Membrane filtration is a crucial wastewater treatment method that uses semipermeable barriers to separate contaminants from water. It relies on transmembrane pressure to force water through pores while retaining particles based on size, charge, or other properties.

Different membrane types, from microfiltration to reverse osmosis, target specific contaminant sizes. Factors like pore size, pressure, and fouling affect performance. Understanding these principles helps optimize treatment efficiency and maintain membrane longevity in wastewater applications.

Membrane Filtration Principles

Principles of membrane filtration

• Membrane filtration separates contaminants from water or wastewater using a semipermeable membrane
• Membrane acts as a selective barrier allowing water and certain solutes to pass through while retaining others based on size, charge, or other properties
• Driving force for membrane filtration is transmembrane pressure (TMP) which forces water through the membrane pores
• Membrane filtration categories based on particle or molecule size removed:
  • Microfiltration (MF) removes particles 0.1-10 μm (bacteria, protozoa, suspended solids)
  • Ultrafiltration (UF) removes particles and macromolecules 0.01-0.1 μm (viruses, proteins, colloids)
  • Nanofiltration (NF) removes multivalent ions and organic molecules 0.001-0.01 μm (hardness, pesticides, herbicides)
  • Reverse osmosis (RO) removes monovalent ions and small organic molecules < 0.001 μm (salts, small organic compounds, metal ions)

Pore size and solute rejection

• Membrane pore size determines particle or molecule size that can pass through
  • Smaller pore sizes result in higher rejection of solutes and particles
• Pressure is the driving force for membrane filtration
  • Higher pressure increases water flux through the membrane
  • Excessive pressure can cause membrane compaction and reduced permeability
• Solute rejection measures the membrane's ability to remove specific contaminants influenced by pore size, solute size, charge, and interactions with the membrane material
• As membrane pore size decreases, higher pressure is required to maintain water flux, but solute rejection increases
  • MF and UF membranes operate at lower pressures (0.1-10 bar) with lower solute rejection
  • NF and RO membranes operate at higher pressures (5-80 bar) with higher solute rejection

Factors in membrane performance

• Flux is the rate of water flow through the membrane per unit area ($L/m^2/h$) influenced by pressure, temperature, and feed water quality
  • Higher flux indicates better membrane performance but can lead to increased fouling
• Permeability measures the membrane's ability to allow water to pass through ($L/m^2/h/bar$) influenced by membrane material, pore size, and surface properties
  • Higher permeability allows for greater water production at a given pressure
• Selectivity refers to the membrane's ability to preferentially allow certain components to pass through while rejecting others determined by pore size, charge, and interactions with solutes
  • Higher selectivity results in better contaminant removal and improved permeate quality
• Membrane fouling is a major factor affecting performance over time
  • Fouling occurs when particles, colloids, or organic matter accumulate on the membrane surface or within pores
  • Fouling leads to decreased flux, increased pressure requirements, and reduced membrane lifespan
  • Pretreatment, cleaning, and proper operating conditions can help mitigate fouling

Dead-end vs cross-flow filtration

• Dead-end filtration:
  1. Feed water flows perpendicular to the membrane surface
  2. All feed water passes through the membrane, and contaminants accumulate on the surface
  3. As the filter cake builds up, resistance to flow increases, and flux decreases
  4. Requires frequent backwashing or membrane replacement to maintain performance
  5. Suitable for feeds with low suspended solids content
• Cross-flow filtration:
  1. Feed water flows parallel to the membrane surface
  2. A portion of the feed water (permeate) passes through the membrane, while the remaining concentrate (retentate) flows across the surface
  3. The cross-flow action helps to sweep away accumulated contaminants, reducing fouling
  4. Allows for continuous operation with less frequent cleaning or membrane replacement
  5. Suitable for feeds with higher suspended solids content
• Comparison:
  • Dead-end filtration is simpler and less energy-intensive but more prone to fouling
  • Cross-flow filtration is more complex and energy-intensive but provides better fouling control and longer membrane lifespan
  • Choice between dead-end and cross-flow filtration depends on feed water quality, desired permeate quality, and operational considerations