Exploring the vast space has always been one of the greatest dream for all humanbeings. The International Space Station (ISS) is currently working as a test platform for experimenting various essential equipment for further deep-space exploration and colonization. Because of limited space for storing water on ISS, Water Recovery System (WRS) is heavily depended to produce recycled drinkable water from collected wastewater. As one important component of WRS, the thermal catalytic reactor is designated to remove all Volatile Organic Compounds (VOCs) which is difficult to be dissolved by other treatments. However, the high temperature and pressure working environment makes some of its fragile parts need continuous replacements leading to increased cost and potential risks. Herein, we propose a photocatalytic microfluidic reactor (PMFR) which can be operated at normal temperature and pressure to replace the thermal catalytic reactor. For this titanium (Ti) PMFR, micropillar arrays covered with nanoporous titania (NPT) are designed to attain increased surface area to volume ratios, reduced diffusion lengths, and greater uniformity of irradiation. For the first time, the microelectromechanical systems vii i (MEMS) techniques are used to fabricate a Ti-based microfluidic reactor. For the first degradation study with organic dye Methylene Blue, the micropillar reactor (MP) was found to dramatically outperform its planar counterpart and the 50 μm deep MP (MP50) presented at least two times higher degradation activity than other reported PMFRs. The second study introduced a VOC half-life estimation model designed based on the steady-state concentration of hydroxyl radicals (·OH). This radical was considered as the main reactant of the photocatalytic degradation of VOCs, and its concentration was measured by using nitrobenzene as probe. Half-lives for all VOCs existed in the Ersatz wastewater proposed by NASA were estimated, and results indicated that the MP50 was capable of dissolving most of these VOCs at or under around 100 seconds. The third and last study looked into the MP50 degradation performance with two common stubborn VOCs – ethanol and 1,4-dioxane. Results showed that MP50 eventually fully mineralized about 11.78% of ethanol. As for 1,4-dioxane, MP50 successfully degraded around 78.83% of it with a reaction rate constant two to three orders of magnitude higher than that of other bulk reactors reported in literatures. These findings demonstrate a great potential for the micropillar PMFR to replace thermal catalytic reactor on ISS and even be used for more varieties of water purifications.

Producing enough drinkable water for long-term deep space missions is a great challenge. A small scale photocatalytic microreactor with nano-porous TiO2 coated titanium micropillar arrays is designed to solve the problem. The COMSOL Multiphysics is used to simulate several parts of the microreactor. The first objective is to model the tree-branched bifurcating flow distribution design and the diamond-shaped flow distribution design, then subsequently compare the pressure drop between the two. Based on simulation results, the tree-branched bifurcating flow distribution design can provide a longer residence time for the waste liquid due to the lower pressure drop inside the distribution channels which increases the photocatalytic efficiency. However, the extension of channels increases the area that this design takes up, thus reducing the size of the reactor chamber. The diamond-shaped distributor is more compact in size because of its geometry which gives more space to the reactor chamber. However, the pressure drop in the microreactor increases about 256% compared to the pressure drop of tree-branched bifurcating distributor which reduces the residence time. The second objective is to simulate photon interaction among micropillars and light intensity along the micropillars under UV irradiation, then find how the micropillar height influences the light intensity on the micropillar. According to simulation, light intensity on the 50 µm height micropillar creates an ideal photocatalytic efficiency. For the 100 µm height micropillar, 50% of the micropillar has low light intensity which causes low photocatalytic efficiency. And 66.67% of the 150 µm height micropillar’s surface has low light intensity. As the height of the micropillar increases, a larger percentage of the micropillar’s surface will have low light intensity. This leads to a decrease in photocatalytic efficiency.