reverse osmosis, which is used to separate dissolved species. Ultrafiltration is capable of removing 90 percent of a specific size particle while operating under a pressure differential of 10 to 100 psi. Ultrafiltration membranes are constructed by utilizing an ultrathin polymer layer of controlled, fine pore structure, which is supported by a thicker, stronger, but more porous layer. Separation is controlled by the thin layer, and mechanical integrity is supplied by the thick layer. Polymers that have been successfully employed at the thin layer include cellulose, nylon, polyvinylchloride (PVC), polysulfone, and acrylonitrile copolymers. Design is a complex process, and pilot plant testing for specific applications is usually required.

Given that ultrafiltration operates as a physical screening process, removal of a particle of some specific size can be established by the pore size of the membrane itself. The important variables are the flux rate of the solvent through the membrane and the ability of the membrane to resist fouling (which leads to flux reduction) (Klinowski, 1989). As solvent particles flow through the membrane, a layer of particles that cannot get through will build up on the surface and impede the passage of solvent. For this reason, the mixture on the upstream side of the membrane is kept flowing along the surface (i.e., operated in a cross-flow geometry), thus creating a shear flow at the surface that helps sweep away the particles. This works only up to a point and requires expenditure of energy to pump the mixture. In addition, particles may become attached to the surface and within the membrane, and cleaning procedures must be worked out (usually on line). Therefore, the subtle separations made possible by ultrafiltration membranes carry substantial engineering challenges for large-scale applications. Even so, ultrafiltration is widely used in industry today (e.g., food processing and paint manufacture), and applications in the waste treatment area are appearing.

Ultrafiltration offers the ability to concentrate liquids that are present in shipboard waste streams, e.g., black water, gray water, and bilge water. Bilge water is an interesting case. Navy ships are equipped with parallel plate separators that are effective at separating oil from water. However, if detergents are present (as from cleaners), these separators lose their effectiveness because of emulsion formation. Ultrafiltration can separate these emulsions.

Ultrafiltration is a promising technology for inclusion on board Navy ships as a treatment process for liquid waste. Treatment of black water, gray water, and bilge water could all utilize membrane systems for concentrating some aspect of these waste streams. The methods are sufficiently well employed in industry that long development times would not be required.

Advanced Oxidation—Semiconductor Photocatalysis

Semiconductors (e.g., TiO2, ZnO, Fe2O3, CdS, and ZnS) can act as sensitizers for light-induced redox processes because of their electronic structure, which is characterized by a filled valence band and an empty conduction band (Hoffmann et al., 1995). When a photon possesses an energy that matches or exceeds the bandgap energy of the semiconductor, an electron is promoted from the valence band into the conduction band, leaving a hole behind. Excited-state conduction-band electrons and valence-band holes can recombine and dissipate the input energy as heat, get trapped in metastable surface states, or react with electron donors and electron acceptors adsorbed on the semiconductor surface or within the surrounding electrical double layer of the charged particles.

In the absence of suitable electron and hole scavengers, the stored energy is dissipated within a few nanoseconds by recombination. If a suitable scavenger or surface defect state is available to trap the electron or hole, recombination is prevented and subsequent redox reactions may occur. The valence band holes are powerful oxidants, whereas the conduction band electrons are good reductants. Most organic photodegradation reactions use the oxidizing power of the holes either directly or indirectly; however, to prevent a buildup of charge, one must also provide a reducible species to react with the electrons. In contrast, on bulk semiconductor electrodes, only one species, either the hole or electron, is available for reaction because of band bending. However, in very small semiconductor particle suspensions, both



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