MABR on Mars?
As part of the NASA Big Idea Challenge 2019, a Mars life support habitat greenhouse was conceptualized by various university teams.
The summer interns selected by NASA then built out the winning concept further, and MABR was selected as part the liquid effluent treatment system.
Interesting that this makes sense to the next crop of NASA scientists, how long will it take for wastewater engineers back on earth to catch on?
Udi Tirosh from Fluence was also referenced at the bottom of the paper for the energy usage calculation.
[119] U. Tirosh, "Membrane Aerated Bioflim Reactor," Fluence Corporation, 2018.
3.11. Waste Processing
A bioregenerative life support system recovers resources from waste products. The combined MIH and
CYBELE systems will produce three major streams of waste: liquid human waste, solid human waste, and
solid inedible biomass. In addition to this, there will be greywater from hygiene processes and circulated
nutrient solution from the hydroponics system. The waste management mechanism is shown in Figure 15.
3.11.1. Human Liquid Waste and Greywater
Human liquid waste requires its own nutrient recovery process. The assumed rate of urine production
is 1.42 L d-1, with a total nitrogen content of 11g d-1.For a four-person crew, this translates to 44g N d-1,
with the majority of this being in the form of urea (NH2CONH2) [108].
Urea hydrolyzes into ammonia (NH3) and ammonium (NH4+) depending on the pH and temperature
[109]. While plants can use nitrogen in this form, it is inefficient and can be phytotoxic in high levels even
while correcting for pH [110].
In the interest of efficiency and safety, CYBELE uses a membrane-aerated bioreactor (MABR) to
further process ammonium into nitrate (NO3-), in addition to breaking down Dissolved Organic Carbon
(DOC) that often contributes to biofouling of downstream processes. MABR systems have been extensively
studied for application in human space exploration. A full-scale model, the Counter-diffusion Membrane
Aerated Nitrifying Denitrifying Reactor (CoMANDR) at the Texas Tech University has shown the ability
of such systems to process both urine and hygiene waste [111].
Being an aerated system, MABR consumes a significant amount of oxygen. According to Terada et al.
(2003), ammonia oxidizing bacteria (AOB) in MABR can consume 921 g O2 / day. In addition to
nitrification, MABR can simultaneously denitrify, producing N2 gas which can help maintain the pressure
of CYBELE [112]. The nitrification to denitrification ratio can be adjusted with pH and oxygen
concentration. It is unlikely that the denitrified nitrogen will be fixed back into the nutrient solution by the
soybean crops, whose symbiotic root nodulation infected with Bradyrhizobium japonicum will be inhibited
by the NO3- already present in the hydroponic system [110].
There are several challenges that need to be addressed before these systems can be applied in CYBELE.
It is possible that particular processes such as nitrification will require inoculation with ammonia-oxidizing
bacteria (AOB such as Nitrosomonas) and nitrite-oxidizing bacteria (NOB such as Nitrobacter), which are
slow-growing and require a ramp up period after dormancy [112]. This bioreactor could be established on
board the transit vehicle prior to crew arrival. However the mass, volume, and retention time associated
with such a bioreactor will need to be addressed. In addition, CoMANDR has shown nitrification
percentages greater than 60%, and primarily produced nitrites (NO2-) as opposed to NO3-. It is not
unreasonable, however, for the nitrification efficiency and the NO3- production rate of MABRs to improve,
potentially through synthetic biology methods of incorporating complete ammonia oxidation (comammox)
[109]. Comammox bacteria are capable of converting NH4+ into NO3- without an intermediary organism,
and could potentially decrease the level of denitrification that leads to nitrogen loss.
Alternatively, if MABR systems do not reach the appropriate Technology Readiness Level (TRL) for
CYBELE, the liquid waste management system could incorporate the novel ammonia regenerative system
from the Kennedy Space Center. This system uses Magnesium Phosphate dibasic trihydrate (MgHPO4),
which precipitates struvite from ammonia under standard temperature and pressure [113]:
MgHPO4⋅3 H2O (s) + NH3 (1) + 3 H2O (l) → MgNH4 PO4⋅6 H2O (s)
The process is regenerative in that the above reaction can be reversed to reclaim the ammonia and the
substrate at elevated temperatures:
MgNH4 PO4⋅6 H2O (s) → MgHPO4⋅3 H2O (s) + NH3 (g) + 3 H2O (g)
Once separated from the liquid waste stream, the recovered NH3 can be turned into NH4+ by adjusting
the temperature and pH, and then into NO3- via a compact axenic nitrification bioreactor. Given the highly
compact and regenerative nature of the system, the struvite regenerator could be activated onboard the
transit vehicle to begin nutrient recovery prior to CYBELE activation.
A significant challenge associated with urine processing for hydroponics is the elevated salt level.
According to Subbarao et al, Na+ concentrations of 100 mM are sufficient to kill most plants [114]. The
starting point of a typical Hoagland solution has 1.2 ppm of Na+, which is approximately 0.052 mM.
Throughout the 600-day duration of the mission, the crew of four is assumed to produce 400 mmol day-1 of
Na+[108]. As the nutrient solution volume is 16,300 L in CYBELE, this means that by the end of the
mission, the sodium concentration will increase by approximately 15 mM. As most of the crops in CYBELE
are not specifically salt-resistant, even concentrations of 25-50 mM can inhibit growth [115]. To minimize
risk, the crew should have a combination of a low-sodium diet and periodic usage of electro dialysis (ED)
or ion exchange to separate out Na+ from the rest of the salts [116]. Another technology to consider is a
combination of an Electro Deionization (EDI) process with an aluminum-based tribocharger to separate out
specific ions from dry brine based on their triboelectric properties [117]. This option is still in conceptual
phase and would need to be developed further for implementation.
The liquid waste processing system is shown in Figure 17. Liquid waste in the form of urine and
greywater is first collected in a pre-treatment tank, which allows for urea to hydrolyze into ammonia. The
effluent is then sent to a distillation tank, where a significant portion of the water is collected and polished
for use in the hydroponics system and MIH. The remaining effluent could either be sent through a struvite
regeneration system to recover ammonia, from which it will be fed to an axenic bioreactor of nitrifying
organisms, or fed directly into the MABR system. After being processed with an ion-exchange membrane,
the effluent will be sanitized with UV and corrected with a computer-controlled nutrient solution injector
into the hydroponics system.
Other liquid human waste in the form of greywater can be simultaneously processed in the above
system. Based on the NASA SPP 30262 Space Station ECLSS Architectural Control Document, there will
be 26 L / person of liquid waste in addition to the urine, in the form of flush water, hygiene, laundry/dish
water, and latent water in the system [118]. This will mean that including urine and greywater, the liquid
waste processing system will be processing on average 110 L d-1 of liquid waste from the MIH. The entire
estimated volume of the liquid waste processing system is approximately 1.5 m3, and is expected to be
stowed beneath the floor in the MIH as it also contains the human waste collection system for the mission.
The crew maintenance required for the system is expected to be minimal, primarily comprising of routine
checks on the influent/effluent tanks and sensors.
In terms of power consumption, the CYBELE MABR will use approximately 1 W continuously, given
that the industry standard is 0.267 kW/m3 for MABRs [119]. Based on verification with Lunar Palace – 1,
a closed environment human habitation experiment in China that employed membrane bioreactors and a
low pressure distillation system, the power consumption of the human liquid waste management system is
expected to be less than 500 W, with the majority of the demand coming from the low pressure distillation
system [120]. Further calculations will be required to determine the power consumption of the ammonia
regeneration system.
http://bigidea.nianet.org/wp-content/uploads/2019/10/CYBELE-final-2019-10-18.pdf
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