Advanced Water Treatment - ISS Institute · Advanced Water Treatment ... Micro ‐ and...

Maria D. Kennedy, PhD LN0424/10/1 Sergio G. Salinas Rodríguez, MSc Prof. Jan C. Schippers, PhD, MSc

Advanced Water Treatment Low pressure membrane technology

Maria D. Kennedy, PhD Lecture Notes LN0424/10/1 Sergio G. Salinas Rodríguez, MSc Prof. Jan C. Schippers, PhD, MSc

Advanced Water Treatment Low pressure membrane technology

Table of contents

Title Page

number

Introduction to desalination and membrane related technologies 3

Basic Principles of Ultrafiltration & Microfiltration 41

Micro ‐ and Ultrafiltration elements, modules and systems 64

Membrane Fouling of MF and UF Systems 96

Membrane Cleaning of MF and UF Systems 108

1

“Introduction to desalination and membrane related technologies”

Prof. Jan C. Schippers, PhD, MScMaria D. Kennedy, PhDSaroj Sharma, PhD

Delft – April 2010

Here is certainly a water crisis?

2

René Magritte

3

However there is plenty of water, namely seawater and fresh water

3

Water consumption and water cycle (2008)

Direct and indirect water

consumption equals: 9,000 x 109 m3 /yearp q , /y

Abstraction: 4,000 x 109

Precipitation on land: 100,000 x 109

Evaporation: 60,000 x 109

Available: 40,000 x 109

Seawater: 13,700,000,000 x 109

4

4

Global water use per person

Average water use per person equals about 1,300 m3/year g p p q , /y

80 % agriculture;

0.5 – 5 % domestic consumption;

15 – 20 % industrial.

Total desalination capacity of 45 million per day

(16.5 billion per year), covers virtually the need of fresh water for 16 million people or 0.2 %.

5

Water use

Today the average direct and indirect water use (water footprint) in the world equals:

1,300 m3 per person per year or

9,000 billion m3 per year by 6.7 billion people.

Remark: Scarcity exists, if less than 750 m3 per person per day is available

6

Total available renewable fresh water resources on earth equals:

6,600 m3 per person/year or 16.5 m3/day or 40,000 x109 m3/year

5

Plenty of fresh water on earth, however:

Rain fall is not evenly divided;y ;

Population is not evenly divided;

Water use is not evenly divided.

7

As a result water stress

8

6

Water stress

Many countries are running out of water and many more will run out, because of:

Abstracting more that renewable sources during many years due to:

Population growth:

h h ld l d

9

By the year 2050 the world population is expected to increase with 50%.

Increasing income;

Groundwater and recharge

10

7

How to solve these local water stresses?

Increasing (water related) productivity in agriculture and i dindustry;

Reducing leakages in public water supply;

Further increasing water reuse;

Water transportation over large distances;

D li ti f b ki h t t d t t d

11

Desalination of brackish, treated waste water and seawater.

Water Transportation over large distances

In 2002 the Spanish Government launched the Spanish National Hydrological Plan.

To meet water countries demands by transferring water from areas where it is “in excess” to other areas with a water deficit.

A 912 km pipeline from Ebro river in the north to the south and east for agriculture and tourist development had to be constructed;

Project has been cancelled for environmental, energy and cost reasons.

Seawater desalination plants have been constructed and are under

12

Seawater desalination plants have been constructed and are under construction.

Construction of an additional pipeline in California from the north has been cancelled for similar reasons

8

Spanish Hydrological Plan 2002

13

Desalination Techniques

Distillation:

Multi‐Effect (ME) and Multi Stage Flash Distillation (MSF); Applied for Seawater desalination;

Reverse osmosis

Applied for Sea, Brackish, waste water and fresh water

l d l

14

Electrodialysis

Applied for brackish, waste water and fresh water

9

Desalination Capacity

Source: Sabine Lattemann

15

Distillation

16

10

Simple distillation unit

17

Energy consumption for distillation is much higher e.g.:

To raise 1L water 1 oC in temperature 4.2 kJ/L energy is needed.

The heat of vaporization at 100 oC equals 2256 kJ/L.

Consequently the heat required for evaporation of 1 m3

water of 25 oC amounts about 2600 MJ (2.6 GJ/m3).

World energy prices are ranging from $ 5, $ 10 to $15 for coal, natural gas and crude oil respectively.

As a consequence, simple (single effect) distillation would cost $ 13 to 39 per m3 water, which is far to costly.

18

11

Single effect distillation

19

Multiple use of evaporation heat

In these techniques the heat in the evaporated seawater is (re) used to evaporate sea water.

To reduce the energy consumption:

Multi effect distillation (MED)

and

20

Multi stage flash distillation (MSF) are developed

12

Multi effect distillation

21

In each effect steam (vapor) is condensed on one side of a tube and the heat of condensation derived from this is

Multi effect evaporators

utilized to evaporate saline water on the other side of the tube wall.

The subsequent use and reuse of the heats of vaporization and condensation reduces the heat consumption significantly.

In practice up to about 9 times the steam is “reused” and In practice up to about 9 times the steam is reused and low value steam from power plants is used.

So the energy cost will drop to approx. $ 1.4 to 4.3. If low value steam from power plants is used, further reduces the energy cost to $ 0.8 to $ 2.4 per m3 water.

22

13

Multi effect distillation plant in Al Hidd Capacity: 273,000 m3/day

23

Seawater reverse osmosis is gaining ground at the expense of distillation

70

Source: Global Water Intelligence

.

60

50

40

30

20

Membrane

24

1980 85 90 95 2000 05 10 15

20

10

0

Thermal FORECAST

14

Trends in seawater desalination

Why is seawater reverse osmosis gaining ground at the expense of distillation?

Answer: The energy consumption and investment costs are lower.

25

Reverse osmosis

26

15

Reverse osmosis makes use of membranes, with small pores e.g., flat sheets, capillaries or tubes made of organic

What is reverse osmosis?

polymers.

Water is forced to flow through these pores with the help of (high) pressure to overcome:

ti

27

osmotic pressure

hydraulic resistance of the membrane.

Salts can not pass the small pores (are rejected).

Membrane elements

Membranes are assembled to membrane elements and placed in pressure vessels.

28

16

29

30

17

31

Largest Sea Water Reverse Osmosis Plant (Ashkelon, Israel, 2005) ‐ Capacity: 330,000 m3/day

32

18

Desalination capacity and Dutch Water Supply

Capacity in the WorldCapacity in the World

Brackish and seawater 50 x 106 m3/day

Sea water (total)

Reverse Osmosis

30 x 106 m3/day

10 x 106

Dutch Water Supply Companies 3 x 106 m3/day

33

Spain, Israel, Australia are in the process of expanding their seawater reverse osmosis capacity rapidly.

Today

Spain is aiming at a capacity of 500,000 m3/day (200x106 m3/year);

Australia and Israel will arrive at 1.5 million m3/day (500x106

m3/year);

Studies in United States indicate that seawater desalination is inevitable.

Water Authority of South Nevada is considering to build a seawater desalination plant in Mexico.

Several plants are under design and construction.

China, Jordan, Morocco, Yemen, Chile and other countries are considering seawater desalination to solve the fast growing water shortages.

34

19

Water use in excess of natural supply (average annual)

Source: World water Report 2

35

Where might desalination be needed?

36

20

Annual Renewable Water Resources

2500

RC

ES

500

1000

1500

2000

NU

AL

RE

NE

WA

BL

E W

AT

ER

RE

SO

U

(m3 p

er c

apit

a)

Water scarcity

37

0

Ku

wai

t

UA

E

Sau

di A

rab

ia

Lib

ya

Sin

gap

ore

Yem

en

Isra

el

Om

an

Alg

eria

Tu

nis

ia

Eg

ypt

Mo

rocc

o

Ind

ia

Iran

Ch

ina

AN

N

Source: DesalData 2008

Group Type Form of Energy

Desalination Technologies

Distillation processes

• Multi‐stage flash evaporation (MSF)

• Multi‐effect evaporation (MED)

• Vapor compression (MVC or MED‐TVC)

• Heat and electrical

• Heat and electrical

• Mechanical or electrical

Membrane processes

• Reverse Osmosis

• Nanofiltration

• Electrodialysis

• Electrical

• Electrical

• Electrical

38

y

Ion exchange • Cation and anion exchange • Chemicals for regeneration

21

Normal operation range of desalting technologies

39

Source Technology

Applications of Desalination Technologies

Source Technology

Seawater• Distillation

• SW Reverse Osmosis

Brackish/Fresh• Reverse Osmosis

• Electrodialysis

Low salinity water (polishing) • Ion exchange

40

y (p g) g

Hard water and colored water • Nanofiltration

22

50,000,000Total

ED(Electrodialysis)

Total

Total desalination capacity installed and under construction

10 000 000

20,000,000

30,000,000

40,000,000

Capacity (m

3/d)

ED (Electrodialysis)

MED (Multi‐effect distillation)

NF (Nanofiltration)

MSF (Multi‐stage flash)

RO (Reverse osmosis)

RO

MSF

0

10,000,000

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Year

MEDEDNF

41


30 000 000 Total Total

Seawater desalination in the world, installed and under construction

10,000,000

15,000,000

20,000,000

25,000,000

30,000,000

Capacity (m

3/d)

Total

MED (Multi‐effect distillation)

NF (Nanofiltration)

MSF (Multi‐stage flash)

RO (Reverse osmosis)

Total

RO

MSF

0

5,000,000

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010Year

MED

NF

42


23

20

Desalination capacity in the world by sub‐region

0

5

10

15

20

Capacity (Mm3/d)

43


25ED EDI

Desalting plants in the World: Technologies

5

10

15

20

Cap

acit

y (M

m3 /d

)

ED EDI

MED MSF

NF RO

44

0

World America Europe Asia Pacific Middle East& Africa


24

100

Seawater desalting plants: Technologies

40

60

80

rcen

tag

e b

y te

chn

olo

gy

(%)

Distillation Reverse Osmosis Others

0

20

World America Europe Asia Pacific Middle Eastand Africa

Per

45


Brackish and Seawater Desalination

World Europe Middle East

Total installed capacity 50.4 Mm3/day 6.9 Mm3/day 26.9 Mm3/day

Distillation processes 41 % 16 % 65 %

Reverse osmosis 50 % 75 % 31 %

46


Others (NF, ED, EDI, etc) 9 % 9 % 4 %

25

Seawater Desalination

World Europe Middle East

Total installed capacity 31 Mm3/day 3.9 Mm3/day 22.3 Mm3/day

Distillation processes 64 % 21 % 79 %

Reverse osmosis 32 % 73 % 19 %

O h ( ) % 6 % 2 %Others (NF,ED, EDI etc) 4 % 6 % 2 %

47


Gulf

Southern Europe

Total World Seawater RO by Region

Australia & Pacific

West Asia

Latin America

Japan, Korea, Taiwan

Caribbean

North America

Rest Middle East

East Asia

North Africa

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Rest of Africa

Northern Europe

Australia & Pacific

Capacity (Mm3/d)

48


26

Seawater

Total World Desalination by Source

Pure water

Wastewater

River water

Brackish water

0 5 10 15 20 25 30 35

Brine

Capacity (Mm3/d)

49


Seawater

Total World Desalination by Source

Pure water

Wastewater

River water

Brackish water

0 5 10 15 20 25 30 35

Brine

Capacity (Mm3/d)

50


27

Seawater

Total World Reverse Osmosis by Source

Pure water

Wastewater

River water

Brackish water

Seawater

0 2 4 6 8 10

Brine

Capacity (Mm3/d)

51


Seawater

Reverse Osmosis by Source in Middle East

Pure water

Wastewater

River water

Brackish water

Seawater

0 1 2 3 4 5

Brine

Pure water

Capacity (Mm3/d)

52


28

Seawater

Reverse Osmosis by Source in Europe

Pure water

Wastewater

River water

Brackish water

Seawater

0 1 2 3 4 5

Brine

Pure water

Capacity (Mm3/d)

53


Six different membrane technologies are applied for the production of drinking and industrial water.

Membrane Technologies

These technologies are :

electrodialysis (ED)

electrodionization (EDI)

reverse osmosis (RO)

fil i ( ) nanofiltration (NF)

ultrafiltration (UF)

microfiltration (MF)

54

29

Membrane Process Main Application

Membrane Processes

Reverse Osmosis Desalination: Sea water

Desalination: Brackish Water

Electrodialysis Desalination: Brackish Water

Nanofiltration Removal: Sulphate, Hardness and natural organic matter

Ultra/Microfiltration Removal: Suspended and ColloidalUltra/Microfiltration Removal: Suspended and Colloidal matter/ Disinfection

55

Micro‐ and ultrafiltration

56

30

Definition Micro‐ and Ultrafiltration

Micro‐ and Ultrafiltration are:

Pressure or vacuum driven separation processes in which:

• Suspended matter (particles larger than 1 μm) are rejected;

• Colloidal matter (particles < 1 μm and > 0.001 μm) are (partly) rejectedrejected.

5757

What are micro‐and ultrafiltration membranes?

MF and UF membranes are:

Thin sheets;

or

Capillaries with thin walls;

Allowing to pass water under influence of differential ( )pressure (pressure or vacuum)

5858

31

Pore size

MF membranes have pores in the range of:

0.1 – 0.2 μm

UF membranes have pores in the range of:

0.01 – 0.05 μm

or less (down to approximately 0.005 μm)

5959

Types of MF/UF membranes

Tubular membranes (5 – 20 mm);

Flat membranes;

Spiral wound membranes;

Capillary membranes dominate the market;

60

Capillary membranes (0.5 – 5 mm) diameter;

Others play a minor role;

60

32

Drinking water production

Removal

Main large scale applications Micro‐ and Ultrafiltration

• Microbials e.g., Cryptosporidium, bacteria, viruses;

• Suspended and colloidal matter;

• Algae.

Pre‐treatment reverse osmosis and nanofiltration;

Removal

• Suspended and colloidal matter; Reduction SDI• Suspended and colloidal matter; Reduction SDI.

Waste water treatment/Water reuse

Removal

• Bacteria;

• Suspended and colloidal matter.

61

General

location: Clay Lane, United Kingdom

i i 2002

Ultrafiltration Clay Lane, Norit MT/X‐Flow

status: in operation 2002

source: ground water abstracted from

karstic (limestone) soil under the

influence of river water

capacity: 6,750 m3/h

Functions

disinfection: removal of cryptosporidium cysts

because chlorination and ozonation are

not effective.

62

33

Capillary membrane elements placed in a vessel

63

Clay Lane (under construction 2002)

64

34

MF/UF capacity for drinking water

65

Source: Panglish, 2007

Pore size membranes

µm nm

microfiltration 0 02 10 20 10 000

NB: Pores in reverse osmosis membranes are smaller than in nanofiltration membranes.

microfiltration 0.02 – 10 20 – 10,000

ultrafiltration 0.005 – 0.02 5 – 20

nanofiltration < 0.001 < 1

reverse osmosis < 0.001 < 1

The table “Energy consumption and pressure” shows that membranes with smaller pores require higher pressure and consequently more energy.

66

35

Whether particles can pass a membrane or (partially) not (are partially or fully rejected) depends on:

Rejection

Firstly

size of the particles;

size of the pores in the membranes;

So mechanisms of sieving is governing the process

67

Rejection

In addition

electrical charge of membrane pores;

nature membrane material;

electrical charge of particles (in particular for ions);

diffusion coefficient particles (ions);

process conditions e.g.,

t t• temperature

• salinity

• filtration rate (flux e.g., L/m2h)

68

36

dissolved < 0.001 µm (<1 nm)

colloidal 0 001 1 µm (1 1000 nm)

Size particles (diameter)

colloidal 0.001 – 1 µm (1 – 1000 nm)

suspended > 1 µm (> 1000 nm)

Size of inorganic ions (including attached water molecules)

H+ 0.053 nm H2O 0.33 nm

K+ 0 25 Cl‐ 0 24

69

K+ 0.25 Cl‐ 0.24

Na+ 0.37 NO3‐ 0.26

Ca2+ 0.62 HCO3‐ 0.42

Mg2+ 0.7 SO42‐ 0.46

Microbe µm

Size of microbials

µ

Algae > 10

Giardia cysts 5 – 15

Cryptosporidium oocysts 3 – 5

Coliform bacteria 0.1 – 10Coliform bacteria 0.1 10

Viruses 0.02 – 0.03

70

37

Removal

Removal RO NF UF MF ED

Inorganic compounds

‐mono valent: Na+, Cl‐ + +/‐ ‐ ‐ +

‐ di valent: SO42‐, Ca2+ + + ‐ ‐ +

Organic compounds

‐ synthetic organic compounds + + ‐ ‐ ‐

71

‐ natural organic matter + + ‐ ‐ ‐

Micro‐organisms + + + + ‐

Suspended / colloidal matter + + + + ‐

Energy consumption and pressure

bar kWh/m3 Heat

UF/MF 0.5 – 2 0.1 – 0.2 ‐UF/MF 0.5 2 0.1 0.2

NF 5 – 10 0.3 – 0.5 ‐

RO Brackish 10 – 20 0.5 – 1 ‐

ED ‐ 0.5 – 10 ‐

RO Seawater 50 – 90 2 – 4 ‐

72

Distillation

Price

‐1 – 4

$ 0.05‐0.1/kWh

0.16 GJ/m3

$ 5‐15/GJ

38

From the start of all five membrane technologies energy was a major issue

Energy consumption membrane technologies

Electrodialysis makes use of an electrical current.

Energy consumption is proportional with the removal of salt (ions).

Reverse osmosis, nanofiltration and ultra‐ and microfiltration are pressure driven membrane techniques.

Water is forced to flow through small pores in RO,NF,UF andWater is forced to flow through small pores in RO,NF,UF and MF membranes are:

• thin sheets

or

• tubes, capillaries with a thin wall

73

Energy Intensity Range (kWh/m3)

Energy intensity of the water cycle

Water Use Cycle Segments Low High

Water supply & conveyance 0.0 3.7

Water treatment (conventional) 0.03 0.05

Brackish water desalination 1.0 3.0

Seawater desalination 3.5 4.2

Water distribution 0.2 0.3

Waste water collection/treatment 0.3 1.2

Waste water discharge 0.0 0.1

Recycled treatment & distribution 0.1 0.3

74

Source: IDA Desalination Yearbook 2007‐2008

39

Euro/m3

Cost indications

Euro/m3

Seawater reverse osmosis 0.50 – 1.00

Brackish water reverse osmosis 0.25 – 0.50

Electrodialysis 0.25 – 0.50

Nanofiltration 0.15 – 0.25

Ultra/microfiltration 0.05 – 0.10

75

40

“B i P i i l f“Basic Principles of Ultrafiltration & Microfiltration”

Prof. Jan C. Schippers, PhD, MScM i K d PhDMaria Kennedy, PhD


Definition Micro‐ and Ultrafiltration

Micro‐ and Ultrafiltration are:

Pressure or vacuum driven separation processes in which:

• Suspended matter (particles larger than 1 μm) are rejected;

• Colloidal matter (particles < 1 μm and > 0.001 μm) are (partly) rejected.

2

41

What are micro‐and ultrafiltration membranes?

MF and UF membranes are:

Thin sheets;

or

Capillaries with thin walls;

Allowing to pass water under influence of differential ( )pressure (pressure or vacuum)

3

Pore size

MF membranes have pores in the range of:

0.1 – 0.2 μm

UF membranes have pores in the range of:

0.01 – 0.05 μm

or less (down to approximately 0.005 μm)

4

42

Pore size UF membranes

UF membranes have the ability to retain (dissolved) larger organic molecules;

That is why historically UF membranes have been characterized by molecular weight cut‐off (MWCO);

The concept of MWCO (95 % of a target compounds rejected) is a measure of the removal characteristic of membranes in terms of molecular mass (weight) rather than size;size;

Small particles are morphologically difficult to define (and measure). So it is useful to apply MWCO for UF membrane characterization;

5

Pore size UF membranes

Typically MWCO levels for UF membranes range from 1,000 to 500,000 Daltons;

Most UF membranes used for water treatment have approximately 100,000 MWCO;

A MWCO of e.g. 100,000 means that a reference polymer ( k ) i h l l i h f 100 000 D l i(marker) with a molecular weight of 100,000 Dalton is rejected for 95%;

Dextran is commonly used as a marker.

6

43

Pore size distribution

All membranes have a distribution of pore sizes;

This distribution will vary according to the membrane material and manufacturing process;

Nominal pore size is equal to average pore size;

Absolute pore size is equal to maximum pore size

7

Pore size of membranes 50 and 150 kDa

20

Source: Boerlage

4

8

12

16

20

Frequency (%

) 50 kDa 150 kDa

0

4

0 6 9 12 15 19 22 26 30 40 70

Pore size (nm)

8

44

Surface Porosity

The surface porosity is the part of surface which is “covered” by pores.

Porosity can be measured by analyzing processed images obtained ffrom microscopic analyses such as scanning electron microscopy (SEM), transmission electron microscopy (TEM) or atomic force microscopy (AFM) as well as pore size and pore size distribution.

9

Membrane Materials

MF & UF membranes are made of:

Organic polymers;

Inorganic materials such as ceramic, glass or metal.

Membranes made of organic polymers are dominating the market

10

45

Polymeric membranes

Synthetic organic polymeric membranes can divided into two classes i.e., hydrophobic and hydrophilic.

Hydrophilic polymers such as cellulose and its derivatives have been used widely for the manufacture of MF and UF membranes.

Hydrophobic membranes such as: polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyethylene (PE), or polypropylene( )(PP) are commonly used for MF and UF membranes as well;

Hydrophilic means that the membrane “likes” water and hydrophobic means that it “hates” water.

11

MF Membrane Materials

Hydrophilic

Polymeric materials

Hydrophobic

Polymeric materialsPolymeric materials

Cellulose esters

Polyamide

Polycarbonate Polysulphone/

Polyethersulphone

Polymeric materials

Polyvinylidene fluoride

Teflon;

Polypropylene;

Polyethylene;y p

Poly(ether‐imide)

12

46

Symmetric Membranes

Have pores of uniform size throughout.

Their thickness is ca. 10‐200 µm. µ

Resistance to mass transfer (hydraulic resistance) is determined by the total membrane thickness ‐ the thinner the membrane the higher the permeability.

13

Asymmetric Membranes

Have a very dense top layer with a thickness of 0.1‐0.5 µm supported by a porous sub‐layer with a thickness of 50‐150 µm. The pores change in size over the depth of the membrane.

These membranes combine the high selectivity of a dense membrane with the high permeability of a thin membrane.

14

47

Cross‐section of an asymmetric polysulphone UF membrane

15

Composite Membranes

Composite membranes are ‘skinned’ asymmetric membranes. However, the top‐layer and sometimes the support layer originate from different polymeric materials.

The support layer is usually already an asymmetric membrane on which a thin dense layer is deposited (of another material).

16

48

Removal efficiency

Whether particles can pass a membrane or not, depends mainly on:

size of the particles and their flexibility;

size of the pores;

pore size distribution

l h h So size exclusion mechanism or sieving mechanism is assumed to be dominant.

17

Water Flow and Water Flux in Membranes

18

49

Water Flow

Membranes have narrow pores;

RO, NF, UF and MF are pressure driven processes;, , p p ;

Pressure is needed to force water to flow through narrow pores;

RO membranes have the smallest pores;

MF membranes have the largest pores;

Consequently RO needs the highest pressure and MF needs the lowest pressure;

19

Water Flow

Quantity of water flowing through a membrane

(Q in e.g., m3/h) is proportional to:( g , / ) p p

Pressure: P

More correct: “difference in feed pressure

and product (permeate) pressure” ∆P

Membrane surface area : A

Permeability of the membrane: Kw

20

50

Water Flow

Qw = ∆P∙A∙Kww w

Qw : water flow (m3/h) or (L/h)

∆P : pressure difference (bar)

A : surface area membrane (m2)

Kw : membrane permeability (m3/m2.h.bar) or (L/m2.h.bar)

21

Water Flux

Water flux is the water flow per m2 membrane area or filtration rate (m3/m2.h or m/h or L/m2.h)

Jw : water flux (m3/m2h or L/m2.h)

w ww

Q P A KJA A

22

w wJ P K

51

Temperature and permeability

The permeability of membranes depends on viscosity of water and as a consequence on the temperature;

ηt = η20C ∙ (1.03)(20 – t)

As a consequence:

Kwt = Kw20C / (1.03)(20 ‐ t)

23

Question

A UF plant is filtering clean water (so no fouling) at a p g ( g)capacity of 200 m3/h at temperature of 25 oC and 0.2 bar.

What will be the required pressure, when the temperature drops to 5 oC?

24

52

Characterizing membrane performance

Normalized permeability and normalized clean water flux at 20 oC and 1 bar, are commonly used to characterize the performance of MF/UF membranes and are expressed as:

L/m2.bar at 20 oC

There is no difference since:

Jw20 = ΔP∙Kw20at 1 bar

Jw20 = Kw20 (normalized flux at 1 bar and 20 oC)

Remark: X‐Flow UF membrane have a Normalized clean water flux of approximately 1000 L/m2.bar.

25

Water Flux: Hagen Poiseuille Law

Hagen‐Poiseuille Law can be used, when the feed water is free of fouling components and we assume laminar flow in

: membrane porosity (‐)

r : membrane pore size (m)

P : Pressure (Pa)

x : membrane thickness (m)

the very small capillary pores in the membrane.

2

8

ε r ΔPFlux J

μ Δx τ

26

x : membrane thickness (m)

: membrane tortuosity (‐)

: water viscosity (Pa.s)

8μ Δx τ

53

Question

A UF plant equipped with membranes with pores of 0.1 µm and runs at constant capacity. The required pressure is 0.2 bar.

What will be the required pressure when the pores in the membranes are replaced with pores of 0.05 µm?

27

Resistance versus permeability

Frequently the concept “resistance” is used instead of permeability.

Resistance is the inverse of Permeability

Resistance = 1 / Permeability

So: Jw = ΔP∙Kw or

Jw = ΔP/η ∙ RmWhere:

η = μ is viscosity

Rm = resistance membrane

28

54

Increasing membrane resistance due to fouling

When water – containing suspended/colloidal matter – is filtered through a MF or UF membrane. The membrane resistance will increase due to depositing of suspended and colloidal particles on and/or in the pores.

As a result:

Th fl ill d h h i k The flux will decrease, when the pressure is kept constant;

The required pressure will increase, when the flux (filtration rate/capacity) is kept constant, which is common practice.

29

Increasing membrane resistance due to fouling

Membrane resistance will increase due to:

Blocking of membrane pores completely or partly by particles and/or organic polymers;

Depositing of a growing layer (cake) on the membrane surface by particles and/or organic polymers

30

55

Resistance‐in‐series model

Total membrane resistance comprises:

The resistance of the membrane (Rm)m The resistance due to particles deposited inside pores or blocking the pore entry (Rb)

The resistance due to particles forming a cake (Rc)

Total resistance total m b cR R R R

31

Hence, flux=

total m b c

P PJ

µ R µ R R R

Blocking & cake filtration mechanisms

Blocking filtration start first and is associated with rapid drop in flux!

Cake filtration will follow and is associated with slower rate of flux decline!

Slow drop

rapid drop

32

56

Filtration Mechanisms in MF/UF

Complete Blocking

One particle in water completely blocks one 1 pore or more pores, with no superposition of particles

Standard Blocking

Particles are deposited on the internal pore walls, decreasing the pore volume.decreasing the pore volume.

33

Filtration Mechanisms in MF/UF

Intermediate BlockingThe probability that particles can settle on other particles previously deposited and already blocking p p y p y gthe pores or particles can directly block membrane area.

Cake FiltrationEach particle can settle on other particles previously deposited and already blocking the pores but there is no room for particles to directlypores, but there is no room for particles to directly block membrane area.

34

57

Filter cakes

The cake can be classified as compressible or incompressible depending on the nature of the particles, such as shape, particle size distribution and rigidity.

Incompressible cake: can withstand cumulative drag stresses or any other external stresses without any significant structural changes (specific cake resistance will be constant over time and as the porosity is pressure and time independent).

Compressible cake: the drag stress will lead to a rearrangement of particles within the cake as internal stresses increase which means that the porosity of the cake will be at its minimum value.

35

Pore blocking

Mathematical equations describing pore blocking are rather complicated;

f However the most simple equation illustrates fairly good the process:

Where:

n = number of particles per liter;

V = feed volume;

N number of pores per m2 membrane surface;

P n VJ 1 ‐

R A N

N = number of pores per m2 membrane surface;

η = viscosity;

A = membrane surface area

This equation assumes that each particle can block one pore completely

36

58

Cake filtration at constant flux

J = ΔP∙Kw Frequently the concept of resistance R is used, instead of q y p ,

permeability:

Kw= 1/η∙Rt

Where:

η = viscosity of the water

Rt = total resistance is sum of resistance membrane (Rm), pore blocking (Rp) and cakeformation (Rc)

Rt = Rm + Rp + Rc

37


1 ΔPJ

R R R

If we assume that pore blocking does not play a dominant role in (in a certain stage), then fouling is mainly due to cake

m p C

t

R R R

1 ΔP

R

formation.

As a consequence:

38

m C

1 ΔPJ

R R

59


c

I VR

A

Where:

I = is a measure of the fouling characteristics of the (particles in the) water.

The value of I is a function of the nature of the colloids and is proportional to the concentration.

rk = resistance cake per mg cake per m2 membrane (mg/m2)

V/A = total filtered volume per m2

c = concentration of suspended/colloidal matter.39

kI c r

Effect particle size on resistance

According Carman – Kozeny the specific resistance of a cake with spherical particles is:

rk = 180∙(1 ‐ ε) / (ρ∙d2∙ε3)

Where:

ε = porosity cake

ρ = density particles

d = diameter particles

40

60


Substitution:

V/A = J . t because J = constant

in: Rc = I ∙ V/A = I ∙ J ∙ t

results in :

m

1 ΔPJ .

η (R I.J.t)

41


or

ΔPt = η.Rm .J + η.I.J2.tt η m η

So ΔPt (differential pressure at time t) is linear proportional with time and proportional with (flux)2 or J2. When no compression of the cake occurs.

As a consequence, flux has a very dominant effect on the development of ΔPt (pressure after filtering tminutes).

42

61

Questions

A UF plant runs at a constant flux of 50 L/m2.h at 20 oC.

At the start 0.1 bar is needed.

After 30 minutes filtration the pressure has been increase to 0.2 bar;

What will happen when we operate the plant at 100 L/m2.h?

Wh t ill b th t th t t d ft 30 i t

43

What will be the pressure at the start and after 30 minutes filtration?

Compressible & Incompressible Filter Cakes

Porosity

PressurePorosity

Pressure

ε

ε0

PressurePressure ε

ε

TimeTime

44

0 ,(n = compressibilityindex)

0, incompressiblecake

0, compressiblecake

nP

n

n

62

Remedial actions after fouling

Different measures are taken in practice e.g.,

backwashing with:

• water;

• air;

• water supported with air;

”flush and soak” (enhanced backwashing) with e.g., sodium hydroxide sodium hypochlorite acid hydrochloric ascorbic

45

hydroxide, sodium hypochlorite, acid hydrochloric, ascorbic, oxalic, detergents;

Cleaning in place (CIP) with chemicals, including circulation.

63

Prof. Jan C. Schippers, PhD, MSc

“Micro ‐ and Ultrafiltration elements, modules and systems”


25

RO

UF/MF

RO

Total capacity membrane technology in the world, installed/under construction

5

10

15

20

Mm

3/d

ay

UF/MF

ED

NF

UF/MF

2

0

5

1975 1980 1985 1990 1995 2000 2005

Year

UF/MF

ED

NF

Source: Wangnick 2004; Furukawa, 2002

64

Applications Micro‐ and Ultrafiltration

drinking water production;g p ;

waste water reuse;

industrial water production;

membrane bioreactor systems (emerging);

pre‐treatment for reverse osmosis systems (emerging).

Remark: Capillary membranes dominate currently the market.

3

Industrial11%

Other5%

Global installed Micro and Ultra Capacity

Drinking water

Wastwater Reuse21%

11% 5%

4

water63%

Source: David H. Furukawa (2002)

65

Types of MF/UF membranes

Tubular membranes (5 – 20 mm);

Flat membranes;;

Spiral wound membranes;

Capillary membranes (0.5 – 5 mm) diameter;

Capillary membranes dominate the market;

5

Others play a minor role;

—CH2—n

—CF2—n o

oo

oH

oAc

CH2oAc

oAcS

Polymeric Membrane MonomersSource: G. Amy

—CH2—CF2—

O||

S||

O

o

n

Polyethersulphone (PES)

O||

—O—C—O—

CH3|

C|

CH3 n

Polycarbonate (PC)

Polyethylene (PE)

Polyvinylidene fluoride (PVDF)

n

—CH2—CH—CH3

n

Teflon

oo

CH2oAc oH

Cellulose Acetate (CA)Ac: OCOCH3

nPolyphenylene‐Sulphide (PPS)

n

6

Polyethersulphone (PES)

O||

S||

O

CH3|

C|

CH3

o o

nPolysulphone (PSf)

Polycarbonate (PC)

—CH2—CH—OH

3Polypropylene (PP)

n

Polyvinylalcohol(PVOH)

—CH2—CH—CN n

Polyacrylonitrile(PAN)

66

Membrane Properties

(Clean Water) Permeability (CWP);

Pore Size or Molecular Weight Cutoff (MWCO);

Hydrophobicity (or Hydrophilicity);

Surface/Pore Charge;

7

Chemical Tolerance (pH, Chlorine).

Cross flow filtration

“Cross flow” filtration versus “Dead end” filtration

Historically tubular membranes were used and operated in “Cross flow” mode, to control membrane fouling;

The higher the cross flow velocity, the lower the rate of fouling;

At high cross flow velocities a major part of the

8

suspended/colloidal particles will not deposit on the membrane surface, due to high shear forces;

High cross velocities result in high energy consumption, due to high head losses.

67

Relative flux decline (Δf) in tubular UF membranes due to Poly Styrene Latices (80 nm) in Dead‐end and Cross flow mode

“Cross flow” filtration versus “Dead end” filtration

Dead end filtration

Has been introduced by the end of the eighties;

All suspended/ colloidal matter rejected is deposit on the membrane surface;

Fouling is mainly controlled by very frequent backwashing;

Energy consumption is much lower than in “Cross flow”

10

Energy consumption is much lower than in “Cross flow” mode;

68

Cross flow and dead end filtration

Backwashing/Cleaning

Hydraulic Backwash

every 30/45 min.

water, air, water/air or air scour with water;

Chemically Enhanced Backwash (CEB)

once per day/week;

chlorine (hypochlorite) and/or high pH;

12

low pH.

Cleaning‐in‐Place (CIP)

every week/month/year.

69

Pressure or suction

Driving force and energy consumption

pressure e.g., 0.5 – 2 bar in “vessel” type of modules

suction e.g., max. 1 bar in “immersed” type of systems

Energy Consumption

up to 0.2 kWh/m3 in: dead end mode

up to 5 kWh/m3 in: tubular systems operating in cross flow mode.

13

Filtration inside/outside versus outside/inside

In capillary membrane filtration two different mode of filtration are applied namely;

Inside to outside filtration,

and

Outside to inside filtration

k d / d d h b

14

Remark: In outside/inside mode the active membrane surface area in an element is larger. However backwashing is more critical.

70

Constant pressure versus constant flux

Initially MF/UF plants were operation at constant pressure;

Nowadays almost all plants run at constant flux.

Question: Why?

15

MF ‐ UF modules and systems

A great variety of micro‐ and ultrafiltration modules and systems are on the market.

Leading companies are e.g., Aquasource / Ondeo – Degremont – Suez

Hydranautics/Nitto

Kubota

Memcor (Siemens)

Mitsubishi

P ll/A hi Pall/Asahi

X‐Flow /Norit

Zenon/GE;

Gradually several other companies entered this market e.g., Dow Inge, Hyflux, Koch, Membrana etc.

16

71

Aquasource

capillary membranes;

filtration: inside to outside;

pressurized system up to 2 bar;

filtration mode:

dead end, or

semi dead end when activated carbon is added.

backwashing:

water (e g with chlorine or hydrogen peroxide)water (e.g. with chlorine or hydrogen peroxide)

outside to inside

largest element:

membrane area: 125 m2

capacity: 10 m3/h at flux of 80 L/m2‐h

17

Aquasource

18

72

Question

Why are there different fiber bundles in the elements?

19

Membrane surface area increase

20

125 m2 (1344 ft2) module‐DN 450

73

DN450 Aquasource UF modules

21

Question

What is the capacity of this plant?

22

74

Cross‐flow

23

Dead‐end

24

75

Hydranautics

capillary membranes;

filtration: inside to outside;;

filtration mode: dead end;

backwashing:

water ( e.g., chlorine or hydrogen peroxide);

outside to inside;

largest element/module

membrane area: 46 m2

capacity: 3.7 m3/h at 80 L/m2‐h

25

HYDRAcap Module

HYDRAcap 40: 30 m2 (320 ft2)

HYDRAcap 60: 46 m2 (500 ft2)

26

76

Ultrafiltration seawater pre‐treatment RO in Kindasa (Saudi Arabia)

27

Inge

capillary membranes of PESM, multibore;

filtration: inside to outside

pressurized system

filtration mode: dead end

Backwashing

water

outside to inside

largest element:

diameter: 25 cm length: 1.5 m

surface area: 50 m2

placed in horizontal vessels

diameter : 25 cm (inside) length : 1.7 m

capacity at 80 L/m2h : 4 m3/h28

77

Inge – Multi‐bore membranes and Rack units

29

Kubota

flat plate membrane elements;

filtration: outside to inside;;

vacuum inside (immersed);


backwashing: no

cleaning by air scour outside during no filtration (no vacuum)

largest element: 0.8 m2

maximum number of elements per module : 200

capacity at 10 L/m2h : 1.6 m3/h

application: Membrane Bio Reactor

30

78

Kubota membrane element

Vertically‐mounted, 0.8 m2 flat plate element

0.1 ‐ 0.4 microns pore size nominal

Development of “dynamic layer of protein and cellular material provides effective pore size of less than 0 01 microns”than 0.01 microns

31

Source: S. Judd

Kubota membrane module

150 ‐ 200 elements per module

200 element module used for >30 L∙s‐1 flow

32

Source: S. Judd

79

Memcor (Siemens)

capillary membranes of Poly Propylene;

filtration: outside to inside;;

pressurized systems (and submerged);


backwashing:

air (7 bar);

inside to outside;

largest element/module: 38 m2

diameter: 12 cm

capacity at 100 L/m2h : ~ 4 m3/h

33

34

80

Memcor Microfiltration units treating groundwater in Jordan, capacity 800 m3/h

35

Memcor unit

36

6x15 elements

‐ Diameter 10 cm‐ Length 100 cm

81

Backwash with air

38

Mitsubishi

capillary membranes

filtration: outside to inside

vacuum inside (immersed)


Backwashing

W t Water

inside to outside

40

82

Mitsubishi pilot plant

41

42

83

Pall

capillary membranes of PVDF and PAN;

filtration: outside to inside

pressurized system


Backwashing

Water

inside to outside

enhanced with air scour outside enhanced with air scour outside

largest element :

diameter : 16.5 cm length : 2.3 m 50 m2

placed in vertical vessels

diameter : 16 cm (6 inch) length : 2.3 m43

Pall

44

84

Upper HeadPotting

Module and Fibers

Fibers

Fiber Cut Off

0.1 micron nominal

Large ultrafiltration range

M t i l

45

Housing

Lower HeadPotting

Material PVDF

– Flexible and Mechanically Strong

– Chemically Stable

– Resistant to Oxidants

Unique backwash with water d i

Backwash

Treated water inlet

and air

Possible reagent addition

Combination of different phases

Wastewater outlet

46

Airinlet

85

Microza System, 300 m3/hChandler, Arizona

47

Questions

Why are the vessels of Memcor and Pall placed in a vertical position?

Why are the vessels of X‐Flow placed in a horizontal position?

Will a membrane (placed in a tube) filtering in id /i id d f i l i h b k hi

48

outside/inside mode, function properly with backwashing with water only? Why?

86

X ‐ Flow

capillary membranes of PES;


pressurized system


Backwashing

water

outside to inside

largest element:

diameter: 20 cm length: 1.5 m surface area: 34 – 40 m2

placed in horizontal vessels

diameter : 20 cm (inside) length : 6 m

capacity at 80 l/m2h : 10 ‐12.5 m3/h

49

Ultrafiltration

type: capillary membranes

concept: XIGAp

housing fit in standard vessels up to 6 m length

50

87

Question

Wh t i th f ti f th h i t l t b l t thWhat is the function of the horizontal tubes, close to the product tube?

51

52

88

Sulaibiya Ultrafiltration plant for pre‐treatment RO in Kuwait, 15,000 m3/h (treated domestic waste water)

53

Source: F. Knops Norit MT

X‐Flow/Stork

tubular membranes


pressurized system

filtration mode:

dead end, or

cross flow ( up to 5 m/s)

Backwashing

water

outside to inside

remark: in cross flow mode ‘ no backwashing ‘

Application

e.g., membrane bioreactor systems and landfill leachates

54

89

External loop configuration

NORIT Cross flow MBR

source: bioreactor

function: effluent polishing

application: process water

capacity: 85 m3/h

industry: dairy industry

L i I l d Location: Ireland

55

Side stream: Pumped and Airlift Norit X‐Flow

56

Source: S. Judd

90

Norit Airlift Membrane Bio Reactor for waste water treatment

57

Zenon (General Electric)

capillary membranes of PVDF;

filtration: outside to inside;

suction inside (immersed);

filtration mode : dead end;

backwashing;

Water

inside to outside

enhanced with air scour outside

largest cassette (module);

surface area: 370 m2

applications: industrial and drinking water, waste water and Membrane bioreactor systems;

58

91

The ZW‐4000 ZeeWeed® Cassette

Surface area: 370 m2

Dimensions

Height: 140 cm

Width: 60 cm

Depth: 160 cm

Packing density: 210 m2/m3

59

ZENON

®

The “ZeeWeed®” membrane cassette

60

92

ZENON

®

The ZeeWeed® Hollow Fibre

61

Cassette Trains

62

ZENON

®

93

Ceramic Modules

Ceramic Materials (e.g., aluminium oxide; α‐Al2O3 & γ‐ Al2O3)

Ceramic Tubes (Tubular)

Dead Endw/Periodic Backwash

Similar to hollow fiber (larger diameter)

Monolith Multi‐channel tubular membrane element

63

MF plant equipped with Ceramic membranes

64

94

Attributes/claims of Ceramic Membranes

High flux at relatively low pressure;

Very high backwash flux possible (50 x);

High durability against oxidants, strong acids, bases, and temperature (can be aggressively cleaned);

65

Hydrophilic membrane surface;

Claims long life/wear‐resistant (~ 20 Year)

95

“Membrane Fouling of MF & UF Systems”

Maria D. Kennedy, PhD


Membrane Fouling

One of major problems in operating of membrane processes is membrane fouling.

Membrane fouling is referred to as the flux decline of a membrane filter caused by the accumulation f i i i hof certain constituents in the

feed water on the surface of the membrane or in membrane matrix.

2

96

Removal of foulants by backwashing

TMP increases to maintain constant flux ‐ due to accumulation and/or adsorption of material onto the membrane surface (fouling)

f f The fraction of pressure that can be recovered by backwashing describes the “backwashable fouling”

“non‐backwashable fouling” is characterized by the increase in pressure after backwashing

3

How fouling affects membrane flux

The effect of membrane fouling can be examined through a simplified model – Hagen‐Poiseuille equation:

Where:

J: flux

ε: porosity of the membrane (ratio of the membrane pores area to total membrane area),

d mean pore diameter of the membrane dp : mean pore diameter of the membrane,

ΔP: trans‐membrane pressure,

δ: effective thickness of the membrane,

µ: viscosity of fluid.

4

97


When a membrane is fouled, porosity decreases, hydraulic diameter decreases, and effective thickness increases.

f It has been reported that pores appeared to be more preferable sites for adsorption (Jucker and Clark, 1994). That may explain why organic fouling typically causes more severe flux decline than particle/colloidal fouling (Lahoussine‐Turcaud et al, 1990), which is most likely to foul the membranes through mechanisms of pore blocking and cake formation.

It should be noted that the increase in membrane thickness due to the accumulation of foulants is not physical thickness of the fouling layer, but a hydraulically equivalent of an increase in thickness of clean membrane. This is because fouling layer and clean membrane may have different permeability.

5


In reality, more than one type of membrane fouling may occur simultaneously. In addition, the relationship of flux decease and the changes in ε, DH and δ cannot be established because they are very difficult to measure experimentally.

Therefore, alternative procedures have to be established to quantify the impacts of fouling. One commonly used approach is to lump all factors affecting flux exceptapproach is to lump all factors affecting flux except transmembrane pressure into one resistance term to form so‐called resistance model:

6

98

Classes of Membrane Foulants

Four classes of membrane fouling exist:

Inorganic fouling/scaling

Particulate/colloidal fouling

Microbial fouling

Organic fouling

7

Inorganic fouling & scaling

Scaling is caused by the accumulation of inorganic precipitates, such as calcium carbonate calcium sulphate bariumcarbonate, calcium sulphate, barium sulphate, and metal (e.g., iron, aluminium, & silica) oxides/hydroxides on membrane surfaces or within the pore structure.

Precipitates are formed when the concentration of chemical species exceeds their saturation concentration. Scaling is a major concern in RO NF as inorganic salts are rejected.

8

99

Inorganic fouling and scaling in MF/UF

For microfiltration (MF) and ultrafiltration (UF), inorganic fouling due to concentration polarization is much less profound as ions are not rejected by MF/UF membranes However fouling can occur due to interactions betweenMF/UF membranes. However, fouling can occur due to interactions between ions and other fouling materials (i.e., organic polymers) via chemical bonding.

Some pretreatment processes for membrane filtration such as coagulation and oxidation, if are not designed or operated properly, may introduce metal hydroxides on membrane surface or within pore structure.

Inorganic fouling/scaling can be a significant problem for make‐up water of caustic & sodium hypochlorite solutions. In particular, Enhanced Backwashing involves the automatic injection of a chemical (e.g., sodium hypochlorite at pHinvolves the automatic injection of a chemical (e.g., sodium hypochlorite at pH 10‐12) into the backwash water. Since dissolved calcium is not rejected by MF/UF membranes, the backwash water (permeate) may be supersaturated with calcium carbonate at pH 10‐12, and precipitation of calcium carbonate can occur!!

9

10-4 10-3 10-2 10-1 1 101 102 103

10-1 1 101 102 104 105 106103

m

nm

1 102 103kDa

Particulate & Colloidal Fouling – IUPAC definition

1 10 10kDa

fatty acidscarbohydratesamino acidshydrocarbons

molecules

polysaccharides

suspended particles

bacteria

phytoplanktonviruses

humic acid

Dissolved

fulvic acid

3.5 kDa 0.45 m

Particulates

Colloidal (IUPAC)

NON‐COLL. COLLOIDAL

10

100

Particulate/colloidal fouling in UF/MF

Algae, bacteria, and certain natural organic matter fall into the size range of particles and colloids.

The flux decline caused by the accumulation of biologically inert particles and The flux decline caused by the accumulation of biologically inert particles and colloids on the membrane surface is largely reversible by hydraulic cleaning measures such as backwash and air flushing/scrubbing.

Irreversible fouling by particles and colloids may occur if they have smaller size relative to the membrane pore size. Therefore, those particles and colloids can enter and be trapped within the membrane structure matrix, and not easily be cleaned by hydraulic cleaning.

11

Microbial/Biological Fouling

Microbial fouling is a result of formation of biofilms on membrane surfaces. Once bacteria attach to the membrane, they start to multiple d d t ll l l t i b t (EPS) t fand produce extracellular polymetric substances (EPS) to form a

viscous, slimy, hydrated gel.

EPS typically consists of heteropolysaccharides and have high negative charge density. This gel structure protects bacterial cells from hydraulic shearing and from chemical attacks of biocides such as chlorine.

Severity of microbial fouling is greatly related to the characteristics of the feed water. Water quality parameters that indicate the potential of q y p pmicrobial fouling are classified into three categories:

(a) Parameters indicating the abundance of microbes

(b) Parameters indicating nutrient availability,

(c) Parameters indicating environmental conditions for microbial growth

12

101

Organic fouling in MF/UF

Surface water (lake, river) typically contains higher NOM than ground water, with exceptions. For source water high in NOM, organic fouling is believed to be the most significant factor contributed to flux decline (Mallevialle et al

NOM

be the most significant factor contributed to flux decline (Mallevialle et al., 1989; Lahoussine‐Turcaud et al., 1990).

MF/UF usually remove insignificant amount of organic matter, as measured by dissolve organic carbon (DOC). DOC as an indicator for organic fouling is neither proper nor adequate.

Hydrophobic

- Strong acid : fulvic & humic acid - Weak acid : alkyl monocarboxylic acid & dicarboxylic acids

- Weak hydrophilic acids : Hydroxy acid, sugar- acids & sulfonic acids

- Hydrophilic nuetrals : polysaccharides, low MW alkyl- alchols& amides- Hydrophilic bases : low MW alkyl amines and amino- acids

Transphilic Hydrophilic

13

Organic compounds from degradation of plants and animals in aquatic environments.

CO

OH

C

C OH

O

Caboxyl Alcohol

E h

Natural Organic Matter (NOM)

Humic substances (humic/fulvic acids) contain mainly carboxylic and phenolic groups. Comprise over 50% of dissolved organic carbon (DOC).

Non humics are composed of proteins, amino acids and carbohydrates. They account for 20‐40% of DOC. O

HO C

OH

OH

HOOH

C O

OHOH

O C

OH

O C

C OH

O

C OH

OH

OH

O C

OH

O C

OH O

C OH

O

C OH

O

C OH

HC=O

OH

C O

OC C

OC C R

C NH2

Carbonyl

Phenolic

Ether

Esher

Amine

Thurman, 1985

O

C OHO

HO C OH

OH

O C

OH

C O OH

C O

O

OHO

HO C

OH

O C

OH

C O

O

C OH

O

C OH

O

C OH

HO

OH OH

OH

O

C OH

Type structure of fulvic acid (Schnitzer and Khan, 1972)

COOH COOHCOOH

HO

OH OH

HOR-CH

[HC-CH]4

HC=O

O O

H

R-CH

C=O

NH

NH

N

OH

COOH

COOHO

O

CH

CH

O

O

N

O O

CH2

HC=O

OO

O O

H

OHO

Type structure of humic acid (Stevenson, 1982)

14

102

NOM Characterization

Important characteristics that affect the interaction of NOM with a membrane are hydrophobicity (aromatic) versus hydrophilicity( li h ti ) t f th NOM i (MW dMW di t ib ti ) f th

pH

12

9-OH → -O￣￣

(aliphatic) nature of the NOM, size (MW and MW distribution) of the NOM, and it’s content in terms of functional groups.

NOM can be characterized by size, structure and functionality (charge density)

Size of NOM: NOM size is expressed as the molecular weight (MW)

NaOH

-COOH → -COO￣￣

3

the molecular weight (MW)

SUVA is an index of aromaticity(hydrophobicity) of water, and is defined as (SUVA = UVA254/DOC)

Charge density of NOM

15

Fraction categories Definitions

H i d F l i id

NOM characterization by LC‐OCD (Liquid Chromatography ‐ Organic Carbon Detection)

Humics (HS)Humic and Fulvic acids (1,000~20,000 Da)

Building Blocks (HS‐Hydrolysates)

Weathering and oxidation products of Humics (300~500 Da)

Low Molecular Weight (LMW) Organic‐Acids

the summaric fraction for all aliphatic low‐molar‐weight organic acids(< 350 Da)

LMW Neutralsalcohols, aldehydes, ketones(< 350 Da)

Polysaccharides(including Amino

Sugars, polypeptides and proteins)

Associated with peptides or proteins and originated from algae and bacteria(>20,000 Da)

16

103

Interaction between fouling materials and membranes

Electrostatic and hydrophobic/hydrophilic interactions between membranes and fouling materials have a significant bearing on membrane fouling ‐ this is particularly true of more difficult fouling problems caused by adsorption of natural organic matter and biopolymers on the membrane.

The balance between the forces of electrostatic repulsion and hydrophobic adhesion determines the outcomes of membrane fouling, as well as the efficiency of chemical cleaning.

17

Hydrophobic Interactions

Hydrophobic interactions can be described as “like attracts likes”. That is, there is a natural tendency of attractionbetween membranes and solutes with similar chemical structures.

Hydrophobic attraction results from van der Waals force between molecules

Hydrophobic adhesion is an important mechanism for fouling dominated by NOM because of the high molecularfouling dominated by NOM because of the high molecular weight of NOM compared to their charge density –providing great potential for hydrophobic adhesion

18

104

Hydrophobic/Hydrophilic membrane materials

Hydrophobicity of membrane media is usually characterized by (water) contact angle. The greater the contact angle, the more hydrophobic of a membrane medium ismembrane medium is.

A plot of data from Cheryan (1998) is depicted ibelow, which represents an approximate order of hydrophobicity of various membrane media.

PAN ‐ polyacrylonitrile,

RC ‐ regenerated cellulose,

PS – polysulfone,

PES – polyethersulfone,

PVDF – polyvinylidene fluoride,

PP – polypropylene

19

Impact of Membrane Material on Fouling

Hydrophilic membrane materials (and membrane coatings) with a highly negative surface charge are important in preventing adsorption of (natural) organic matter & colloids.

Membrane permeability is very sensitive to the number of large pores and their loss via plugging, adsorption etc., results in rapid flux decline. Therefore, membrane porosity, pore size distribution, surface roughness & charge are very important.

The nature, size & charge of solutes in the feed water is important as colloids, suspended solids, macrosolutes, microsolutes adsorb or plug pores on the membrane surface.

20

105

pH and electrostatic charge repulsion effects

Surface charge of membrane media is the result of ionization of particular functional groups on the membrane surface (e.g., g p ( g ,carboxyl and amine). Because ionization of a functional group depends on pH, surface charge is also pH‐dependent. In the pH range of natural water, most membranes appear to have a neutral to negative net surface charge.

Aquatic humic substances are generally polyprotic acids (Malcolm, 1985). Major acidic functional groups include carboxylic and phenolic functional groups. Over two thirds of acidic functional groups dissociate at pH of natural water; therefore colloids, particles, and dissolved organic matter typically carry negative charges at the pH of natural water.

21

pH and electrostatic charge repulsion effects

Therefore, there is a tendency of electrostatic repulsion between membranes with a negative surface charge and NOM constituents with a high charge density!!

Conceptual sketch of solute and particle rejection at charged membrane surface (Braghetta et.al., 1997)

22

106

Role of pH, ionic strength and divalent ions in membrane fouling

Conditions other than pH may also affect the interactions between fouling materials and membranes. For example, p ,high ion strength of a solution can compress “double electric layer” of colloids, which could reduce their repulsion to negatively charged membranes.

Another example is divalent cations, which can act as “salt bridge” between a negatively charged membrane and other negatively charged species in the fluid by charge neutralization. It has been reported that high ion strength and high calcium concentration increased the tendency of membrane fouling (Clark and Jucker, 1993; Hong, 1996).

23

Conceptual model of Membrane Fouling

For membrane fouling dominated by the adsorption of natural organic matter, and dominated by microbial causes to a less extent, the fouling d l i b ill t t d b i l t l d l b land cleaning can be illustrated by a simple conceptual model as below.

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107

“MF & UF MEMBRANE CLEANING: backwashing, flushing & chemical cleaning”

Maria D. Kennedy, PhD


Cleaning is an integral part of membrane process operation, and has a profound impact on the performance &

Membrane Cleaning

economics of membrane processes

2

108

Categories of Fouling

Membrane fouling is a complicated phenomenon and typically resulted from multiple causes.

(a) inorganic fouling/scaling

(b) particle/colloidal fouling

(c) microbial fouling

(d) organic fouling

3

Hydraulic Cleaning (Backwashing with permeate)

Membrane Cleaning

Hydraulic Cleaning (Chemically Enhanced Backwashing whereby a chemical is added to the permeate)

Flushing with feed water and feed water/air mixtures

Chemical Cleaning

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109

Effects of various operating strategies against different types of fouling

Effects of operating strategy

Type of fouling

p g gy

Hydraulic Cleaning / Backwashing Feed Chlorination Feed Acidification

Chemical Cleaning

Inorganic ‐ ‐ ++ ++

Particulate ++ ‐ ‐ ++

Microbial + ++ +* ++

Organic ‐ + ‐ ++g

Chemical cleaning is an effective control strategy for all types of membrane fouling.

5

Backwashing is performed by automatically by reversing the flow of permeate (ca. every 20 – 40 min), from the permeate tank into

Hydraulic Cleaning (Backwashing)

the membrane for a duration of ca. 30 sec.

It is generally effective in removing particle cakes from the membrane surface, and it can also remove foulants from the membrane interior, particularly when performed with a chemical cleaning solution (Enhanced Backwash).

The ideal situation regarding backwash flux and frequency is to use a high flux as frequently as possible. However, such practice results in a very low net flux, as permeate is consumed in backwashing ‐ It is therefore desirable to optimise the backwash flux & frequency.

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110

TMP

Fouling reversibility by hydraulic & chemical cleaning

1. Flushing & Backwashing

2. Enhanced BW with NaOH pH 12

Slope = rate of BW fouling

Slope = rate of nBW foulingReversible F&BW

Reversible EBW

nBW1

BW1

nBW2

BW2

1

2

TMPf

BW with permeate

Irreversible

TMP0

nBW1

Time

(source: adapted from Peavy et al., 1984)

7

How does backwashing work?

(a) At initial of BW

40

60

80

100

Turb

idity

(FN

U)

85

90

95

100

x R

esto

ratio

n (%

)

Pb= 1.6 bar

(b) During BW

0

20

0 1 2 3Ti me d u r i n g BW (min )

80

85

Flu

Turbidity Res toration

8

111

Filtration Process

9

Backwash Process

10

112

50

100

BW BW CEB

Backwashing Process

‐100

‐50

0

50

flow

Filtration Filtration Filtration Filtration

‐250

‐200

‐150

time

11

Reversibility of fouling by backwashing

Fouling is not always fully reversible by backwashing in surface water treatment

applications (J = 180 L/m2.hr, BW every 30 mins for 30 sec)

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113

20

25

humics

building blocks

PPrere‐‐filtered 0.45filtered 0.45µµmm

LC‐OCD minor.

What causes UF/MF membrane fouling in surface water treatment?

0

5

10

15

0 20 40 60 80 100

retention time (min)

Inte

nsity

OC

polysaccharides

building blocks

LMW acids and LMW humics

LMW neutrals


Feed pre-filtered 0.45um Permeate after UF

Comparing LC‐OCD results of UF feed water & permeate showed high rejection of polysaccharides, suggesting they are major foulants. Humics, LMW acids, building blocks & neutrals amphiphilics were also rejected.

13

30

h i

What is removed by backwashing a UF membrane treating surface water?

0

5

10

15

20

25

0 20 40 60 80 100

Inte

nsity

OC

humics

building blocks


LMW neutrals

polysaccharides


Backw ash colloidal NOM colloidal NOM permeate

Backwashing with permeate removed polysaccharides. LMW acids, building blocks and neutral amphiphilics may have contributed to non‐backwashable) fouling

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114

Enhanced Backwashing

Enhanced Backwash: a low dose (ca. 200 ppm) of oxidant or disinfectant is automatically injected into the permeate during backwashing in order to enhance cleaning. An enhanced backwash is performed evry 4‐8 hrs for about 10‐15 minutes.

An enhanced backwash comprises 3 steps: firstly a backwash with permeate (approx 30 s) is performed to remove accumulated particles from the hollow fibersremove accumulated particles from the hollow fibers. Secondly, a short soak (10‐15 mins) with a low dose of oxidant/disinfectant to remove adsorbed foulants from the membrane and finally another short backwash (with permeate) to remove the chemicals from the systems.

15

Chemically Enhanced Backwash

16

115

50

100

BW BW CEB

Backwashing Process

‐100

‐50

0

flow

Filtration Filtration Filtration Filtration

‐250

‐200

‐150

time

17

Backwashing Enhanced Backwash

Backwashing Optimization

Backwashing

optimization

Backwashing frequency

Enhanced Backwash

Optimization

Chemical dose

Backwashing pressure/flux

Backwashing duration

Frequency

Soakage time

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116

100100 100

Effect of BW Pressure/Flux & Time

25

50

75

85

90

95

C /

P R

ati

o (%

)

Flu

x R

esto

rati

on

(%)

85

90

95

Flu

x R

esto

rati

on

(%

)

080

1 2.5 4 8Pressure Ratio (Pb/Pf)

t=0.5min t=1min t=2min

t=0.5min t=1min t=2min

80

0.5 1 2BW Duration (min)

PR=1 PR=2.5 PR=4 PR=8

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Backwashing & chemical cleaning reduce net flux in MF/UF systems

Net flux (J ) = V V VNet flux (J ) = V V V

Gross flux (Jgros) = Vf

tf .Am

Gross flux (Jgros) = Vf

tf .AmVf = filtrate vol.

tf = filtration time

Am = membrane area

Vbw = vol. backwash

Net flux (Jnet) = Vf - Vbw - Vcc

(tf + tbw +tcc)Am

Net flux (Jnet) = Vf - Vbw - Vcc

(tf + tbw +tcc)Am

Vcc = vol. cleaning

tbw = backwash time

tcc = chem. clean time

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117

70

Backwashing Optimisation: Pressure, Duration & Water Consumption

Vf ‐ VbwJnet = (tf + tbw)Am

30

40

50

60

70

et F

lux (L

/m2.h

) • reduce BW pressure/flux

20

1 2.5 4 8

N

Pressure Ratio (Pb / Pf)

t=0.5min t=1.0min t=2.0min

• reduce BW frequency

• reduce BW time .

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Water Recovery = Q * 100Water Recovery = Q * 100

Water Recovery : Single stage UF

100% 85‐98%

Feed Filtrate

Water Recovery = Qpermeate * 100

Qfeed

Water Recovery = Qpermeate * 100

Qfeed

2‐15%

Feed

Concentrate

Filtrate

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118

Water Recovery: Dual Stage UF

100%

2‐15%

Feed

Concentrateprimary UF

Filtrate

98‐99,9%

Concentratesecondary UF

0,1‐2%

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Flushing: water & air/water mixtures

Particles deposited on the membrane surface can be removed with a turbulent flow with feed water (or permeate) parallel to the membrane surface. This cleaning method is called forward flush (cross flush). The velocity with which the forward flush is performed is much higher than the velocity during filtration.

Recently a new technique has been introduced the

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Recently a new technique has been introduced, the AirFlush® was introduced . This technique makes use of air to create higher turbulence compared to a water flush

119

Forward flushing with feed water can also be used to flush material out of the hollow fibres.

Forward Flushing with water

System efficiency increases as down‐time is short (3‐10 sec) & product water is not consumed.

Feed eedwater

25

What is happening during CF?

95 % of turbidity removed within 3‐5 sec

50

100

150

200

250

Turbidity (FNU) BW PeriodsCF Periods

Above 5 sec: almost no further flux restoration.

0

1 10 100

Time (sec)

26

120

Effect of Crossflush Velocity

70

80

Flux

(L/h

.m2)

v = 0.8 m/s v = 1.6 m/s v = 1.8 m/sCh.C BWBW BWCh.C

CWF

60

0 2 3 4 5 6 7Time (hr)

27

)

Combining CF & BW Improves Cleaning

BW (0.5): P=0.5bar, 0.5min

85

90

95

100

Flux

res

tora

tion

(%)

25

50

75

100

C/P

(%)

BW (1.0): P=1.0bar, 1.0min

CF+BW : simultaneous

CF/BW : CF followed by BW

80

B.W.(0.5) B.W.(1.0) B+C C/B

F

0

Flux Res. C/P

28

121

6570

CHC CHC

O NLY BW O PERATIO N

Efficiency with & without crossflushing

Net Flux (BW & CHC)

ca. 41 L/m2‐hr

20253035404550556065

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18T im e ( h o u r )

Flux

(L/m

2.h)

at 2

0oC

CHC

CHC

CHC

T =5. 6 h o u r T =5. 1 h o u r T =4 . 2 h o u r

80CF / B W O P ERATIO N

Net Flux (CF/BW & CHC)

ca. 47 L/m2‐hr

20

30

40

50

60

70

80

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18T i m e ( h o u r )

Flux

(L/m

2.h)

at 2

0oC

C h e m ic a lC le a n in g

T =14 . 4 h o u r

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Pipe center line

Liquid slug

Forward Flushing with water & air

Vf

w

Vb

Liquid slug

air slug

Liquid film

Liquid slug

Two‐phase, gas‐liquid flow increases turbulence within hollow fibers!

Wall shear stress and superficial liquid velocity will increase!

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122

Flow regime in vertical upward two‐phase flow

Source:

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Source:

Verberk, 2005

Distribution of velocity

Tubular membrane module (3”).

During a forward flush at an average water velocity of 0.24 m/s.

Capillary membrane module (3”).

During a forward flush at an average water velocity of 1.01 m/s.

Water distribution during water and air flush at uL,s = 0.2 m/s and uG,s = 0.65 m/s

32

123


Average water velocity = 0.195 m/s. Average air velocity = 0.419 m/s.

Average water velocity = 0.92 m/s.Average air velocity = 1.119 m/s.

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Removal percentage of suspended solids at constant water velocity of 0 2 m/s and varying air velocities (T= 20oC)


uL,s (m/s) uG,s (m/s)Removal

percentage (%)

0.2 0 9

0 2 0 1 49

velocity of 0.2 m/s and varying air velocities (T= 20oC)

0.2 0.1 49

0.2 0.3 74

0.2 0.4 72

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124

Air/Water flushing in dead‐end UF systems

More turbulence is created by the water and air flow compared to single phase liquid flow. From experiments, the optimal values were 0.2 m/s for the water velocity and 0.3 m/s for the air velocity (Verberk, 2002).

Pressure drop experiments showed that the head loss when using a two‐phase flow is lower than when the module is flushed with water.

The water and air flush enables only the removal of material

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The water and air flush enables only the removal of material accumulated on the membrane surface. Material in the membrane pores is not removed by the combined water and air two‐phase flow. Therefore this cleaning method should always be combined with a back flush.

Considerations when flushing with air and water in UF hollow fiber systems

Equal distribution of water and air over the cross sectional area of a membrane module is important to have the same cleaning conditions in every membrane in a module.

Verberk (2005) found that water is well distributed over the cross sectional area of a membrane module when a forward flush is performed. All membranes are flushed with almost the same velocity.

When an AirFlush® is performed some membranes areWhen an AirFlush® is performed some membranes are flushed with water and air, while other membranes have a stagnant water level.

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125

Chemical cleaning is performed when flushing and/or backwashing cannot restore the flux.

Chemical Cleaning

Compared to an enhanced backwash, the chemical dose is higher when performing a chemical cleaning (ca. 400 ppm), the duration of chemical cleaning is longer i.e. few hours and the frequency of chemical cleaning is usual much lower (approx 1 per week).

The enhanced backwash can be fully automated as it isThe enhanced backwash can be fully automated as it is performed simulatneously with the routine backwash. However, chemical cleaning involves labour to make up the chemicals, fill and flush the system etc,.

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Chemical Cleaning of MF/UF systems

Once the cause(s) of membrane fouling is identified, various cleaning chemicals can be used to remove fouling

Category Major Functions Typical Chemicals

Caustic Hydrolysis, solubilization NaOH

Oxidants/Disinfectants Oxidation, disinfection NaOCl, H2O2, peroxyacetic acid

various cleaning chemicals can be used to remove fouling materials from the membrane and to restore the membrane flux.

Acids Solubilization Citric, nitric, hydrochloric acid

Chelating Agents Chelation Citric acid, EDTA

SurfactantsEmulsifying, dispersion, surface conditioning

Surfactants, detergents

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126

Chemical Cleaning: Caustic

Caustic is typically used to clean membranes fouled by organic and microbial foulants. The function of caustic is 2‐fold: (1) hydrolysis, and (2) solubilization.

There are a number of organic materials including polysaccharides, and proteins can be hydrolyzed by caustic. Tertiary structures of proteins are likely to be disrupted and proteins are reduced to peptides Fats and oils also reactproteins are reduced to peptides. Fats and oils also react with caustic through saponification, generating water‐soluble soap micelles.

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2 2

pre‐filtered 0.45µm Colloidal NOM

Identification of foulants removed by chemical cleaning (caustic)

0

1

0 20 40 60 80 100

Inte

nsity

OC

polysaccharides

building blocks


LMW neutrals

0

1

0 20 40 60 80 100

Inte

nsity

OC

polysaccharides

building blocks


LMW neutrals

retention time (min) retention time (min)

Neutral amphiphilics, building blocks and LMW polysaccharides appeared responsible for non‐backwashable fouling.

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127

Chemical Cleaning: Oxidants

Most common oxidants used for membrane cleaning include chlorine and hydrogen peroxide.

The oxidation of organic polymers generates more oxygen containing functional groups such as ketone, aldehyde, and carboxylic acids. The existence of these functional groups generally increases hydrophilicity of their parent compounds Therefore oxidation reduces the adhesion ofcompounds. Therefore, oxidation reduces the adhesion of fouling materials to membranes.

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Why Chemical Cocktails ?

Oxidants are often mixed with caustic to form a cleaning “cocktail”. There are three reasons to mix oxidants, specifically chlorine with caustic:

To enhance cleaning efficiency. The mixture provides a synergy for NOM dominated fouling because the fouling layer tends to have more open structure at caustic conditions This synergy provides more access to chlorineconditions. This synergy provides more access to chlorine to reach inner layer of fouling materials, facilitates the mass transfer and reactions between chlorine and fouling materials, and enhances the cleaning efficiency.

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128

1 Fl hi & B k hi

TMP

Fouling reversibility by hydraulic & chemical cleaning

1. Flushing & Backwashing

2. Enhanced BW with NaOH pH 12

Slope = rate of BW fouling

Slope = rate of nBW foulingReversible F&BW

Reversible EBW

I iblnBW1

BW1

nBW2

BW2

1

2

TMPf

BW with permeate

Irreversible

TMP0

nBW1

Time

(source: adapted from Peavy et al., 1984)

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Why Cleaning Cocktails?

To control of excess oxidation to membranes and other module components. At acidic conditions, chlorine is such a strong oxidant that the potential of damage to membrane and other filter components increases to a great extent. Mixing chlorine with caustic will prevent excess oxidation by chlorine.

To simplify the equipment and operation of membrane cleaning Both caustic and oxidants are needed for efficientcleaning. Both caustic and oxidants are needed for efficient membrane cleaning. Mixing them allows the cleaning to be conducted in one step.

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Membrane Cleaning: acids and chelating agents

Acids are used primarily to remove scales and metal dioxides from fouling layers. When a membrane is fouled by iron oxides, citric acid is very effective because it not only dissolves iron oxides precipitates, but also forms complex with iron.

If there is coexistence of divalent cations (calcium, for example) and natural organic matter, the “salt bridge” effect of divalent cations can cause a denser and moreeffect of divalent cations can cause a denser and more adhesive fouling layer. The removal of divalent cations by either acids or chelating reagent such as EDTA can also improve the cleaning of membranes fouled by organic foulants (Hong and Elimelch, 1997).

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Effect of chemical cleaning on Membrane permeability after fouling by Delft Canal water

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Membrane Cleaning: surfactants

Surfactants are compounds that have both hydrophilic and hydrophobic structures & they can form micelles with fat, oil, and proteins in water and help to clean the membranes fouled by these materials.

Some surfactants may also interfere with hydrophobic interactions between bacteria and membranes (Paul and Jeffery, 1984; Ridgway et al., 1985, Ridgway, 1988; Rosenberg and Doyle 1990)Rosenberg and Doyle, 1990).

In addition, surfactants can disrupt functions of bacteria cell walls. Therefore, surfactants affect fouling dominated by the formation of biofilms.

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Effect of Operating Parameters on Chemical Cleaning Efficiency

Membrane cleaning is conducted through chemical reactions between cleaning chemicals and fouling materials, therefore factors that affect mass transfer and chemical reactions such as concentration, temperature, length of cleaning period, and hydrodynamic conditions all affect cleaning efficiency.

Chemical compatibility of membrane and other filterChemical compatibility of membrane and other filter components and systems limits the type and the maximum allowable concentration of a chemical to be used during cleaning.

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Conclusions

Pilot plant tests are required to optimize the:

1. flux, duration and frequency of backwashing,

2. the chemical concentration, duration and frequency of chemically enhanced backwashing

3. the frequency and velocity of (air/water) flushing, and

4. the chemical concentration, frequency and conditions of chemical cleaningchemical cleaning.

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Advanced Water Treatment - ISS Institute · Advanced Water Treatment ... Micro ‐ and...

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