W. Richard Bowen and Nidal Hilal 4

140 5. AFM AND DEvELOPMENT OF (BIO)FOULINg-REsIsTANT MEMBRANEs
surface, restricting the efficiency of treatment processes which are dependent
upon membrane filtration. Fouling becomes evident in the characteristic
flux decline that results from increased resistance to flow due to
the deposited material. Fouling analysis is difficult because the hydrodynamic
aspects of concentration polarisation, as well as the physical
chemistry of solute–solute and solute–membrane interactions must be
considered.
The formation of a fouling layer on the membrane surface, referred to
as surface fouling, is the dominant fouling mechanism. Surface fouling
involves the initial deposition of foulants of organic or inorganic origin
on the membrane surface and the subsequent growth of a fouling layer
that adversely influences membrane performance.
Modification of the membrane surface can alter the surface chemistry
and subsequently the surface charge properties of the membranes. The
effectiveness of membranes significantly depends on their molecular and
structural architecture, i.e. functional group distribution (2D or threedimensional
[3D] distribution and uniformity), shielding of functional
groups and their accessibility, and membrane morphology. Therefore,
membranes containing different functional groups and characterised by
different structural architectures are of special interest for the advance of
their application in different areas. In their development, profound structure
characterisation is of particular importance in providing an insight
into the relationships between membrane performance and structure.
Changes in surface morphology (roughness, pore size and pore size distribution
[PSD]) and surface charge due to modification of membrane
surfaces have been reported by several researchers [1–3].
The degree of hydrophobicity of both membrane and solutes determine
the degree of fouling of membranes. It has been reported that
protein fouling is greater for hydrophobic membranes than hydrophilic
membranes [4]. However, the charge and hydrophobicity effects are
functions of environmental conditions such as the salt concentration
and the solution pH. The effect of charge decreases with increasing salt
concentration due to a resultant decrease in the extent of the electrical
double layer.
Several authors have described natural organic matter (NOM) as
one of the major membrane fouling agents in microfiltration, ultrafiltration
and nanofiltration of surface water [5]. The chemical characteristics
of the water in which the humic substances are dissolved, such as pH
and ionic strength, are primary determinants in membrane fouling due
to their effects on the conformational structure and charge of the humic
substances.
As bacterial surfaces contain proteins, polysaccharides and other
biopolymers that can affect their adhesion to another surface, several