5.2 Separation mechanisms and process parameters

Microfiltration separates particles from fluids using porous membranes. This process relies on size exclusion, surface adsorption, and cake formation. Understanding these mechanisms is crucial for optimizing separation efficiency and managing fouling issues in water treatment applications.

Key process parameters include transmembrane pressure, flux, and permeability. These factors influence separation performance and energy consumption. Operational modes like cross-flow filtration help control fouling, while proper membrane cleaning and replacement strategies ensure long-term system reliability.

Separation Mechanisms

Particle Retention and Accumulation

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• Size exclusion separates particles larger than the membrane pore size while allowing smaller particles and fluid to pass through
• Surface adsorption occurs when particles adhere to the membrane surface due to chemical interactions or electrostatic forces
  • Adsorbed particles can reduce membrane permeability and contribute to fouling
• Cake formation happens when retained particles accumulate on the membrane surface forming a dense layer (cake layer)
  • The cake layer acts as an additional filtration barrier increasing resistance to fluid flow
  • Cake formation is more prominent in dead-end filtration compared to cross-flow filtration
• Concentration polarization is the accumulation of retained solutes near the membrane surface creating a concentration gradient
  • High solute concentration at the membrane surface reduces permeate flux and can lead to membrane scaling (mineral precipitation)

Factors Affecting Separation Efficiency

• Membrane pore size distribution determines the size range of particles that can be effectively retained
  • Narrow pore size distribution improves size-based separation selectivity
• Particle size and shape influence their retention and tendency to cause fouling
  • Smaller particles can enter and block membrane pores while larger particles form cake layers
  • Elongated or irregular shaped particles are more likely to cause pore plugging compared to spherical particles
• Feed solution properties such as pH, ionic strength, and presence of organic matter affect particle-membrane interactions and fouling propensity
  • High ionic strength can compress the electrical double layer around particles promoting aggregation and cake formation
  • Organic matter adsorption on the membrane surface can alter its hydrophilicity and charge affecting particle adhesion

Process Parameters

Driving Force and Permeation Rate

• Transmembrane pressure (TMP) is the pressure difference across the membrane that drives fluid flow and particle separation
  • Increasing TMP enhances permeate flux but also promotes fouling and concentration polarization
  • Optimal TMP balances productivity and fouling minimization
• Flux represents the volumetric flow rate of permeate per unit membrane area (L/m²·h)
  • Flux depends on TMP, membrane permeability, and feed solution properties
  • Flux decline over time indicates membrane fouling or concentration polarization
• Permeability quantifies the membrane's intrinsic ability to allow fluid flow under a given TMP (L/m²·h·bar)
  • Higher permeability membranes require lower TMP to achieve a target flux
  • Permeability is affected by membrane material, pore size, and surface properties

Separation Efficiency and Selectivity

• Rejection represents the percentage of a specific solute or particle that is retained by the membrane
  • Rejection is calculated as (1 - Cp/Cf) × 100%, where Cp and Cf are permeate and feed concentrations, respectively
  • High rejection indicates effective removal of the target species
  • Rejection can vary for different solutes or particles depending on their size, charge, and interactions with the membrane
• Membrane selectivity refers to its ability to preferentially allow passage of certain components while retaining others
  • Selectivity is influenced by membrane pore size distribution, surface charge, and affinity towards different species
  • Highly selective membranes are desirable for achieving specific separations (virus removal, protein fractionation)

Operational Modes

Flow Configuration and Fouling Control

• Cross-flow filtration involves feeding the solution parallel to the membrane surface creating a shear force that sweeps away accumulated particles
  • Cross-flow reduces cake formation and concentration polarization compared to dead-end filtration
  • Retentate is continuously recirculated to maintain high cross-flow velocity and fouling control
• Dead-end filtration feeds the solution perpendicular to the membrane surface without any retentate flow
  • Dead-end mode is simpler and more compact but suffers from rapid fouling due to particle accumulation
  • Periodic backwashing or membrane replacement is required to restore permeate flux
• Fouling is the accumulation of retained particles, solutes, or organic matter on the membrane surface or within its pores
  • Fouling mechanisms include pore blocking, cake formation, and biofilm growth
  • Fouling leads to flux decline, increased TMP requirement, and deterioration of permeate quality
  • Fouling control strategies involve cross-flow operation, pretreatment (coagulation, adsorption), and membrane cleaning (backwashing, chemical cleaning)

Membrane Regeneration and Replacement

• Membrane cleaning is performed to remove foulants and restore permeate flux
  • Physical cleaning methods include backwashing, air scouring, and ultrasonic cleaning
  • Chemical cleaning uses acids, bases, oxidants, or enzymes to dissolve and detach foulants
  • Cleaning effectiveness depends on foulant type, cleaning agent selection, and cleaning conditions (concentration, temperature, duration)
• Membrane replacement is necessary when fouling becomes irreversible or membrane integrity is compromised
  • Frequent membrane replacement increases operational costs and process downtime
  • Membrane life can be extended by proper pretreatment, fouling control, and cleaning optimization