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c p ( x ). Permeate ( Q p , c p , out ). Feed ( Q f , c f ). c c ( x ). Concentrate ( Q c , c c , out ), Retentate, Rejectate. Membrane Applications in Drinking Water Treatment. Pressure-Driven Membrane Processes. Separate by size and chemistry Concentration, Porosity Effects.

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slide2

cp(x)

Permeate(Qp, cp,out)

Feed(Qf, cf)

cc(x)

Concentrate (Qc, cc,out),

Retentate, Rejectate

pressure driven membrane processes
Pressure-Driven Membrane Processes
  • Separate by size and chemistry
  • Concentration, Porosity Effects
other driving forces
OTHER DRIVING FORCES
  • Charge Gradient (Electrodialysis)
  • Concentration Gradient (Dialysis)
  • Temperature Gradient (Thermoosmosis)
slide6

PRESSURE GRADIENT

PORE DIAMETER

REMOVAL EFFICIENCY

MEMBRANE DESIGNATION

slide7

Membrane Separations for Application to Drinking Water Treatment

Macro

Micro

Ionic Range

Molecular Range

MacroParticleRange

Molecular Range

ParticleRange

Size, Microns

0.001

(nanometer)

0.01

100

1000

0.1

10

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

slide9

1 mm

The Two Meanings of Filtration:2. Porous Membrane Filtration

membrane geometry
Membrane Geometry

Hollow Fibers

MF/UF

Spiral Wound

NF/RO

inorganic synthetics
INORGANIC SYNTHETICS

Ceramics

Glass

Metallic

  • Excellent thermal stability
  • Withstands chemical attack
two mf uf configurations
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.

slide19

Raw Water

Pump

Permeate

2-12 psi

HF

Wasting

Air

Immersed Membranes with Gentle Crossflow

cascade system
CASCADE SYSTEM

FEED

PERMEATE

RETENTATE

slide23

A

Qf

QP

FEED

PERMEATE

Cf

CP

QR

RETENTATE

CR

TMP = “Transmembrane pressure (difference)”

Flux (“LMH” or “GFD”) = Qp / A

(Contaminant) Rejection (%) = 1 -Cp/Cf

Recovery (%) = Qp/Qf

slide25

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:

slide27

Flow Through Porous Membranes

Darcy-Weisbach Eqn:

For Laminar Flow:

For Steady Flow Through a Pore:

Hagen-Poiseuille Eqn:

slide28

Flow Through Porous Membranes

Resistance (kg/m2-s):

Membrane Resistance (m-1):

slide29

Flow Through Porous Membranes

Resistivity:

Permeability for overall flow:

Permeability for individual species:

slide30

1 mm

Contaminant Rejection by Open Pores (Clean Membrane)

slide31

A

B

Pore

Membrane

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.

slide33

Problems Caused by NOM

Membrane Fouling

Interference w/Activated Carbon

+Cl2

DBPs

nom fouling of an mf membrane
NOM Fouling of an MF Membrane

Gel Surface

Gel Cross-Section

Membrane

Membrane support

Note: <3% Removal of NOM from Feed

heated aluminum oxide particles haops
Heated Aluminum Oxide Particles (HAOPs)

Al2(SO4)3+NaOHpH 7.0

110 oC, 24 hrs

progressive nom deposition on the haops layer
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
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
slide41

Transport Through Water-Selective, Dense (“Non-Porous”) Membranes

cw,p

Osmosis of water

55.5

cw,f

55.0

Pressure profile for P=0 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.

slide42

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.

slide43

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

P = p

cw,f

55.0

P = 0

cs,eff,f

0.56

cs,f

0.555

cs,p

Solute, 90% rejection

0.055

slide44

Applying a DP >p causes water to move in the opposite direction from passive osmosis, hence is called reverse osmosis. For P ~3000 kPa,ceffincreases by ~3%, so:

cw,eff,f

56.5

cw,p

Reverse osmosis

55.5

cw,f

55.0

P > p

cs,eff,f

0.57

P = 0

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.

performance and modeling of dense membranes
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 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