Membrane fouling

1
Membrane fouling

• Adsorption and deposition of material present in
  the feed on the membrane leading to loss of
  efficiency of the separation process.
• In constant TMP membrane filtration fouling
  results in
• Reduction in permeate flux
• Alteration in solute transmission
• In constant Jv membrane filtration fouling
  results in
• Increase in TMP
• Alteration in solute transmission

2
Effects of membrane fouling
3
Factors affecting membrane fouling

• Physicochemical properties of the membrane, e.g.
  hydrophobicity, electrostatic charge, reactive
  groups
• Physicochemical properties of the solute,
  molecular weight, hydrophobicity, electrostatic
  charge, shape
• Membrane morphology, i.e. pore size, pore shape,
  straight pores vs. pore network
• The operating parameters, e.g. TMP, permeate
  flux, system hydrodynamics
• Concentration polarization
• The physicochemical parameters of the feed
  solution, e.g. solute concentration, pH, salt
  concentration
• Membrane operation history

4
Mechanisms of membrane fouling

• External fouling
• When the adsorption/deposition takes place on
  the external surface, the hydraulic permeability
  and solute transmission characteristics are
  altered due to
• Increase in the effective membrane thickness
• Blockage of pore entrance
• Constriction of pore entrance
• Internal fouling
• When the adsorption or deposition takes place
  within the pores, the hydraulic permeability and
  solute transmission characteristics are altered
  due to
• Internal blockage of pore
• Internal constriction of pore

5
External fouling
6
Ultrafiltration Mode of operation
7
Microfiltration

• Microfiltration separates micron-sized particles
  from fluids using membranes having sub-micron
  sized pores.
• Microfiltration membranes retain particles by a
  purely sieving mechanism.
• Microfiltration are similar to those used in
  ultrafiltration.
• Typical permeate flux values are higher than in
  UF
• TMP used are lower than in UF.
• Microfiltration can be operated either in
  dead-ended (normal flow) mode or cross-flow mode.

8
Microfiltration Applications

• Cell harvesting from bioreactors
• Virus removal for solutions
• Clarification of fruit juice and beverages
• Removal of cells from fermentation media
• Water purification
• Air filtration
• Sterilization

9
Concentration polarization and fouling in
microfiltration

• Concentration polarization in microfiltration is
  different from that in UF
• Fouling in microfiltration can be caused by
  particles as well as solutes
• Particles have much lower diffusivity than
  consequently the extent of Fickian back diffusion
  is negligible
• Concentration polarization scheme based on a
  stagnant film model is not feasible since the
  built-up layer in microfiltration frequently
  exceeds the hydrodynamic boundary layer thickness
• The accumulated particles are brought back to
  bulk solution primary by
• Shear induced diffusion
• Shear induced erosion
• Particle-particle interactions
• Inertial lift mechanism

10
Enhancement of microfiltration processes

• Back flushing and pressure pulsing are used to
  remove
• accumulated particles from the membrane surface
• Flux enhancement can be achieved in
  microfiltration
• processes using the same techniques as in
  ultrafiltration
• processes
• The actual mechanisms involved in flux
  enhancement are different in microfiltration and
  ultrafiltration processes

11
Microfiltration equations