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Applied Design

Considering alternative applications of human feces on Mars

The current solid human waste management system on the International Space Station (ISS) includes dehydration and storage. This means that the system attempts to recover all the moisture from feces and afterward stores the remaining solid material in the vacuum bags which are either thrown out into space, where the solids will burn upon re-entry into the Earth's atmosphere, or shipped back to Earth. It is essential to recover as much liquid from the feces as possible, as water is a precious resource in space and should ideally be fully recycled.

A few applications for the use of feces on Mars are being considered; however, no system for complete recycling of water and nutrients from human waste has been proposed yet. Dr. Pascal Lee (NASA Ames Research Centre) mentioned that one major proposed application for human feces on Mars is ionic radiation shielding. Feces are suitable for this purpose since they contain many hydrogen atoms which create a neutron shield. Keeping these comments in mind, we have proposed to use torrefaction (mild pyrolysis) as a way to process the solid remains from solid-liquid separation . The advantage of employing torrefaction as our solid by-product treatment process is the production of chemically stable char, which does not support biological activity and can be used as radiation shielding, building material, and substrate for food production, which are other proposed applications for solid human waste, and as a storage of carbon and hydrogen. Additionally, torrefaction recovers pyrolytic water (chemically bound water), resulting in greater overall water recovery, which is essential to close the water system loop.

By considering other applications for solid human waste, we were able to better integrate our project with NASA's plans.

Developing the process specifically for space travel

Having the goal for developing the process for Mars, we decided to contact professionals in the space industry to get their advice and guidance. Col. Chris Hadfield (former CSA astronaut) and Dr. Matthew Bamsey (runner-up to become a Canadian astronaut and Chief Systems Engineer at the German Aerospace Center) have both highlighted the importance of stating our assumptions regarding the environment, capacity, human waste generation rates, mission duration, and power availability. For more information on how we used their advice please visit the Integrated Human Practices page.

In order to gain a better understanding of space travel, mission parameters, and general functioning of the human body, the engineering students on the team examined the Life Support Systems Baseline Values and Assumptions Document (BVAD) (Anderson et al., 2015) which was recommended to us by Dr. Bamsey. The document was developed by the National Aeronautics and Space Administration (NASA) in order to provide a set of guidelines to be used by researchers, thus allowing the proposed systems to be easily comparable and ensure that they are capable of withstanding the same duration, loads, and environmental conditions.

In order to develop a solution for Mars, it was crucial to take into account the comments from consulted professionals:

Analysis of the existing solid human waste management processes in space is important to ensure that the proposed system doesn’t compromise other applications of waste
Consideration of the volumetric, power and weight footprint was crucial
Creation of an easily accessible and maintainable system was necessary
Ensuring automatization would improve ease of use and save astronaut's time

Assumptions developed by the Process Development team

Crew Size: The maximum (6 people) crew size from the BVAD is used for our calculations.

Duration: Our system is developed for the Nominal Surface habitat duration from the BVAD: 600 days (Anderson et al., 2015)

Table 1: Martian missions duration (adapted from Anderson et al., 2015)

Feces Production: The maximum value for the feces mass (150 g/CM-d) and volume (150 mL/CM-d) was used to design the system parameters and load capacity, while the nominal value (123g/CM-d and 123ml/CM-d) is used to make predictions for water recovery, VFA production and PHB production (Anderson et al., 2015).

Note: CM-d denotes crew member per day.

Table 2: Average production of feces per crew member per day. (adapted from Anderson et al., 2015)

	Minimal	Nominal	Maximum
Mass (g/CM-d)	95.5	123	150
Volume (mL/CM-d)	95.5	123	150

Energy Availability: Another important consideration suggested by both Dr. Pascal Lee and Dr. Bamsey was the power sources available on Mars. It was noted that the power requirement of our system should be attainable using the current power production technology developed for Mars. It should be no larger than the power requirements of other systems currently used on the ISS or suggested for the Martian application.

Based on BVAD (Anderson et al., 2015), current solar technologies can provide 28-100 kW of electricity. The nuclear solutions have the power output ranging from 16-550 kW. We can assume 100 kW of electricity to be generated by one solar panel. However, Dr. Lee had assured the team that assuming the availability of nuclear power generator on Mars is valid since their high energy production would eventually be employed on Mars.

Process component selection criteria

The proposed PHB production system cannot be imagined as a single component system, hence it was important to create a way of comparing different proposed processes and evaluate the feasibility of implementing them in space and on Mars. To do so, the team employed the Equivalent System Mass (ESM) Guidelines (Jones et al., 2016).

ESM is the tool often used by NASA to evaluate advanced life support systems or their individual components. It allows one to convert parameters like power, cooling requirements, volume, and crew time commitment to a single unit of mass (kg). This is achieved by multiplying each requirement and parameter by a given equivalency factor. The aim of the analysis is to provide each system in consideration with an ESM number in kg and then choose the system with a lower value since it would be a more economical and feasible solution. The ESM formula used by our team is a simplification of an original formula found in the ESM Guidelines document published by NASA:

We chose to remove the crew time parameter from the equation due to the difficulty in estimation and lack of data; however. we examined the maintenance requirements for all systems outside of the equation.

Table 3: Mars Mission Infrastructure “Costs” (assuming surface time parameters) (Jones et al., 2016)

ESM Equivalency Factors (Martian surface time)	Unit	Nominal Value
Shielded volume	kg/m³	216.5
Unshielded volume	kg/m³	9.16
Power	kg/kW	87
Thermal control	kg/kW	146
Crew time	kg/CM-h	0.466

According to the Baseline Values and Assumptions for Advanced Life Support Systems, the nominal long-term duration of Martian missions is 600 days, and therefore the ESM calculations would be performed with this duration in mind.

The other parameters which were considered when choosing a system component were ease of maintenance, spares and consumables requirements, and byproduct stability and usability. As such, the process was more favourable if it could be fully automated and if it would produce stable byproducts which could be used on Mars.

Feasibility of system implementation in space

The following table allows easy comparison of the power requirements of the system to the currently employed processes on Mars. It can be noted for example that the total power Environmental Control and Life Support System (ECLSS) is 5.31 kW assuming constant operation, while our safe estimation for the system power requirements is around 2.8 kW, with only 0.566 kW of power being used continuously.

Table 4: Power requirements of systems currently employed on Mars.

Design iterations

The design was constantly changing and evolving based on the experimental results, system requirements updates, and advice received from experts. Visit the Process pages to learn about iterations of different stages of the system.

Total system ESM Estimations

The following table summarizes the design parameters and calculates the total system ESM. The Data Sources tab outlines the origins of each design parameters.

Table 5: Calculations of the total system ESM

Process Stage	Mass (kg)	Consumables Mass (kg)	Power (kW)	Volume (m³)	Consumables Volume (m³)
Storage tank + pumps	56₍₁₎	0	1.8₍₂₎	0.049₍₃₎	0
VFA fermentater tank	20₍₄₎	0	0.02₍₅₎	0.008₍₆₎	0
Centrifugal separation +filter	25₍₇₎	0	2.237₍₈₎	0.0138₍₉₎	0
Stirred-tank bioreactor	15₍₁₀₎	0	0.38₍₁₁₎	0.1153₍₁₂₎	0
Self-cleaning filter	16₍₁₃₎	0	0.186₍₁₄₎	0.03₍₁₅₎	0
Extraction	41.5₍₁₆₎	0	0.6219₍₁₆₎	1.021₍₁₆₎	0
Totals:	173	0	5.245	0.237	0
ESM (kg)	897

(1), (2), (3), (4), (5), (6) Visit the VFA fermentation page for the detailed descrption of apparatus and the calculations for each of the parameters.

(7) Mass = mass of table top centrifuge (5 kg) + mass of the pump (30 kg)

(8) Russel Finex brochure on the centrifugal separators

(9) Russel Finex centrifugal separator brochure

(10) Soulre Xcellerex bioreactor Site Preparation Guide. The total mass of the whole system is 100 kg (including hardware, scales, cables and vessel) yet the instructions say that vessel can be lifted by hands, while the laptop needs to be mechanically placed (much heavier) --> the mass of the vessel is assumed to be 15 kg.

(11) 100 V * 3.8 A = 0.38 kW. Soulre Xcellerex stirred tank bioreactor Site Preparation Guide. The power includes the analytical equipment power.

(12) Soulre Xcellerex Bioreactor brochure

(13), (14), (15) Eaton Self-cleaning filter brochure, page 36.

(16) Visit the Extraction page for the ESM estimations calculations

The following table summarizes different International Space Station support systems and their ESM parameters and values. The table is provided for the comparison purposes.

Table 6: ISS support systems with their ESM parameters and values.

It can be noted that the estimated system ESM compares well to the other available systems on the International Space Station. For example, the total ESM of the Water Processor (WP) is 5,324 kg. The feces collection and treatment would be a subsystem of the process, meaning that it would only take a small fraction of the ESM value.

Hence, the estimated value of 897 kg seems realistic and feasible for application in space.

WORKS CITED

Anderson, M. S., Ewert, M. K., Keener, J. F., & Wagner, S. A. (2015). Life Support Baseline Values and Assumptions Document. Nasa/Tp-2015-218570, (March), 1–220. http://doi.org/CTSD-ADV-484 A

Jones, H., Fisher, J., Delzeit, L., Flynn, M., & Kliss, M. (2016). Developing the Water Supply System for Travel to Mars. Presented at the 46th International Conference on Environmental Systems

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