Membrane Applications in Drinking Water Treatment

1
(No Transcript)
2
(No Transcript)
3
Membrane Applications in Drinking Water Treatment
4
Pressure-Driven Membrane Processes

• Separate by size and chemistry
• Concentration, Porosity Effects

5
OTHER DRIVING FORCES

• Charge Gradient (Electrodialysis)
• Concentration Gradient (Dialysis)
• Temperature Gradient (Thermoosmosis)

6

PRESSURE GRADIENT
PORE DIAMETER
REMOVAL EFFICIENCY
MEMBRANE DESIGNATION

7
Membrane Separations for Application to Drinking
Water Treatment
Macro
Micro
Ionic Range
Molecular Range
Macro Particle Range
Molecular Range
Particle Range
Size, Microns
0.001 (nanometer)
0.01
100
0.1
10
1000
1.0
Molecular Weight (approx..)
100
100,000
500,000
1,000
Viruses
Dissolved Salts (ions)
Bacteria
Relative Sizes
Algae
Organics (e.g., Color , NOM, SOCs)
Cysts
Sand

Clays
Silt
Asbestos Fibers
Reverse Osmosis
Ultrafiltration
Separation
Conventional Filtration (granular media)
Microfiltration
Process
Nano filtration
8
(No Transcript)
9
The Two Meanings of Filtration2. Porous
Membrane Filtration
10
40 PDMAEMA-60 PFOMA Thin-film Composite NF
Membrane (Polysulfone Support Layer)
11
Membrane Geometry
Hollow Fibers MF/UF
Spiral Wound NF/RO
12
(No Transcript)
13
Tubular Elements

14
Spiral Elements

15
(No Transcript)
16
INORGANIC SYNTHETICS

• Ceramics
• Glass
• Metallic
• Excellent thermal stability
• Withstands chemical attack

17
PLATE AND FRAME

18
Two MF/UF Configurations

• Encasedmembrane system
• Submerged membrane system

Filtrate
Membrane
Pressure Vessel(s)
FeedWater
Pump
Pump supplies positive pressure to PUSH water
through membrane media.
FeedWater
Membrane
Filtrate
Open Tank
Pump
Pump suction PULLS water through membrane media.
19
Immersed Membranes with Gentle Crossflow
20
(No Transcript)
21
NF RO Scottsdale Water Campus
22
CASCADE SYSTEM
FEED

PERMEATE
RETENTATE
23
A
Qf
QP
PERMEATE
FEED
Cf
CP
QR
RETENTATE
CR

TMP Transmembrane pressure (difference) Flux
(LMH or GFD) Qp / A (Contaminant)
Rejection () 1 - Cp/Cf Recovery () Qp/Qf
24
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
25
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
26
Example. What is the average velocity of solution
toward a membrane, if the flux is 50 LMH?
27
Flow Through Porous Membranes
Darcy-Weisbach Eqn
For Laminar Flow
For Steady Flow Through a Pore
Hagen-Poiseuille Eqn
28
Flow Through Porous Membranes
Resistance (kg/m2-s)
Membrane Resistance (m-1)
Process Typical VolumetricFlux, (L/m2-h) Typical MembraneResistance, Rm (m-1)
Microfiltration 100-250 1x1011 1x1012
Ultrafiltration 30-150 1x1012 1x1013
Nanofiltration 20-50 1x1013 1x1014
Reverse osmosis 5-40 5x1013 1x1015
29
Flow Through Porous Membranes
Resistivity
Permeability for overall flow
Permeability for individual species
30
Contaminant Rejection by Open Pores (Clean
Membrane)
31
Contaminant Rejection by Open Pores (Clean
Membrane)
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 DP or J.
32
Membrane Fouling
33
Problems Caused by NOM
Membrane Fouling
Interference w/Activated Carbon
Cl2
DBPs
34
NOM Fouling of an MF Membrane
Gel Surface
Gel Cross-Section
Membrane
Membrane support
Note lt3 Removal of NOM from Feed
35
(No Transcript)
36
Heated Aluminum Oxide Particles (HAOPs)
Al2(SO4)3NaOH?pH 7.0
110 oC, 24 hrs
Particle Size Range 1.520 mm, mean 5 mm
Point of Zero Charge pH 7.7
BET Surface Area 116 m2/g
Aluminum Content 25 (Al(OH)3?H2O)
37
Transmembrane pressure with varying HAOPs surface
loadings
38
DOC Concentrations in Permeate
39
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
40
Summary Performance and Modeling of Porous
Membranes

• Solution flux proportional to DP, 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

41
Transport Through Water-Selective, Dense
(Non-Porous) Membranes
cw,p
Osmosis of water
55.5
cw,f
55.0
Pressure profile for P0 everywhere
cs,f
0.555
cs,p
Solute, 90 rejection
0.055
With no DP, the concentration gradients drive
water toward the feed and contaminants toward the
permeate.
42
Increasing pressure increases the effective
concentration of any species. For an increase of
DP, the effective concentration is
For water
At 25oC
At DP 3000 kPa
Result Even a large DP increases effective
concentrations by only a few percent.
43
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, p. Permeate is often
approximated as pure water. In this example, p
is a pressure that increases ceff by 1. Note
that ceff of the solute also increases by 1.
cw,p
cw,eff,f
Osmosis eliminated
55.5
55.5
cw,f
55.0
cs,eff,f
0.56
cs,f
0.555
cs,p
Solute, 90 rejection
0.055
44
Applying a DP gtp 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,eff,f
56.5
cw,p
Reverse osmosis
55.5
cw,f
55.0
P gt p
cs,eff,f
P 0
0.57
cs,f
0.555
cs,p
Solute, 90 rejection
0.055
Although increasing DP causes the same
percentage increase in ceff for water and
solute, it has a much bigger effect on Dceff for
water than for solute.
45
(No Transcript)
46
(No Transcript)
47
Performance and Modeling of Dense Membranes

• Water flux occurs by diffusion, and is
  proportional to DP-Dp, because changing DP has
  big effect on Dcw,eff
• Solute flux occurs by diffusion, and is
  proportional to Dci, because changing DP has
  small effect on Dci,eff
• Conclusion changing DP 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 cant 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