Environmental Engineering Reference
In-Depth Information
membrane materials: (i) nanostructured ceramic membranes; (ii) thin ilm nanocomposite
organic membranes; and (iii) organic-inorganic materials. An excellent review of nano-
engineered membranes was prepared by Pendergast and Hoek.
21
Ceramic membranes
have the advantage of being the most mechanically stable at high pressures and able to
handle mixed waste streams that contain surfactants, disinfectants, and other potential
fouling agents. Most nanostructured ceramic membranes are based on zeolite materials
that consist of a three-dimensional cross-linked (Si/Al)O
4
tetrahedral framework with
interconnected cavities that create a cage-like structure, and allow for the movement of
water and ions through an interconnected pore architecture.
22
The Si/Al ratio of the zeolite
is the most important factor in determining the chemical stability and hydrophobic prop-
erties.
23
Increasing the relative Si content lowers surface charge and water permeability.
24
Separation is most often based on a molecular sieving mechanism making the pore size
and framework density the primary factors in separation. A variety of zeolite materials
have been nanoengineered to tailor the framework and the internal surfaces to promote
separations. For instance, anionic sites can be introduced in the zeolite matrix to allow ion
exchange type of interactions. This level of chemical and morphological control on the
nanoscale allows for separations to be achieved similar to those with reverse osmosis.
25
Many nanostructured ceramic zeolite membranes have been developed that have the abil-
ity to desalinate water by promoting water permeability and rejecting ion lux.
26
Although
this is a notable achievement, oil-in-water treatment relative to bilge water management
does not generally require reverse osmosis levels' separation. Toward shipboard man-
agement applications, membrane permeability is more important than molecular-scale
selectivity in separation. As a result, organic thin ilms supported on high-lux ceramic
materials have been developed for oily wastewater treatment that optimizes lux and rejec-
tion of oil. Peinemann and colleagues initially demonstrated the use of coblock polymers
and self-assembly to achieve highly ordered porous membranes overlaying nanocylinder-
based supports.
2 7, 2 8
Asatekin and Mayes used an amphiphilic copolymer comb composed
of polyacrylonitrile-
graft
-poly(ethylene oxide) to modify UF membranes for improved
resistance to fouling.
29
Results indicated that comb-like polymer-modiied membranes
allowed for ~42% chemical oxygen demand removal from reinery wastewater and could
be cleaned by physical means, making them attractive for use. Faibish and Cohen reported
a similar approach by covalent modiication of a zirconia-based ceramic UF membrane
with poly(vinylpyrrolidone).
30
Linkage of the organic polymer to the zirconium was
accomplished using a silane modiication of the surface. The resulting membranes were
effective at separating microemulsions and demonstrated improved resistance to fouling
by organics. Permeate lux of native membranes irreversibly declined upon exposure of
anionic surfactants; however, polymer thin ilm modiication prevented fouling under the
same conditions.
Two Spanish groups examined the use of high-porosity ceramic membranes specii-
cally for bilge water treatment. Benito and coworkers used a multilayer membrane with
a tubular cordierite support with either α-Al
2
O
3
or γ-Al
2
O
3
layers deposited on the sur-
face by dip-coating or the sol-gel process, respectively.
31
Reduction of dispersed oil levels
in simulated bilge water to <15 ppm was achievable to pressures up to 0.7 MPa. García
et al. tested organically modiied ceramic membranes manufactured by Orelis and PCI
Membrane systems in a pilot-scale system to separate hexane and water.
32
The ceramic
substrate membrane was zirconia coated with polyethersulfone. The results from these
tests are most insightful as they provide data for long-term use and performance. Initially,
the membranes performed well with almost zero hexane in the permeate low. However,
hexane luxes increased when the membranes were exposed to solvents, indicating partial
