17. Life Support Systems

The Life Support Systems on DL4 were chosen in an attempt to close the system loop as much as possible.  This was achieved by choosing subsystems that produced by-products that were compatible with the inputs of the surrounding systems.  The most important, and hardest to control, aspects of the life support system are its biological components, including humans, agricultural plant life, recreational plant life, biological nitrification and biological waste decomposition.  All figures for the life support systems are based on the approximate input and output rates of the first three of these.  This allows other more technological systems to be controlled depending on the conditions at that time.

The overall system was divided into thirteen main subsystems.  These were:

1.       Urine Processing

2.       Waste Processing

3.       Nitrogen Makeup

4.       Temperature Control

5.       Humidity Control

6.       Water Monitoring

7.       Water Processing

8.       Oxygen Generation

9.       Carbon Dioxide Concentration

10.   Carbon Dioxide Removal

11.   Trace Contaminant Control

12.   Particulate Removal

13.   Atmospheric Monitoring

17.1      Urine Processing

Urine is 95% water (ref. 24), the rest being made up of more than 100 various organic and inorganic substances.  These include urea (13-20g/L), sodium chloride (8-12g/L) and various acids (up to 3g/L).  All these contaminants must find their way back into the loop in order to maintain a closed system.  Options considered to attain this included Biological Decomposition, TIMES (Thermoelectric Integrated Membrane Evaporation System), AES (Air Evaporation System), VCD (Vapour Compression Distillation), and VAPCAR (Vapour Phase Catalytic Ammonia Removal).

Although all systems had their advantages and disadvantages VAPCAR was considered to be most capable of producing high quality water and operating with little or no need for re-supply.  The other options were ruled out for a number of reasons: Biological Decomposition required the growth of micro-flora at 60°C, giving rise to fears of bacterial epidemics and denitrification; TIMES required pre-treatment with ozone, which is unstable and hazardous, and its small Nafion tubes could easily be blocked; AES, although providing 100% water recovery, periodically required replacement wicks, produced a foul smell and did not return the 5% contaminant portion of urine to the system; and finally, VCD, although coming a close second, still required pre- and post-treatment, and its water quality varied depending on the number of organic compounds that co-condensed with the water.

VAPCAR is based on a catalytic chemical process, which eliminates the need for expendable chemicals and oxidises impurities to innocuous gases that can be used to replace the settlement’s atmosphere.   Fig. 17.1a shows a simplified flow diagram of the VAPCAR process.  Urine is initially fed through the evaporator, which consists of a bundle of hollow fibre membranes made from perflourinated ion-exchange polymer.  Here the urine passes through the interior of the fibres and is vaporised through their exterior.  Next the feed vapour is put through the first catalyst bed.  This consists of 0.5% platinum on alumina oxidation catalyst pellets and operates at about 523K.  The bed oxidises ammonia to nitrous oxide (N₂O) and nitrogen while volatile hydrocarbons are oxidised to carbon dioxide and water.  This bed then leads to a condenser-separator where the freshly processed water is removed from the system.  The remaining feed is passed through the second catalyst bed, consisting of 0.5% ruthenium on alumina catalyst pellets, which operates at 723K.  Here the nitrous oxide is decomposed to nitrogen and oxygen, which can be used to replenish the settlement’s atmosphere.  The remaining recycle and vapour loops in the system are maintained above pasteurisation temperature (347K), using wet heat, in order to prevent microbial growth and thus increase water quality (ref. 18).

VAPCAR produces near potable water, which only requires slight alteration of its pH to meet the necessary standards (detailed later in 17.5 Water Monitoring).  Therefore, the water produced can either be put into plant circulation for further purification or placed after being tested in potable water storage for human consumption, depending on the needs at the time (see fig. 17.12a).  The remaining outputs are mostly gases that can go directly into the atmosphere.  Some problems with the compressor system within VAPCAR have arisen, however these should be solved shortly and given the futuristic nature of this project are not seen as posing any significant difficulties.

17.2            Waste Processing

Waste products within a life support system mainly consist of faeces and food preparation wastes.  Due to the use of plants as a food source on board DL4, a large amount of inedible plant material will have to be catered for in order to return the carbon to the plants.  Inedible plant material will take up 98% of the total dry solid waste.  The remaining waste will be produced in semi-solid, solid or mixed forms from humans and from concentrates from the atmosphere and water purification systems.

For the large percentage of inedible plant material that must be dealt with aerobic biological decomposition (mainly composting) was chosen.  This produces liquid and gas effluents that are, for the most part, compatible with plant growth and less toxic than the effluents produced by alternative physio-chemical systems.  The system’s production of nitrates instead of ammonia reduces denitrification, although some nitrification will be necessary for any ammonia produced and for the phytotoxic NO₂.

Anaerobic decomposition was dismissed on the basis that it produces methane, which, although capable of being used as a fuel, is incompatible with any of the other life support subsystems.  Aerobic decomposition does still produce some difficulties however.  Like all biological systems it is difficult to control with slow start up and turnover times and the risk of producing epidemic micro-organism populations.  These problems can be overcome by altering the bioreactor’s flux rate, temperature, pH levels and the incoming waste composition and thus controlling the organisms’ equilibrium.  Even if difficulties arise in the bioreactor it is easily repaired due to its relative simplicity.

Another system for waste treatment will also be in place to deal with more toxic materials, which are less compatible with biological decomposition.  For instance, human faeces, which although easily subjected to biological decomposition, produce a foul smell and its decay is difficult to control safely.  Therefore, physio-chemical systems, such as: Incineration, SAC (Starved Air Combustion), the RITE (Radio Isotope Thermal Energy) Process, Wet Oxidation, and SCWO (Super Critical Wet Oxidation) were considered.

SAC, although using less oxygen than complete incineration was dismissed as it produces toxic fumes; the RITE process, although utilising the safer catalytic oxidation method, also used a plutonium heat source making its servicing a risk on such a long term project; Wet Oxidation required a detailed knowledge of the input material in order to predict the compounds produced, while SCWO was chosen as it is capable of handling all settlement waste, including the removal of trace contaminants directly from the process atmosphere.  This ability to handle all spacecraft waste offers a useful alternative to VAPCAR should any difficulties arise with the urine processing subsystem.

SCWO makes use of water in its super critical state (above 647K and 2.21 x 10⁷Pa) as a medium for the oxidation of usually insoluble compounds.  These insoluble compounds become soluble in super critical water (including O₂), allowing the oxidation of all wastes in a single phase, assuming sufficient oxygen is available in the process atmosphere and the reactor is above 922K and 2.53 x 10⁷Pa.  The temperature allows for extremely short reaction times with complete oxidation of organic compounds, including trace contaminants, to carbon dioxide, hydrogen, and nitrogen, in less than one minute.  With proper design the heat generated by oxidation can be used to maintain the reactor temperature thus requiring little external energy (ref. 18).

As in VAPCAR, the SCWO process produces a potable water supply, which can then be either further purified or sent to potable water storage for human consumption.  Some difficulties have been noted with reactor material corrosion, leading to contamination of the outputs, however, again as in VAPCAR, these problems are not seen as long term dilemmas.

All the details above regarding Waste Processing deal specifically with Life Support Systems.  Additional subsystems may have to be implemented for specific wastes produced under unusual conditions, i.e. – lab work (see Trace Contaminant Control).

17.3            Nitrogen Makeup

Nitrogen will be necessary in the atmosphere as an inert gas, in order to make up the necessary pressure within the settlement.  Apart from this, nitrogen will be necessary for plant growth as it forms part of their protein makeup.  Although it is necessary for plants it is poisonous to humans, if metabolised, and most amino acids consumed undergo deamination in the liver.  This process involves the removal of nitrogen from the molecules and then its excretion as urea in urine.  Thus the return of human consumed nitrogen to the atmosphere is achieved through the VAPCAR process (detailed earlier).  Some more lost nitrogen is returned to the atmosphere through SCWO while the rest is contained within the inedible plant material and food processing wastes.  This is not returned directly to the atmosphere but instead, through biological decomposition, takes the form of nitrous oxides and a little ammonia (see fig. 17.12a).

These compounds can be fed through a biological system of Nitrification, which would convert Ammonia (NH₃) to Nitrite (NO₂) through bacterial oxidation (i.e. – by Nitrosammonas), and Nitrite to Nitrate (NO₃) again through bacteria (i.e. – Nitrobacter).  Nitrate (some of which is produced directly by biological decomposition of wastes) is usable by plants and readily converted to plant proteins, nitrites however are phytotoxic and must go through this process.  If this process does not produce enough nitrogen for plant usage, nitrogen fixation can take place directly from atmospheric nitrogen by symbiotic bacteria such as Rhizobium in conjunction with legume crops.  On the other hand, if too many nitrates are produced, resulting in a loss of atmospheric nitrogen, denitrification of nitrates can take place through bacteria such as T. Denitrificans.  This safety feature can be set up as a negative feedback loop as the level of atmospheric N₂ rises or falls.  All these biological systems can be controlled as with the biological decomposition of wastes, through the alteration of the organisms’ equilibrium.

Although the above systems should recycle all nitrogen and close the loop, any loss of atmosphere through leakage would be most noticeable in nitrogen, due to its relatively high partial pressure in the atmosphere.  However, apart from trying to prevent leakage, such losses cannot be made up from resources onboard the settlement, therefore some re-supply may be required periodically, possibly from a cryogenically stored supply or from future discoveries of asteroid sources.

17.4            Temperature and Humidity Control

Due to DaedalusaL4’s’ location at L4, providing sunlight 99% of the time, direct solar heating should provide sufficient heat for the human habitat.  However this will still require control to prevent overheating and to transfer the heat to those areas that will receive the minimum of direct solar heating.  This can be achieved by using a CHX (Condensing Heat Exchanger).  Luckily this system can also remove moisture from the atmosphere extending its capability into humidity control.

The temperature must be maintained at a suitable ‘shirt sleeve’ temperature (20-25°C) to give a comfortable working and living environment in the human habitat.  This temperature will have to be varied for different areas of the settlement such as laboratories, agricultural areas and industrial areas.  However, the humidity must also be maintained at a suitable level for human habitation (about 40%), to allow the transpiration of agricultural plants, and to prevent the condensation of water on instruments.

First, in a CHX, the air temperature is lowered to below dew point by passing it over an enclosed coolant water loop.  This water loop can then be used to transfer the collected heat to other areas of the settlement that require it.  Meanwhile, the air is cool enough so that a condensate film forms from the water-laden air.  This film of water is then removed from the subsystem through small holes called ‘slurper holes’ (see fig. 17.4a).  The main stream of air continues on back to the atmosphere, while liquid water is precipitated out of the condensate.  This subsystem can be used to remove transpiration water from the atmosphere surrounding plants and thus produces a potable water supply.  Varying either the number or size of the slurper holes can control the process.  It will probably prove easier to vary the number of holes, closing off some when the humidity is low enough.

Other options considered for humidity control included: Molecular Sieves, Membrane Separation and Water Vapour Electrolysis.  Molecular Sieves and Membrane Separation were dismissed due to their complex fine design compared to the technological simplicity of CHX.  Water Vapour Electrolysis offered no advantages in terms of humidity control over CHX apart from its small size, however it was considered as a backup portable emergency oxygen generation unit and as such would also function as a humidity control device.

Using CHX, the amount of heat energy trapped on DL4 can be regulated and transferred between different sections of the settlement.  However, sufficient heat energy must initially be provided to maintain the shirt sleeve climate before the CHX can transfer it.  In order to calculate the amount of heat energy required, the power balance of the station must be found.

A power balance is influenced by the energy per unit time created within the object, energy per unit time coming from outside and energy per unit time given to outside. Within our solar system, the sun is the only major source of energy outside, however to attain a power balance the planetary reflected solar input has to be taken into account.  Thus the power balance (ref. 18) becomes:

where,

solar input to spacecraft:                            

planetary-reflected solar input:                    

internally generated power:                        

power radiated from spacecraft to space:    

power radiated from spacecraft to planet:   

planetary albedo:                                       

spacecraft surface absorptivity:                  

spacecraft surface emissivity:                     

view factor spacecraft-to-space:                

temperature of the spacecraft:                    

space background temperature:                  

solar energy flux:                                        = 1,390W/m²

area of spacecraft:                                    

projected area of the spacecraft:                

view factor spacecraft-to-planet:                

temperature of the planet:                          

Stefan Boltzmann constant:                        

Assuming that K (in reality, K) and , in equilibrium condition the energy balance becomes:

Located at L4, approximately 384,400km from Earth,  and  are negligible and the equation may be simplified to:

yielding the equilibrium temperature of the spacecraft:

For DL4, this equation can now be used to calculate the temperature of the torus given a few assumptions. Firstly, it is assumed that the settlement’s hull is made entirely of aluminium.  Therefore, the surface absorptivity, (ref. 44) and, the surface emissivity,  (ref. 80).  Also, taking the approximate internal power output () of the torus as 69,007.5kW (see below), the surface area[1] () as 5,260,499.146m² and the projected surface area[2] () as 1,674,349.504m² and using 5.6710^-8W/(m²K⁴) a value for  can be deduced:

487.5468837K or @ 214.4°C

This value is clearly far above our desired torus ‘shirt sleeve’ temperature and so must be reduced drastically.  This has been catered for to a certain by the thermal radiators mounted on the central sphere, however at the above temperature, the water coolant of the CHX will vaporise and thus be useless at transferring heat to the radiator.  Therefore, additional design features must be considered.

Firstly, it must be remembered that the hull is not made entirely from aluminium, it is, in fact, covered in meteorite shielding, made of windows and, in places, surrounded by a sheath of radiation shielding.  Therefore, if it is assumed that the majority of the hull is coated in Mylar, Kevlar and foam, a slightly more suitable equilibrium temperature can be deduced, with one slight alteration.  The addition of a white ‘beta-cloth’, Teflon coated fibreglass, layer to the settlement adjusts the values of  and  to 0.05 and 0.5 respectively.  This changes dramatically, yielding:

181.573K or @ –91.6°C

Once again, this is clearly not suitable, however now at least we can more easily adjust the temperature by adding power to the settlement, transferring heat through the CHX (once sufficient power has been given to liquefy the coolant), and radiate excess heat through the thermal radiator.

Using the equation:

the amount of extra power required (x) to warm the settlement can be deduced, assuming 69,007,500W have already been supplied through everyday activities and the desired temperature () is 23°C or 296.15K.

This extra power can easily be provided by electric heating throughout the torus, however an additional power source has gone unnoticed in the above calculations.

The central sphere will be devoted to novel industry and research methods and thus the  value for this area is unknown.  However, given the high temperature and power requirements of material refinement and construction, it can be assumed that this value will be high.  Therefore, the independent temperature of the central sphere in equilibrium is anticipated to exceed the desired shirt-sleeve climate.  This means that such heat could be monitored and transferred through the CHX system to the torus habitat to increase the temperature within this area, thereby reducing the dependency of the settlement on purposeful heating (although some will still be required), while introducing the torus as a heat sink and lowering dependency on the thermal radiator. 

It should however be noted that the value used for the torus  above is extremely approximate.  It was formulated on the basis of a 27,203,000W requirement for food processing services, a similar figure for torus industry and research, and an extra home power consumption of 1,360.15W/person.  In addition to this an average value of 100W was taken as the power generation of the average resident.  (This figure was arrived at by taking the average amount of food consumed per day as 2,000Cal: 2,000Cal/day = 2,000kcal/day = 2,000,000cal/day = 8.37Mjoule/day = 349kjoule/hour = 97joule/sec = 97W.)  Thus the total figure of 69,007,500W (27,203,000W + 27,203,000W + 13,601,500W + 1,000,000W) was deduced.  This value will change over time, given different industrial activities, the diurnal range and human metabolic rates.  Thus constant monitoring and control of the heat being transferred by the CHX system will be required, along with careful alignment of the ‘slurper’ holes to prevent over-dehumidification, given such extensive use of the system.

17.5      Water Monitoring

Water Monitoring must take place to ensure that all potable and hygiene water satisfies the following physical parameters (table 17.5a), and maintains safe levels of chemical and biological contaminants (table 17.5b):

table 17.5a  - Physical parameters for potable and hygiene water (ref. 18)

table 17.5b – Chemical and biological contaminant limits in potable and hygiene water (ref. 18)

In order to ensure these parameters are met, the pH, ammonia content, total organic carbon (TOC), electrical conductivity and microbial concentration must be frequently monitored.  Microbial content can be monitored indirectly by measuring the iodine content of the water, thus ensuring the constant presence of a biocide.  A suitable procedure for water quality monitoring is given in fig. 17.5a.  This could easily be implemented before water produced from CHX, VAPCAR and SCWO subsystems are inputted into water storage.

A decision that is open to opinion is whether or not to fluoridate drinking water. Fluoridation has been used for many years to help prevent dental caries. Recently, there has been a change in public opinion. There are fears that fluorine in drinking water can cause other health risks. We considered the arguments for and against fluorination from many sources (ref. 86,87,89).

The American Dental Association is still for the fluoridation of community water supplies. In their 1999 ‘Fluoridation Facts’ report, they stated:

“Fluoridation is considered beneficial by the overwhelming majority of the health and scientific communities as well as the general public.

·      Fluoride helps prevent tooth decay. All ground and surface water in the U.S. contains some naturally occurring fluoride. If a community’s water supply is fluoride-deficient (less than 0.7 parts fluoride per million parts water) fluoridation simply adjusts the fluoride’s natural level, bringing it to the level recommended for decay prevention (0.7-1.2 parts per million).

·      Fluoridation is a community health measure that benefits children and adults. Simply by drinking optimally fluoridated water, members of a community benefit, regardless of income, education or ethnicity — not just those with access to dental care.

·      Fluoridation protects over 360 million people in approximately 60 countries world-wide, with over 10,000 communities and 145 million people in the United States alone.

·      As with other nutrients, fluoride is safe and effective when used and consumed properly. From time to time, opponents of fluoridation have questioned its safety and effectiveness. None of these charges has ever been substantiated by generally accepted science. After 50 years of research and practical experience, the overwhelming weight of scientific evidence indicates that fluoridation of community water supplies is both safe and effective.” (ref. 88)

We found that there was little conclusive evidence that the low fluoridation levels that are employed to prevent dental caries pose any significant threat to health. Any health risks evolved from poor control of the levels of fluorine in the water, and in toothpaste given to children.  We decided to design for the fluoridation of drinking water, with strict controls and monitoring.  If in the meantime, conclusive proof can be found that the low levels of fluorine used to prevent dental caries have a significant adverse effect on people’s health, then the design could be adapted accordingly.

17.6      Water Processing

Water processing is necessary in order to supply the inhabitants with water, usable both as potable and hygiene water.  Most of the current physio-chemical methods for recycling this supply involve filtration, which leads to a loss of materials and the need for replacement filters (i.e. - Reverse Osmosis, Multifiltration and Ultrafiltration).  However, about 40% of the potable drinking water needs (about 1.61L/person/day, excluding all water in food, 1.15L, and food preparation water 0.79L, 0.04L of which is never consumed due to evaporation) of DL4 have already been catered for in earlier sections (17.1 Urine Processing and 17.2 Waste Processing) as VAPCAR and SCWO combined will produce about 0.45L potable water/person/day.  These sources only require testing and under certain circumstances, limited processing, before being ready for human consumption and thus supply a relatively quick recycling loop for water.  However, this still leaves the remaining drinking water (0.7L/person/day), all the food preparation water (0.79L/person/day) and hygiene water (0.36L oral + 1.81L hand and face wash + 5.44L shower + 12.47L clothes wash + 5.44L dish wash = 25.52L/person/day, ref. 31) to be catered for.  Plants, during their growth season, also require an additional 0.45L/plant/day in order to photosynthesise and maintain turger pressure through osmosis (hence, on average everybody consumes 1.15L of water per day through food).  Using the CHX temperature and humidity control system extensively, plant transpiration water can be taken from the atmosphere and used as a potable source of water.  This source of clean water will easily cover the remaining needs of DL4, which are not catered for by SCWO and VAPCAR. 

However, a surplus of water will be maintained onboard, leading in total to one weeks consumable supply being stored.  This surplus will be utilised for industrial purposes (if these processes do not incorporate water recovery then water will have to be shipped to the settlement along with the other materials being refined) and will cover the time lag experienced due to the slow biological nature of the water recycling system.  It will also be an emergency water source and could help with fire suppression or be used for water electrolysis and the emergency supply of oxygen, for respiration, or hydrogen, as a fuel.  Based on the approximate figures that: 30% of agricultural area will be taken up with plants capable of growing at a density of up to 2,000 plants/m², although these will more likely be grown at about 1,300 plants/m²; 40% will be used for vine like plants and legumes growing at about 750 plants/m²; and the remaining 30% will be used for low density crops at about 20 plants/m² (ref. 49), then, given an agricultural area of 869,056m², with only 217,300m² of this being in season, and therefore of real significance for this calculation, 68,047,084L of potable water will be required by the plants per day (leaving a wide margin for error and for those plants not in season).  In addition to this 284,100L will be required by the inhabitants per day for hygiene and metabolic processes.  Therefore a total of 478,318,288L of potable water will be kept on DL4 (this does not include recreational water for swimming pools and natural waterways).  On top of this figure a further half-week’s water will be stored within the waterways and lakes of the recreational areas.  This large amount is necessary to compensate for the inevitable mixing between recreational and potable water in the atmosphere and allow for the slow recycling loop that will return such water to the recreational areas (which will no doubt require monitoring and topping up from potable stores).  This extra water will bring the total stored water onboard to 717,477,432L.

17.7      Oxygen Generation

Oxygen aboard spacecraft is currently ‘scavenged’ from spent fuel tanks and supplied from a stored cryogenic source, however in order to maintain a CELSS (Closed Environment Life Support System), it will be necessary to recycle oxygen used in human respiration.  Physio-chemical means of achieving this involve the electrolysis of water vapour to produce hydrogen and oxygen.  The oxygen then would go into the atmosphere while the hydrogen would be used to reduce carbon dioxide, thus producing a fresh source of water to undergo electrolysis.  This process takes care of both oxygen generation and carbon dioxide removal, however leaves carbon out of the cycle at the end, leaving no other option but its oxidation through SCWO to CO₂.  Ignoring the final oxidation of carbon, the process would yield 0.74kg of oxygen per person per day (based on an average 1kg carbon dioxide produced per person per day, ref. 18).  This does not completely close the loop, as approximately 0.85kg of oxygen are required per person per day (ref. 31), however such a system could be used in emergencies or when large numbers of the inhabitants congregate in an enclosed area.

Such systems include the use of Water Vapour Electrolysis, which, as mentioned earlier, performs the second duty of lowering humidity, but also the more promising Direct Carbon Dioxide Electrolysis.  This solid electrolyte system can electrolyse both carbon dioxide and water vapour directly from breath, however the simultaneous electrolysis of water leads to the release of hydrogen into the atmosphere making this an unsafe process for use over long periods of time (the hydrogen would be removed at a later stage by the trace contaminant control systems).  Initial electrolysis produces oxygen but also carbon monoxide, which then passes through a catalytic bed that decomposes it to solid carbon and carbon dioxide. The CO₂ then passes through the electrolyte again.  This process is only used as a backup system and thus is not included in the life support flow diagram (fig. 17.12a), however hydrogen-carbon dioxide reduction and water vapour electrolysis are, as sinks for the hydrogen produced through SCWO.  The water vapour electrolysis in this case is also only in use as a backup system in case of oxygen depletion, otherwise the water produced from the previous CO₂ reduction can be stored in the potable water store (while the carbon produced passes through SCWO).

The main system for oxygen generation onboard DL4 will be from agricultural and recreational plants.  For agricultural reasons there will already be, at any one time, four overall growing ‘seasons’ for agricultural plants.  These will relate roughly to spring, summer, autumn and winter, allowing for the year round production of all foodstuffs.  The ‘seasons’ will be further divided into climates, however this has been dealt with in more detail in 9.3 Agriculture Area Layout.  For life support reasons, the four ‘season’ areas will be doubled, thus creating two springs, two summers, and so on.  One out of every two of these will approximate daytime conditions, and thus the production of oxygen, while the other will approximate night-time conditions, and thus the production of carbon dioxide (see fig. 17.12a). Plants in the recreational area will grow according to the seasons within the human habitat.  These can more or less be discounted from oxygen generation as they will be, in the main, fully grown trees (eventually), grass, and flowering plants, all of which are less effective at decreasing carbon dioxide levels in the atmosphere.

Therefore, in terms of oxygen generation, the main plants, at any one time, to be considered will be the agricultural daytime plants undergoing their spring season.  These plants, in the height of their photosynthesis year, will be capable of producing 0.85kg of oxygen for every 1.12kg carbon dioxide fed to them.  This 0.85kg of oxygen supplies the needs of one person for one day, however the average person only produces 1kg of carbon dioxide per day (ref. 18).  This gap between supply and demand should however be overcome due to other sources of CO₂ onboard DaedalusaL4, such as the ‘night-time’ plants, SCWO, the recreational plants, biological waste decomposition and VAPCAR.  Thus, assuming the extra CO₂ needs are met and not exceeded an agricultural plant growing area of 217,300m² (including non-food products grown in the agricultural section – cotton, flax) will be capable of recycling the necessary oxygen for 10,000 residents back into the atmosphere.  However, there are other sinks for oxygen onboard DaedalusaL4, such as SCWO, VAPCAR, TCC (Trace Contaminant Control – Reactive Bed Plasma, see 17.10), Biological Nitrification and Biological Waste Decomposition, along with others.  The addition of 651,900m² for those plants that are out of season (not at the height of their photosynthesis year) will easily meet these needs with the help of the ‘recreational plants’.  Assuming a total population presence and allowing a generous buffer zone to compensate for any failed crops, this brings the agricultural area up to 869,200m², although not all of this may be used at the one time, depending on oxygen consumption, population and the ‘preference crops’ being grown.  Floor space taken up on the main 1g level will actually be half this figure (» 434,600m²) as the agricultural section will be double stacked (with about 5-6m between levels in order to incorporate robotic harvesting technology and tall crops like maize) to maximise space.

Other rejected oxygen generation options were: Static Feed Electrolysis – required asbestos for the electrodes –, Superoxides – required expendable chemicals –, and Cryogenic or high Pressure Storage – not an option in CELSS.

17.8      Carbon Dioxide Concentration

Carbon Dioxide Concentration is necessary in order to supply a CO₂ rich airflow to the plant habitats, which will act as the principal carbon dioxide sinks.  Such methods of concentration will also ensure that carbon dioxide levels do not exceed safe limits (above 0.4kPa partial pressure) within the human habitats.

In order to concentrate carbon dioxide, a process of Electrochemical Depolarisation Concentration (EDC) was chosen.  The EDC reacts hydrogen (from SCWO) and oxygen with carbon dioxide in an electrochemical cell to produce an anode, CO₂ rich, airflow containing a little hydrogen, and a cathode, low CO₂, airflow, which returns to the atmosphere.  The little hydrogen in the anode exhaust can then be used to carry on the process or to reduce carbon dioxide and produce water for further electrolysis.  The carbon dioxide rich air stream can be used to revitalise the atmosphere of the daytime plant habitats.

Due to the nature of the hydrogen/oxygen reaction EDC produces a DC current, as in a fuel cell.  This can be used to power other CELSS functions.  The reactions also generate heat, most of which is carried away by the various air streams, however a coolant water loop is also necessary.  As shown in fig. 17.8a a typical cell consists of two electrodes separated by an aqueous electrolyte matrix (Cs₂CO₃).

The major problem with EDC is the additional loads on oxygen generation and humidity control.  However, with plants as oxygen generators this subsystem will only improve their efficiency in such a role and the CHX systems should be able to cope with the added moisture (providing more potable water for human consumption).  Other options considered included: Molecular Sieves, Solid Amine Water Desorption (SAWD) and LiOH.  Two possible molecular sieves were the 4- and 2- Bed Molecular Sieves.  Both of these were promising in their previous usage, however the beds required removal for desorption and regeneration in near vacuum conditions, making direct CO₂ supply to the plant habitats difficult.  The SAWD process was ruled out by its consumption of hygiene water and requirement for replacement Amine Beds.  Finally, LiOH was clearly not acceptable, as it is a non-regenerable process, requiring expendable chemicals.

17.9      Carbon Dioxide Removal

Carbon dioxide removal is necessary to maintain the health of the crew.  However, it must be carried out by a means that returns the carbon to the regeneration loop.  A number of methods for doing this have already been outlined in Oxygen Generation.  These were Water Vapour Electrolysis and subsequent carbon dioxide reduction, Direct CO₂ Electrolysis, and Biological Exchange by Plants.  The latter of these three will compose the principal method for CO₂ removal onboard DL4, as detailed previously.

17.10    Trace Contaminant Control and Particulate Removal

Trace contaminants originate from the metabolism of residents, the out- and off-gassing of cabin materials (plastics, insulation, adhesives, paints), thermal degradation, leakage and spills.  Residents must be protected from these in order to prevent hazardous contamination through airborne biological and chemical compounds.  The purpose of TCC (Trace Contaminant Control) is to do this in a manner that involves the conversion of these contaminants to harmless gases in non-consumable ways.  Acceptable levels of these contaminants are provided in Atmosphere Monitoring and Control (table 17.11a).

For the purposes of TCC, two main options were considered.  A conceptual design, incorporating a number of current technologies such as charcoal beds (one regenerable and one non-regenerable), filters, sorbents (such as LiOH), and catalytic oxidisers, was rejected as it was not compatible with the ideal of a CELSS (the non-regenerable charcoal bed removed materials from the loop and other features, such as the sorbents, also required replacements and re-supply).  Instead a Low Temperature Plasma Process consisting of Reactive Bed Plasma was chosen.

In this system contaminants are oxidised at low temperatures (394K) using a combination of plasma and catalyst.  Process air is passed through an annular (ring-shaped) reactor where it is partially ionised into plasma by an alumina catalyst.  The air then passes into the main reaction chamber where plasma-generated high-energy and, subsequently produced, species (active oxygen) oxidise toxic materials.  The Reactive Bed Plasma (RBP) includes treatment of toxic reaction products (i.e. – nitrous oxides).  Luckily the RBP also acts as an electrostatic precipitator to collect and deactivate particulate matter.

Despite some problems due to electromagnetic interference and its high power requirements the RBP system should be capable of acting as a long-term, low temperature TCC subsystem.  Even if problems do arise with such a new technology the RBP will be operating simultaneously with SCWO, which deactivates all trace contaminants in the process atmosphere it takes in.

17.11    Atmospheric Monitoring

Atmospheric Monitoring must take place to ensure that all the above systems for atmosphere regeneration are functioning properly, especially those dealing with trace contaminant control and carbon dioxide removal.  For this purpose two systems were chosen, in addition to any special sensors that will be necessary for specific gases, in labs etc.

The first system, called FTIR (Fourier Transform Infrared) Spectroscopy, involves passing infrared radiation through a sample of cabin air then creating an IR absorption spectrum with an interferometer.  This signature reveals the constituents of the air sample.  The system is unfortunately incapable of distinguishing between related compounds and does not detect some compounds, which do not absorb IR radiation, e.g. – those of hydrogen and chlorine.  Due to these disadvantages this system will be used in conjunction with an Ion Trap Mass Spectroscopy/Mass Spectroscopy (MS/MS).  This fills in what gaps FTIR leaves but is incapable of distinguishing substances of identical molecular weight.

Once in the ion trap of MS/MS the cabin air sample is ionised before being separated on the basis of molecular weight and analysed by an ion detector.  The more current Gas Chromatograph leading to Mass Spectrometer (GC/MS) system was rejected as it requires trace contaminant concentration before analysis whereas MS/MS offers the same detailed information in the fastest time.  This system along with Trace Contaminant Control must ensure that trace contaminant levels are maintained below the safety levels recommended aboard the Space Shuttle (see table 17.11a).

The atmospheric composition of the main gases (CO₂, O₂, N₂) will vary across the settlement depending on the habitat (i.e. – slightly higher carbon dioxide levels in plant habitats to maximise photosynthesis).  However, in the main human habitat, the atmosphere will closely resemble that found at sea level on Earth.  Therefore, a partial pressure of 22.7kPa of oxygen will be maintained to ensure proper alveolar functions and to prevent losses in blood cell mass or the growth of bacterial epidemics.  Carbon dioxide should be kept at a partial pressure below 0.4kPa in order to ensure the safety of the crew and maximise human habitat plant photosynthesis.  The idea of a decreased partial pressure of nitrogen was rejected on the basis that this would lower the atmosphere’s resistance to fire and that the overall reduction in atmospheric pressure would cause a safety hazard when it came to operating some of the high pressure systems onboard (i.e. – SCWO and VAPCAR).  Therefore, a partial pressure of 53.2kPa was chosen for nitrogen.  This unfortunately must be made up initially from terrestrial sources as we are not aware of any significant nitrogen sources on asteroids or the Moon.

table 17.11a – Safe Trace Contaminant Levels (US Space Shuttle)

DL4’s atmospheric oxygen supply will be harvested from lunar soils.  This process involves the crushing and screening of the ore, the processing of the ore and the liquefaction and integration of oxygen into DL4’s atmosphere (ref. 35).  Tailings produced, given a mere 10% ilmenite – FeTiO₃ – (the compound from which water will be initially removed through reduction with hydrogen[3], and then from which oxygen will be harvested by electrolysis, ref. 19) recovery rate from the soil, allows for such surplus materials to then be stored and processed for industrial metal extraction and for the creation of radiation shielding.  Therefore, as all mined lunar soil will be put to use on DaedalusaL4 and given the settlement’s receipt of almost constant sunlight, it makes sense to perform lunar oxygen extraction directly from regolith (collected on the Moon) onboard the settlement during its construction phase.  The need for a constant power source arises as the power-up and -down stages of the harvesting process lower the efficiency of the system to around 45% thus possibly requiring nuclear power to make the process viable.  Constant sunlight will also provide for a solar furnace (although some problems have been envisaged due to mirror and window contamination).  The processes carried out onboard DL4 can be easily automated or tele-operated and thus the production of oxygen for the settlement, as it is being built, should not increase the work force greatly.

17.12    Conclusion

Success in the closure of the various subsystems of Life Support is measured in K values, K representing the closure itself.  For every substance recycled:

where E is the rate of consumption in the system, e is the rate of (incomplete) closure, and I is a constant (ref. 30).  The goal, K = I – 1, is unfortunately unreachable, but asymptotically approaching it will be every biologists’ favourite game on DL4 and critical to achieving a self-sustaining existence.  The proposed subsystems above along with our agricultural proposals will hopefully provide the maximum closure possible to attain this dream.

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