Membrane and Desalination Technologies - TCE Moodle Website

Desalination of Seawater by Reverse Osmosis 579
Colloidal fouling is caused by the accumulation of particles and macromolecules on, in, and
near a membrane. Materials accumulated on the membrane surface form an additional layer of
resistance to permeation. Early work on colloidal fouling of RO membrane used to treat feed
water indicated that subparticles of 5 mm contribute more substantially to fouling than larger
particles (35). It was postulated that particles are subjected to higher velocity and shear force
on the membrane surface as particle size increases. Therefore, larger particles tend to be swept
away in bulk flow rather than deposits on the membrane surface. In addition to surface
deposition, some particles may be small enough to penetrate and remain within the pores of
the membrane (36). In a study on the influence of colloidal fouling on salt rejection of TFC
RO, a sharp decrease in permeate flux and significant decline in salt rejection with increasing
concentration factor were observed under conditions in which colloidal fouling occurred (37).
The adsorption of organic matter on membrane surfaces is detrimental to permeate flux
and affects the salt rejection of membranes. The negatively charged functional groups on
organic foulants have an affinity for the charged membrane surface, thereby forming a
permeate-resistant layer. Organic foulants also interact with colloidal foulants. Polyphenolic
compounds, proteins, and polysaccharides bind colloids and particles and increase their
cohesion to the membrane surface (35). In addition, the biochemical interaction between
organics and microorganisms promote biofilm formation and growth. Insufficient knowledge
on the composition of dissolved organics in water makes the control of organic fouling
difficult (38).
Inorganic fouling is caused by the deposition of iron, silica, aluminum, calcium, phosphorus,
and sulfate. The fouling mechanism on the membrane surface can be caused by the
concentration polarization effect. A concentrated boundary layer is created on the separation
surface as product water passes through the membrane. Within this boundary layer, the high
concentration causes the salts to precipitate and suspended solids initiate the deposition on the
membrane surface, thereby leading to scaling and fouling (36). Scale deposition on and into
RO membranes impairs the hydrodynamic conditions of the feed flow. When fouling conditions
are not controlled properly, scaling becomes a self-sustaining phenomenon. Under
severe concentration polarization, channeling, failure of RO performance, and damage to
membrane surface occur.
Biofouling is the term given to the adhesion of microorganisms and growth of biofilm on
the membrane surface. In addition to the detrimental effects of increased transmembrane
pressure and decreased permeate flux, biofouling may cause chemical degradation of the
membrane material. This could result from direct enzymatic biodegradation of the membrane
surface or by generation of extreme local pH that may hydrolyze the membrane polymer (39).
Such fouling can significantly reduce the membrane lifetime.
The ratio of carbon/nitrogen/phosphorus has important influences on the rate of biofilm
development (40). It has been reported that membranes that suffered severe biofouling were
found to contain a high percentage (typically >60%) of organics. Laboratory characterization
of membrane biofilms found that a typical biofilm contains
l 90% moisture,
l 50% total organic matter,