Membrane and Desalination Technologies - TCE Moodle Website

212 L.K. Wang and R. Menon
As discussed previously, a complete MBR system (Fig. 5.4), developed by ONDEO
Degremont, is based on external or in-series configuration, where the membrane units
follow, and are situated outside the bioreactor. This added special feature may keep the
biological process in the bioreactor and the liquid–solid separation process (clarification) in
the membrane module separate, avoiding interferences and enabling individual process
optimization.
2.4. Membrane Module Design Considerations
Membrane processes are characterized by two basic process parameters: (a) flux, which is
the rate of transport of solvent or solution through the membrane and (b) rejection, which is
the degree of separation of a particular feed component.
There are five major variables that affect the two basic process parameters: (a) driving
force in terms of applied trans-membrane pressure and/or electric voltage/current; (b) flow
velocity, which affects turbulence and mass transfer coefficient; (c) process water temperature,
which has effects on physical properties such as density, viscosity, diffusivity, osmotic
pressure, surface tension, and others; (d) feed stream characteristics in terms of particle
concentration, particle size, viscosity, molecular weight, molecular configuration, ionic
charges, and fouling potential; and (e) membrane module in terms of materials, pore
sizes, membrane configuration, membrane ionic charges, and feed compatibility (1).
There are basically six different designs of membrane modules: (a) tubular modules with
channel diameters greater than 3 mm; (b) hollow-fiber or capillary modules made of selfsupporting
tubes, usually 2 mm or less in internal diameters; (c) plate modules; (d) spiralwound
modules; (e) pleated sheet modules; and (f) rotary modules. The latter four module
designs use flat sheets of membrane in various configurations (1).
In selecting a particular membrane module and a particular membrane process, the major
criteria are (a) feed stream characteristics, which affect the biocompatibility of the membranes;
(b) flux requirements, which are controlled by the volumetric rate of a feed stream; (c)
rejection requirements, which decide the process objectives and treatment efficiencies; and
(d) cost requirements, which are affected by energy consumption, membrane replacement
cost, and operating and cleaning costs.
Biocompatibility of the membrane relates to the interaction between the membrane module
and the feed stream. Major biocompatibility factors include (a) stability to extremes in
temperature, pressure, and pH, especially under cleaning and sanitizing conditions; (b)
membrane–solute interactions, which affect the rate of fouling, cleaning, yields, and rejection
of individual feed substances; and (c) acceptability of the membrane as a contact material for
the final product, which essentially implies using membrane materials that are inert and do not
leach out any toxic substances from the membrane into the final product. In this regard, there
are new generations of membranes, made of expensive inorganic materials, such as ceramics,
stainless steel, carbon–zirconia, etc.
MF membranes are made of a wide range of inorganic materials (such as alumina,
zirconia–carbon composites, carbon–carbon composites, ceramics, stainless steel, silica,