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