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Understanding hydrodynamics in membrane bioreactor systems for wastewater treatment:two-phase empirical and numerical modelling
and experimental validation
Nicolás Ratkovich
Faculty of Bioscience EngineeringGhent University
May 3rd 2010, Ghent - Belgium
2
IntroductionWaste water treatment processes• Goals
- Produce clean effluent- Recover nutrients and energy from waste stream
• Biological treatment - Conventional Activated Sludge (CAS) – Gravity-based separation
Air
Influent
Bioreactor
Effluent
Settler
3
IntroductionWaste water treatment processes (cont.)• Biological treatment (cont.)
- Membrane Bioreactor (MBR) – Filtration-based separation
Immersed
Side-streamAir
Influent
Bioreactor
Effluent
Membrane
Air
Influent
Bioreactor
Effluent
4
IntroductionWaste water treatment processes (cont.)• Comparison (pros & cons)
CAS MBR
Sludge production ↑ ↓
Effluent quality ↓ ↑
Disinfection ↓ ↑
Footprint ↑ ↓
Problem Settling Fouling
Energy consumption ↓ ↑
Cost ↓ ↑
5
IntroductionMBR economics
6
IntroductionMBR economics• Energy
- MBR > CAS- MBR ≈ CAS + TT
• Total cost- MBR > CAS- MBR ≈ CAS + TT
• Effluent quality- MBR > CAS- MBR = CAS + TT
TT: Tertiary treatment (polishing)
O&
M c
osts
bre
akdo
wn
($/m
3 )
0.15
0.10
0.05
0.00
CAS MBR CAS+TT
Cote et al. (2004)
Energy optimization (air sparging)
7
IntroductionMembrane fouling (drawback)• Caused by attachment of…
- suspended solids and - soluble substances
• Mechanisms of fouling:
Pore blocking
Cake build up
Clean membrane
Resistances
Relaxation
Mem
bran
e
Operation
Filtration
Backwash
8
IntroductionAir sparging• Used as fouling control• Gas-liquid (two-phase) flow in
vertical tubes• Slug flow
• Advantages- Airlift (buoyancy)- Scouring effect (shear stress)
Gas slug
Air flow
9
Biology
Filtration
Hydrodynamics Particle size distribution
Influent
TMP - Flux
Effluent
Motivation
• Membrane Fouling- Scouring effect (shear)- Particle removal
• Energy consumption- Aeration (air sparging)- Viscosity
10
Outline
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
12
IntroductionSlug flow• 3 Zones• Large shear stress values• Dynamic shear stress
*Taha & Cui, 2006
Falling film zone
Wake zone
Liquid slug zone
13
Experimental set-upTube 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
14
Experimental set-up at UBC (SP)Electrolyte solution• Cathode (probes):• Anode (pipe fitting):
Shear probe (magnitude)
( ) ( ) −− →+ 46
36 CNFeeCNFe
( ) ( ) eCNFeCNFe 36
46 +→ −−
Shear stress
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0 0.2 0.4 0.6 0.8 1
Time (s)
Volta
ge (V
)
Probe 1
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)
Shea
r str
ess
(Pa)
Probe 1
15
Experimental set-up at UBC (SP)2 Shear probes (flow direction)
Two voltage signals
Gas slug Gas slug
Shear stress
16
Experimental measurements
Experimental measurements• i.e. 0.11 - 0.06 m s-1 (water-N2)
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
4 4.5 5 5.5 6 6.5 7
Time (s)
Shea
r str
ess
(Pa)
Shear probe
17
Experimental measurements
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
-3 -2 -1 0 1 2 3
Shear stress (Pa)
Freq
uenc
y (-)
Liquid slug
Gasslug
Shear stress Histogram (SSH)• i.e. 0.11 - 0.06 m s-1 (water-N2)
18
Experimental measurements
Liquid 0.1 l·min-1
0
0.1
0.2
0.3
0.4
0.5
-3 -2 -1 0 1 2 3
Shear stress (Pa)
Freq
uenc
y
Gas 0.1 l/minGas 0.2 l/minGas 0.3 l/min
19
Computational Fluid DynamicsCFD model• Numerical methods and algorithms• Analyze problems that involve fluid flows• Interaction of liquid-gas
20
Computational Fluid DynamicsSlug 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)
21
Validation SP and HSC with CFD
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
Time (s)
Phas
e, V
eloc
ity (m
/s),
Shea
r str
ess
(Pa)
PhaseVelocityShear stress
wall
22
Validation SP and HSC with CFDLiquid slug: well predictedGas slug: shifted to the left
Liquid 0.1 l·min-1
0
0.1
0.2
0.3
0.4
0.5
-3 -2 -1 0 1 2 3
Shear stress (Pa)
Freq
uenc
y
Exp 0.1 l/minExp 0.2 l/minExp 0.3 l/minSim 0.1 l/minSim 0.2 l/minSim 0.3 l/min
23
Validation SP and HSC with CFDTB rising velocity (9.9 mm)
• Theoretical values- C = 1.2- k = 0.35
y = 1.00x + 0.41R2 = 0.99
y = 1.04x + 0.30R2 = 0.99
0
0.2
0.4
0.6
0.8
1
1.2
0 0.1 0.2 0.3 0.4 0.5 0.6
Um/(gd)0.5
UTB
/(gd)
0.5
HSCSim
( ) ( )k
dgu
Cdg
U mTB += 5.05.0
[ ] 5.0345.02.1 dguu mTB +=
24
Shear Stress HistogramsEmpirical 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
0.05
0.1
0.15
0.2
0.25
0.3
-3 -2 -1 0 1 2 3
Shear stress (Pa)
Freq
uenc
y
0
0.05
0.1
0.15
0.2
0.25
-3 -2 -1 0 1 2 3
Shear stress (Pa)
Freq
uenc
y
0
0.05
0.1
0.15
0.2
0.25
0.3
-3 -2 -1 0 1 2 3
Shear stress (Pa)
Freq
uenc
y
= +
Gas slug Liquid slug
25
Bimodal distributionSSH• liquid-gas flow rates that equilibrates the peaks
0
0.05
0.1
0.15
0.2
0.25
0.3
-3 -2 -1 0 1 2 3
Shear stress (Pa)
Rel
ativ
e fr
eque
ncy
(-)
0.1 - 0.43 l/min0.2 - 0.49 l/min0.3 - 0.54 l/min0.4 - 0.58 l/min0.5 - 0.63 l/minLiq - gas
26
16000
17000
18000
19000
20000
21000
22000
0 0.1 0.2 0.3 0.4 0.5 0.6
Liquid flow rate (L/min)
Tota
l pre
ssur
e dr
op (P
a)
Gas flow 0.0 L/minGas flow 0.1 L/minGas flow 0.2 L/minGas flow 0.3 L/min
Pressure drop and energy consumptionPressure drop
7 %
27
0
0.05
0.1
0.15
0.2
0.25
0 0.1 0.2 0.3 0.4 0.5 0.6
Liquid flow rate (L/min)
E pum
p (W
)
Gas flow 0.0 L/minGas flow 0.1 L/minGas flow 0.2 L/minGas flow 0.3 L/min
Pressure drop and energy consumptionPump power
7 %
28
0
0.05
0.1
0.15
0.2
0.25
0 0.1 0.2 0.3 0.4 0.5 0.6
Liquid flow rate (L/min)
E blo
wer
(W)
Gas flow 0.0 L/minGas flow 0.1 L/minGas flow 0.2 L/minGas flow 0.3 L/min
Pressure drop and energy consumptionBlower power
70 %
29
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 0.1 0.2 0.3 0.4 0.5 0.6
Liquid flow rate (L/min)
E tot
al (W
)
Gas flow 0.0 L/minGas flow 0.1 L/minGas flow 0.2 L/minGas flow 0.3 L/min
Pressure drop and energy consumptionTotal power
25 %
30
Pressure drop and energy consumptionOptimal bimodal SSH• Pressure drop: ↓ 4 %• Pump power: ↑ 2 %
↑ gas flow ↓ fouling does not result in large increase in energy consumption
13000
13500
14000
14500
15000
15500
16000
0 0.1 0.2 0.3 0.4 0.5 0.6
Liquid flow rate (L/min)
Tota
l pre
ssur
e dr
op (P
a)
0
0.1
0.2
0.3
0.4
0.5
0.6
Ener
gy c
onsu
mpt
ion
(W)
Total pressure dropEpumpEblowerEtotal
• Blower power: ↑ 9 %• Total power: ↑ 2 %
31
PhD thesis structure
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
33
Norit airlift systemMembrane module• Length 3 m• 700 tubes• ID 5.2 mm
34
Membrane Module (1 tube)Membrane tube• 3D single UF tube
- Hydrodynamics- Filtration- Single phase flow
• Membrane resistance- Viscous resistance (Darcy’s law)- Inertial resistance
Permeateoutlet
waterinlet
wateroutlet
Membranetube
Outsidevolume
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)
36
Air diffusersTwo types• Ring aerator
• Disk aerator
Water inlet
Ringaerator
outlet
Diskaerator
37
Air diffusersRing aerator
Module
Red 0.05 – Blue 0 volume fraction of air
Air diffuser
Disk aeratorModule
Air diffuser
Inlet of membrane module
38
Module + air diffuserModule + air diffuser• Ring aerator:
- Air near the wall
• Disk aerator:- Air in the bulk
Red 0.2 – Blue 0 volume fraction of air
Diffuser0.5 m
MembraneModule
3 m
Disk Ring
Disk aerator
Ringaerator
Water inlet
39
Outline
40
Objectives
Sludge rheology• Viscosity• Delft Filtration Characterization method (DFCm) unit• Activated sludge rheological model
41
Sludge RheologyViscosity• 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
42
Activated sludge• Pseudoplastic (Power-law)
- or
Sludge RheologyViscosity (cont.)• Relation between shear stress ( ) and shear rate ( ):
- Newtonian (e.g. water, oil)- Non-Newtonian (e.g. blood, toothpaste, ketchup & activated sludge)
τ γ
Flow behaviour index (Pa·s)Flow consistency index (-)
nkγτ =
k
n
1−= nkγη ( )TSSfnk =&
43
Sludge RheologyRotational rheometers:• 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:
- Measurement on-site• Can the DFCm unit be used as a
tubular rheometer?
Pout
Pin
CFV
TMP J
CFVSa
mpl
e
44
Sludge RheologyViscosity in a tube
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0 2 4 6 8 10 12 14 16 18 20
TSS (g/l)
App
aren
t vis
cosi
ty (P
a s)
Experimental dataThis workRosenberger et al. (2006)Pollice et al. (2007)Water
45
ConclusionsModelling 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.
46
ConclusionsSludge 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
47
PerspectivesSludge 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.
48
Thank you for your attention