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Biological filters and their use in potable water filtration systems in spaceflight conditions
Accepted Manuscript
Biological filters and their use in potable water filtration systems in spaceflight conditions Starla G. Thornhill , Manish Kumar PII: DOI: Reference:
S2214-5524(17)30105-0 10.1016/j.lssr.2018.03.003 LSSR 169
To appear in:
Life Sciences in Space Research
Received date: Revised date: Accepted date:
14 October 2017 16 February 2018 4 March 2018
Please cite this article as: Starla G. Thornhill , Manish Kumar , Biological filters and their use in potable water filtration systems in spaceflight conditions, Life Sciences in Space Research (2018), doi: 10.1016/j.lssr.2018.03.003
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Biological filters and their use in potable water filtration systems in spaceflight conditions
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Starla G. Thornhill and Manish Kumar* Department of Biology, Texas State university, San Marcos, Texas -78666, USA Starla G. Thornhill email: [email protected] *Corresponding author
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Dr. Manish Kumar Tel: 1-512-245-8968 Fax: 1-512-245-8713
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Email: [email protected]
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Abstract Providing drinking water to space missions such as the International Space
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Station (ISS) is a costly requirement for human habitation. To limit the costs of water transport, wastewater is collected and purified using a variety of physical and chemical means. To date, sand-based biofilters have been designed to
function against gravity, and biofilms have been shown to form in microgravity
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conditions. Development of a universal silver-recycling biological filter system that is able to function in both microgravity and full gravity conditions would
reduce the costs incurred in removing organic contaminants from wastewater by limiting the energy and chemical inputs required. This paper aims to propose the
on manned spaceflights.
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use of a sand-substrate biofilter to replace chemical means of water purification
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Keywords: Biofilters, microgravity, manned space flight, life support systems,
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biological filters, water filtration systems
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Abbreviations EPS: extracellular polymeric matrix, GAC: granular activated charcoal, OM: organic material, PVP: polyvinylpyrrolidone, AgNP: silver
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nanoparticles, LSMMG: low shear modeled microgravity, RWV: rotating wall vessel, HARV: high aspect ratio vessel
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1. Introduction On manned space missions such as the International Space Station (ISS),
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transportation of materials, including drinking water, is prohibitively costly at approximately $10,000 per pound of payload (Jones, 2015). To limit transport costs for water delivery, wastewater in the form of sewage waste and ambient
humidity, caused by astronaut sweat and breath, is collected and purified to serve
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as drinking water (Shaw and Barreda, 2008). There are two water filtration systems on ISS. The Russian Potable Water Dispenser (PWD) (Figure 1)
equipped on the Zvezda module provides support for a crew of three and serves as the backup filtration system (Shaw and Barreda, 2008). The American
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Environmental Control and Life Support System (ECLSS), equipped on the Tranquility module provides drinking support for six crew members (Shaw and
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Barreda, 2008). Both systems function by utilizing chemical and physical
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methods of treatment (Carrasquillo, 2005). Chemical methods include iodine sterilization (Wong et al., 2010), ion exchange chromatography, and catalytic
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oxidation (Carrasquillo, 2005), while physical methods include removal of
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microorganisms using 0.2 micron filters (Wong et al., 2010). Though the current methods are generally effective at wastewater clean-up
(Wong et al., 2010; Pierson et al., 2012), supplies must be replenished periodically to continuously provide a supply of clean drinking water to the crew in residence. At the cost of $10,000 per pound, transport of treatment chemicals
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and filters is costly. A larger crew could be supported by providing an additional water filtration system which would incur increased costs in transportation of
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chemical treatments and replacement filters. In addition to the significant financial requirements for water treatment, iodine as a sterilization method poses a health risk to potential astronauts as a causative agent for hyperthyroidism and
allergic (sensitivity) reactions (Zareba et al., 1995). Biofilters have been used for
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centuries to provide clean water for public use (Huisman and Wood, 1974) and
are a viable and cost-effective option for replacement of chemical means of water purification. In addition to the limited crew support, there is a biofilm in the outflow pipe downstream of current treatment and filtration methods that has
Bornemann et al., 2015).
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persisted after multiple sterilization attempts (Mermel, 2012; Pierson et al., 2012;
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2. Biological Filters
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Biofilms are surface-associated communities of bacteria and other microorganisms surrounded by an extracellular polymeric matrix (EPS). This EPS
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acts as a barrier between the microbes and the environment, protecting them from stressors and allowing sequestration of nutrients (Flemming et al., 2007).
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Biological filters (biofilters) containing biofilms have been synthesized using various substrates including sand, soil (Healy et al, 2010), bark, charcoal (Dalameh et al., 2014), and granular activated charcoal (GAC) (Gibert et al., 2013) as the biofilm attachment surface. Biofilters first came into use in 1804,
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when John Gibb of Paisley, Scotland designed a sand filter with the purpose of selling his waste water to the public (Huisman and Wood, 1974). By 1830, the
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practice had been adopted by Chelsea Water Company, creating the first historical instance of publically available treated water (Huisman and Wood, 1974). Substrates such as sand in biofilters like Gibb’s function by acting as an
attachment surface for biofilms. In a traditional biofilter, substrate is packed into a
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column and seeded with activated sludge: a community of microbes that are
capable of degrading organic contaminants, including phosphorous-, nitrogen-, and sulfur-containing compounds. In addition to removing organic material, biofilters have also been found to remove over 90% of the initial contaminant
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bacterial load (Healy et al., 2010).
Each of the various materials that can serve as a substrate for biofilm
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growth have benefits and drawbacks for use in spaceflight. Bark and activated
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charcoal substrates have been shown to support greater species diversity than sand substrates, leading to removal of more contaminating material. Unfortunately,
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bark substrate runs the risk of leeching other organic materials into the water, and granular charcoal could potentially pose a fire risk on ISS. Marble is non-
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flammable, biologically inert, and has been shown to be an effective substrate, but sand shares those same qualities and results in more biofilm formation, likely due to the increased surface area (Kadaverugu et al., 2016).
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Sand-based biological filters have been shown to be effective in removing various types of organic material (OM) and in denitrification of wastewater (Vega
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et al., 2014), including removal of compounds that tend to be recalcitrant or persistent in water that has been treated by activated sludge (Escola et al., 2015). Biofilms using sand filters are also effective in removing phosphates, sulfates, nitrates, and nitrites, as well as fecal coliforms (Bai et al., 2013; Kahn et al.,
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2015). Alkalinity has also been found to be reduced, which is important in
clearing urine-containing wastewater since the reduction of nitrogen is inhibited under strongly alkaline conditions (Kahn et al., 2015). In addition, as mixed communities, biofilms occupying biofilters are able to be adaptable based on the
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composition of waste water and can result in a more robust purification of variable types of waste than chemical means of filtration (Bai et al., 2013).
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3. Biofilm Growth in Spaceflight Conditions
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The most effective way to seed a biofilter is using activated sludge. While certain species have been shown to remove specific contaminants completely
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(Zayadan et al., 2014; Alegbeleye et al., 2016; Ruffell et al., 2016; Xiao et al., 2016; Zhao et al., 2016) studies have shown that greater species richness is
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correlated with increased removal of contaminant material (Dalameh et al., 2014). Activated sludge is produced in large water treatment facilities, consisting of a vast variety of organisms sourced from the wastewater, and is commonly utilized in municipal water filtration systems. One of the largest difficulties in using a
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biological system as a method of filtration in space is the low gravity environment, termed microgravity. For water filtration in a microgravity
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environment, activated sludge can potentially be grown in media representative of the wastewater generated on ISS and serially passaged in modeled microgravity
bioreactors (Schwarz et al., 1992) until the community stabilizes. This will allow
for strains that are stable in microgravitational conditions to dominate the biofilm,
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providing the opportunity for scientists to study and potentially modify those organisms to ensure more complete purification of wastewater. Adapting the biofilm prior to spaceflight will also reduce the likelihood and effect of community changes once microgravity is established.
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Information on the ability of bacteria to form biofilms under spaceflight conditions is very sparse, though some studies have demonstrated the formation
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of biofilms in space (McLean et al., 2001; Rosenzweig et al., 2009; Kim et al.,
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2013) and in monocultured species that are capable of growth in microgravity (Rosenzweig et al., 2009; Castro et al., 2011).
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4. PVP Coated Silver Nanoparticles as Antibiotics It has long been known that silver has antimicrobial properties (Rogers,
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1913). Recently, it has been found that this antibiotic activity is also active in spaceflight conditions (Wong et al., 2010). Polyvinylpyrrolidone (PVP) coated (“capped”) silver nanoparticles (AgNP) also display antibiotic properties (Bryaskova et al., 2011) against a wide range of bacterial species (Bryaskova et
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al., 2011; Bahtia et al., 2016) by rapid release of the colloidal silver at bactericidal concentrations into the environment (Stobie et al., 2008; Bryaskova et al., 2011;
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Bhatia et al., 2016). Importantly, release of colloidal silver occurs without releasing the entire delivered volume of AgNP (Dobias and Bernier-Latmani,
2013). In addition to antibacterial activity, PVP coated AgNP has also been found to have antifungal activity (Pereira et al., 2014; Silva et al., 2014). Of particular
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importance, PVP capped AgNPs are capable of passing through biofilters intact, with little adsorption to the biofilm (Li et al., 2013). Unreleased AgNPs are also
able to be recovered from solution by mixing AgNP with magnetic nanoparticles and applying a magnetic field (Mwilu et al., 2014).
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5. Proposal for Biofilter Design
We propose the design of a biofilter capable of functioning in a
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microgravity environment using the above principles. Activated sludge sourced
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from a local water treatment plant should be inoculated into a low-shear modeled microgravity (LSMMG) bioreactor to select for organisms capable of growth in
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microgravitational conditions. The LSMMG bioreactor is a rotating wall vessel (RWV) that rotates at a set rate to establish solid body rotation of a liquid media,
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possible due to the low shear environment in the High Aspect Ratio Vessel (HARV) (Nickerson et al., 2014). Incubation of the activated sludge will allow the community of microorganisms to adjust to the modeled microgravity conditions. Organisms that are incapable of growth in modeled microgravity will
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die off, allowing the organisms capable of growth to flourish. The adapted activated sludge should be used to seed a packed sand column. Additionally, the
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sand column would need to be designed with a fine mesh screen on either end of the column to prevent sand from passing out of the column. Wastewater should be passed through the column until the biofilm is established, approximately one month.
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In the assembled biofiltration system, the wastewater, from which large
particulate matter has already been removed, would pass through the intake pipe to the AgNP introduction point, where PVP-coated silver nanoparticles would be added to the wastewater. Immediately after addition of AgNP, the water will flow
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through the biofilter at an experimentally derived rate that is maintained by pressurizing the water flow. The cleaned water will exit the biofilm, and the
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AgNP will release some of the silver, allowing sanitization of the water. Magnetic
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nanoparticles will be released into the water, which will sequester the AgNP. A magnet on one side of the outtake pipe will then collect the aggregates of AgNP
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and magnetic nanoparticles and direct them to a recycling pipe that will collect the AgNP. The aggregates will flow into a holding container filled with a non-
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polar buffer that will allow the separation the AgNP from the magnetic nanoparticles, as well as stabilize the PVP to preserve the AgNP. The magnetic particles will be recycled back into the outtake pipe to collect more AgNP, and the AgNP will be recycled back to the intake pipe to treat more wastewater
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(Figure 2). Released colloidal silver will travel with the treated water to a secondary “recoating” biofilter seeded with bacteria capable of recoating colloidal
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silver with PVP, such as Lysinibacillus varians (Bhatia et al., 2016). This biofilter would rotate, applying a centrifugal force to the recoated magnetic particles (Siekmann and Johann, 1976), allowing them to move to the borders of the biofilter where they can be recollected magnetically by residual magnetic
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nanoparticles.
In addition, careful engineering of the orientation of components to switch to filtration based on the gravitational pressure exerted on a planetary surface would result in a filtration system that can be removed from a travel rocket and be
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implemented in a permanent shelter, particularly useful in future colonization efforts for other planets like Mars. In this case, the system should be designed so
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that once it is present on land, in gravity, the pressurizing input can be powered
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down, allowing the slow sand filter to work via gravity. This would reduce the energetic input required to produce clean drinking water in newly established
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colonies. This would also allow for the size of the crew able to be supported on each colonizing mission to be increased with each rocket that arrives to the
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colony. This type of biofilter should reduce operating costs for spaceflight missions by 1) removing the need for membrane-type filters that require frequent changing, 2) eliminating energy-intensive methods of wastewater cleaning, and 3) preventing the requirement for iodine sterilization or other single-use chemical
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methods or treatment. This type of water filter would require less maintenance and fewer changes of replaceable parts, like physical filters, while providing a
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safe, purified source of drinking water for both ISS and any future space-flight missions. Acknowledgements
We would like to thank RCJ McLean and the rest of the Slime Gang for their
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support.
Funding: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Author Disclosure Statement: No competing financial interests exist.
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Figure Legends
Figure 1. Schematic of the PWD on-board the ISS. Overview of the design of
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the Russian PWD currently supplying potable water on-board ISS. Courtesy
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NASA. (One column)
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Figure 2. Theoretical proposal of new biofilter for spaceflight. Movement of
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water through the system is indicated by dashed arrows. (Two column)
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Biological filters and their use in potable water filtration systems in spaceflight conditions Starla G. Thornhill , Manish Kumar PII: DOI: Reference:
S2214-5524(17)30105-0 10.1016/j.lssr.2018.03.003 LSSR 169
To appear in:
Life Sciences in Space Research
Received date: Revised date: Accepted date:
14 October 2017 16 February 2018 4 March 2018
Please cite this article as: Starla G. Thornhill , Manish Kumar , Biological filters and their use in potable water filtration systems in spaceflight conditions, Life Sciences in Space Research (2018), doi: 10.1016/j.lssr.2018.03.003
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Biological filters and their use in potable water filtration systems in spaceflight conditions
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Starla G. Thornhill and Manish Kumar* Department of Biology, Texas State university, San Marcos, Texas -78666, USA Starla G. Thornhill email: [email protected] *Corresponding author
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Dr. Manish Kumar Tel: 1-512-245-8968 Fax: 1-512-245-8713
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Email: [email protected]
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Abstract Providing drinking water to space missions such as the International Space
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Station (ISS) is a costly requirement for human habitation. To limit the costs of water transport, wastewater is collected and purified using a variety of physical and chemical means. To date, sand-based biofilters have been designed to
function against gravity, and biofilms have been shown to form in microgravity
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conditions. Development of a universal silver-recycling biological filter system that is able to function in both microgravity and full gravity conditions would
reduce the costs incurred in removing organic contaminants from wastewater by limiting the energy and chemical inputs required. This paper aims to propose the
on manned spaceflights.
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use of a sand-substrate biofilter to replace chemical means of water purification
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Keywords: Biofilters, microgravity, manned space flight, life support systems,
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biological filters, water filtration systems
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Abbreviations EPS: extracellular polymeric matrix, GAC: granular activated charcoal, OM: organic material, PVP: polyvinylpyrrolidone, AgNP: silver
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nanoparticles, LSMMG: low shear modeled microgravity, RWV: rotating wall vessel, HARV: high aspect ratio vessel
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1. Introduction On manned space missions such as the International Space Station (ISS),
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transportation of materials, including drinking water, is prohibitively costly at approximately $10,000 per pound of payload (Jones, 2015). To limit transport costs for water delivery, wastewater in the form of sewage waste and ambient
humidity, caused by astronaut sweat and breath, is collected and purified to serve
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as drinking water (Shaw and Barreda, 2008). There are two water filtration systems on ISS. The Russian Potable Water Dispenser (PWD) (Figure 1)
equipped on the Zvezda module provides support for a crew of three and serves as the backup filtration system (Shaw and Barreda, 2008). The American
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Environmental Control and Life Support System (ECLSS), equipped on the Tranquility module provides drinking support for six crew members (Shaw and
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Barreda, 2008). Both systems function by utilizing chemical and physical
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methods of treatment (Carrasquillo, 2005). Chemical methods include iodine sterilization (Wong et al., 2010), ion exchange chromatography, and catalytic
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oxidation (Carrasquillo, 2005), while physical methods include removal of
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microorganisms using 0.2 micron filters (Wong et al., 2010). Though the current methods are generally effective at wastewater clean-up
(Wong et al., 2010; Pierson et al., 2012), supplies must be replenished periodically to continuously provide a supply of clean drinking water to the crew in residence. At the cost of $10,000 per pound, transport of treatment chemicals
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and filters is costly. A larger crew could be supported by providing an additional water filtration system which would incur increased costs in transportation of
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chemical treatments and replacement filters. In addition to the significant financial requirements for water treatment, iodine as a sterilization method poses a health risk to potential astronauts as a causative agent for hyperthyroidism and
allergic (sensitivity) reactions (Zareba et al., 1995). Biofilters have been used for
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centuries to provide clean water for public use (Huisman and Wood, 1974) and
are a viable and cost-effective option for replacement of chemical means of water purification. In addition to the limited crew support, there is a biofilm in the outflow pipe downstream of current treatment and filtration methods that has
Bornemann et al., 2015).
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persisted after multiple sterilization attempts (Mermel, 2012; Pierson et al., 2012;
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2. Biological Filters
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Biofilms are surface-associated communities of bacteria and other microorganisms surrounded by an extracellular polymeric matrix (EPS). This EPS
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acts as a barrier between the microbes and the environment, protecting them from stressors and allowing sequestration of nutrients (Flemming et al., 2007).
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Biological filters (biofilters) containing biofilms have been synthesized using various substrates including sand, soil (Healy et al, 2010), bark, charcoal (Dalameh et al., 2014), and granular activated charcoal (GAC) (Gibert et al., 2013) as the biofilm attachment surface. Biofilters first came into use in 1804,
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when John Gibb of Paisley, Scotland designed a sand filter with the purpose of selling his waste water to the public (Huisman and Wood, 1974). By 1830, the
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practice had been adopted by Chelsea Water Company, creating the first historical instance of publically available treated water (Huisman and Wood, 1974). Substrates such as sand in biofilters like Gibb’s function by acting as an
attachment surface for biofilms. In a traditional biofilter, substrate is packed into a
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column and seeded with activated sludge: a community of microbes that are
capable of degrading organic contaminants, including phosphorous-, nitrogen-, and sulfur-containing compounds. In addition to removing organic material, biofilters have also been found to remove over 90% of the initial contaminant
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bacterial load (Healy et al., 2010).
Each of the various materials that can serve as a substrate for biofilm
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growth have benefits and drawbacks for use in spaceflight. Bark and activated
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charcoal substrates have been shown to support greater species diversity than sand substrates, leading to removal of more contaminating material. Unfortunately,
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bark substrate runs the risk of leeching other organic materials into the water, and granular charcoal could potentially pose a fire risk on ISS. Marble is non-
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flammable, biologically inert, and has been shown to be an effective substrate, but sand shares those same qualities and results in more biofilm formation, likely due to the increased surface area (Kadaverugu et al., 2016).
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Sand-based biological filters have been shown to be effective in removing various types of organic material (OM) and in denitrification of wastewater (Vega
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et al., 2014), including removal of compounds that tend to be recalcitrant or persistent in water that has been treated by activated sludge (Escola et al., 2015). Biofilms using sand filters are also effective in removing phosphates, sulfates, nitrates, and nitrites, as well as fecal coliforms (Bai et al., 2013; Kahn et al.,
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2015). Alkalinity has also been found to be reduced, which is important in
clearing urine-containing wastewater since the reduction of nitrogen is inhibited under strongly alkaline conditions (Kahn et al., 2015). In addition, as mixed communities, biofilms occupying biofilters are able to be adaptable based on the
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composition of waste water and can result in a more robust purification of variable types of waste than chemical means of filtration (Bai et al., 2013).
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3. Biofilm Growth in Spaceflight Conditions
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The most effective way to seed a biofilter is using activated sludge. While certain species have been shown to remove specific contaminants completely
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(Zayadan et al., 2014; Alegbeleye et al., 2016; Ruffell et al., 2016; Xiao et al., 2016; Zhao et al., 2016) studies have shown that greater species richness is
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correlated with increased removal of contaminant material (Dalameh et al., 2014). Activated sludge is produced in large water treatment facilities, consisting of a vast variety of organisms sourced from the wastewater, and is commonly utilized in municipal water filtration systems. One of the largest difficulties in using a
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biological system as a method of filtration in space is the low gravity environment, termed microgravity. For water filtration in a microgravity
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environment, activated sludge can potentially be grown in media representative of the wastewater generated on ISS and serially passaged in modeled microgravity
bioreactors (Schwarz et al., 1992) until the community stabilizes. This will allow
for strains that are stable in microgravitational conditions to dominate the biofilm,
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providing the opportunity for scientists to study and potentially modify those organisms to ensure more complete purification of wastewater. Adapting the biofilm prior to spaceflight will also reduce the likelihood and effect of community changes once microgravity is established.
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Information on the ability of bacteria to form biofilms under spaceflight conditions is very sparse, though some studies have demonstrated the formation
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of biofilms in space (McLean et al., 2001; Rosenzweig et al., 2009; Kim et al.,
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2013) and in monocultured species that are capable of growth in microgravity (Rosenzweig et al., 2009; Castro et al., 2011).
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4. PVP Coated Silver Nanoparticles as Antibiotics It has long been known that silver has antimicrobial properties (Rogers,
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1913). Recently, it has been found that this antibiotic activity is also active in spaceflight conditions (Wong et al., 2010). Polyvinylpyrrolidone (PVP) coated (“capped”) silver nanoparticles (AgNP) also display antibiotic properties (Bryaskova et al., 2011) against a wide range of bacterial species (Bryaskova et
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al., 2011; Bahtia et al., 2016) by rapid release of the colloidal silver at bactericidal concentrations into the environment (Stobie et al., 2008; Bryaskova et al., 2011;
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Bhatia et al., 2016). Importantly, release of colloidal silver occurs without releasing the entire delivered volume of AgNP (Dobias and Bernier-Latmani,
2013). In addition to antibacterial activity, PVP coated AgNP has also been found to have antifungal activity (Pereira et al., 2014; Silva et al., 2014). Of particular
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importance, PVP capped AgNPs are capable of passing through biofilters intact, with little adsorption to the biofilm (Li et al., 2013). Unreleased AgNPs are also
able to be recovered from solution by mixing AgNP with magnetic nanoparticles and applying a magnetic field (Mwilu et al., 2014).
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5. Proposal for Biofilter Design
We propose the design of a biofilter capable of functioning in a
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microgravity environment using the above principles. Activated sludge sourced
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from a local water treatment plant should be inoculated into a low-shear modeled microgravity (LSMMG) bioreactor to select for organisms capable of growth in
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microgravitational conditions. The LSMMG bioreactor is a rotating wall vessel (RWV) that rotates at a set rate to establish solid body rotation of a liquid media,
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possible due to the low shear environment in the High Aspect Ratio Vessel (HARV) (Nickerson et al., 2014). Incubation of the activated sludge will allow the community of microorganisms to adjust to the modeled microgravity conditions. Organisms that are incapable of growth in modeled microgravity will
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die off, allowing the organisms capable of growth to flourish. The adapted activated sludge should be used to seed a packed sand column. Additionally, the
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sand column would need to be designed with a fine mesh screen on either end of the column to prevent sand from passing out of the column. Wastewater should be passed through the column until the biofilm is established, approximately one month.
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In the assembled biofiltration system, the wastewater, from which large
particulate matter has already been removed, would pass through the intake pipe to the AgNP introduction point, where PVP-coated silver nanoparticles would be added to the wastewater. Immediately after addition of AgNP, the water will flow
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through the biofilter at an experimentally derived rate that is maintained by pressurizing the water flow. The cleaned water will exit the biofilm, and the
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AgNP will release some of the silver, allowing sanitization of the water. Magnetic
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nanoparticles will be released into the water, which will sequester the AgNP. A magnet on one side of the outtake pipe will then collect the aggregates of AgNP
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and magnetic nanoparticles and direct them to a recycling pipe that will collect the AgNP. The aggregates will flow into a holding container filled with a non-
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polar buffer that will allow the separation the AgNP from the magnetic nanoparticles, as well as stabilize the PVP to preserve the AgNP. The magnetic particles will be recycled back into the outtake pipe to collect more AgNP, and the AgNP will be recycled back to the intake pipe to treat more wastewater
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(Figure 2). Released colloidal silver will travel with the treated water to a secondary “recoating” biofilter seeded with bacteria capable of recoating colloidal
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silver with PVP, such as Lysinibacillus varians (Bhatia et al., 2016). This biofilter would rotate, applying a centrifugal force to the recoated magnetic particles (Siekmann and Johann, 1976), allowing them to move to the borders of the biofilter where they can be recollected magnetically by residual magnetic
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nanoparticles.
In addition, careful engineering of the orientation of components to switch to filtration based on the gravitational pressure exerted on a planetary surface would result in a filtration system that can be removed from a travel rocket and be
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implemented in a permanent shelter, particularly useful in future colonization efforts for other planets like Mars. In this case, the system should be designed so
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that once it is present on land, in gravity, the pressurizing input can be powered
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down, allowing the slow sand filter to work via gravity. This would reduce the energetic input required to produce clean drinking water in newly established
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colonies. This would also allow for the size of the crew able to be supported on each colonizing mission to be increased with each rocket that arrives to the
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colony. This type of biofilter should reduce operating costs for spaceflight missions by 1) removing the need for membrane-type filters that require frequent changing, 2) eliminating energy-intensive methods of wastewater cleaning, and 3) preventing the requirement for iodine sterilization or other single-use chemical
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methods or treatment. This type of water filter would require less maintenance and fewer changes of replaceable parts, like physical filters, while providing a
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safe, purified source of drinking water for both ISS and any future space-flight missions. Acknowledgements
We would like to thank RCJ McLean and the rest of the Slime Gang for their
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support.
Funding: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Author Disclosure Statement: No competing financial interests exist.
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Figure Legends
Figure 1. Schematic of the PWD on-board the ISS. Overview of the design of
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the Russian PWD currently supplying potable water on-board ISS. Courtesy
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NASA. (One column)
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Figure 2. Theoretical proposal of new biofilter for spaceflight. Movement of
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water through the system is indicated by dashed arrows. (Two column)
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