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LOW-TEMPERATURE, LIQUID-PHASE PROCESSES 132 ⢠cleavage of the amide linkage followed by reduction of the resulting cleavage products; and ⢠cleavage of the methylphosphorus bond of the phosphonate to produce methane (Harkhess, 1986; Schowanek and Verstraete, 1990a,b) or biodegradation of the alcohols (the hazards associated with the methane, the main constituent of natural gas, would need to be addressed). Chemical Hydrolysis and Bioremediation of Mustard The direct biodegradation of mustard agents containing sulfur is not promising because there are no corresponding microbial or enzyme-based systems. Unlike the G agents and VX, mustard compounds are toxic to most biological systems. For this reason, initial chemical processing, possibly to form thiodiglycol, might be used followed by biological degradation to eliminate possible regeneration of agent by reaction with HCl. Mustard has reportedly been hydrolyzed by Ca(OH)2, yielding thiodiglycol as the primary product. The thiodiglycol can be degraded by two different strains of recently isolated bacteria (Pseudomonas sp. and Alcaligenes xylosoxidans) that are able to use thiodiglycol as their sole source of carbon and sulfur. Mustard from 1-ton containers at Aberdeen Proving Ground was used to demonstrate that chemical hydrolysis by NaOH with NH3, followed by biodegradation, was directly applicable to chemical agent stockpiles. The resulting culture medium was determined by bacterial toxicity studies to be nontoxic (Harvey and DeFrank, 1992). Bioremediation of Explosives and Energetics Bioremediation of explosive and energetic materials has been demonstrated for dilute, purified materials (Kaplan, 1993). However, applications to the explosive and energetic materials in the stockpile do not seem expeditious because of the burnable characteristics of these materials and the ease by which they can be destroyed through combustion. Engineering Prospects Numerous steps are required to assess the progression from scaling up of biological concepts to the practical engineering systems for chemical demilitarization.
LOW-TEMPERATURE, LIQUID-PHASE PROCESSES 133 Direct destruction of GB and VX. Further evaluations of the efficacy of enzymes and cellular-based systems for direct destruction of GB and VX should focus on several areas: ⢠identifying appropriate enzymes and cellular systems capable of detoxifying VX; ⢠defining the maximum extent of reaction (percent agent destruction) achievable for selected representatives of each potential system; ⢠defining the maximum aqueous concentration of chemical agent that each selected system can treat; and ⢠determining the usable life of each enzyme or cellular system and the quantities required for practical application. If the results of these investigations encourage further studies, subsequent research should also focus on the following areas: ⢠defining a suitable reactor configuration, including reaction and reactor kinetics, to determine reactor size and processing time requirements for bioprocessing; ⢠determining the production requirements of the enzymes or cellular systems in quantities sufficient for scaleup; and ⢠defining the quantities and characteristics of process effluents, including exhausted enzymes or cells, and nonbiodegradable organic species and salts. Biochemical processing would most likely involve either enzymes or whole cells dispersed in dilute chemical agent or immobilized as a catalyst bed through which the chemical agent would flow. The whole cells method would most likely require production of smaller quantities of enzymes or cells. In either case, considerable effort may be required to produce large quantities of purified enzymes. Application of enzyme deactivation for the treatment of nerve agent using whole cells may be a viable option. Specific concerns over use of whole cells in this application would be (1) the sorption of non-deactivated agent to cellular material (biomass) resulting in residual toxicity problems; (2) potentially more rapid deactivation of enzymes; and (3) greater limitations on operating conditions (temperature, pH, agent concentration, etc.) for whole cell systems rather than isolated enzymes. Whether to use purified enzymes or whole cell systems should be based on the ease of enzyme purification and the relative activities of the two systems. Use of the agent (non-deactivated) as the carbon and phosphorus source for microbial growth would most likely be unattractive because of the relatively slow growth rates (compared to deactivation rates of either purified enzymes or those present in whole cells)
LOW-TEMPERATURE, LIQUID-PHASE PROCESSES 134 and the attendant requirement that the cell growth reactor be configured to contain active agent. Incineration of the resulting whole suspension is unattractive because of the large quantifies of water that would have to be incinerated. Biodegradation of chemical degradation products. Future investigations for biodegradation of chemical detoxification processes should initially focus on identifying organisms or mixed microbial populations with broad degradation capabilities for the defined categories of reaction products of selected processes. Success of this secondary degradation will be determined by the development of suitable microbial consortia capable of carrying out the sequence of biodegradation steps necessary for treatment of the mixed-reaction products. Scaleup of potential biodegradation processes could be accomplished by using well-established fermentation and biodegradation technology. The most likely approach would use a series of sequencing batch reactors, as is common in the biodegradation waste treatment industry, or batch fermentation, as practiced in the biotechnology industry (Irvine and Ketchum, 1989). In either case, a series of batch reactors would need to be operated in parallel to permit the greatest process efficiency (this allows a reactor to start operation at a high substrate concentration and continue until the lowest residual concentrations are achieved). These engineering goals must be balanced with complete process control; all reactor contents should be tested before transport or final disposal. The design of a biodegradation reactor will depend primarily on whether the initial chemical reaction is carried out in aqueous or organic solution. Preliminary process design estimates for aqueous chemical reactions suggest that a typical batch bioreactor size would be 10,000 gallons and that two reactors operating in tandem would be required for the biodegradation of residual products from the chemical processing of 1 ton of agent per day. The biodegradation of reaction products from chemical reactions carried out in organic solvents (methanol or ethanolamine) would require approximately a 10-fold increase in reactor size to accommodate biodegradation of the carrier solvent. (To facilitate biodegradation the carrier solvent most likely would have to be diluted with water to less than 10 percent by weight.) Waste streams. Biodegradation of chemical reaction products would result in the following process waste streams: ⢠process waste water, including nondegradable process reaction or biodegradation products and neutralization salts; ⢠sludge from produced microbial cell mass; and ⢠gaseous bioreactor effluents.
LOW-TEMPERATURE, LIQUID-PHASE PROCESSES 135 Process waste water could most likely be treated through reverse osmosis or evaporation to remove the salts accumulated from pH neutralization and halogen ion released through agent degradation. Recovered water could be recycled into the chemical reaction process or biodegradation process steps. The resulting dry salt stream could be disposed of by conventional waste-disposal practices after the absence of residual toxicity is confirmed. Sludge from microbial cell mass could be disposed of by using conventional commercial facilities for disposal of waste water treatment sludge disposal. Approximately 600 pounds (dry weight) of sludge could be expected for each ton of organic solute degraded. Gaseous bioreactor emissions (CO2, O2, N2, and methane [CH4]) would result from aeration of the bioreactors and the CO2 produced by organic solute mineralization. Emissions can be minimized by using either oxygen enrichment or pure oxygen instead of air to supply oxygen to the reactor, depending on microbial sensitivity to oxygen concentration and reactor design. Initial estimates are that between 20,000 and 40,000 cubic feet of oxygen (at standard temperature and pressure) would be required for each ton of organic solute biodegraded. This quantity is small enough to permit complete capture and testing of gaseous emissions before release. Effluent CO2 could be captured in alkaline solution to eliminate gaseous discharge. Effluent CH4 would need to be managed by the industrial processes commonly used for potentially flammable gaseous emissions. Developmental status. Limited investigation has been carried out to date on the potential for biodegradation of chemical reaction products. The following steps would be required for process development and scaleup: ⢠identifying microorganisms capable of biodegrading specific chemical reaction products; ⢠determining the maximum initial concentration of reaction products that can be degraded without adverse effects on the microbial community; ⢠determining the maximum extent of biodegradation achievable; ⢠determining biodegradation stoichiometry and rates; and ⢠developing process-control strategies. Some DOD agencies have established University Research Initiative programs involving several research centers for biodegradation to address the basic science and engineering of biodegradation of environmental contaminants. This joint work, which is being performed at the Army Research Office/Texas A&M University, Office of Naval Research/University of Washington (Seattle) and Advanced Research Projects Agency/Rutgers University, may help address some of the questions above.
