Membrane fouling

Membrane fouling is a process where solute or particles deposit onto a membrane surface or into membrane pores in a way that degrades the membrane's performance. It is a major obstacle to the widespread use of this technology. Membrane fouling can cause severe flux decline and affect the quality of the water produced. Severe fouling may require intense chemical cleaning or membrane replacement. This increases the operating costs of a treatment plant. There are various types of foulants: colloidal (clays, flocs), biological (bacteria, fungi), organic (oils, polyelectrolytes, humics) and scaling (mineral precipitates).^[1]

Fouling can be divided into reversible and irreversible fouling based on the attachment strength of particles to the membrane surface. Reversible fouling can be removed by a strong shear force of backwashing. Formation of a strong matrix of fouling layer with the solute during a continuous filtration process will result in reversible fouling being transformed into an irreversible fouling layer. Irreversible fouling is the strong attachment of particles which cannot be removed by physical cleaning.^[2]

Factors affecting membrane fouling

Factors that affect membrane fouling:

1. Membrane properties such as pore size, hydrophobicity, pore size distribution and membrane material.
2. Solution properties such as concentration, the nature of the components and particle size distribution.
3. Operating conditions such as pH, temperature, flow rate and pressure.

Measure of membrane fouling

Flux and transmembrane pressure (TMP) are the best indicators of membrane fouling. Under constant flux operation, TMP increases to compensate for the fouling. On the other hand, under constant pressure operation, flux declines due to membrane fouling.

Fouling control

Even though membrane fouling is an inevitable phenomenon during membrane filtration, it can be minimised by strategies such as cleaning, appropriate membrane selection and choice of operating conditions.

Membranes can be cleaned physically, biologically or chemically. Physical cleaning includes sponges, water jets or backflushing using a permeate. Biological cleaning uses biocides to remove all viable microorganisms, whereas chemical cleaning involves the use of acids and bases to remove foulants and impurities.

Another strategy to minimise membrane fouling is the use of the appropriate membrane for a specific operation. The nature of the feed water must first be known; then a membrane that is less prone to fouling with that solution is chosen. For aqueous filtration, a hydrophilic membrane is preferred.

Operating conditions during membrane filtration are also vital, as they may affect fouling conditions during filtration. For instance, crossflow filtration is always preferred to dead end filtration, because turbulence generated during the filtration entails a thinner deposit layer and therefore minimises fouling.

See also

• Water purification

References

1. ^ Baker, R.W. (2004). Membrane Technology and Applications, England: John Wiley & Sons Ltd
2. ^ Choi, H., Zhang, K., Dionysiou, D.D.,Oerther, D.B.& Sorial, G.A. (2005) Effect of permeate flux and tangential flow on membrane fouling for wastewater treatment. J. Separation and Purification Technology 45: 68-78.