The MBR Book: Principles and Applications of Membrane

24 The MBR Book
Feed
in the flow of water through the membrane (i.e. the flux) at a given transmembrane
pressure (TMP), or conversely an increase in the TMP for a given flux (reducing the
permeability, which is the ratio of flux to TMP). These phenomena are collectively
referred to as fouling. Given that membrane fouling represents the main limitation
to membrane process operation, it is unsurprising that the majority of membrane
material and process research and development conducted is dedicated to its characterisation
and amelioration (Section 2.3).
Fouling can take place through a number of physicochemical and biological mechanisms
which all relate to increased deposition of solid material onto the membrane surface
(also referred to as blinding) and within the membrane structure (pore restriction
or pore plugging/occlusion). This is to be distinguished from clogging, which is the filling
of the membrane channels with solids due to poor hydrodynamic performance. The
membrane resistance is fixed, unless its overall permeability is reduced by components
in the feedwater permanently adsorbing onto or into the membrane. The resistance
imparted by the interfacial region is, on the other hand, dependent on the total amount
of fouling material residing in the region. This in turn depends upon both the thickness
of the interface, the feedwater composition (and specifically its foulant content) and the
flux through the membrane. The feedwater matrix and the process operating conditions
thus largely determine process performance.
2.1.2 Membrane materials
Figure 2.2 Schematic of membrane
Retentate
Permeate
There are mainly two different types of membrane material, these being polymeric
and ceramic. Metallic membrane filters also exist, but these have very specific applications
which do not relate to membrane bioreactor (MBR) technology. The membrane
material, to be made useful, must then be formed (or configured) in such a
way as to allow water to pass through it.
A number of different polymeric and ceramic materials are used to form membranes,
but generally nearly always comprise a thin surface layer which provides the required
permselectivity on top of a more open, thicker porous support which provides mechanical
stability. A classic membrane is thus anisotropic in structure, having symmetry
only in the plane orthogonal to the membrane surface (Fig. 2.3). Polymeric membranes
are also usually fabricated both to have a high surface porosity, or % total surface pore
cross-sectional area (Fig. 2.4), and narrow pore size distribution to provide as high a
throughput and as selective a degree of rejection as possible. The membrane must also
be mechanically strong (i.e. to have structural integrity). Lastly, the material will
normally have some resistance to thermal and chemical attack, that is, extremes
of temperature, pH and/or oxidant concentrations that normally arise when the