Life support systems that support human activities even in future far and long-duration human space exploration need to be regenerative in order to minimize the supplies. JAXA is conducting research and development of next-generation water recovery systems to purify the waste water to drinkable water. Our system uses electrolysis under high temperature and high pressure, and electrodialysis to decompose organic carbon and deionize, unlike NASA's distillation and catalytic oxidation methods. The challenges of liquid handling under microgravity are profound, particularly in understanding the unique behavior of bubbles in space compared to the ground. We performed a technical demonstration on the ISS, assuming that water electrolysis exhibits gravity dependence due to bubble formation during the process. In the demonstration experiment, a sub-scale water treatment system was used to purify the artificially synthesis urine as raw water to be treated. At first, it was operated under the same parameters as those that had been previously verified on the ground. Next step, in order to confirm the effect of microgravity, data was obtained by changing the flow rate and electrolytic current. As a result, it was found that water electrolysis could be performed under microgravity conditions with almost the same performance as on the ground, and the flow rate thresholds that indicate microgravity effects were also obtained. In order to better understand the phenomenon, we modeled the bubble behavior and discussed the experimental results in conjunction with the analysis.
Matsumoto et al. (2024) studied this question.