Environmental Engineering Reference
In-Depth Information
using a three-layer membrane system composed of a conventional nonwoven microibrous
support overlaid with an electrospun nanoiber midlayer and a hydrophilic nanocomposite
top layer. 52 The nanoibers composing the midlayer were electrospun poly(vinyl alcohol)
cross-linked with glutaraldehyde to increase structural integrity. The top layer consisted
of either a polyether- b -polyamide copolymer or poly(vinyl alcohol) hydrogel embedded
with surface-oxidized multiwall carbon nanotubes. These membranes showed high lux
(330  L/m 2 h) and achieved 99.8% rejection of emulsiied oil. Tests using simulated bilge
water were done using soybean oil emulsiied with a nonionic surfactant (Dow Corning
193 luid). The nanocomposite membranes exhibited lux rates signiicantly higher than
commercially available Pebax ® 1074 (polyethyleneoxide- b -polyamide 12 copolymer)-coated
membranes. Eficiency was >99.5% rejection of emulsiied oil. Additional designs using cel-
lulose also showed good effectiveness with simulated bilge water. 53
Continued work on optimizing the lux, separation eficiency, and mechanical stability
of nanoiber-based ilters should further improve performance. In many respects, these
membranes will compete with mesh designs. One of the questions concerning electrospun
ibers is the throughput rate at which membranes can be achieved. Manufacturing large
mats of electrospun membranes currently takes substantial amounts of time. Material
costs are relatively low, so if fabrication rates can be improved there may be utility of
nanoibrous UF membranes in large-scale applications such as bilge water management.
7.4 Adsorbents
An alternative approach to physical separation by centrifugation or membrane-based sys-
tems is the use of high-capacity sorbents to remove organics from water streams. Such pro-
cesses differ from locculation in that dissolved chemical additives are not added when using
sorbents. Chemical locculent residues may be part of the discharged water, which may have
limitations depending on discharge regulations. Given the high levels of oil in bilge water,
adsorbents are currently economical for only secondary treatment or polish steps and would
be used after skimming or even centrifugation to remove hard-to-treat emulsions. One of the
advantages of sorbents is the ability to remove dissolved oil or other organics, a process that
can signiicantly improve water quality. Activated carbon is the most widely used sorbent
material. A disadvantage of sorbents is the need to store the media pre- and post-use, as well
as having logistics for disposal and resupply at port. In many ports of call, proper disposal
facilities may be lacking. Use of water treatment media requires that a sorbent have high
capacity, long working lifetime, and the potential for regeneration onboard. Nanotechnology
advances have led to new materials that have functional capabilities beyond traditional
media such as activated carbon for use in bilge water treatment.
7.4.1 Surface-Modified Mineral Particles
Selective absorption of oil from water has been accomplished using superhydrophobic
calcium carbonate powders by Arbatan and coworkers. 54 Extraordinary levels of hydro-
phobicity can be achieved through a combination of surfaces that are nonpolar (low sur-
face energy) and have microscale and nanoscale surface roughness. Typically, a material
is deemed superhydrophobic when the contact angle (i.e., the angle between the edge of
a drop of water and the surface) is >150° and contact angle hysteresis is <5°. Precipitated
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