk 0.5 m
aerator
Disk Ring
38

39. Outline
39

40. Objectives
Sludge rheology
• Viscosity
• Delft Filtration Characterization method (DFCm) unit
• Activated sludge rheological model
40

41. Sludge Rheology
Viscosity
• It describes a fluid's internal resistance to flow
• Why is it important…
- To characterize hydraulic regime near membrane.
- Design of equipment (e.g. mixing, pumping, aeration devices)
k
41

42. Sludge Rheology
Viscosity (cont.)
• Relation between shear stress (τ ) and shear rate (γ):
- Newtonian (e.g. water, oil)
- Non-Newtonian (e.g. blood, toothpaste, ketchup & activated sludge)
Activated sludge
• Pseudoplastic (Power-law)
τ = kγ n
- or
η = kγ n −1
k & n = f (TSS )
k Flow behaviour index (Pa·s)
n Flow consistency index (-)
42

43. Sludge Rheology
Rotational rheometers: Pout
• Torque is correlated to viscosity
• Drawback
- Measurement ex-situ
- Eddies formation
Tubular (capillary) rheometers:
• Pressure drop is correlated to viscosity
• Drawback
- Large sludge samples
• Advantage:
TMP
Sample
- Measurement on-site J
• Can the DFCm unit be used as a
tubular rheometer? Pin
CFV CFV
43

44. Sludge Rheology
Viscosity in a tube
0.016
Experimental data
This work
0.014 Rosenberger et al. (2006)
Pollice et al. (2007)
0.012 Water
Apparent viscosity (Pa s)
0.01
0.008
0.006
0.004
0.002
0
0 2 4 6 8 10 12 14 16 18 20
TSS (g/l)
44

45. Conclusions
Modelling of slug flow
• SSH used to represent slug flow
• First peak (liquid slug) is properly captured by the CFD model
• Second peak (gas slug) is shifted to the left
• SSH with two balanced peaks is desirable
- To decrease/control fouling
- However, more energy is required
Modeling of airlift MBR (modelling exercise)
• Step-wise extrapolation was made for the tube-bundle (700) and
membrane resistance.
• Two types of diffusers were modeled
- Disk aerator provides a better dispersion of air within the module than
the ring aerator.
45

46. Conclusions
Sludge rheology
• A new rheological model for MBR activated sludge is presented
based on the data collected using the DFCm.
• It was found that the previous models underestimate the data
collected from different MBR plants.
- Difference in sludge composition and apparatus used
46

47. Perspectives
Sludge rheology
• Model that includes floc structure, size, strength, etc.
Two-phase flow
• Varying thermo-physical properties to study coalescence effects.
Shear stress for non-Newtonian liquids
• Electrolyte solution mix with a non-Newtonian liquid (e.g. CMC)
Air diffusers
• To study the air distribution in non-Newtonian fluids.
47

48. Thank you for your
attention
48