Understanding hydrodynamics in membrane bioreactor systems for wastewater treatment:two-phase...

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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