cc(x)cc(x) cp(x)cp(x) feed (q f, c f ) permeate (q p, c p,out ) concentrate (q c, c c,out ),...
TRANSCRIPT
Physical, chemical, and/or electrical driving force
Permeate
Solute or particle rejection
Feed or concentrate
Semi-permeable (selective) membrane
Accumulated, rejected material, migrating back to bulk solution
cc(x)
cp(x)Feed(Qf, cf)
Permeate(Qp, cp,out)
Concentrate (Qc, cc,out),Retentate, Rejectate
Membrane Applications in Drinking Water Treatment
Pressure-Driven Membrane Processes
• Separate by size and chemistry• Concentration, Porosity Effects
OTHER DRIVING FORCES
• Charge Gradient (Electrodialysis)• Concentration Gradient (Dialysis)• Temperature Gradient (Thermoosmosis)
PRESSURE GRADIENT
PORE DIAMETER
MEMBRANE DESIGNATION
REMOVAL EFFICIENCY
RelativeSizes
SeparationProcess
Molecular Weight (approx..)
Size, MicronsIonic Range
0.001(nanometer)
Molecular Range
0.01
MacroMolecular Range
0.1 1.0
MicroParticle Range
10 100
Macro Particle Range
1000
100 1,000 100,000 500,000
BacteriaVirusesDissolved Salts(ions)
Algae
Clays Silt
Asbestos Fibers
Cysts Sand
Conventional Filtration (granular media)
Organics (e.g., Color , NOM, SOCs)
Microfiltration
Ultrafiltration
Nanofiltration
ReverseOsmosis
Membrane Separations for Application to Drinking Water Treatment
Membrane cross section
(b)
The Two Meanings of Filtration:2. Porous Membrane Filtration
1 m
1 m
PDMAEMA-b-PFOMA layer
Microporous Polysulfone Support
40% PDMAEMA-60% PFOMA Thin-film Composite NF Membrane (Polysulfone Support Layer)
Membrane Geometry
Spiral Wound
NF/RO
Hollow Fibers
MF/UF
Tubular Elements
Spiral Elements
(a)
INORGANIC SYNTHETICS
Ceramics
Glass
Metallic
• Excellent thermal stability
• Withstands chemical attack
PLATE AND FRAME
Two MF/UF Configurations
• Encasedmembrane system
• Submerged membrane system
Pump supplies positive pressure to PUSH water through membrane media.
FeedWater
Filtrate
Pump
PressureVessel(s)
Membrane
Pump suction PULLS water through membrane media.
FeedWater
Filtrate
Pump
OpenTank
Membrane
Permeate
HF
Air
Raw Water Pump
2-12 psi
Wasting
Immersed Membranes with Gentle Crossflow
NF & RO Scottsdale Water Campus
CASCADE SYSTEM
RETENTATE
PERMEATE
FEED
PERMEATEFEED
RETENTATE
Qf
Cf
A
QP
CPQR
CR
TMP = “Transmembrane pressure (difference)”
Flux (“LMH” or “GFD”) = QQpp / A / A
(Contaminant) Rejection (%) = 1 CCpp/C/Cff
Recovery (%) = Qp/Qf
Membrane Geometry
Approximate Packing Density (m2/m3)
Capillary 5000-8000
Spiral wound 700-2000
Hollow fiber 1000-2000
Flat (plate and frame)
200-500
Tubular 100-300
Membrane Process Transmembrane Pressure, ∆Ptot (kPa)
System Recovery (%)(a)
Microfiltration 10 to 100 90 to 99+
Ultrafiltration 50 to 300 85 to 95+
Nanofiltration 200 to 1500 75 to 90+
Reverse Osmosis 500 to 8000 60 to 90
(a) Defined as the ratio of permeate flow rate to feed flow rate
LP gh
2
MF
3 2
kg m-s15,000 Pa 1.0
Pa1.53 m
kg m1000 9.81
m sL
Ph
g
2
6
RO
3 2
kg m-s4.5x10 Pa 1.0
Pa459 m
kg m1000 9.81
m sL
Ph
g
Example. What height would a column of water have to be to exert a pressure equal to 15 kPa? 4500 kPa?
Solution. From fluid mechanics:
Therefore:
Example. What is the average velocity of solution toward a membrane, if the flux is 50 LMH?
3
2
L 1 m cm cm50 100 5.0
m -h 1000 L m hVJ
Flow Through Porous Membranes
2
2f
L vh f
D g
For Laminar Flow:
64 / Ref
fP gh
Darcy-Weisbach Eqn:
For Steady Flow Through a Pore:
Hagen-Poiseuille Eqn:
2
8pore
pore
r PJ
L
2
8pores pore
memmem mem
A r PJ
A
Flow Through Porous Membranes
Driving Force
Flux
P
J
RResistance (kg/m2-s):
P
J
R
RMembrane Resistance (m:
Process Typical Volumetric
Flux, (L/m2-h)
Typical Membrane
Resistance, Rm (m1)
Microfiltration 100-250 1x1011 – 1x1012
Ultrafiltration 30-150 1x1012 – 1x1013
Nanofiltration 20-50 1x1013 – 1x1014
Reverse osmosis 5-40 5x1013 – 1x1015
Flow Through Porous Membranes
/ mem
mem
P
J
RResistivity:
1
Resistivity /V
Vmem
Jk
P
Permeability for overall flow:
/i
imem
Jk
P
Permeability for individual species:
1 m
Contaminant Rejection by Open Pores (Clean Membrane)
A B
Membrane
Pore
Contaminant Rejection by Open Pores (Clean Membrane)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0Particle-to-Pore Diameter Ratio, i
Par
tic
le R
eje
cti
on
, R
i
Flat
Parabolic
Modifiedparabolic
Velocity Profile at Entrance
Increasing driving force increases flux of both water and contaminants. So, rejection of a given type of particle by a clean membrane is predicted to be independent of P or J.
Membrane Fouling
0
20
40
60
80
100
120
0 60 120 180 240 300
Time (min)
Tra
nsm
emb
ran
e P
ress
ure
(kP
a)
Backwash
Irreversible Fouling
Backwash & Chemical Cleaning
Hydraulically Irreversible Fouling
Problems Caused by NOM
Membrane Fouling
O
OHHO
HOOC COOH
OH
OH
O
COOH
COOHO
OH
HOOC
OO
O
HO
O
O
OHDBPs
+Cl2
Interference w/Activated
Carbon
NOM Fouling of an MF Membrane
Note: <3% Removal of NOM from Feed
Gel Surface
Membrane
Gel Cross-Section
Membrane support
0.0.E+00
1.0.E+12
2.0.E+12
3.0.E+12
4.0.E+12
5.0.E+12
6.0.E+12
7.0.E+12
8.0.E+12
9.0.E+12
1.0.E+13
0 50 100 150 200 250 300 350 400
Vsp (L/m2)
Rf (
m-1
)
FR: 1.64×1010
FR: 4.6×1010
Heated Aluminum Oxide Particles (HAOPs)
Al2(SO4)3+NaOHpH 7.0 110 110 ooC, 24 hrsC, 24 hrs
Particle Size Range: m, mean ~5 m, mean ~5 mm
Point of Zero Charge: pH 7.7pH 7.7
BET Surface Area: 116 m116 m22/g/g
Aluminum Content: ~25% (Al(OH)~25% (Al(OH)33HH22O)O)
0
50
100
150
200
250
300
0 1,000 2,000 3,000 4,000 5,000 6,000 7,000
Specific volume filtered, V sp (L/m2)
Tran
smem
bran
e pr
essu
re (k
Pa)
0
4.5
9
18
HAOPssurface loading
(g/m 2 as Al)
Transmembrane pressure with varying HAOPs surface loadings
DOC Concentrations in Permeate
0.0
0.2
0.4
0.6
0.8
1.0
0 1,000 2,000 3,000 4,000 5,000 6,000 7,000
Specific volume filtered, Vsp (L/m2)
No
rmal
ized
DO
C
0
4.5
9
18
HAOPssurface loading
(g/m2 as Al)
* Filtration ended at TMPs of 35~40 psi
Progressive NOM Deposition on the HAOPs Layer
Vsp: 0 L/m2 1,200 L/m2 3,600 L/m2
4,700 L/m2 7,000 L/m2 7,000 L/m2
Summary: Performance and Modeling of Porous Membranes
• Solution flux proportional to P, inversely proportional to resistance• Resistance of clean membrane can be estimated from basic fluid
mechanics• If contaminant rejection is primarily due to geometrical factors, it is
expected to be insensitive to applied pressure and flux• In practice, resistance of accumulated rejected species quickly
overwhelms that of membrane (fouling)• Frequent backwashing reduces, but does not eliminate fouling• In drinking water systems treating surface water, NOM is often a
major fouling species, even though only a small fraction of the NOM is rejected
• Approaches to reduce fouling by NOM and other species are the focus of active research
Transport Through Water-Selective, Dense (“Non-Porous”) Membranes
With no P, the concentration gradients drive water toward the feed and contaminants toward the permeate.
cw,f
55.0
0.555
55.5
0.055 Solute, 90% rejection
Osmosis of water
Pressure profile for P=0 everywhere
cw,p
cs,f
cs,p
Increasing pressure increases the “effective” concentration of any species. For an increase of P, the effective concentration is:
, exp ieff i i
V Pc c
RT
18 g/mol L0.018
1000 g/L molwV
6 17.45x10 kPawV
RT
At P= 3000 kPa: , exp 1.023ieff w w w
V Pc c c
RT
For water:
At 25oC:
Result: Even a large P increases effective concentrations by only a few percent.
The pressure required to bring the effective concentration of water up to the concentration of pure water (and thereby stop diffusion) is the osmotic pressure, . Permeate is often approximated as pure water. In this example, is a pressure that increases ceff by ~1%. Note that ceff of the solute also increases by ~1%.
cw,f 55.0
0.555
55.5
0.055
cw,p
Solute, 90% rejection
Osmosis eliminatedcw,eff,f
55.5
P = 0
P =
0.56 cs,f
cs,p
cs,eff,f
Applying a P > causes water to move in the opposite direction from passive osmosis, hence is called reverse osmosis. For P ~3000 kPa, ceff increases by ~3%, so:
cw,f 55.0
0.555
55.5
0.055
cw,p
Solute, 90% rejection
Reverse osmosis
cw,eff,f
56.5
P = 0
P >
0.57
Although increasing P causes the same percentage increase in ceff for water and solute, it has a much bigger effect on ceff for water than for solute.
cs,f cs,p
cs,eff,f
Permeate
Concentration increase due to solute rejection and slow diffusion back to bulk solution
Concentration increase in bulk concentrate due to selective water removal
Permeate
Highest salt concentrations occur right next to membrane, where precipitation (‘scaling’) is
most likely
Performance and Modeling of Dense Membranes
• Water flux occurs by diffusion, and is ~proportional to P, because changing P has big effect on cw,eff
• Solute flux occurs by diffusion, and is ~proportional to ci, because changing P has small effect on ci,eff
• Conclusion: changing P increases water transport more than solute transport, and so increases rejection (different from porous membranes)
• Fouling also occurs on dense membranes, mostly by NOM and precipitation (scaling); reduced by “anti-scalants”
• Dense membranes can’t be backwashed, because required pressures would be too high; therefore, major effort is usually devoted to pre-treatment to remove foulants
• Approaches to reduce fouling are the focus of active research