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Transcript of 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
2
“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
Source: DesalData 2008
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
Source: DesalData 2008
23
20
Desalination capacity in the world by sub‐region
0
5
10
15
20
Capacity (Mm3/d)
43
Source: DesalData 2008
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
Source: DesalData 2008
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
Source: DesalData 2008
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
Source: DesalData 2008
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
Source: DesalData 2008
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
Source: DesalData 2008
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
Source: DesalData 2008
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
Source: DesalData 2008
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
Source: DesalData 2008
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
Source: DesalData 2008
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
Source: DesalData 2008
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
Delft – April 2010
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
Cake filtration at constant flux
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
Cake filtration at constant flux
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
Cake filtration at constant flux
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
Cake filtration at constant flux
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”
Delft – April 2010
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);
filtration mode: dead end;
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);
filtration mode: dead end;
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)
filtration mode: dead end
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
filtration mode: dead end
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;
filtration: inside to outside
pressurized system
filtration mode: dead end
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
filtration: inside to outside
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
Delft – April 2010
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
How fouling affects membrane flux
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
How fouling affects membrane flux
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
Delft – April 2010
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
4
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.
6
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)
12
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
retention time (min)
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 acids and LMW humics
LMW neutrals
polysaccharides
retention time (min)
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
14
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
18
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
19
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
20
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 .
21
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%
23
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
24
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
29
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!
30
122
Flow regime in vertical upward two‐phase flow
Source:
31
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
Distribution of velocity
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.
33
Removal percentage of suspended solids at constant water velocity of 0 2 m/s and varying air velocities (T= 20oC)
Distribution of velocity
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
35
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.
36
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,.
37
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
38
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.
39
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 acids and LMW humics
LMW neutrals
0
1
0 20 40 60 80 100
Inte
nsity
OC
polysaccharides
building blocks
LMW acids and LMW humics
LMW neutrals
retention time (min) retention time (min)
Neutral amphiphilics, building blocks and LMW polysaccharides appeared responsible for non‐backwashable fouling.
41
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.
42
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.
43
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)
44
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.
45
129
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).
46
Effect of chemical cleaning on Membrane permeability after fouling by Delft Canal water
47
130
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.
48
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.
49
131
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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132