2.3 Membrane fouling and cleaning strategies

Membrane fouling is a critical challenge in wastewater treatment. It occurs when unwanted materials accumulate on membrane surfaces, reducing performance and increasing costs. Understanding fouling types and mechanisms is crucial for effective membrane operation and maintenance.

Cleaning strategies are essential to combat membrane fouling. Physical, chemical, and biological methods can be employed to remove foulants and restore membrane performance. However, balancing cleaning frequency with membrane lifespan and operational costs requires careful consideration of trade-offs.

Membrane Fouling

Membrane fouling and performance

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• Accumulation of unwanted materials (foulants) on membrane surface or within pores leads to decline in performance
• Reduces permeate flux requiring higher transmembrane pressure to maintain desired flux
• Decreases permeate quality allowing passage of contaminants through fouled membrane
• Accelerates membrane degradation increasing frequency of cleaning and replacement
• Shortens overall lifespan of membrane

Types of membrane fouling

• Organic fouling caused by accumulation of organic compounds (NOM, humic substances, proteins) forms gel layer on membrane surface
• Inorganic fouling caused by precipitation of inorganic compounds (CaCO3, CaSO4, SiO2) forms scales on membrane surface
• Biofouling caused by growth and attachment of microorganisms (bacteria, fungi, algae) forms biofilm on membrane surface

Mechanisms of fouling formation

• Pore blocking where particles or solutes smaller than membrane pores deposit inside pores
• Cake layer formation where particles or solutes larger than membrane pores accumulate on surface
• Concentration polarization increases concentration of rejected solutes near membrane surface
• Membrane properties (pore size and distribution, surface roughness and charge, hydrophobicity) impact fouling propensity
• Feed water characteristics (concentration and type of foulants, pH and ionic strength, temperature) affect fouling
• Operating conditions (transmembrane pressure, cross-flow velocity, recovery rate) influence fouling formation

Membrane Cleaning Strategies

Strategies for fouling mitigation

• Physical cleaning methods:
  1. Backwashing reverses flow direction to dislodge foulants
  2. Air scouring introduces air bubbles to create turbulence and remove foulants
  3. Membrane relaxation temporarily stops filtration process allowing foulants to diffuse back into bulk solution
• Chemical cleaning methods:
  1. Acidic cleaning uses low pH solutions to remove inorganic foulants
  2. Alkaline cleaning uses high pH solutions to remove organic foulants
  3. Oxidative cleaning uses oxidizing agents (chlorine, hydrogen peroxide) to degrade organic foulants
• Biological cleaning methods:
  1. Enzymatic cleaning uses enzymes to break down specific organic foulants
  2. Biocidal cleaning uses biocides to inactivate and remove microorganisms

Trade-offs in fouling control

• Frequent cleaning cycles maintain high permeate flux and quality but reduce membrane lifespan due to increased wear and tear
• Pretreatment processes reduce foulant load on membrane but increase capital and operational costs
• Higher transmembrane pressure required to overcome fouling increases energy consumption for pumping
• Cleaning processes require additional energy for pumping and heating cleaning solutions
• More frequent membrane replacement needed due to fouling-induced degradation increases costs
• Increased use of cleaning chemicals and pretreatment additives raises operational expenses
• Downtime during cleaning cycles reduces overall plant productivity