- Email: [email protected]
Waste system implications for mars missions
Available online at www.sciencedirect.com Pergamon www.elsevier.com/locate/asr
SCISNCC
DIRECT*
doi: lO.l016/SO273-1177(03)00012-7
WASTE SYSTEM IMPLICATIONS FOR MARS MISSIONS
A. E. Drysdale and S. Maxwell
The Boeing Company 100 Boeing Way, Titusville, FL 32780, USA
ABSTRACT
Waste technologies for Mars missions have been analyzed, considering equivalent system mass and interface loads. Storage or dumping seems most appropriate for early missions with low food closure. Cornposting or other treatment of inedible biomass in a bioreactor seems most attractive for moderate food closure (50 - 75%). Some form of physicochemical oxidation of the composted residue might be needed for increased food closure, but oxidation of all waste does not seem appropriate due to excess of production of carbon dioxide over demand. More comprehensive analysis considering interfaces with other mission systems is needed. In particular, in-situ resource utilization is not considered, and might provide resources more cheaply than waste processing. 0 2003 COSPAR. Published by Elsevier Science Ltd. All rights reserved.
INTRODUCTION Any human space mission will result in the production of waste materials. For short missions, special provision need only be made for hazardous wastes. For longer missions, the mass and volume of waste builds up, and provision must be made to deal with it. Initially innocuous waste may also become noxious if it is left untreated for a time.
Some missions will have special requirements and constraints on dumping wastes. There was some degradation of Skylab thermal control coatings from off-gassing, for example, and this probably resulted in the attempt to minimize dumping of waste gases Ikom the International Space Station (ISS). On a Mars mission, a primary interest is whether life exists there, or has ever existed there. If a mission deposits wastes on the surface, micro-organisms (or even, potentially, larger animals) may be introduced that could be mistaken for native life or its remains, and which could destroy the native ecology before it has been studied. Biochemicals could be deposited that could further confuse this issue. Thus, special care must be taken to avoid contaminating the surface of Mars with either living or dead micro-organisms or chemical traces of them. TYPES AND QUANTITIES
OF WASTE
Waste comes in many varieties, and both type and quantities depend on the mission scenario. Wastes might include human wastes, wastes from the life support system, and wastes from other systems. Human wastes would include waste water, feces, urine, hair, exfoliated skin, and carbon dioxide. Generally, for any space mission, water and carbon dioxide will be removed and possibly regenerated. Though the focus Adv. Space Res. Vol. 31, No. 7, pp. 1791-1797.2003 Q 2003 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177103 $30.00 + 0.00
119?
A E. Drysdale and S. Maxwell
of this paper is on solid waste, though this would include some water. Wastes from the life-support system could include used ion exchange cartridges, air filters, spare parts, food packaging, uneaten food, possibly brine from water processing systems, and inedible biomass from any plant production systems used. Wastes from other systems might include extra vehicular activity (EVA) expendables such as maximum absorbency garments, used carbon dioxide removal cartridges (and even regenerable ones would have a limited life), used batteries, and other used spares, paper, and general trash. The quantities of waste depend on the life support scenario. A recent life support workshop defined the waste quantities as shown in Table 1 [Keener and Gertner, 20001. These data are for 75% food closure’, but may not adequately identify all types of waste. The various waste streams might be segregated for processing as not all technologies can handle all types of waste. Used spares, for example, are not identified, but they would probably be kept separate from the wastes identified, as different methods of disposal would be appropriate. If the inedible biomass was to be used for secondary production of food, such as production of sugars and single cell oil, or crops such as mushrooms or fish as has been suggested elsewhere [e.g. Strayer et al, 1990, Hunter et al, 19971, that would almost certainly be kept separated from the human metabolic waste for both aesthetic and practical reasons. Table 1. Waste Model from the Houston Waste Workshop, April, 2000: Option 32
Miscellaneous HISTORICAL
0.07
(plastics)
0.07
APPROACHES
Previous long-duration missions have either stored waste or returned it. Skylab, for example, stored trash in what was normally the Saturn SIVE3 oxygen tank, and the trash eventually burned up when Skylab reentered. Long-duration Russian missions, Salyut and Mir, were supplied by unmanned Progress vehicles, and these were filled with trash and re-entered. The International Space Station (ISS) is expected to use a combination of return on the shuttle and re-entry in Progress and other expendable vehicles. There are, however, no overboard venting provisions Corn ISS trash stowage, so wet-trash storage could be a problem due to offgassing into the cabin. Furthermore, no mission has, as yet, approached the three-year mission duration without resupply as is anticipated for a journey to Mars. Thus, much greater concerns are expected for trash disposal for a mission to Mars. In addition, no previous missions have considered planetary protection. Even when the Apollo astronauts went to the Moon, there was no great concern that dumping trash would contaminate the Lunar surface. When missions go to Mars, however, care must-be taken not to contaminate the Martian surface. POTENTIAL PROCESSING
OPTIONS
Generally, there are three options for dealing with waste: store it, dump it, or convert it into something else. These are not necessarily mutually exclusive, and storage may be needed to either buffer unsteady trash flow for continuous waste processes or to accumulate trash for batch processes. In addition, the ’Food closure is taken to be the percentage in dry weight of food consumed by the crew that is produced locally, rather than being supplied from Earth. ’ Waste models for five mission options were identified in the document. Option 3 was for a 600-day Mars surface mission.
Waste System for Mars Missions
1793
waste must often be handled in some fashion for transfer from where it is generated to where it is processed or stored. Storing most kinds of solid waste will require some pre-treatment, if only to minimize the production of noxious gases and odors. This treatment might be chemical or physical. If chemical, the chemicals must be carried in sufficient supply, would need to be mixed with the waste, and could be hazardous themselves. Physical treatment such as drying would also inhibit most changes during storage. It could make more water available for use, though the water would have to be purified first. Radiation could also be used for sterilization. This might be competitive for inhibiting microbial action, but would not inhibit chemical or physical action and would not promote recovery of water. Storage requires space, but that may not be a major problem, particularly if a trash compactor is used, as the waste was stowed somewhere for carriage from Earth prior to use. There would still be problems with storage planning, particularly with avoiding mixing storage locations for food and waste. Storage is not, however, likely to be an option when it increases the payload for any major propulsive maneuver. Propulsion is costly for a Mars mission. Thus, store and return of wastes from Mars is unlikely to be used, as this would increase mission costs dramatically. Dumping waste does eliminate most of the problems with stowage, or at least minimize stowage by limiting it to short term only. Dumping looks attractive for transit out and back. However, the destination of the dumped wastes does need to be considered. If a negligible impulse (delta V) is imparted to a waste packet, it will arrive at about the same place as the vehicle. Thus, the waste might end up on Mars also, though this is less likely for rapid-transit missions which require a velocity change for capture. If the waste does end up on Mars, entry should vaporize the waste, but breakdown may be incomplete unless the package breaks into pieces small enough to ensure thorough heating. Large chunks of meteoroids have been observed to reach the surface of the Earth while still cold inside. Providing a sufficient delta V to ensure that the waste misses Mars could be costly. Dumping into the Sun is certainly not feasible for most missions for this reason. Dumping might require less rigorous treatment than for stowage during transit, but more rigorous treatment on the surface of Mars. At the least, dumped waste is likely to need to be sterilized to avoid introducing terrestrial micro-organisms to Mars and packaged to avoid chemical contamination. Similar options exist for pre-treatment of dumped wastes as for stowed wastes. Conversion of trash could vary from drying (to recover water), through pyrolysis (to recover additional water and perhaps some other chemicals), to oxidation (to recover additional chemicals and minimize the mass - ash - to be dumped). It could also include conversion of certain wastes, notably inedible biomass, into foodstuffs. Conversion into other useful materials would reduce the mass of these materials to be delivered, together with the packaging and other delivery costs, and would reduce the mass to be disposed of. Production of paper products, activated carbon, and plant growth media have been considered, but most such products would be more suited for a larger, longer term mission than those currently being considered. Drying and pyrolysis seem quite attractive. Energy is required, but the equivalent system mass (ESMP impact would not be huge, particularly if heat exchangers, waste heat, or concentrated sunlight are used. The water recovered is likely to be important in minimizing the mass of water that needs to be shipped, including use on the surface of Mars for EVA cooling and producing EVA oxygen. The hydrogen produced in generating oxygen horn water would also increase the fraction of oxygen that can economically be recovered from carbon dioxide by the Sabatier process4 if a carbon dioxide reduction system is used. 3 ESM is the real mass plus the associated mass penalties. See below. 4 The Sabatier process is the catalytic reduction of carbon dioxide by hydrogen, producing water and methane.
1794
A. E. Dryadale and S. Maxwell
Oxidation is less attractive than non-oxidative processes for early missions to Mars. Until a significantly closed food loop is created, with bioregeneration accounting for over 50% of the food consumed, there is not likely to be a surplus of oxygen to use for oxidation, and removing the carbon dioxide produced would be an additional drain on the air regeneration system. Thus, if waste was oxidized in this scenario, additional air revitalization capacity (more equipment, power, and supplies) would be required and the overall water balance needs to be considered. However, for later missions to Mars, when facilities will exist on Mars with a planned lifetime of many years, bioregeneration is expected to become economically attractive and the degree of food closure to approach 100%. The plants will then need more carbon dioxide than the crew will be producing, as the harvest index is always below 100%. Whether the additional carbon dioxide can most effectively be recovered by oxidizing the inedible biomass, by oxidizing other waste, or by extracting it from the Martian atmosphere remains to be determined. Certainly carbon dioxide is readily available on Mars, and extraction should involve little more than pumping it to the pressure required. Food closure is a complex topic in itself, but a high degree of closure seems to be economically attractive only for missions of several years, perhaps over a decade, in duration. In that case, there are other waste issues to also consider, notably recovery of plant waste mineral content. With a mission having significant food closure, as the food closure is increased to exceed the harvest index averaged over all the crops, more carbon dioxide is needed than is produced by the crew. As identified above, this could come from oxidizing trash or from in situ resource utilization (ISRU). For intermediate degrees of food closure, biological waste processing systems seem attractive for a number of reasons. They do not require high temperatures or pressures, they are robust, self-starting, and for this application the limited completion of processing would not be a problem. The remaining waste could be sterilized, dried, and dumped. (The mass of the system would, of course, be maintained by supply of food.) As the degree of food closure gets closer to lOO%, some form of physicochemical oxidation could be required. However, even with total food closure, there is no apparent reason to oxidize all of the solid waste. In the example used (Table l), only half the waste is inedible biomass. If all the trash is oxidized, twice as much oxygen must be found as would be available, and twice as much carbon dioxide would be generated as would be required by the plants. As a significant benefit, high temperature oxidation would stabilize the material, perhaps breaking it down to a form that could be dumped overboard without degrading the planetary environment. SELECTION OF OPTIONS FOR A MARS MISSION Our typical trade study approach has been documented elsewhere [e.g. Drysdale et al, 19991. Equivalent system mass (ESM) is the sum of the actual masses and the mass penalties for a technology over the entire mission. The waste approach that offers the lowest ESM while meeting mission requirements is considered to be the best approach. Thus, the ESM must include provision for adequate safety and reliability (and these requirements have not been rigorously defined at this point) as well as capacity for the expected waste streams. ESM provides a useful initial approach, and the Advanced Life Support Project metric [Drysdale and Hanford, 19991 is based on it, but is not likely to be the only criterion used. The waste technologies identified at the Houston waste workshop were evaluated and the ESM for each technology was calculated for a 600-day Mars surface mission based on Option 3, with 75% bioregeneration (Table 2). The data from this workshop are preliminary. Some of the data seem unreasonably large or small. Some data are missing. In particular, crew-time requirements and logistics support were generally not identified. Logistics support could be significant for some options, such as incineration, which might require quantities of supplied items for treating flue gases. Furthermore, this is only a preliminary evaluation, as the waste system needs to be evaluated as part of an entire mission. For
179.5
Waste System for Mars Missions
example, the technology and EVA schedule will be significant determinants of the need for additional water, which is a critical question. The in-situ resource utilization (ISRU) approach must be defined to determine the cost of additional and alternative resources such as oxygen and water.’ Table 2. Preliminary Sizing Data for Waste Processing Technologies
Composting, 7 day
345
1.25
0.01
0.08
354
119
1,000 kg 02
Continuous incineration Electrochemical oxidation Leach/anaerobic Magnetically assisted gasification Peroxide oxidation
303 230
4.53 2.80
8.42 750
9.37 700
172 50
1.16 2.00 0.16
COZ,water, compost 1,670 153 4,300 kg 02 COZ,water, ash 112,000 10,300 4,300 kg 02 COZ,water, ash
1.00
0.08 1.oo
180 208
30 19
Minerals 4,300 kg 02 COZ,water, ash
0.75
0.95
129
12
9,100 kg
COZ,water, ash
H202
Plasma arc Pyrolysis Pyrolysis with sub critical water Single cell protein Super critical wet oxidation Storage
1,150 43 6
4.01
80 613
1.40 0.29
25
1.00
67.8 0.60 2.00
1.10
38.1 0.40 2.00
9,600 121 313
899 11 45
4,300 kg 02 CO2, water, ash COZ,water, ash COz, water, ash
83 709
46 64
Small Protein, CO2 4,300 kg 02 COZ,water, ash
127
2
Pre-treatments were not included in our analysis, though some pre-treatments had an ESM as high as some of the primary treatments. Size reduction, for example, had an ESM estimated as from 500 kg (dry) to 2,700 kg (wet). Some pretreatments would be less costly. Drying, for example, had an estimated ESM of about 45 kg. Sterilizing (requiring a higher temperature - resulting in more massive equipment, more power and cooling) was estimated as 58 kg ESM. Not all of these technologies would address all of the waste. In particular, acid hydrolysis, single cell protein production, and leaching would primarily apply to inedible biomass, and possibly to paper. Thus, the ESM per kg feed per day may be a better indication of the effectiveness of a technology than the overall ESM for that approach. Also, the breakdown of waste is not always complete. Biological 5 Since this paper was written, considerable quantities of water have been discovered on Mars. However, the form, purity, and accessibility of this water remain to be determined.
I796
/\ E. Drysdalc md S. Maxwell
oxidation of inedible biomass typically breaks down only 50% of the biomass. down eventually, but over much longer timeframes.
The remainder will break
Of the options assessed, storage seems to have the lowest ESM, 27 kg, for early missions. Thus, even with drying or sterilizing, storage seems quite attractive. Dumping was not considered at the workshop but would also require sterilizing (in this instance for a Mars surface mission), but would not require as much volume as storage, and would perhaps be similar to storage in ESM. Pyrolysis has a somewhat higher ESM, of about 12 1 kg, but would release additional water and could thus be attractive if a positive water balance is not achieved by drying alone. Some related options such as magnetically assisted gasification are not much higher ESM, and might be useful for specific applications (in this case, operation in a weightless environment). Oxidative approaches varied from ESM only somewhat higher than pyrolysis (e.g. 310 kg for batch incineration) to extremely high (e.g. 112,000 kg for electrochemical oxidation). There may be specialized applications for the higher-cost approaches, but they seem lacking in promise for general waste treatment. However, it is at least possible that the technologies giving these high values were not sized correctly. The requirement for oxygen is not considered in Table 2, and this would make oxidative technologies very unattractive for these early missions, adding up to 4 metric tonnes to the ESM, depending on how completely the waste was oxidized and the form in which the oxygen was provided. A similar large quantity of carbon dioxide would be generated that would have to be dealt with, perhaps by removal and dumping. The ESM for resizing the air revitalization system to deal with these loads was not addressed, but based on waste mass would be 3 to 4 times the size of what would be required for the crew alone, and would be significant. Previous estimates of the ESM of the air system for a surface mission have been about 3 tonnes, but this would not scale linearly with the load. Evidently, oxidation would only be a good option if there was an additional reason to require it, particularly if the interface loads demanded it. A view of this approach is provided by Pisharody et al, 2002. However, they did not address water poor missions. Once some or all of the waste must be oxidized, several additional waste-disposal technologies become attractive. Biological wastes, including paper products and some cloth, can be oxidized biologically, using either an aqueous or solid phase bioreactor. Biological treatments have the advantages of making recovery of plant minerals easier, avoiding the need for high temperatures and pressures, and in some cases providing a degree of pathogen reduction, but are slower than physicochemical processes and typically only oxidize part of the biomass within a reasonable time. The remaining wastes, including undigested inedible biomass, can be oxidized physicochemically in a variety of ways. Super critical wet oxidation and plasma incineration offer quite complete destruction of wastes. However, even more oxygen is then needed, and more carbon dioxide is produced, more than is needed unless the base is being expanded rapidly in size. The masses of oxygen and carbon dioxide involved have been calculated for several scenarios. Our suggested approaches and the results of this analysis are presented in Table 3. Note that not all of the water recovered is additional available water. Some of it is water that would have been provided to the plants for growth. However, most of it must be recovered if plant growth is to be feasible, and it is indicative of the load on the waste system. Some of the water recovered is newly available water, produced from waste that had previously been stored as food, paper, or trash. Some of these differences reflect the different amount of plant biomass with different degrees of food closure. As the food closure increases, more water must be recycled, but less additional water is available from supplies consumed.
1797
Waste System for Mars Missions
This analysis did not consider the availability of ISRU, nor the economic cost of the products of ISRU vs the cost of materials recovered from trash. Table 3. Recommended Approaches and Interface Implications
food closure Total waste oxidation
pyrolyze and dump Incinerate
5,500
7,600
42,000
CONCLUSIONS For a near-term Mars mission, storage or dumping appears to be the most favorable approach for dealing with waste based on preliminary analyses. In either case, the waste would need to be dried to recover water and possibly sterilized. Recovery of water by pyrolysis is attractive, but the mass of water needed for closing the water loop is undetermined at this time. For a Mars base with a planned life of greater than 10 years, cornposting of inedible biomass appears to be the most attractive option, with other wastes being stored or dumped as before. As the Mars base grows in size and as the food loop approaches 100% closure, some form of physicochemical oxidation of the residue after cornposting seems likely to be attractive. The best approach to this oxidation cannot be determined at this time. ACKNOWLEDGEMENTS This paper was supported by the NASA contract NAS 9-98119. We would also like to thank the people who provided data through the waste workshop, to the workshop organizers, and to the people who did the initial organizing of the data. REFERENCES Drysdale, A.E., M. Ewert, and A. J. Hanford, Eauivalent System Mass Studies of Missions and Concepts, SAE 1999-01-2081, 1999. Drysdale, A.E., and A. J. Hanford, Advanced Life Support Research and Technology Development Metric - Baseline, CTSD-ADV384, JSC 39503,1999. Hunter, J.B., S. Lin, A. E. Drysdale, and Y. Vodovotz, Prospects for Single-Cell Oil Production in a Lunar Life Support System, SAE 972365,1997. Keener, J., and B. Gertner, Reference Missions and Waste Model Document, Reference summary provided for the waste workshop held in Houston, April, 2000. Pisharody, S., K. Wignarajah, and J. Fisher, men Penaltv for Waste Oxidation in an Advanced Life Support System - A Systems Approach, ICES2002-Ol-2396,2002. Strayer, R. F., M.A. Brannon, and J.L. Garland, Use of inedible wheat residues from the KSC-CELSS breadboard facility for production of fungal cellulase, In: MacElroy (ed) NASA Tech Mem., 102277, 1990.
SCISNCC
DIRECT*
doi: lO.l016/SO273-1177(03)00012-7
WASTE SYSTEM IMPLICATIONS FOR MARS MISSIONS
A. E. Drysdale and S. Maxwell
The Boeing Company 100 Boeing Way, Titusville, FL 32780, USA
ABSTRACT
Waste technologies for Mars missions have been analyzed, considering equivalent system mass and interface loads. Storage or dumping seems most appropriate for early missions with low food closure. Cornposting or other treatment of inedible biomass in a bioreactor seems most attractive for moderate food closure (50 - 75%). Some form of physicochemical oxidation of the composted residue might be needed for increased food closure, but oxidation of all waste does not seem appropriate due to excess of production of carbon dioxide over demand. More comprehensive analysis considering interfaces with other mission systems is needed. In particular, in-situ resource utilization is not considered, and might provide resources more cheaply than waste processing. 0 2003 COSPAR. Published by Elsevier Science Ltd. All rights reserved.
INTRODUCTION Any human space mission will result in the production of waste materials. For short missions, special provision need only be made for hazardous wastes. For longer missions, the mass and volume of waste builds up, and provision must be made to deal with it. Initially innocuous waste may also become noxious if it is left untreated for a time.
Some missions will have special requirements and constraints on dumping wastes. There was some degradation of Skylab thermal control coatings from off-gassing, for example, and this probably resulted in the attempt to minimize dumping of waste gases Ikom the International Space Station (ISS). On a Mars mission, a primary interest is whether life exists there, or has ever existed there. If a mission deposits wastes on the surface, micro-organisms (or even, potentially, larger animals) may be introduced that could be mistaken for native life or its remains, and which could destroy the native ecology before it has been studied. Biochemicals could be deposited that could further confuse this issue. Thus, special care must be taken to avoid contaminating the surface of Mars with either living or dead micro-organisms or chemical traces of them. TYPES AND QUANTITIES
OF WASTE
Waste comes in many varieties, and both type and quantities depend on the mission scenario. Wastes might include human wastes, wastes from the life support system, and wastes from other systems. Human wastes would include waste water, feces, urine, hair, exfoliated skin, and carbon dioxide. Generally, for any space mission, water and carbon dioxide will be removed and possibly regenerated. Though the focus Adv. Space Res. Vol. 31, No. 7, pp. 1791-1797.2003 Q 2003 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177103 $30.00 + 0.00
119?
A E. Drysdale and S. Maxwell
of this paper is on solid waste, though this would include some water. Wastes from the life-support system could include used ion exchange cartridges, air filters, spare parts, food packaging, uneaten food, possibly brine from water processing systems, and inedible biomass from any plant production systems used. Wastes from other systems might include extra vehicular activity (EVA) expendables such as maximum absorbency garments, used carbon dioxide removal cartridges (and even regenerable ones would have a limited life), used batteries, and other used spares, paper, and general trash. The quantities of waste depend on the life support scenario. A recent life support workshop defined the waste quantities as shown in Table 1 [Keener and Gertner, 20001. These data are for 75% food closure’, but may not adequately identify all types of waste. The various waste streams might be segregated for processing as not all technologies can handle all types of waste. Used spares, for example, are not identified, but they would probably be kept separate from the wastes identified, as different methods of disposal would be appropriate. If the inedible biomass was to be used for secondary production of food, such as production of sugars and single cell oil, or crops such as mushrooms or fish as has been suggested elsewhere [e.g. Strayer et al, 1990, Hunter et al, 19971, that would almost certainly be kept separated from the human metabolic waste for both aesthetic and practical reasons. Table 1. Waste Model from the Houston Waste Workshop, April, 2000: Option 32
Miscellaneous HISTORICAL
0.07
(plastics)
0.07
APPROACHES
Previous long-duration missions have either stored waste or returned it. Skylab, for example, stored trash in what was normally the Saturn SIVE3 oxygen tank, and the trash eventually burned up when Skylab reentered. Long-duration Russian missions, Salyut and Mir, were supplied by unmanned Progress vehicles, and these were filled with trash and re-entered. The International Space Station (ISS) is expected to use a combination of return on the shuttle and re-entry in Progress and other expendable vehicles. There are, however, no overboard venting provisions Corn ISS trash stowage, so wet-trash storage could be a problem due to offgassing into the cabin. Furthermore, no mission has, as yet, approached the three-year mission duration without resupply as is anticipated for a journey to Mars. Thus, much greater concerns are expected for trash disposal for a mission to Mars. In addition, no previous missions have considered planetary protection. Even when the Apollo astronauts went to the Moon, there was no great concern that dumping trash would contaminate the Lunar surface. When missions go to Mars, however, care must-be taken not to contaminate the Martian surface. POTENTIAL PROCESSING
OPTIONS
Generally, there are three options for dealing with waste: store it, dump it, or convert it into something else. These are not necessarily mutually exclusive, and storage may be needed to either buffer unsteady trash flow for continuous waste processes or to accumulate trash for batch processes. In addition, the ’Food closure is taken to be the percentage in dry weight of food consumed by the crew that is produced locally, rather than being supplied from Earth. ’ Waste models for five mission options were identified in the document. Option 3 was for a 600-day Mars surface mission.
Waste System for Mars Missions
1793
waste must often be handled in some fashion for transfer from where it is generated to where it is processed or stored. Storing most kinds of solid waste will require some pre-treatment, if only to minimize the production of noxious gases and odors. This treatment might be chemical or physical. If chemical, the chemicals must be carried in sufficient supply, would need to be mixed with the waste, and could be hazardous themselves. Physical treatment such as drying would also inhibit most changes during storage. It could make more water available for use, though the water would have to be purified first. Radiation could also be used for sterilization. This might be competitive for inhibiting microbial action, but would not inhibit chemical or physical action and would not promote recovery of water. Storage requires space, but that may not be a major problem, particularly if a trash compactor is used, as the waste was stowed somewhere for carriage from Earth prior to use. There would still be problems with storage planning, particularly with avoiding mixing storage locations for food and waste. Storage is not, however, likely to be an option when it increases the payload for any major propulsive maneuver. Propulsion is costly for a Mars mission. Thus, store and return of wastes from Mars is unlikely to be used, as this would increase mission costs dramatically. Dumping waste does eliminate most of the problems with stowage, or at least minimize stowage by limiting it to short term only. Dumping looks attractive for transit out and back. However, the destination of the dumped wastes does need to be considered. If a negligible impulse (delta V) is imparted to a waste packet, it will arrive at about the same place as the vehicle. Thus, the waste might end up on Mars also, though this is less likely for rapid-transit missions which require a velocity change for capture. If the waste does end up on Mars, entry should vaporize the waste, but breakdown may be incomplete unless the package breaks into pieces small enough to ensure thorough heating. Large chunks of meteoroids have been observed to reach the surface of the Earth while still cold inside. Providing a sufficient delta V to ensure that the waste misses Mars could be costly. Dumping into the Sun is certainly not feasible for most missions for this reason. Dumping might require less rigorous treatment than for stowage during transit, but more rigorous treatment on the surface of Mars. At the least, dumped waste is likely to need to be sterilized to avoid introducing terrestrial micro-organisms to Mars and packaged to avoid chemical contamination. Similar options exist for pre-treatment of dumped wastes as for stowed wastes. Conversion of trash could vary from drying (to recover water), through pyrolysis (to recover additional water and perhaps some other chemicals), to oxidation (to recover additional chemicals and minimize the mass - ash - to be dumped). It could also include conversion of certain wastes, notably inedible biomass, into foodstuffs. Conversion into other useful materials would reduce the mass of these materials to be delivered, together with the packaging and other delivery costs, and would reduce the mass to be disposed of. Production of paper products, activated carbon, and plant growth media have been considered, but most such products would be more suited for a larger, longer term mission than those currently being considered. Drying and pyrolysis seem quite attractive. Energy is required, but the equivalent system mass (ESMP impact would not be huge, particularly if heat exchangers, waste heat, or concentrated sunlight are used. The water recovered is likely to be important in minimizing the mass of water that needs to be shipped, including use on the surface of Mars for EVA cooling and producing EVA oxygen. The hydrogen produced in generating oxygen horn water would also increase the fraction of oxygen that can economically be recovered from carbon dioxide by the Sabatier process4 if a carbon dioxide reduction system is used. 3 ESM is the real mass plus the associated mass penalties. See below. 4 The Sabatier process is the catalytic reduction of carbon dioxide by hydrogen, producing water and methane.
1794
A. E. Dryadale and S. Maxwell
Oxidation is less attractive than non-oxidative processes for early missions to Mars. Until a significantly closed food loop is created, with bioregeneration accounting for over 50% of the food consumed, there is not likely to be a surplus of oxygen to use for oxidation, and removing the carbon dioxide produced would be an additional drain on the air regeneration system. Thus, if waste was oxidized in this scenario, additional air revitalization capacity (more equipment, power, and supplies) would be required and the overall water balance needs to be considered. However, for later missions to Mars, when facilities will exist on Mars with a planned lifetime of many years, bioregeneration is expected to become economically attractive and the degree of food closure to approach 100%. The plants will then need more carbon dioxide than the crew will be producing, as the harvest index is always below 100%. Whether the additional carbon dioxide can most effectively be recovered by oxidizing the inedible biomass, by oxidizing other waste, or by extracting it from the Martian atmosphere remains to be determined. Certainly carbon dioxide is readily available on Mars, and extraction should involve little more than pumping it to the pressure required. Food closure is a complex topic in itself, but a high degree of closure seems to be economically attractive only for missions of several years, perhaps over a decade, in duration. In that case, there are other waste issues to also consider, notably recovery of plant waste mineral content. With a mission having significant food closure, as the food closure is increased to exceed the harvest index averaged over all the crops, more carbon dioxide is needed than is produced by the crew. As identified above, this could come from oxidizing trash or from in situ resource utilization (ISRU). For intermediate degrees of food closure, biological waste processing systems seem attractive for a number of reasons. They do not require high temperatures or pressures, they are robust, self-starting, and for this application the limited completion of processing would not be a problem. The remaining waste could be sterilized, dried, and dumped. (The mass of the system would, of course, be maintained by supply of food.) As the degree of food closure gets closer to lOO%, some form of physicochemical oxidation could be required. However, even with total food closure, there is no apparent reason to oxidize all of the solid waste. In the example used (Table l), only half the waste is inedible biomass. If all the trash is oxidized, twice as much oxygen must be found as would be available, and twice as much carbon dioxide would be generated as would be required by the plants. As a significant benefit, high temperature oxidation would stabilize the material, perhaps breaking it down to a form that could be dumped overboard without degrading the planetary environment. SELECTION OF OPTIONS FOR A MARS MISSION Our typical trade study approach has been documented elsewhere [e.g. Drysdale et al, 19991. Equivalent system mass (ESM) is the sum of the actual masses and the mass penalties for a technology over the entire mission. The waste approach that offers the lowest ESM while meeting mission requirements is considered to be the best approach. Thus, the ESM must include provision for adequate safety and reliability (and these requirements have not been rigorously defined at this point) as well as capacity for the expected waste streams. ESM provides a useful initial approach, and the Advanced Life Support Project metric [Drysdale and Hanford, 19991 is based on it, but is not likely to be the only criterion used. The waste technologies identified at the Houston waste workshop were evaluated and the ESM for each technology was calculated for a 600-day Mars surface mission based on Option 3, with 75% bioregeneration (Table 2). The data from this workshop are preliminary. Some of the data seem unreasonably large or small. Some data are missing. In particular, crew-time requirements and logistics support were generally not identified. Logistics support could be significant for some options, such as incineration, which might require quantities of supplied items for treating flue gases. Furthermore, this is only a preliminary evaluation, as the waste system needs to be evaluated as part of an entire mission. For
179.5
Waste System for Mars Missions
example, the technology and EVA schedule will be significant determinants of the need for additional water, which is a critical question. The in-situ resource utilization (ISRU) approach must be defined to determine the cost of additional and alternative resources such as oxygen and water.’ Table 2. Preliminary Sizing Data for Waste Processing Technologies
Composting, 7 day
345
1.25
0.01
0.08
354
119
1,000 kg 02
Continuous incineration Electrochemical oxidation Leach/anaerobic Magnetically assisted gasification Peroxide oxidation
303 230
4.53 2.80
8.42 750
9.37 700
172 50
1.16 2.00 0.16
COZ,water, compost 1,670 153 4,300 kg 02 COZ,water, ash 112,000 10,300 4,300 kg 02 COZ,water, ash
1.00
0.08 1.oo
180 208
30 19
Minerals 4,300 kg 02 COZ,water, ash
0.75
0.95
129
12
9,100 kg
COZ,water, ash
H202
Plasma arc Pyrolysis Pyrolysis with sub critical water Single cell protein Super critical wet oxidation Storage
1,150 43 6
4.01
80 613
1.40 0.29
25
1.00
67.8 0.60 2.00
1.10
38.1 0.40 2.00
9,600 121 313
899 11 45
4,300 kg 02 CO2, water, ash COZ,water, ash COz, water, ash
83 709
46 64
Small Protein, CO2 4,300 kg 02 COZ,water, ash
127
2
Pre-treatments were not included in our analysis, though some pre-treatments had an ESM as high as some of the primary treatments. Size reduction, for example, had an ESM estimated as from 500 kg (dry) to 2,700 kg (wet). Some pretreatments would be less costly. Drying, for example, had an estimated ESM of about 45 kg. Sterilizing (requiring a higher temperature - resulting in more massive equipment, more power and cooling) was estimated as 58 kg ESM. Not all of these technologies would address all of the waste. In particular, acid hydrolysis, single cell protein production, and leaching would primarily apply to inedible biomass, and possibly to paper. Thus, the ESM per kg feed per day may be a better indication of the effectiveness of a technology than the overall ESM for that approach. Also, the breakdown of waste is not always complete. Biological 5 Since this paper was written, considerable quantities of water have been discovered on Mars. However, the form, purity, and accessibility of this water remain to be determined.
I796
/\ E. Drysdalc md S. Maxwell
oxidation of inedible biomass typically breaks down only 50% of the biomass. down eventually, but over much longer timeframes.
The remainder will break
Of the options assessed, storage seems to have the lowest ESM, 27 kg, for early missions. Thus, even with drying or sterilizing, storage seems quite attractive. Dumping was not considered at the workshop but would also require sterilizing (in this instance for a Mars surface mission), but would not require as much volume as storage, and would perhaps be similar to storage in ESM. Pyrolysis has a somewhat higher ESM, of about 12 1 kg, but would release additional water and could thus be attractive if a positive water balance is not achieved by drying alone. Some related options such as magnetically assisted gasification are not much higher ESM, and might be useful for specific applications (in this case, operation in a weightless environment). Oxidative approaches varied from ESM only somewhat higher than pyrolysis (e.g. 310 kg for batch incineration) to extremely high (e.g. 112,000 kg for electrochemical oxidation). There may be specialized applications for the higher-cost approaches, but they seem lacking in promise for general waste treatment. However, it is at least possible that the technologies giving these high values were not sized correctly. The requirement for oxygen is not considered in Table 2, and this would make oxidative technologies very unattractive for these early missions, adding up to 4 metric tonnes to the ESM, depending on how completely the waste was oxidized and the form in which the oxygen was provided. A similar large quantity of carbon dioxide would be generated that would have to be dealt with, perhaps by removal and dumping. The ESM for resizing the air revitalization system to deal with these loads was not addressed, but based on waste mass would be 3 to 4 times the size of what would be required for the crew alone, and would be significant. Previous estimates of the ESM of the air system for a surface mission have been about 3 tonnes, but this would not scale linearly with the load. Evidently, oxidation would only be a good option if there was an additional reason to require it, particularly if the interface loads demanded it. A view of this approach is provided by Pisharody et al, 2002. However, they did not address water poor missions. Once some or all of the waste must be oxidized, several additional waste-disposal technologies become attractive. Biological wastes, including paper products and some cloth, can be oxidized biologically, using either an aqueous or solid phase bioreactor. Biological treatments have the advantages of making recovery of plant minerals easier, avoiding the need for high temperatures and pressures, and in some cases providing a degree of pathogen reduction, but are slower than physicochemical processes and typically only oxidize part of the biomass within a reasonable time. The remaining wastes, including undigested inedible biomass, can be oxidized physicochemically in a variety of ways. Super critical wet oxidation and plasma incineration offer quite complete destruction of wastes. However, even more oxygen is then needed, and more carbon dioxide is produced, more than is needed unless the base is being expanded rapidly in size. The masses of oxygen and carbon dioxide involved have been calculated for several scenarios. Our suggested approaches and the results of this analysis are presented in Table 3. Note that not all of the water recovered is additional available water. Some of it is water that would have been provided to the plants for growth. However, most of it must be recovered if plant growth is to be feasible, and it is indicative of the load on the waste system. Some of the water recovered is newly available water, produced from waste that had previously been stored as food, paper, or trash. Some of these differences reflect the different amount of plant biomass with different degrees of food closure. As the food closure increases, more water must be recycled, but less additional water is available from supplies consumed.
1797
Waste System for Mars Missions
This analysis did not consider the availability of ISRU, nor the economic cost of the products of ISRU vs the cost of materials recovered from trash. Table 3. Recommended Approaches and Interface Implications
food closure Total waste oxidation
pyrolyze and dump Incinerate
5,500
7,600
42,000
CONCLUSIONS For a near-term Mars mission, storage or dumping appears to be the most favorable approach for dealing with waste based on preliminary analyses. In either case, the waste would need to be dried to recover water and possibly sterilized. Recovery of water by pyrolysis is attractive, but the mass of water needed for closing the water loop is undetermined at this time. For a Mars base with a planned life of greater than 10 years, cornposting of inedible biomass appears to be the most attractive option, with other wastes being stored or dumped as before. As the Mars base grows in size and as the food loop approaches 100% closure, some form of physicochemical oxidation of the residue after cornposting seems likely to be attractive. The best approach to this oxidation cannot be determined at this time. ACKNOWLEDGEMENTS This paper was supported by the NASA contract NAS 9-98119. We would also like to thank the people who provided data through the waste workshop, to the workshop organizers, and to the people who did the initial organizing of the data. REFERENCES Drysdale, A.E., M. Ewert, and A. J. Hanford, Eauivalent System Mass Studies of Missions and Concepts, SAE 1999-01-2081, 1999. Drysdale, A.E., and A. J. Hanford, Advanced Life Support Research and Technology Development Metric - Baseline, CTSD-ADV384, JSC 39503,1999. Hunter, J.B., S. Lin, A. E. Drysdale, and Y. Vodovotz, Prospects for Single-Cell Oil Production in a Lunar Life Support System, SAE 972365,1997. Keener, J., and B. Gertner, Reference Missions and Waste Model Document, Reference summary provided for the waste workshop held in Houston, April, 2000. Pisharody, S., K. Wignarajah, and J. Fisher, men Penaltv for Waste Oxidation in an Advanced Life Support System - A Systems Approach, ICES2002-Ol-2396,2002. Strayer, R. F., M.A. Brannon, and J.L. Garland, Use of inedible wheat residues from the KSC-CELSS breadboard facility for production of fungal cellulase, In: MacElroy (ed) NASA Tech Mem., 102277, 1990.