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TRANSCRIPT
Effects of Membrane Age and Chemical Cleaning on Membrane Properties and Performance in
Drinking Water Treatment
Yi He1, Jatin Sharma1, Ram Bogati1, Carl Goodwin2, Kerri Marshall2 and B. Q. Liao1
1 Department of Chemical Engineering, Lakehead University
2 Environmental Division, City of Thunder Bay
1
October 28 , 2011 City of Thunder Bay, ONTARIO
1
Overview
Discussions and Conclusions
Results
Materials and Method
Introduction
Ongoing Research
2
Introduction
Ultrafiltration (UF) is considered as a very promising process to supply high quality drinking water:
Remove suspended solids, colloidal material
and pathogenic microorganisms ;
The quality of the produced water is constant;
Small footprint, decreased requirements for
pretreatment, and easy to scale up and operate.
UF applied in drinking water production has increased rapidly for the past decade:
More stringent drinking water regulations;
Degradation of water environment;
Ever-declining cost of the UF membranes.
3
Fundamentals of membrane fouling and controls
Deposition
Adsorption (solutes or
colloids)
Cake layer formation
Fouling Material
•Biofouling (Bacteria) •Organic Fouling (humics, protein, polysaccharides) •Inorganic Fouling (metal ions and their compounds)
Removable fouling, irremovable fouling and irreversible fouling
4
Factors affecting Membrane Fouling and Controls
Membrane
Fouling
Hydraulic conditions:
(shear stress, aeration
intensity, flow velocity)
Membrane materials
(pore size, surface
roughness, charge,
and hydrophobicity)
Membrane module
design
(flat sheet vs. hollow fiber)
Operational conditions
(flux, cycle length,
backpulse, cleaning
conditions)
Characteristics of raw water
(Temp., pH, suspended
solids, organic molecules
(NOM), inorganic constituents)
5
Membrane fouling - Summary
Mechanisms:
The accumulation of materials on membrane surface (such as cake/gel formation),
adsorption on the membrane surface,
and adsorption/deposition of solute within membrane pores
Disadvantage:
Permeate flux decline and/or trans-membrane pressure increases
rising operating and asset management costs
Control:
Backwash or backpulse
Clean-in-place (CIP) is routinely performed in water treatment plant
CIP may degrade the membrane
6
Lakehead University City of Thunder Bay – Environment Division Parternship
Student Projects
3 year study at the Bare Point Water Treatment Plant
Pilot and Full Scale Operation
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Scope of work – 3 year study
Characterize the membrane age effect
Permeability, Fiber morphology, FTIR and tensile strength analysis of accumulated operating time expressed in years’ membranes
Examine current CIP strategy
Measure the organic and inorganic matter remaining on the membranes
Simulation Chemical Cleaning Study CIP
Characterize the portion if any the membrane cleaning agents effect membrane mechanical and chemical properties
8
Are the membranes fouling?
Are the membranes aging?
What’s Occurring at the Plant
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50
70
90
110
130
150
170
190
Pe
rme
abili
ty (
lmh
r/b
ar)
Membrane characterization: Permeability
Fig. 1 Post CIP Permeability Change of Plant Membranes
Initial 177 Lm-2h-1/bar decreased to ~100 Lm-2h-1/bar
Permeability loss of about 40%
Some fouling caused permeability loss exists
10
Membrane characterization: Morphography
Figure. 2
SEM photograph of membrane at 1700× magnification:
(a) virgin, (b) 2 years and (c) 3 years
(c)
The virgin membrane surface was very smooth
The used fiber’s surface degrades and appears to become rough
(b) (a)
(b) (a)
11
Membrane characterization: Tensile strength
Fig. 3 Tensile strength of different years’ membrane
o The mechanical property of membrane deteriorated with increasing the operating time
o Hypothesis: Caused by the daily operation (Mechanical Stress) and chemical cleaning
0.0
2.0
4.0
6.0
8.0
10.0
0 2 3
Ten
sile
str
engt
h (
MP
a)
Period (yrs)
12
Membrane characterization: Surface function groups
0
0.04
0.08
0.12
0.16
0.2
500 800 1100 1400 1700 2000
Wavelength (cm-1)
Ab
sorb
ance
(A
IU)
Virgin
2 yrs
3 yrs841
881
1078
1178
1238
1277
1749
1402
Peak value (cm−1) Type of vibration
841 CH2 rocking
1178,1238 CF2 stretching
881,1277 PVDF fingerprint
1070,1402 CH2 wagging
1749 Carbonyl peak
Fig. 4 FTIR spectra of virgin and used membranes from water treatment plant
Partial scission of the PVDF functional group polymer of used membranes
No additional absorption peak appeared on the used membranes
Table 1. Identification of the main peaks from FTIR spectrum
13
Foulant characterization: Organic/Inorganic matters
0
40
80
120
160
200
Al Ca Fe Mg Mn
Metal ions
Extr
act/
mem
bra
ne
(m
g/m
2)
0 year
2 years
3 years
Fig. 6 Organic carbon adsorbed on the membrane
Fig. 5 Metal ions detected on different years’ membrane
Increase the concentration of organic matter on the surface of the membrane with respect to age
A number of inorganic elements were found on 2 and 3 years’ membranes
These foulants could be converted from reversible to irreversible fouling which contributed to a gradual decrease of membrane flux
0.00
0.05
0.10
0.15
0.20
0.25
0 2 3
Org
an
ic C
arb
on
(g
/m2)
14
Are the membranes being affected by sodium hypochlorite?
Are the membranes being effected by citric acid? Does the CIPs account for of all the membrane aging?
What’s Occurring during CIPs A Simulation Study in the lab
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Materials and methods Membrane
UF polyvinylidene fluoride (PVDF) hollow fiber membrane
Virgin membranes were harvested from the new membrane module
Membrane characterization
Tensile strength: Dual-Range Force Sensor
Membrane function group: ATR-FTIR
Simulation Chemical Cleaning Study CIP
Sodium Hypochlorite at 500 mg/L for 6 hrs once per month Citric Acid at 200 mg/L for 5 hrs every other month at water treatment plant
Four treatment series
(1) tap water sample every 36 hrs
(2) sodium hypochlorite sample every 36 hrs
(3) citric acid sample every 15 hrs
(4) sodium hypochlorite/citric acid sample every 36 hrs hypo 15hrs citric
16
17
Simulation study – Isolating the CIP Effect: Tensile strength
Fig. 7 Tensile strength of different chemical treatment: (a) Tap water and (b) Citric acid treatment
No change when the membranes were only exposed to tap water
Citric acid treatment had insignificant impact on the membrane tensile strength
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3
Ten
sile
str
engt
h (
MP
a)
Simulated period (yrs)
(a) Tap water treatment
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3
Ten
sile
str
engt
h (
MP
a)
Simulated period (yrs)
(b) Citric acid treatment
18
Simulation study – Isolating the CIP Effect: Tensile strength
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3
Ten
sile
str
engt
h (
MP
a)
Simulated period (yrs)
(c) Sodium hypochlorite treatment
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3
Ten
sile
str
engt
h (
MP
a)
Simulated period (yrs)
(d) Sodium hypochlorite/Citric acid treatment
Fig. 7 Tensile strength of different chemical treatment: (c) Sodium hypochlorite
and (d) Sodium hypochlorite/Citric acid treatment
Sodium hypochlorite treatment had some effect on the tensile strength
Citric acid treatment had insignificant impact on the membrane tensile strength
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Simulation study – Isolating the CIP Effect: ATR-FTIR spectroscopy
0
0.04
0.08
0.12
0.16
0.2
500 800 1100 1400 1700 2000
Ab
sorb
ance
(A
IU)
Wavelength (cm-1)
(a) Tap water treatment
Virgin
1 yrs
2 yrs
3 yrs
Fig. 8 FTIR spectrum of different chemical treatment: (a) Tap water and (b) Citric acid treatment
No change while the membranes were only immersed in tap water for short time
Citric acid did not attack the membrane too much as the change of FTIR spectra was insignificant Citric acid hardly degrades the membrane function groups
0
0.04
0.08
0.12
0.16
0.2
500 800 1100 1400 1700 2000
Ab
sorb
ance
(A
IU)
Wavelength (cm-1)
(b) Citric aicd treatment
Virgin
1 yrs
2 yrs
3 yrs
20
Simulation study – Isolating the CIP Effect: ATR-FTIR spectroscopy
0
0.04
0.08
0.12
0.16
0.2
500 800 1100 1400 1700 2000
Ab
sorb
ance
(A
IU)
Wavelength (cm-1)
(c) Sodium hypochlorite treatment
Virgin
1 yrs
2 yrs
3 yrs
0
0.04
0.08
0.12
0.16
0.2
500 800 1100 1400 1700 2000
Ab
sorb
ance
(A
IU)
Wavelength (cm-1)
(d) Sodium hypochlorite/Citric acid treatment
Virgin
1 yrs
2 yrs
3 yrs
Fig. 8 FTIR spectrum of different chemical treatment: (c) Sodium hypochlorite and
(d) Sodium hypochlorite/Citric acid treatment
Sodium hypochlorite had adverse effect on membrane surface chemistry
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Discussion: mechanical property
The tensile strength at break after 3 years’, membranes at water treatment plant was 5.0 MPa
The results of simulated chemical cleaning study was only 5.7 Mpa (3 yrs exposure) Therefore, Degradation of mechanical properties of the aging membranes was due to both the mechanical fatigue stressors of daily operation and chemical cleanings
0.0
2.0
4.0
6.0
8.0
10.0
0 2 3
Ten
sile
str
engt
h (
MP
a)
Period (yrs)
Tensile strength of different years' membrane from water treatment plant
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3
Ten
sile
str
engt
h (
MP
a)
Simulated period (yrs)
Simulated study: Sodium hypochlorite/Citric acid treatment
22
Discussion: function groups of membrane
0
0.04
0.08
0.12
0.16
0.2
500 800 1100 1400 1700 2000
Ab
sorb
ance
(A
IU)
Wavelength (cm-1)
Simulated study: Sodium hypochlorite/Citric acid treatment
Virgin
1 yrs
2 yrs
3 yrs
0
0.04
0.08
0.12
0.16
0.2
500 800 1100 1400 1700 2000
Ab
sorb
ance
(A
IU)
Wavelength (cm-1)
FTIR spectra of virgin and used membranes from water treatment plant
Virgin
2 yrs
3 yrs
841
881
1078
1178
1238
1277
1749
1402
The deterioration of surface chemistry of the hollow fiber membranes appears to be greater in the membranes from the plant compared to the simulated chemical cleaning study membranes
This agrees with the tensile strength results
Chemical cleaning using sodium hypochlorite played the dominant role in the deterioration of membrane properties
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Conclusions
1) Membrane aging as determined by loss of tensile strength and changes in the surface chemistry FTIR peaks has been shown to decrease with operational time in a Water Treatment Plant.
2) Sodium Hypochlorite exposure contributes to membrane aging.
3) Citric Acid exposure has no or limited effect on membrane aging.
4) Cleaning chemical exposure does not account for all of the increase in membrane aging. The mechanical stressors of TMP, backpulses and aeration agitation have some positive correlation to membrane age.
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Ongoing Research: Accelerated Membrane fouling Longer Production Cycles
80
100
120
140
0 5 10 15 20 25 30 35 40
Pe
rme
ab
ilit
y(L
MH
/ba
r)
Operation Time(hrs)
Permeability Drop versus operation time
Permeability
Initial 143 Lm-2h-1/bar decreased to 97 Lm-2h-1/bar
Permeability loss of about 34%
Some foulants caused permeability loss exists
Fig. 9 Permeability change with the operating time
(b) Membrane module after operation
(a) Membrane module before operation
25
0
20
40
60
80
100
120
140
160
180
Befor Cleaning After Hypo clean After Citric acid Clean
Pe
rme
ab
ilit
y(L
MH
/ba
r)
Chemical Cleaning
Comparison of permeability before and after clean
Fig. 10 Permeability recovery by chemical cleaning
Ongoing Research: Chemical Cleaning
Initial 97 Lm-2h-1/bar increased to 140 Lm-2h-1/bar after
sodium hypo cleaning
Permeability further increased to 157 Lm-2h-1/bar after
citric acid cleaning
Foulants are being removed by chemical cleaning
(a) Membrane module before cleaning
(b) Membrane module after cleaning 26
0
5
10
15
20
25
30
35
40
01 Sept 30 Aug 02 Sept 08 Sept 09 Sept 12 Sept 16 Sept 15 Sept
Ch
an
ge
in
pe
rme
ab
ilit
y(L
MH
/ba
r)
Date of experiment
Prod cycle
No prod cycle
Unexpected Result Chemical Cleaning All Cleans were Hypo followed by Citric
Running the pilot plant between cleans essentially prevented citric acid from cleaning the
membrane
Speculation? Organics are attaching to the inorganics and preventing the citric acid from
dissolving the inorganics into solution and off the membrane surface
27
Ongoing Research: Future Work
Next steps:
Sodium hypochlorite cleaning
Optimize the chemical dosage, soaking time
Citric acid cleaning
Optimize the chemical dosage, soaking time and pH value
Recovery (ie Production time between backpulse/backwash)
Determine the relationship between fouling method and chemical cleaning
Outcomes:
Optimal chemical conditions will be determined (Seasonal?)
Reduce the potential deterioration of membrane properties
Reduce the cost of chemical cleaning and increase membrane life span
28
Acknowledgements
The authors thank the financial support of the
Northern Ontario Heritage Fund Corporation (NOHFC) (Emerging Technology Fund)
Green Municipal Fund (GMF)
City of Thunder Bay.
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Thank you for your attention!
Discussion/Questions
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