Membrane and Desalination Technologies - TCE Moodle Website

314 J. Paul Chen et al.
biofilm formation and growth. Insufficient knowledge on the composition of dissolved
organics in water makes the control of organic fouling difficult.
Inorganic fouling is caused by deposit of iron, silica, aluminium, calcium, phosphorus and
sulphate (Fig. 7.23a). The fouling mechanism at 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 can start to deposit on the
membrane surface leading to scaling and fouling (58). Scale deposition onto 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 (60). Under
severe concentration polarization, channelling, 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 (Fig. 7.23c). Sewage carries a very high load of potentially pathogenic
and nonpathogenic microorganisms in addition to chemical pollutants and nutrients (61).
Besides the detrimental effects of increasing 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 hydrolyse the membrane polymer (61–63). Such fouling can
significantly reduce the membrane lifetime.
The formation of biofilm involves three steps:
1. Formation of a conditioning film composed of macromolecules, proteins, etc.
2. Primary bioadhesion by microorganisms
3. Biofilm development
A recent study has shown that some RO membranes are more prone to bioadhesion than
others. The study involved a bioadhesion assay, which utilizes a model bacterium, SW8,
known to adhere to membranes. Examination of bacteria adhered to the membranes using
optical microscopy revealed that membranes that are less susceptible to bioadhesion are
hydrophilic in nature (62, 63). Further investigation with municipal wastewater carried out on
RO simulators that consisted of flat sheet membranes to simulate spiral-wound module
revealed biofilm characteristics under field emission scanning electron microscope. Microorganisms
cover the surface of all types of RO membranes used in the experiment to a density
of about 2.25 10 8 cells/cm 2 (62). Bacteria of different shapes (mostly rod-shaped) with
dimensions between 1 and 3 mm were observed. The organisms appeared to excrete extracellular
polymeric substances (EPSs).
The ratio of carbon to nitrogen to phosphorus (C:N:P) has important influences on the rate
of biofilm development. It has been reported that membranes which 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