Membrane and Desalination Technologies - TCE Moodle Website

276 J. Paul Chen et al.
The MF membranes are usually made from natural or synthetic polymers such as cellulose
acetate (CA), polyvinylidene difluoride, polyamides, polysulphone, polycarbonate,
polypropylene and polytetrafluoroethylene (PTFE) (13). Some of the newer MF membranes
are ceramic membranes based on alumina, membranes formed during the anodizing of
aluminium, and carbon membrane. Glass is being used as a membrane material. Zirconium
oxide can also be deposited onto a porous carbon tube. Sintered metal membranes are
fabricated from stainless steel, silver, gold, platinum and nickel, in discs and tubes. The
properties of membrane materials are directly reflected in their end applications. Some
criteria for their selection are mechanical strength, temperature resistance, chemical compatibility,
hydrophobility, hydrophilicity, permeability, permselectivity and the cost of membrane
material as well as manufacturing process.
Two process modes – namely, dead-end and cross-flow modes – are widely used for
microfiltration (14). For the dead-end mode, the entire solution is forced through the
membrane. The substances to be separated are deposited on the membrane, which increases
the hydraulic resistance of the deposit. The membrane needs to be renewed as soon as the
filtrate flux no longer reaches the required minimum values at the maximum operation
pressure. This mode is mostly used for slightly contaminated solutions, for example, production
of the ultra-pure water. For the cross-flow mode, the solution flows across the membrane
surface at a rate between 0.5 and 5.0 m/s, which prevents the formation of a cover layer on
the membrane surface. A circulation pump produces the cross-flow velocity or the shear
force needed to control the thickness of the cover layer. The system is most widely used
for periodic back flushing, where part of the filtrate is forced in the opposite direction
at certain interval, and breaks up the cover layer. The normal operating pressure for this
mode is 1–2 bars.
A MF has a wide range of applications in water and wastewater treatment. The most
promising opportunity for MF is the treatment of the municipal water, allowing it to be
disinfected without any disinfectants such as ozone and chlorine. A recent study showed that
MF membranes can also remove viruses from contaminated surface water. Since viruses are
much smaller than the pores in a MF membrane, the finding has been attributed to the viruses
being adsorbed on the clay particles, which are large enough to be caught by the microfilter
(16). Treatment of municipal sewage is a potential market for MF. Engineers have tried
to shift the treatment of sewage to distributed processing, which consists of many small
sewage treatment facilities. MF can be used to remove heavy metals from waste streams if
pre-treatment chemicals are added to precipitate the metals to particles of filterable size.
Although conventional methods of wastewater treatment may use a similar pre-treatment
mechanism, the final solid/liquid separation by gravity settling is usually not as effective as
membrane filtration. A promising process for the removal of heavy metal ions from aqueous
solutions involves bonding the metals firstly to a special bonding agent and then separating
the loaded bonding agents from the wastewater stream by separation processes. A new hybrid
process of flotation and MF process has been developed to integrate specially designed
membrane modules with a flotation reactor, which can treat heavy metal cationic wastewater
up to portable-grade level (17). This made it possible to combine the advantages of
both flotation and membrane separation while overcoming the limitations. MF is applicable