Membrane and Desalination Technologies - TCE Moodle Website

Treatment of Food Industry Foods and Wastes by Membrane Filtration 243
MF membranes are made of a wide range of inorganic materials (such as alumina,
zirconia–carbon composites, carbon–carbon composites, ceramics, stainless steel, silica,
etc.) and natural and synthetic polymers (such as polypropylene, polycarbonates, polysulfone,
polyvinylchloride, PVC, copolymer – cellulose esters, cellulose acetate, etc.) (15–16).
UF membranes are mainly made of polysulfone-type materials (such as polyether sulfone,
polyphenylene sulfone, sulfonated polysulfone, etc.). This type of membranes is also available
in a wide range of organic materials (such as PVC copolymer, cellulose acetate, etc.) and
inorganic materials (such as ceramic composites, stainless steel, etc.).
Most NF membranes are multiple-layer thin-film composites of synthetic polymers. The
active NF membrane layer usually consists of negatively charged chemical groups. NF
membranes are of porous filter media with an average pore diameter of 2 nm. The nominal
molecular weight cutoff ranges from 100 to 200. The active NF membrane layer can be made
of polyamide, polyvinyl alcohol, sulfonated polysulfone, and sulfonated polyethersulfone.
Salt rejection by NF membranes is mainly due to electrostatic interaction between the ions
and the NF membrane. Rejection of neutral substances is by size (17).
CA and derivatives are widely used as the RO membranes, despite their real and perceived
limitations. Thin-film composite membranes containing a polyamide separating barrier on a
polysulfone or polyethylene supporting layer, generally give better performance for RO
applications with regard to temperature and pH stability and cleanability, but have almost
zero chlorine resistance. In general, these thin-film composite membranes will be the material
of choice for RO applications, unless there is a specific fouling problem with these membranes.
The membranes of UF, NF, and RO have been tried for ED with various results. The use of
ionic membranes (18) in ED and EDR processes has been applied as illustrated in Fig. 6.3.
Table 6.2 shows the special features of four types of membrane systems: tubular membrane
modules, hollow-fiber membrane modules, plate membrane modules and spiral-wound
membrane modules (14). Each design has its own special applications, advantages, and
disadvantages.
The large-bore tubular membrane modules (Fig. 6.4) are suitable for food streams with
high concentration of suspended solids, such as citrus juices and animal waste streams, even
Table 6.2
Special features of membrane equipment
Tubular Hollow fibers Plates Spiral
Channel height (mm) >10