Biomedical Engineering Reference
In-Depth Information
TABLE 7.3
Comparison among Microfiltration, Ultrafiltration, Nanofiltration, and Reverse
Osmosis Membrane Processes
Microfiltration
Ultrafiltration
Nanofiltration
Reverse Osmosis
Pore size
0.01-1.0 μm
0.001-0.01 μm
0.0001-0.001 μm
<0.0001 μm
Materials used
to construct
the
membranes
Ceramics,
polypropylene,
polysulfone,
etc.
Ceramics,
polysulfone,
cellulose
acetate, thin
ilm
composites
Cellulose
acetate, thin
ilm
composites
Cellulose acetate,
thin film
composites
Type of
contaminants
removed
Clay, bacteria,
viruses,
suspended
solids
Proteins,
starch,
viruses,
colloid silica,
organics,
dyes, fat, etc.
Starch, sugars,
pesticides,
herbicides,
divalent
anions,
organics, BOD,
COD,
detergents
Metal cations,
acids, sugars,
aqueous salts,
amino acids,
monovalent
salts, BOD,
COD
Source:
Modified after Wagner J.,
Membrane Filtration Handbook,
2nd Ed., Osmonics Inc., 2001.
With permission.
7.4.2.1 Carbon Nanotube Membranes (CNMs)
Ordered arrays of densely packed, vertically aligned CNTs can be used as
membranes to filter out water impurities, while allowing the water to freely
flow through the filter. CNMs are able to remove almost all kinds of water
contaminants, including bacteria, viruses, and organic contaminants [44].
CMNs are also shown to be effective in desalinating salty water [45]. Table
7.4 compares between the contaminant removal capabilities of CNMs and
some alternative water treatment technologies.
Despite the fact that the pores of CNMs are significantly smaller than
the pores of other membranes, CNMs have shown the same, or even faster,
flow rates. Some CNMs show 10,000 times greater water permeability than
some conventional polycarbonate membranes [44]. The reason behind that
phenomenon is hidden in the smooth interior of the nanotubes, the walls of
which are almost perfectly flat.
CNMs are still relatively expensive but the cost of producing them con-
stantly falls as researchers develop cheaper production methods. It was esti-
mated by Srivastava et al. [45] that CNMs can become much less costly than
other filtration membrane technologies (e.g., reverse osmosis, ceramic and
polymer filters) [45]. CNM filters are already market competitive in Europe
and in the United States but they are still too expensive to apply in develop-
ing countries.
