Membrane and Desalination Technologies - TCE Moodle Website

Treatment of Industrial Effluents, Municipal Wastes, and Potable Water 213
etc.) and natural and synthetic polymers (such as polypropylene, polycarbonates, polysulfone,
polyvinylchloride, PVC copolymer, cellulose esters, cellulose acetate, etc.) (1).
UF membranes are mainly made of polysulfone-type materials (such as polyether sulfone,
polyphenyl sulfone, sulfonated polysulfone, etc.), although they are 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 polyether sulfone.
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.
Cellulose acetate and derivatives are widely used as the RO membrane, 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.
There are four types of membrane equipment: tubular membrane modules, hollow-fiber
membrane modules, plate membrane modules, and spiral-wound membrane modules. Each
design has its own special applications, advantages, and disadvantages (1).
The large-bore tubular membrane modules are suitable for food streams with high concentrations
of suspended solids, such as citrus juices and animal waste streams, even though
the tubular membrane modules have the lowest packing densities and highest energy consumption
among all the modules. The tubular designs with ceramic inorganic membranes are
frequently used in the food processing industries.
The hollow-membrane modules have extremely high packing density (surface area to
volume ratios) and comparatively low energy consumption, and are suitable for comparatively
clean feed streams with low concentrations of suspended solids and macromolecules.
Certain macromolecules display non-Newtonian behavior. Their viscosity will increase
dramatically above certain concentrations, making pumping difficult and reducing mass
transfer within the boundary layer. This will eliminate most hollow-fiber/capillary modules
because they cannot withstand high pressure drops.
Membrane modules utilizing flat sheets (spiral-wound, plate, and pleated sheet modules)
usually have a mesh-like spacer between sheets of membrane. This restricts their use to clear
feed streams containing only fine SS.
Feed streams containing large SS would be treated poorly in spiral-wound modules, owing
to the spacers in their feed channels. On the contrary, spiral-wound membrane modules are
the lowest in capital costs and energy consumption. The trend in the food and beverage
industries in recent years seems to be away from plate modules and toward spiral-wound
modules, with ceramic tubular modules holding their own.