2.2 Types of membrane processes (MF, UF, NF, RO)

Membrane processes in wastewater treatment use different pore sizes to remove contaminants. From largest to smallest, microfiltration, ultrafiltration, nanofiltration, and reverse osmosis target specific pollutants with increasing precision and pressure.

Each process has unique advantages and limitations. Microfiltration removes larger particles, while reverse osmosis can filter out tiny dissolved contaminants. Choosing the right membrane depends on the wastewater composition and treatment goals.

Membrane Processes in Wastewater Treatment

Types of membrane processes

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• Microfiltration (MF)
  • Largest pore size among the four processes ranging from 0.1 to 10 microns
  • Removes suspended solids, bacteria (E. coli), and protozoa (Giardia) using low operating pressure of 0.1 to 2 bar
  • Advantages include low energy consumption, high permeate flux, and effective pretreatment for other processes
  • Limitations include inability to remove dissolved contaminants and susceptibility to fouling
• Ultrafiltration (UF)
  • Smaller pore size than MF ranging from 0.01 to 0.1 microns
  • Removes viruses (norovirus), large organic molecules (proteins), and colloids in addition to MF targets using operating pressure of 1 to 10 bar
  • Advantages include removal of viruses and large organic molecules with moderate energy consumption
  • Limitations include inability to remove dissolved ions and requirement of pretreatment to prevent fouling
• Nanofiltration (NF)
  • Pore size between UF and RO ranging from 0.001 to 0.01 microns
  • Removes divalent ions (calcium, magnesium), small organic molecules (pesticides), and hardness using operating pressure of 5 to 20 bar
  • Advantages include removal of divalent ions and small organic molecules with lower energy consumption than RO
  • Limitations include inability to remove all monovalent ions and potential requirement of post-treatment
• Reverse Osmosis (RO)
  • Smallest pore size among the four processes at less than 0.001 microns
  • Removes monovalent ions (sodium, chloride), dissolved contaminants, and salts using high operating pressure of 10 to 100 bar
  • Advantages include removal of most dissolved contaminants and production of high-quality permeate
  • Limitations include high energy consumption, extensive pretreatment requirements, and generation of concentrated brine

Characteristics of membrane processes

• Pore size
  • MF: 0.1 to 10 microns
  • UF: 0.01 to 0.1 microns
  • NF: 0.001 to 0.01 microns
  • RO: less than 0.001 microns
• Operating pressure
  • MF: 0.1 to 2 bar
  • UF: 1 to 10 bar
  • NF: 5 to 20 bar
  • RO: 10 to 100 bar
• Target contaminants
  • MF: suspended solids, bacteria, protozoa
  • UF: viruses, large organic molecules, colloids
  • NF: divalent ions, small organic molecules, hardness
  • RO: monovalent ions, dissolved contaminants, salts

Selection of membrane processes

• Microfiltration (MF)
  • Suitable for removing suspended solids (sand) and larger microorganisms (algae)
  • Acts as pretreatment for other membrane processes (UF, NF, RO) or conventional treatment (activated sludge)
• Ultrafiltration (UF)
  • Suitable for removing viruses (rotavirus) and large organic molecules (enzymes)
  • Serves as pretreatment for NF or RO processes to prevent fouling
• Nanofiltration (NF)
  • Suitable for removing divalent ions (iron, manganese) and small organic molecules (hormones)
  • Applicable in water softening and partial desalination (brackish water treatment)
• Reverse Osmosis (RO)
  • Suitable for removing most dissolved contaminants (heavy metals) and producing high-quality permeate
  • Applicable in seawater desalination and advanced wastewater treatment (water reuse)