The Aquatic Habitat
On April 10th 2010, at the Kennedy Space Center, President Barack Obama pronounced his "Remarks on Space Exploration in the 21st Century." The President included closed-loop life support systems (LSS) as a technology that "can help improve daily lives of people here on Earth, as well as testing and improving upon capabilities in space." Researchers continue to develop and test regenerative life support technologies that may help reduce the frequency of resupply missions and presumably also reduce the cost of such space habitats in terms of logistics. An example is the commissioning of the Water Processing Assembly (WPA) in the U.S. segment of the International Space Station, which recycles waste liquids, including urine, back into potable water. One subset of regenerative technologies considered are bioregenerative life support systems (BLSS), which make use of biological processes to transform biological by-products back into consumables . An example of such research employs aquatic habitats as small-scale platforms for BLSS research . Aquatic habitats involve biological processes, such as photosynthesis, that regenerate life support resources, such as oxygen. Their reuse of a limited volume of water, their opportunity for isolation from the atmosphere, and their capacity to support life forms make them a candidate for the study of closed-loop life support systems (LSS).
Larger-scale proof of concept projects have been undertaken by public and private organizations to study the sustainability problems and issues that arise from integrating human participants within a variety of life support processes. The main challenge has been the development of subsystems and their integration in a single ecosystem. While some projects have tested single regenerative processes to recycle byproduct into consumables, others have established entire biomes and attempted their integration. Such is the case of the project Biosphere 2 in which, with a volume of 204,000 [m3], attempted to integrate six biomes and a human habitat for a crew of seven or eight participants. A series of experiments were performed in Biosphere 2 during 1991-1994 . Figure 12 shows some of the facilities that have been built for this purpose, including Biosphere 2.
Figure 12: (a) Biosphere 2, (b) Life Support Systems Integration Facility, and (c) Mars Desert Research Station.
These facilities vary in scale and in the reach of the activities they support. The Life Support Systems Integration Facility (LSSIF), displayed in Figure 12(b), contained a volume of 226.5 [m3] in which it was able to support crews of four participants . This facility performed various experiments during 1995-1997 in support of what has come to be known as BIOPlex at Johnson Space Center in Houston, Texas. Volunteer-driven organizations have also pursued initiatives in this direction. Figure 12(C) shows the Mars Desert Research Station operated in Utah by the Mars Society. Although these facilities were effective, there are alternatives to the use of large-scale facilities for closed-loop LSS research. Such alternatives have made use of aquatic habitats for experiments in zoology and physiology in low Earth orbit (LEO) [125, 126, 13, 127, 128], and for ecotoxicological studies in ground-based hardware [129, 130]. Results obtained with the Closed Equilibrated Biological Aquatic System (CEBAS) minimodule in Space Shuttle missions STS-89 and STS-90 show that microgravity does not affect aquatic habitats considerably for exposure periods of up to 16 days . This module also flew in STS-107 , but no results were reported due to the accident of the Space Shuttle Columbia. Researchers from the Chinese Academy of Sciences have employed a Closed Aquatic Ecosystem [131, 132] (CAES) as well for experiments relevant to ecophysiology, a discipline that "seeks to clarify the role and importance of physiological processes in ecological relations of species ." A recent initiative by the Japanese Aerospace Exploration Agency (JAXA) plans to include an aquatic habitat in their International Space Station module, Kibo . Beyond these e orts, very little has been done to make use of aquatic habitats for research on RLSS control and automation.
Given the high costs and difficulties of performing experiments in large-scale RLSS, the first contribution of this work is the use of an aquatic habitat, or aquarium, for experiments relevant to RLSS [61, 135]. The idea builds on the use of aquatic habitats as small-scale platforms for Earth-based and spaceflight LSS research  and applications . Their reuse of a limited volume of water and their capacity to support life forms, such as aquatic animals and plants, make them a candidate for the study of sustainability attributes of larger-scale environmental systems.Aquaria may involve biological processes, such as photosynthesis, that regenerate life support resources, such as oxygen. This further makes them attractive as an option for RLSS research. This particular research platform enables experiments that focus on the process of respiration. Other biological processes take place in the habitat, some of which help to balance the ecosystem by decomposing toxic compounds, like ammonia. The use of this aquatic habitat provides a learning tool to comprehend the challenges and limitations of automation technology in the operation of RLSS and other bioengineering systems. However, the temporal response of life support variables in the habitat is very slow. Therefore, another aspect of this contribution is the mathematical modeling, description, and simulation of the aquatic habitat. The model serves as a numerical testbed for both RLSS and human-automation integration research .
One of the questions addressed by studying RLSS are the mass balances that ensure the correct operation of closed-loop systems in such a way that they may be sustainable over time. Mass balances can focus on a particular consumable or a byproduct associated to a metabolic process of a biological component. Experiments with the aquatic habitat focus on the process of respiration, in which O2 is consumed by 15 snails of the genus Pomacea while exhaling CO2 as a byproduct. Plants of the species Bacopa Monnieri regulate the concentration of CO2 through photosynthesis, enabled by a 6-LED lamp of 300 [lm] and 90° view angle, producing the oxygen needed by snails and bacteria while aiming to maintain acceptable concentration levels in the habitat. Water serves as the medium in which these quantities are stored (dissolved), and through which they are exchanged between the organisms. The habitat consists of a 10-gallon tank divided in four compartments by three separators, as shown in Figure 13; the first two with an opening area of 12.60 [cm2] and the third with 48.00 [cm2]. Further details about the design and construction of the habitat have been discussed in previous work . The first and second compartments contain animals (consumers) and plants (producers), respectively. Snails are fed regularly with sinking algae tablets. The third compartment contains Bio-Fill™ , active carbon, and water filtration foam as the media serving the purpose of biological, chemical and mechanical filtration. The fourth compartment allows access for sensors and the water pump. The sensors used include dissolved oxygen (DO), pH and ORP. The water circulates through the four compartments. The first compartment has a motorized hatch of 10cm x 10cm and an aerator that allow for reconfigurability, making the system open (volatile) or closed (non-volatile) if necessary; this mechanism is triggered as a fail-safe mechanism when the DO levels reach a minimum of 2.0 [mg/L]. The second compartment holds the LED-lamp and gives access to a dosifier pump that provides a sodium bicarbonate solution to increase the carbonate hardness (kH) of the water; the changes in kH are monitored through variations of the pH readings. Measurements from the sensors are processed by a computer/controller operating under LabVIEW® . The controller delivers the control signals that regulate the LED-lamp power via a pulse-width modulation (PWM) board, and also controls the hatch, and the air and dosifier pumps. The control signals can be generated by control laws or driven manually through a graphical user interface (GUI).
Figure 13: (a) Recirculation diagram of the habitat; (b) Physical realization of the habitat.