5. Fouling in membrane operations
Scaling of sparingly soluble salts, biofouling and irreversible fouling
are the major limitations of all membrane operations, which slow
down their use and further development both in desalination and
wastewater treatment. Concentration polarization or temperature polarization
can be responsible for scaling. The higher solute concentration
near the membrane surface, mainly in pressure-driven membrane
processes, reduces the effective driving force and thereby the transmembrane
flux.Moreover, when concentration is too high at themembrane
surface, scaling can occur. Themembranemight act as a heterogeneous
surface for crystallization and exacerbate precipitation on the
membrane surface. In thermal membrane operations, temperature polarization
can have both a positive and a negative effect according to
the nature of the solute in terms of solubility. Adsorption, pore blocking and depositions are other classes of fouling [105]. In particular, fouling
and concentration polarization resulting in high energy consumptions
are the limitations for high recovery RO operations. Fouling can be
prevented through several techniques including pre-treatment of the
feed solution; modification of the membrane surface; the development
of improved membrane materials including zeolite, carbon nanotubes,
and aquaporin membrane [19]; hydrodynamic optimization of the membrane
module; recourse to proper chemical agents for the cleaning; and
back-flushing.