8.1 Types of membrane fouling and their causes

Membrane fouling is a major challenge in water treatment. It occurs when unwanted materials build up on membrane surfaces, reducing efficiency. This section examines the main types of fouling: organic, biofouling, inorganic, and colloidal.

Understanding fouling mechanisms is crucial for developing effective prevention strategies. We'll look at concentration polarization, cake layer formation, pore blocking, and scaling. These processes impact membrane performance and longevity in water treatment systems.

Types of Membrane Fouling

Organic Fouling and Biofouling

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• Organic fouling occurs when organic compounds such as proteins, carbohydrates, and humic substances adsorb onto the membrane surface or within the pores
  • Adsorption of organic matter is driven by hydrophobic interactions, hydrogen bonding, and electrostatic interactions between the foulants and the membrane material
  • Organic fouling leads to a decline in membrane permeability and selectivity over time (humic acids)
• Biofouling is the growth and accumulation of microorganisms on the membrane surface
  • Bacteria, algae, and fungi can form biofilms that create a gel-like layer on the membrane
  • Biofilms increase hydraulic resistance, reduce permeate flux, and can degrade the membrane material through the production of enzymes and metabolic byproducts (Pseudomonas aeruginosa)

Inorganic Fouling and Colloidal Fouling

• Inorganic fouling, also known as mineral scaling, results from the precipitation of sparingly soluble salts on the membrane surface
  • Common scalants include calcium carbonate, calcium sulfate, barium sulfate, and silica
  • Scaling occurs when the concentration of these salts exceeds their solubility limit near the membrane surface due to concentration polarization (calcium carbonate)
• Colloidal fouling is caused by the deposition of suspended particles, such as clays, silica, and metal oxides, onto the membrane
  • Colloidal particles can form a compact cake layer that increases resistance to water permeation
  • The stability and aggregation of colloidal particles are influenced by factors such as pH, ionic strength, and surface charge (silica nanoparticles)

Mechanisms of Membrane Fouling

Concentration Polarization and Cake Layer Formation

• Concentration polarization is the accumulation of retained solutes near the membrane surface, leading to a higher local concentration compared to the bulk solution
  • The increased solute concentration creates a concentration gradient that drives diffusive back-transport of solutes from the membrane surface to the bulk solution
  • Severe concentration polarization can lead to the precipitation of sparingly soluble salts (scaling) and the deposition of suspended particles (colloidal fouling)
• Cake layer formation occurs when rejected particles and macromolecules accumulate on the membrane surface, creating an additional layer of resistance
  • The cake layer increases the overall hydraulic resistance and reduces the effective membrane permeability
  • The structure and compressibility of the cake layer depend on the size, shape, and interactions of the deposited particles (activated sludge)

Pore Blocking and Scaling

• Pore blocking is the occlusion of membrane pores by foulants that are similar in size to the pore diameter
  • Pore blocking can occur through complete pore plugging (standard blocking) or partial pore constriction (intermediate blocking)
  • As pores become blocked, the available membrane area for permeation decreases, leading to a decline in permeate flux (organic macromolecules)
• Scaling is the formation of inorganic precipitates on the membrane surface or within the pores due to the supersaturation of sparingly soluble salts
  • Scaling can occur through surface crystallization (heterogeneous nucleation) or bulk precipitation (homogeneous nucleation) followed by deposition
  • The growth of scale crystals on the membrane surface creates an additional resistance layer that hinders water permeation and can cause irreversible damage to the membrane (gypsum scale)