Membrane and Desalination Technologies - TCE Moodle Website

Potable Water Biotechnology, Membrane Filtration and Biofiltration 479
Size (µm) 0.0001 0.001 0.01 0.1 1.0 10 100
Molecular weight
(Daltons)
200 20,000 200,000
Drinking water
Bacteria
pathogens
Giardia
Cryptosporidium
Viruses
Membrane filtration
process
MCF
MF
Fig. 11.1. Comparison of membrane filtration applications in drinking water treatment process
(adapted from (2)).
2.2. Desalination
Desalination is the area of growth for membrane technology. Various types of membranes
used for desalination include reverse osmosis, nanofiltration, and electrodialysis. Other
techniques such as the distillation process, ion exchange, freezing, and other miscellaneous
processes are also widely employed. Approximately 60% of the worldwide desalting capacity
is attributable to distillation technologies (3). However, the percent of worldwide desalting
capacity by membrane processes has been steadily increasing (4).
To obtain drinking water from brackish water or seawater, the membrane system is a good
alternative treatment method. A combination of ultrafiltration followed by reverse osmosis
can be very cost effective. Ultrafiltration cartridges used as pretreatment can considerably
extend the useful life of reverse osmosis elements. When treating brackish water with
relatively low total dissolved solids (TDS), ultrafiltration can be used as pretreatment to
reverse osmosis and to produce permeate for blending with reverse osmosis product water.
The result is water for distribution that not only tastes good, but is less expensive to produce.
2.3. Control of Disinfection By-Products
Recently there has been a growing interest in employing membrane processes for removal
of precursors to DBPs. The major precursor of DBPs is natural organic matter (NOM). NOM
is normally derived from plant or microbial residues, or is produced in situ in water by life
cycles and by a variety of decomposition pathways (5). NOM can be categorized into humic,
microbial by-products, and colloidal natural organic matter, depending on their source and
characteristics. Microbial by-products are composed of acids, with a relatively high charge
density, polysaccharides, amino sugars, and proteins. Humic material, normally found in
surface water, consists of fulvic acid and humic acid. Fulvic acid has an average molar mass
RO
NF
UF