Physical and chemical processes for solid waste treatment applied to a crewed space habitat

Waste Management & Research (1991) 9, 389-394

PHYSICAL AND CHEMICAL PROCESSES FOR SOLID WASTE TREATMENT APPLIED TO A CREWED SPACE HABITAT* George M. Savage Cal Recovery Inc ., 725C Alfred Nobel Drive, Hercules, California 94547, U .S.A . Solid wastes can be processed for material and energy recovery using a number of unit operations and system approaches. The selection and configuration of unit operations and systems depends upon the characteristics of the wastes to be processed and the uses for recovered secondary materials and for recovered energy forms . The discussion focuses on the types of materials and forms of energy potentially recoverable from solid wastes, waste processing and conversion systems, and design considerations. Key Words-solid waste, space, resource recovery, energy, material recovery, processing, design .

1 . Introduction Many of the strategies and technologies applied terrestrially to the recovery of resources from solid waste are potentially available and feasible for crewed space environments . The strategies can serve to minimize storage requirements for solid wastes generated in the confined environment of space habitats while simultaneously reducing transport-toorbit costs and supplying material and energy resources that are required for space missions . The selection of the appropriate strategies and technological processes depends predominantly upon the characteristics of waste generated aboard space missions and mission requirements for material and energy resources . Terrestrial waste processing systems include those for recovering a variety of secondary materials and forms of energy . Materials that can be mechanically recovered include ferrous, aluminium, glass, mixed paper, and paper pulp . Recoverable forms of energy include low and medium fuel value (BTU-British Thermal Units) gas, steam and electricity, and ethanol . In addition to the use of the latter chemical as a source of energy, ethanol can serve as a chemical feedstock . An overview of some of the alternatives for processing and converting solid waste of terrestrial origin is depicted in Fig . 1 . Certain characteristics of the waste stream and the feasible alternatives for recovered materials and energy forms drive the selection of waste processing and conversion strategies . In space habitats the probable predominant constituents of solid waste are paper and plastic, mostly in the form of packaging . Paper is combustible and biodegradable, and energy recovery is possible via direct combustion or anaerobic processes . The latter conversion process, which is a biological process, is especially of interest in the space environment since anaerobic processes are carried out in the absence of oxygen . Obviously, space is devoid of oxygen, and the supply of oxygen is a valued commodity . Substantial concentrations of plastic in the waste also are conducive to * First presented at the NASA Symposium on Waste Processing in Space for Advanced Life Support, Ames Research Center, Moffett Field, California, September 11-13, 1990 . 0734-242X/91/050389 + 06 $03 .00/0

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Recovered biodegradable/cellulosic feedstocks

Conversion system end product

Waste processing

Fig . 1 . Some potential routes for material and energy recovery from solid waste

energy recovery from waste . However, energy recovery from plastics, since they are not biodegradable, must be accomplished via oxidation, i .e . direct combustion which consumes oxygen . Another key consideration when transferring terrestrial experience to the space environment is the negligible or absent gravitational force . Many of the terrestrial processes for separating and converting wastes depend to a substantial degree upon gravity . For example, flow of material through size reduction equipment and belt conveyance of particles in terrestrial applications depends upon the existence of the gravitational force . Some terrestrial waste processing equipment would have to be modified to perform satisfactorily in zero or near zero gravity . 2 . Waste characteristics In terrestrial applications some key solid characteristics govern the selection of waste processing and conversion alternatives . One key characteristic of waste generated terrestrially is that the paper and plastic content are typically 35-45 and 6-10%, respectively, when expressed on a weight basis . These materials are combustible and therefore of interest where energy recovery is a driving force for utilization of waste derived resources . Historical and contemporary waste processing and conversion strategies are predicted on the range of the terrestrial waste composition shown in Table 1 . Waste processing and resource recovery alternatives applicable for crewed space habitats must take into consideration the characteristics of the wastes as well as the limitation placed upon systems for space missions . The limitations include physical size, weight, and power requirements of the equipment . 2.1 Mechanical processes/front end processing The mechanized recovery of materials from solid waste is accomplished through the utilization of one or more of the following unit operations : • size reduction • air classification • screening



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TABLE 1 waste composition (weight %)

Material category Paper Plastic Yard waste Food waste Other organic Ferrous Aluminum Glass Other inorganic

35-45 6-10 5-30 2-5 5-15 2-6 0 .2-1 5-11 2-5

• magnetic separation • aluminium and glass separation • densification Ancillary equipment used in waste processing systems includes conveyors, electrical equipment, and controls . Brief descriptions of mechanical operations processing are given below . For a more detailed discussion of processing equipment see Savage et al. (1990) . 2 .1 .1 Size reduction

Size reduction is a usual step in waste processing operations . The reduction in particle size eases handling of the waste and renders the dimensions of materials compatible with those of subsequent processing equipment . The consequence of size reduction is that it brings about a degree of uniformity of maximum particle size of diverse waste materials, a necessary circumstance for downstream separation equipment to perform effectively . Generally materials are size reduced to 90% less than 4 to 10 cm, on a weight basis . 2 .1 .2 Air classification

Air classification is a means of separating materials of different densities, particle profiles, or both by subjecting the particles to a moving stream of air . Lighter waste components ("light fraction") are suspended in and carried away by the air stream while the heavier components ("heavy fraction") settle out of the stream . In the air classification of shredded solid waste, paper and plastic tend to be concentrated in the light fraction while metals and glass are concentrated in the heavy fraction . The utility of air classification in a microgravity environment is limited . 2.1 .3 Screening

Screens are used typically in solid waste processing to effect separations of materials on the basis of differences in physical size . Three types of screens are used for sizing particular fractions of unprocessed and size reduced solid waste . The three types are the vibratory flat bed screen, the disk screen, and the trommel screen .



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2 .1 .4 Magnetic separation Magnetic separation is a process employed to recover the ferrous metals from solid waste . There are three conventional configurations, namely, the drum, the magnetic head pulley, and the overhead magnetic belt . The location of the magnetic separator and the type used in the processing system influence the performance of the equipment . Most screens and magnetic separators used on earth depend on gravity . 2.1 .5 Glass and aluminium separation Several processes have been utilized or proposed for aluminium and glass separation . Eddy current separation ejects aluminium particles from a processed waste stream due to electromagnetic flux . For glass recovery, both froth flotation and electronic optical sorting have been used . Aluminium and glass recovery processes are costly, complex and currently developmental . 2 .1 .6 Densification Densification equipment in the form of pelletizers is used to produce dense unit particles of predominantly shredded paper and plastic . The pellets are 2-3 cm in diameter, 3-5 cm in length, and are suitable fuel for direct combustion and thermal gasification systems . The limited processing capability of commercial densifiers is probably not a drawback to their use in space applications . 2 .2 Conversion processes/backend processing 2 .2 .1 Paper pulp recovery Cellulosic fiber derived from waste paper is recoverable from mixed solid waste . The recovery requires processing of mixed waste to recover a process stream rich in paper (dry processing) . Subsequently, the paper-rich stream is pulped and the non-paper contamination is removed using pulp cleaning equipment (wet processing) . The unit operations for dry processing are size reduction, screening, sometimes air classification, and magnetic separation . The wet processing unit operations are pulping, refining, wet screening, centrifugal cleaning and dewatering . The properties of the recovered fiber are a strong function of the grades of paper represented in the waste steam . Typically, the properties of fiber recovered from commercial solid waste exceed those of fiber recovered from residential waste . The reason is that commercial waste generally contains a substantial concentration of corrugated paper, i .e . kraft fibers, in comparison to residential waste which contains substantial proportions of newsprint, i .e . ground wood fibers . 2.2 .2 Ethanol production Ethanol may be produced from mixed solid waste by recovering an enriched cellulosic fiber fraction and subjecting the fiber fraction to biological or chemical processing . The dry processes for the recovery of the enriched cellulosic fiber fraction are similar to those described for dry processing in the case of paper pulp recovery . The processing of the enriched fiber fraction into ethanol is a wet process and is accomplished using acid hydrolysis or enzymatic hydrolysis . In acid hydrolysis the cellulosic fibers are converted to sugars by acid digestion . In enzymatic hydrolysis the conversion of cellulose to sugars is accomplished through the action of enzymes . The conversion of the sugars to ethanol is via fermentation .



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2 .2.3 Anaerobic digestion A medium BTU synthetic gas consisting of methane and carbon dioxide can be produced from a biodegradable feedstock that is recovered from solid waste and subsequently digested in slurry form under anaerobic conditions . The biodegradable feedstock that can be recovered from terrestrial solid waste is composed primarily of paper, yard waste, and foodwaste . The design of the feedstock recovery system is similar to those employed for waste-derived cellulosic feedstocks for ethanol production and for recovery of paper pulp . The heating value of the gas produced as a product of the anaerobic digestion process is about one half that of natural gas, i .e . about 500 BTU per standard cubic foot (17,000 kJ/m3) . 2 .2 .4 Pyrolysis The cellulosic fraction that may be recovered from solid waste can serve as a combustible feedstock for production of low BTU synthetic gas containing carbon monoxide using thermal gasification, i .e . pyrolysis. The preferred form of the cellulosic fraction is densified units : for example pellets or cubettes of the order of 2-4 cm in particle size . Particle (e .g . pellet) densities in the range 900-1100 kg/m3 are required for a satisfactory waste derived pyrolytic fuel . The densified particles result in the formation of a porous bed of fuel within the pyrolytic reactor, thus promoting efficient production of high quality gas and of flow of fuel through the reactor . The resultant heating value of the gas is approximately 100-200 BTU per standard cubic foot (3300-6600 kJ/m 3 ) . 3 . Conclusion A number of strategies employed terrestrially for processing and converting solid waste are applicable to waste management for crewed space habitats . Both material recovery and energy recovery could potentially benefit space missions of long duration where environmentally sound solid waste management would represent a serious technological challenge . Additionally a portion of the solid wastes generated during a mission could be utilized as resources, for example to generate energy for use during the mission . Key design aspects that must be considered in applying terrestrial waste processing technologies to space applications are the absence or near absence of gravity and a practical restriction on the availability of air or oxygen in space . In all likelihood, certain unit operations, such as air classification, would prove impractical because of the absence of or cost of supplying the required working fluid (e .g . air in the case of air classification) or because of size or weight restrictions of the apparatus . Certain conversion systems such as direct combustion and its requirement for oxygen would also be untenable in most instances . On the other hand anaerobic digestion might be attractive as a gas production technology since it is a process that only operates in the absence of oxygen . Some unit operations, e .g . size reduction, would be feasible for the space environment, although some modifications to the equipment might be required to achieve satisfactory performance . There is no question that the design of resource recovery systems for crewed space habitats would represent an engineering challenge . In particular the design of systems to recover materials, energy or both from solid wastes generated in crewed space habitats while meeting mission limitations of size, weight, and energy availability would be an interesting design problem . Also, the designer should bear in mind that the criteria for selection of waste processing alternatives for applications in space may be different than those criteria chosen for waste management on Earth . Consequently, the situation may



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exist in which methods that would be uneconomical on Earth may prove justifiable in applications in space . Reference Savage, G . M . & Diaz, L . F . (1990), Processing of solid waste for material recovery . Proceedings of 1990 National Waste Processing Conference, 417--426 .