Shipboard Pollution Control: U.S. Navy Compliance with MARPOL Annex V (1996)

In Chapter 4, a number of long-range options are presented in the context of future substitutes for incineration. Their purpose is to reduce the volume of solid waste without creating undesirable side effects. Minimization of shipboard space occupied and ease of operation are prime considerations. In this chapter, the committee considers a number of technologies that may contribute to viable strategies for the handling of liquid waste in an integrated waste management system. The selection of only five technology classes is to some extent arbitrary and is intended to give a flavor of the kinds of approaches that may eventually be useful to the Navy. The methods range from biological destruction of organics and pathogens, to separations of suspended solids from water, to destruction of waste materials in suspension or in solution by utilization of light, electrical, and sonic energy.

Biological treatment of wastewater involves contacting the wastewater with mixed microbial cultures under either aerated or air-free conditions. The context of the present discussion is shipboard black water. Conventional sewage treatment plants are the largest operating examples of biological water treatment and are found in nearly every U.S. municipality. These systems accept feedstreams of 300 to 1,000 mg/L dissolved or suspended solids (expressed as dissolved oxygen required to process the complex waste). By microbial action, the organic matter is transformed into low molecular weight species and then converted into mineralized products, carbon dioxide, water, and more microbial cells. An important benefit of aerobic treatment is the destruction of pathogenic (disease-causing) organisms and viruses. Before disposal, the biological mass may be reduced through anaerobic digestion, which produces methane, carbon dioxide, and residual biomass. The latter material (dewatered) may be introduced into landfill or incinerated.

Biological treatment of waste occurs when microbes use constituents in the wastewater for growth. The organic materials in wastewater provide both carbon for new cell material and a source of energy to drive the chemistry. The mechanisms are complex and beyond the scope of this discussion. There are many variations of the technology, most employing both aerobic and anaerobic treatment. A possible strategy for shipboard use is to destroy the pathogens by aerobic digestion and incinerate the dewatered sludge directly, thereby eliminating the anaerobic digestion processes. Large-scale biological oxidation of black water is in current practice in the cruise line industry, as part of its integrated system for waste management. Since large-volume holding tanks are required to achieve the necessary contact time with the microbes, these methods may be less attractive to the Navy. Biological methods are not promising for disposal of oils, greases, paper, and plastic.

Membrane technology has moved forward quickly in recent years because of the use of new technologies for the manufacture of membranes. These membranes are designed to separate species based on particle size down to dimensions as small as 1 to 10 nm. The method is to be distinguished from

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.

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

species are present on the surface. Therefore, careful consideration of both the oxidative and the reductive paths is required.

The application of illuminated semiconductors for the destructive degradation of chemical contaminants has been used successfully for a wide variety of compounds (Hoffmann et al., 1995; Ollis and Al-Ekabi, 1993; Mills et al., 1993), such as alkanes, aliphatic alcohols, aliphatic carboxylic acids, alkenes, phenols, aromatic carboxylic acids, dyes, polychlorinated biphenyls (PCBs), simple aromatics, halogenated alkanes and alkenes, surfactants, and pesticides as well as for the reductive deposition of heavy metals (e.g., Pt^4+, Au^3+, Rh^3+, Cr(VI)) from aqueous solution to surfaces. In many cases, complete mineralization of organic compounds has been reported.

A variety of photochemical reactor configurations have been employed in photodegradation studies and for actual treatment situations. In practical applications of semiconductor photocatalysis, fixed-bed reactor configurations with immobilized particles or semiconductor ceramic membranes may be required. A fixed-bed reactor system allows for the continuous use of the photocatalysts for processing of aqueous-or gas-phase effluents while eliminating the need for post-process filtration coupled with particle recovery and catalyst regeneration. In typical fixed-bed photocatalytic reactors, the photocatalyst is coated on the walls of the reactor, on a solid-supported matrix, or around the casing of the light source. However, these reactors have several drawbacks; most notable are the low surface area-to-volume ratios and inefficiencies introduced by absorption and scattering of light by the reaction medium. Fine particle entrapment can be achieved by immobilization on glass beads, immobilization on walls of reaction vessels or tubes, immobilization on fiberglass or woven fibers, and compression of fine particles into ceramic membranes. Another reactor configuration that appears to be promising uses an optical fiber cable as a means of light transmission to solid-supported TiO₂. Light energy is transmitted to TiO₂ particles, which are chemically anchored onto quartz fiber cores, via radial refraction of light out of the fiber. Operational factors that influence the efficiency of the bundled-array optical fiber reactor are the distance of light propagation down the fiber, the degree of light absorption by the TiO₂ coating of the refracted light, and the ability of the chemical substrates to diffuse into the TiO₂ coating. A TiO₂ coating layer that minimizes the interfacial surface area of the quartz core and TiO₂ particles and operation with incident irradiation angles near 90 º enhance light propagation down the fibers.

Another semiconductor-based treatment scheme involves the use of doped-semiconductor electrodes (anodes), in which Ti(III)/Ti(IV)-Nb(V)/Nb(IV) mixed metal oxides are coated on titanium rods or plates. The semiconductor anodes are coupled with carbon cloth cathodes in an electrochemical cell. This electrolysis system has been shown to be highly efficient for the in situ electrochemical generation of hydroxyl radical at the anode (from hydroxide oxidation) and hydrogen peroxide at the cathode (from dioxygen reduction). Hydroxyl radical and hydrogen peroxide in combination are highly effective for the oxidative degradation of a wide variety of oxidizable chemical contaminants (Weres and Hoffmann, 1994).

Semiconductor photocatalysis systems are currently available on the commercial market. Commercial reactors are offered by IT Corporation, SEMATECH, Solar Kinetics, Nutech, and Solarchem.

The term cavitation refers to the formation and the subsequent dynamic life of bubbles which are filled with vapor and gases, in liquids. Cavitation is brought about by tension produced by pressure variations that may be of acoustic, explosive, hydrodynamic, or thermal origin. Cavitation occurs in water, organic solvents, biological fluids, liquid helium, and molten salts.

The sonochemical degradation of a variety of chemical contaminants in aqueous solution has been previously reported (Kotrounarou et al., 1991, 1992a,b). Substrates such as chlorinated hydrocarbons, pesticides, phenols, and esters are transformed into short-chain organic acids, CO₂, and inorganic ions as the final products. Time scales of treatment in simple batch reactors are reported to range from minutes to

hours for complete degradation. Ultrasonic irradiation appears to be an effective method for the destruction of organic contaminants in water because of localized high concentrations of oxidizing species such as hydroxyl radical and hydrogen peroxide in solution, high localized temperatures and pressures, and the formation of transient supercritical water (Hua et al., 1995).

The application of electrohydraulic cavitation in its various forms for the pyrolytic and oxidative control of hazardous chemicals in water has the potential to become economically competitive with existing technologies in terms of convenience and simplicity of operation.

The chemical effects of ultrasound have been explained in terms of reactions occurring inside, at the interface, or at some distance away from cavitating gas bubbles. In the interior of a collapsing cavitation bubble, extreme but transient conditions are known to exist. Temperatures approaching 5,000 K have been estimated, and pressures of several hundred atmospheres have been calculated. Temperatures on the order of 2,000 K have been estimated for the interfacial region surrounding a collapsing bubble based on observed reactivity. During bubble collapse, which occurs within 100 nsec, H₂O undergoes thermal dissociation to give hydroxy radical and hydrogen atoms. Sonochemical reactions are characterized by the simultaneous occurrence of supercritical water reactions, direct pyrolyses, and radical reactions, especially at high solute concentrations. Volatile solutes such as carbon tetrachloride (Hua and Hoffmann, 1996) will undergo direct pyrolysis reactions within the gas phase of the collapsing bubbles or within the hot interfacial region. In the interfacial regions, combustion and free radical reactions and supercritical water reactions are possible. Pyrolysis and supercritical water reactions in the interfacial region are predominant at high solute concentrations, whereas at low solute concentrations free radical reactions are likely to predominate. In the bulk solution, the chemical reaction pathways are similar to those observed in aqueous radiation chemistry (as induced by aquated electrons, γ rays, or x rays). However, evidence for combustion-like reactions at low solute concentrations has been presented.

Optimization of aqueous-phase organic compound degradation rates within acoustical processors can be achieved by adjusting the energy density, the energy intensity, and the nature and properties of the saturating gas in solution. Observed first-order degradation rate constants increase as the energy density and intensity are increased up to a saturation value. Manipulation of these macroscopic variables should lead to enhancement of the cavitation chemistry, as the number of cavitation bubbles and the chemical events at each bubble are varied. The specific nature of the saturating gas influences the relative proportion of pyrolytic or free-radical reaction steps. Simultaneous acceleration of these pathways results in the maximum destruction rate.

Ultrasonic irradiation is used extensively in manufacturing operations. Ultrasonic processing devices are made by a number of suppliers in the United States and abroad. Many of these suppliers (e.g., Branson, Lewis Corp., Sonics and Materials, Telsonic Ultrasonics, Undatim) are willing to design systems for particular applications. For example, DuPont Merck Pharmaceutical has recently developed (Rouhi, 1995) an ultrasonic tubular resonator for large-scale chemical reactor cleaning.

Pulsed-power discharge into water or water-solid slurries is an electrohydraulic phenomenon characterized by a periodic rapid release of accumulated electrical energy across a submerged electrode gap (1 to 2 cm). The power source is a bank of charged capacitors capable of delivering a high-voltage, high-amperage electrical current to the submerged electrodes at a frequency of 20 kHz. Each electrical discharge produces a short (20 µs) burst of plasma at a high power density within the electrode gap (i.e., 25 kJ). This highly ionized and pressurized plasma has the ability to transfer energy to wastewater via dissociation, excitation, and ionization (e.g., H₂O⁺, OH, H, and so on) with a corresponding increase in temperature. The plasma produces a very high pressure shock wave (> 14,000 atm). If the propagating shock wave is reflected back from either a free surface or from a material with a different acoustic impedance, intense cavitation occurs with the associated chemical changes as described above for the

cavitational effects of ultrasound (pressure waves in the latter case range from 2 to 200 atm depending on applied frequency and power). Thus, a pulsed-power discharge in water provides a highly efficient method for promoting pyrolytic, OH radical, aquated electron, and UV-promoted reactions in water or soil-water slurries. Additional reaction pathways are likely to result from the direct reactions of the rapidly expanding plasma gases with the chemical substrates of interest and from indirect production of OH radicals due to the release of soft x rays and high-energy UV radiation from the energized plasma. In addition, the intense cavitation events induced by the shock waves emanating from the plasma discharge can be used to destroy halogenated hydrocarbons by the above mechanisms, to separate suspended and dissolved solids from water (because of the extremely high pressures that dramatically shift chemical equilibria), and to disinfect wastewater by killing bacteria and viruses.

In conjunction with Pulsed-Power Technologies, Inc., researchers at the California Institute of Technology (Caltech) have designed, constructed and demonstrated the utility of a electrohydraulic discharge reactor (EHDR). For example, more than 80 percent of 2,4-dichloroaniline is degraded in 2 ms of power utilization. In the case of 4-chlorophenol, 40 percent degradation was achieved in 2.4 ms with a series of 120 5-kJ pulsed; the corresponding g-value (i.e., number of molecules converted per 100 eV of input energy) based on the wall voltage utilization is 2.5 × 10⁻³. This value is very favorable when compared with other electrodynamic treatment technologies (e.g., electron beam reactors) which are currently being evaluated. When ozone is added to an air stream flowing into the reaction solution through a small orifice in the discharge electrode, 100 percent degradation of the target compounds is usually achieved in fewer than 40 discharges, and total elimination of total organic carbon is achieved in fewer than 100 discharges (i.e., 2 ms). The EHDR system has the advantage of extremely fast degradation rates for hazardous chemical wastes where the macroscopic rate of degradation is controlled primarily by the repetition rate of the electronic circuit. The repetition rate of the Caltech facility is approximately four discharges per minute; however, rates of 5 to 10 Hz are easily obtained using current engineering technology. At these high repetition rates, residence times in a reactor optimized for industrial applications could be on the order of seconds.

Generally, pulsed-power, cold-plasma reactors are still in the developmental stage. Potential commercial suppliers include Maxwell Industries, Pulsed-Power Technologies, Inc., and General Dynamics.

Impediments to shipboard compatibility are delineated in Chapter 4. Table 6.1 and Table 6.2 give the committee's assessment of compatibility attributes for the longer-range liquid waste-handling technologies discussed in this chapter. An additional attribute, energy requirements (identification of the relative energy demands and energy form to initiate and sustain the waste process technology and operate the machinery required to apply the technology), has been added to these tables.