Water Recovery and Waste Processing
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Water is a ubiquitous resource for human survival, and maintaining a viable water supply for a long-term human presence in space will require life support systems that make use of all organic waste constituents within a habitat or spacecraft. Bioregenerative Life Support Systems (BLSS) coupled with current space-capable wastewater processing and potable water treatment equipment leverages the best possible combination of technologies to achieve near 100% closure of life support architectures. By balancing microbial and plant communities with biotic and abiotic components of the closed environment, BLSS makes use of nearly all wastewater effluent sources by efficiently converting organic matter into nutrients for edible plant mass and subsequent human consumption. Explored are recent experiments and ongoing efforts to better define the detailed components of BLSS, along with a summary of current technologies deployed for waste collection, processing, treatment, and storage in the space environment.
KeywordsBioregenerative Life Support Systems Long-duration life support Spaceflight water recovery Wastewater processing Closed-loop space habitat Water mass balance Direct potable reuse
The problem of waste recycling in life support systems revolves around the need to conserve vital resources by recovering as much consumable material as possible following usage by the crew. This material recovery is achieved by creating a closed-loop life support system architecture capable of reusing as much waste material as possible so as to limit the amount of up-mass or external resource acquisition from the surrounding environment outside the habitat if possible (Anand et al. 2012). Water in particular is a vital resource required for human habitation no matter the destination. Furthermore, if the available water is not purified to the degree necessary to prevent sickness and/or biological poisoning, no amount will support a human population of any size. In the paradigm of terrestrial survival, out of the estimated 1400 million cubic meters of water on Earth, seawater comprises approximately 98%, rendering it undrinkable without significant energy and resources to remove the salt content (Rehmeyer 2006). In this context, it is easy to understand that maintaining an uncontaminated drinking water supply in long-duration spaceflight is a necessity.
While the primary purpose for water in a life support architecture is for human consumption, water is also a ubiquitous medium in which to store and transport oxygen for eventual electrolysis into constituent hydrogen and oxygen gas. Properly separated, the hydrogen gas can be either vented or stored for other spacecraft uses, while the oxygen gas can be directed to a ventilation system to replenish the internal atmosphere of a habitat as a crew consumes it through respiration. This particular use of water requires an even higher purity level beyond needed for liquid consumption by humans, as the current state of water electrolysis technology in life support systems utilizes technology that is highly sensitive to all but the most purified water supplies (Snodgrass et al. 1991). Therefore, the water processing function of a long-term life support system must incorporate a means to purify water beyond potable quality (Schneider and Shull 2017).
Water Mass Balance and Equivalent System Mass (ESM)
Any discussion of water recovery from waste processes aboard an Earth-launched spacecraft or extraterrestrial habitat begins with an understanding of Equivalent System Mass (Jones 2003). While early human spaceflight was of short duration, where fuel cells coupled with compressed gases were often used to supplement onboard water supplies to stock only the amount of water needed for missions of a few weeks (Elitzur et al. 2016), longer duration missions necessitated the incorporation of wastewater recovery and processing to refill water supplies aboard a spacecraft. Currently, the International Space Station (ISS) water reclamation system is derived from crew urine and coupled with water recovered from humidity condensate, comprising the bulk of reprocessed water aboard the station (Kayatin et al. 2016). While this reduces water resupply from Earth by about 65%, the ISS is still reliant upon regular launches of fresh water to supplement its water recycling capacity (Garcia 2016).
Several current spacecraft design concepts utilize Equivalent System Mass (ESM) to demonstrate the increase in mass associated with crew life support for long duration missions. As explained by Swickrath et al. (2011), “ESM quantifies the mass of (life support system) hardware, consumables, and associated infrastructure, along with mass penalties based on power, volume, cooling, and crew time requirements to operate the life support hardware.” Put simply, comparatively low initial life support system mass is needed for short duration missions into space, but as mission duration becomes longer, that initial mass increases. Further, the mass penalty for extending the operational duration of the life support system architecture can itself increase or decrease dependent upon the level of Earth resupply, closure amount, and ability for In-Situ Resource Utilization (Anand et al. 2012).
For early spaceflight, only minimal life support methods were needed for short-duration missions to the Moon and for stays in low-Earth orbit (Wieland 1994). The ESM for these missions not only precluded long duration water reclamation technologies but required less launch mass due to the close proximity to Earth, as the delta-V needed for orbital insertion are less than that for mission such as a Mars transit mission (Jones 2011). By extrapolating ESM to longer periods beyond the short-duration missions of early human spaceflight, not only does the level of complexity of the life support system increase due to added mass from redundancy and contingency consumables, but also with additional systems that utilize closed-loop life support technologies (Swickrath et al. 2011). Therefore, as mission duration increases along with distance away from Earth, mass and volume restrictions preclude current ISS technologies, requiring more advanced methods to process wastewater to useable quality by crew and equipment.
Microgravity Versus Planetary Gravity
The first step in defining a water recovery and processing system for long duration space missions is to determine what environment the system is expected to operate in. Specifically, it is important to know if wastewater must be collected and processed in microgravity, or if a gravity environment is available. If the latter, then terrestrial waste treatment facilities provide a good analog, as much of the same technology used in modern municipal sewage treatment systems can be incorporated into smaller closed-system habitats needed for long duration missions in planetary habitats. However, microgravity provides a problem. Not only does waste collection become more complex (Broyan 2007), but gas entrainment in liquid mixtures can become problematic as buoyancy forces that mediate gas separation in gravity environments do not exist in microgravity (Antar et al. 2007).
In microgravity, there are problems in both wastewater collection and waste processing. For liquid waste, rotary separators are considered the primary technology for limiting gas/air entrainment, as they have been employed on both the shuttle and ISS (Broyan 2007). However, they are subject to clogging from foreign particles and liquids and also prone to mechanical damage. For wastes with additional viscosity and solid masses, regenerable static separators could be employed much as they are in the Russian Service Module toilet aboard ISS but still require a rotary separator. The initial gas separation during urine collection is by no means 100%, and gas entrainment still requires further removal in downstream processes. For the United States On-orbit Segment (USOS) portion of ISS, the urine is pretreated with chemicals and sent to a rotary separator before being stored in a bladder tank.
There are few options for gas liquid separation from urine collection in future microgravity space habitats, so it is likely that rotary separation will continue to be the technology employed. In general, collection of water from fecal waste in microgravity is not considered economical for most life support system technology, as fecal water comprises less than 0.1 kg/crew/day (Wieland 1994). Current microgravity technologies simply collect fecal waste in gas permeable bags and are compacted only on the USOS portion of the ISS before storage and returned to Earth (Broyan 2007). Deep space habitats will have to find a means to store fecal biomass until Earth return, rendezvous with a resupply vehicle, or arrival at a planetary destination for offloading, assuming that planetary protection protocols can be fulfilled. The latter may incorporate reusing the fecal waste for a gravity-based Bioregenerative Life Support System (BLSS) so as to recycle the waste at a later time.
For gravity environments, such as lunar or Mars surface habitats, the need for gas liquid phase separation technology is mitigated by allowing natural gas buoyancy over denser liquids predominate in the collection and processing steps. Baffled settling tanks for urine collection could be employed in surface habitats (Broyan 2007), very similar to a septic system on Earth. It could then either be processed using current ISS methodology as discussed below (resulting in a toxic brine) or be fed into additional waste streams that are treated to produce fertilizers for crop production (Clauwaert et al. 2017). Similarly, fecal biomass may be down-massed from the habitat, with or without water recovery processing through methods such as lyophilization (Swickrath et al. 2011), or recycled as a plant growth medium (Walker and Granjou 2017).
Bioregenerative Life Support Systems (BLSS)
Wastewater, especially that excreted from crew and live payload, contains a multitude of different organic compounds too numerous to target in any one chemical survey (Carter 1998). Complicating the problem is the varying concentration of organic compounds excreted from individual organisms, even of the same species. Currently on ISS, there are chemical and physical separation methods employed to treat liquid waste, such as activated charcoal and ion exchange resin (Kayatin et al. 2016). Additionally, there are chemical transformation methods such as catalytic oxidation to covert low molecular weight organics into purgeable gases, but each of these methods requires constant resupply from Earth. None use low ESM methods (waste recycling) that could be employed for extended missions away from Earth, such as bioregenerative oxidation reactors like those explored by the European Space Agency via the Micro-Ecological Life Support System Alternative (MELiSSA; Clauwaert et al. 2017). While recovery of carbon, nitrogen, and other micronutrients are usually the primary goal of bioreactors, if efficient implementation of these low ESM methods was to be accomplished, they would have the added effect of increasing the quality of wastewater in their processes. For instance, higher plants, algae, and microbes require nutrients such as nitrogen for survival, and some BLSS bioreactors utilize the nitrogen found in human urine to produce fertilizer for these organisms (Clauwaert et al. 2017). While they also metabolize water as well, they also produce water as a byproduct, thus further closing the life support loop.
Primary and Secondary Wastewater Processing
Most terrestrial wastewater processing systems have two levels or water processing, primary and secondary. Primary wastewater treatment entails removing suspended solids, usually through screens, grit tanks, and primary clarifiers that allow settling out of solids (Withgott and Laposata 2014). Secondary treatment involves aeration and microbial treatment of the dissolved and suspended organic matter, converting it to gases such as methane and carbon dioxide, and producing sludge that is further digested via anaerobic bacteria to produce a biosolid waste which must be collected and disposed of (Gikas 2017). Both primary and secondary wastewater processing are energy-intensive in large terrestrial treatment plants, as pumping of water, sludge, and air for microbial degradation requires constant pumping and stirring for effective organic elimination. The energy draw becomes even more expensive when disposal of biosolids is performed using incineration (Gikas 2017).
Elevating the design of terrestrial wastewater treatment to space-based BLSS requires a downsizing of the standard municipal treatment systems mentioned above, which can reduce power consumption, but requires much closer attention to gas processes (e.g., oxygen usage, methane extraction, carbon dioxide removal) and careful monitoring and maintenance of microbial communities since they are more vulnerable to perturbations in smaller aerobic digester systems (Walker and Granjou 2017). Further, such systems may be best suited to planetary gravity operations on the moon or Mars, since establishment and sustainment of stratified microbial communities are paramount in a properly functioning aerobic or anaerobic digester (Ghasimi et al. 2015). However, that is not to say that such communities cannot be implemented in microgravity, but it requires techniques utilizing technology such as biofilm membranes, packed beds, and fluidized beds that can establish microbial communities on shaped substrates rather than within a stratified wastewater column (Landes et al. 2007). With regard to separation of solids, recovery of water through treatment of sludges and solid wastes in BLSS need not utilize high-energy reactors, as composting has been explored as a viable means of recycling biosolids and extracting nutrients for crop production, with water recovery achieved through drying and oxidative processes (Hogan et al. 2001). While many BLSS primary and secondary processors can be implemented in larger, ecologically engineered facilities that effectively utilize bioreactors and algal biomass scrubbers along with constructed treatment wetlands (Blersch et al. 2000), these types of system designs would likely have a high ESM cost and launch mass and are beyond the scope of this essay.
MELiSSA (Micro-Ecological Life Support System Alternative) is an engineered BLSS bioinfrastructure project built by the European Space Agency (ESA) and housed and operated since the 1990s at the University of Barcelona (Walker and Granjou 2017). As a pilot plant project, MELiSSA is composed of five bio-reactors (“compartments”) that carefully balance strains of microbes, plants, and human wastes within biological communities that provide air, drinking water, and edible biomass. The first compartment is a chamber that composts human feces and plant wastes via anaerobic thermophilic bacteria along with microbes similar to what already exists within the gastrointestinal tract of humans. Three other downstream compartments utilize miniaturized microbiological processes containing bacteria optimized for photoheterotrophy, nitrification, and photosynthesis, along with higher plants that are capable of supplying food for crew within a BLSS. Composting functions within MELiSSA contain redundant backups that combine with sewage treatment processes in a controlled fashion to support the crew. By utilizing the metabolic water inputs and outputs from a human crew, and maintaining balance between the air, water, and waste compartments of a habitat/spacecraft, technology provided by MELiSSA has the possibility of reducing the ESM costs of long-term life support within BLSS and reduces the need for constant water resupply from Earth. MELiSSA is an ongoing project with ESA and, as of 2018, continues to produce data vital to the research of BLSS for human space exploration.
Bioreactors such as the ones utilized by MELiSSA and BWP will not likely be stand-alone technologies for water processing within long-duration space habitats and spacecraft. Jones et al. (2016) mentioned that while bioreactors such as BWP are capable obtaining water processing efficiencies of 90–95%, the effluent requires further processing to reach potable water standards. They identified 15 water system technologies as candidates for a Mars transit mission, six of which they calculated ESM for: Air Evaporation System (AES), Multifiltration (MF), Thermoelectric integrated membrane evaporation (TIMES), Vapor Compression Distillation (VCD), Vapor Phase Catalytic Ammonia Removal (VPCAR), and Water Recycling System (WRS). AES is a method that separates water from dissolved solids through evaporation of a brine solution (Pickering et al. 2006). MF is a series of filtration tubes that combine generalized absorption mediums with compound-specific ion exchange resins to remove organic and inorganic contaminants (Kayatin et al. 2016). TIMES is a decades-old wastewater processing technology that utilizes a series of thermoelectric modules and hollow fiber membranes to evaporate water and condense it via heat exchanger (Dehner and Price 1987). VCD is a process that uses a rotary-drum liquid separator in combination with compressing water vapor thus raising the saturation temperature before condensing it (Hutchens and Graves 2004). VPCAR is a combination phase change and catalytic oxidation technique to produce potable water from waste influent (Tomes et al. 2007). WRS is a combination of MF and VCD currently deployed aboard the ISS (Kayatin et al. 2016). AES, TIMES, and VCD can each process urine and urine brines but require that the product effluent be further processed downstream. Multifiltration is capable of processing condensate and wash water but lacks the ability to efficiently process urine directly. VPCAR and WRS are each wholistic in their influent requirements (can accept urine, condensate, and wash water), with effluents that meet potable standards.
While all of these physiochemical (nonbiological) water processing technologies can perform either primary or secondary wastewater processing alone, with some even capable of producing potable water within a spacecraft or extraterrestrial habitat on small scales, in order to reach the “break-even” point for water recovery and waste processing in a long-duration closed-loop BLSS, they will likely need to be coupled with BWP or MELiSSA style biological reactor systems to accomplish it. Further, in order to couple the reactors for the nutrient capture needed for human consumption, plant growth chambers will need to be implemented within BLSS habitats, such as those researched by NASA’s Biomass Production Chamber (BPC) and Lunar Mars Life Support Test Project (LMLSTP), as well as the CEEF (Closed Ecology Experiment Facilities) in Rokkasho, Aomori Prefecture, Japan (Anderson et al. 2017).
Tertiary Wastewater Processing
By the time primary and secondary processing of wastewater is complete in terrestrial facilities, the level of microbial activity, suspended solids, and contaminants such as phosphorus and nitrogen are reduced to levels consistent with local discharge regulations from permitting agencies (Criteria and standards for the national pollutant discharge elimination system 2016). At these levels, the wastewater is clean enough for discharge back into the natural environment or constructed wetlands intended for surface or groundwater recharge (Withgott and Laposata 2014). For BLSS, not only is discharge to the environment not favorable, especially if ISRU disallows the collection or use of extraterrestrial water supplies, but such release would eliminate the ability to keep vital organic compounds within the habitat, thus moving the closed-loop capabilities of the habitat further away from the idealized 100% “break-even” threshold for BLSS. For these reasons, it is more favorable to retain as much processed wastewater as possible inside the habitat for nutrient reclamation and water reuse.
If any level of reuse or drinkability is to be accomplished following primary and secondary wastewater processing, it must undergo additional treatment known as tertiary processing (Gikas 2017). Terrestrial tertiary wastewater processing involves additional disinfection and water polishing steps, usually accomplished through chemical or photo-reactive processes. For instance, ozone, chlorination, and peracetic acid (PAA) treatments have been found to be effective against common water contaminants such as fecal coliform (Mezzanotte et al. 2007). Chlorine dioxide has been found to be effective against pathogenic bacteria (Szabo et al. 2017), while ultraviolet light at different frequencies and wavelengths has found to adequately inactivate the replication processes of microorganisms (Song et al. 2016). Ferrate (VI) salt under low pH has also been shown to oxidize and inactivate microorganisms while also eliminating heavy metals and organic matter (Talaiekhozani et al. 2017).
However, biofilms and established biological treatment of effluents by microbes have been demonstrated to be negatively affected by chemical treatment methods (Simon et al. 2014). This necessitates either the strict isolation of the biotic components of a BLSS from the chemical treatment components or elimination of the latter altogether. While current methods of water purification may eschew any process that forgoes chemical treatment, reducing said treatment to a low-level, point-of-use role may be achieved if tertiary treatment methods incorporate intermediate bio-filtration methods that could capture residual postsecondary wastewater contaminants. Due to BLSS habitat closure needs for retaining of nutrients for plant production, agriculture has been proposed as a means for tertiary water treatment (Cohen 2015). Such treatment of primary wastewater effluent has been accomplished terrestrially over the past several decades with varying degrees of success (Rababah and Ashbolt 2000). Nevertheless, reuse of processed wastewater is generally considered a safe and effective means to alleviate irrigation water supplies in drought-stricken and semi-arid agricultural regions, though salt and heavy-metal retention can pose problems for large-scale operations (Chen et al. 2013). With respect to space habitats, nutrient uptake by plants that provide edible biomass to crew has long been considered an essential element for regenerative life support systems (Wolff et al. 2013). If complimented by final polishing filters such as carbon absorptive mediums and ion exchange, similar to the ISS galley system (Shaw and Barreda 2008), the life of such filters could be extended by tertiary water treatment using intermediate bio-filtration.
Biocide Treatment and Storage
The balance between keeping water drinkable and ensuring that storage containers do not harbor an environment favorable to microbial growth is a confounding problem in life support system architecture. Efforts to find a water additive that allows continued human consumption of stored water while preventing biofilm generation and bacterial contamination at the point-of-use have daunted space programs since the 1950s. Initial short-duration flights did not require extended stays on-orbit, and so, water supplies were kept simple and unrefined, utilizing local terrestrial sources near the spacecraft launch site. For the United States space program, local water supplies were kept relatively free of microbial growth by using municipal tap water that utilized standard chlorination, or chlorine injections into the potable water storage tanks (Willis and Schultz 1987). As the length of duration of on-orbit stays extended from days to weeks, efforts to keep water containers microbe-free began to include additives such as iodine for US water supplies, and colloidal silver for Russian cosmonauts (Yeoman et al. 2008).
Unfortunately, due to sporadic human sensitivity to iodine, NASA was forced to take measures to remove iodine from stored water supplies via the Potable Water Dispenser (PWD) on ISS by incorporation of an Activated Carbon/Ion Exchange (ACTEX) filter at the point-of-use (Shaw and Barreda 2008). Since the Russian segment and USOS utilize different biocide treatments to their water supplies, it has led to a divergence of water storage strategies on ISS, as well as complications when either side of the station must compensate the other for incidental water deficits (Carter et al. 2013). For future systems, NASA’s Advanced Exploration Systems (AES) Life Support Systems (LSS) project is considering a silver-based disinfection system for potable water processing and storage that utilizes a silver dosing unit (Schneider and Shull 2017). While this appears to be the leading long-term technique to maintain stored water quality, the use of both iodine and silver currently on ISS will have to be remedied.
Currently, the water supply system for spacecraft and habitats intended to operate within planetary habitats such as Mars (BLSS or otherwise) has yet to be defined (Jones et al. 2016). The current means for maintaining a stored water supply in space is through the use of Contingency Water Containers (CWCs) aboard ISS, which are 44 liter collapsible water bladders that have been in use since the space shuttle era (Mudgett et al. 2002). The CWCs have a limited storage life for potable water, as they often become entrained with free gas that must be manually expelled, and are certified to store iodinated water for up to 44 months (Carter et al. 2013). As of 2016, NASA has begun work on a Water Storage System (WSS) for ISS to expand the ability of the station to compensate for incidental water imbalances between the Russian segment and the USOS. However, the WSS does not levy new technologies and, instead, required the salvaging of water tanks from the retired Space Shuttles Endeavor and Atlantis (Carter et al. 2016). The shuttle tanks were metal bellows tanks, and while the Orion spacecraft is intended to use similar tanks, they will be incorporated into the service module (SM) portion of the spacecraft and, as such, are a one-mission use as they will be discarded upon Earth re-entry (Peterson and Callahan 2007). Therefore, development of long-term water storage systems for BLSS and other long-duration human habitats should be considered a priority for life support mission planning.
Review and Discussion
For the past 70 years of spacecraft and extraterrestrial habitat design research, little emphasis was placed on the requirements for extended human presence in space with regard to maintaining a viable and recyclable water supply for crew and passengers. Most life support systems currently in operation have maintenance requirements that are burdened by high ESM consumable resources such as replacement membranes and absorption filters, all of which require down-massing of waste material such as brine residue, used media, and spent catalysts. While such requirements will undoubtedly still be needed for any future long-duration human habitat, reducing their mass and increasing lifetime of consumables resources needed to maintain ECLSS operations should be a high priority for designers and engineers. This could be accomplished by augmenting current closed-loop life support system technology with regenerative bioreactor technology that makes use of as much waste material as feasible within the boundary architecture of the habitat or spacecraft.
BLSS shows promise as a means to convert wastewater back into usable water by balancing water recycling, air revitalization, and waste processing functions so that it increases closure to the highest extent possible for achieving a “break-even” point between mass input and mass output. This is achieved through multiple reaction chambers that maintain carefully controlled biological communities of microbes and bacteria that efficiently digest composted human wastes and sewage into usable nutrients for higher plants that provide edible biomass to a human crew. Carbon, nitrogen, oxygen, and hydrogen, along with a multitude of other elements vital to organic life, are transformed and transported via biochemical pathways between the biotic and abiotic compartments of BLSS to achieve a higher level of life support closure with as little mass input as possible. While no level of closure is 100%, and there will still be down-mass requirements outside the habitable environment, extended duration of human life support functions beyond what is currently deployed in space are feasible and within reach for future destinations away from Earth.
Although BLSS is best suited to planetary gravity operations due to difficulties in separating dissolved gaseous from liquid phase water in microgravity, technologies exist that permit at least a small level of biological waste processing without the need for gravitational stratification and settling. Whether their use is deployed as primary, secondary, or tertiary wastewater processing, the biological functions of their either microgravity or planetary-based BLSS will have to be separated from final point-of-use chemical/photo-reactive treatment of product water in order to preserve the synergistic microbial and plant communities. These biocide treatment methods will be crucial and necessary for long-term storage of recycled water supplies, and any future water container development should take into account biocide factors in their designs.
As the age of space exploration reaches its next level of evolution beyond low-Earth orbit, demand for viable long-duration space habitats will only increase, necessitating the development of effective water treatment methods that allow for low equipment and resupply mass while maximizing use of all waste constituents. Biological means for achieving a high level of recycled water purity for human consumption and oxygen generation must be linked together in an effective manner with current space-capable water treatment technologies. While a vast pool of experimental data exists to accomplish this integration, future biologists, engineers, and system designers will ultimately have to rely upon augmentation of tried-and-tested terrestrial wastewater systems for establishing a permanent human presence in space.
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