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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 forexperimenting various essential equipment for further deep-space exploration andcolonization. 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 removeall Volatile Organic Compounds (VOCs) which is difficult to be dissolved by othertreatments. However, the high temperature and pressure working environment makes someof its fragile parts need continuous replacements leading to increased cost and potentialrisks.Herein, we propose a photocatalytic microfluidic reactor (PMFR) which can beoperated at normal temperature and pressure to replace the thermal catalytic reactor. Forthis titanium (Ti) PMFR, micropillar arrays covered with nanoporous titania (NPT) aredesigned to attain increased surface area to volume ratios, reduced diffusion lengths, andgreater uniformity of irradiation. For the first time, the microelectromechanical systemsviii(MEMS) techniques are used to fabricate a Ti-based microfluidic reactor.For the first degradation study with organic dye Methylene Blue, the micropillarreactor (MP) was found to dramatically outperform its planar counterpart and the 50 μmdeep MP (MP50) presented at least two times higher degradation activity than otherreported PMFRs. The second study introduced a VOC half-life estimation model designedbased on the steady-state concentration of hydroxyl radicals (·OH). This radical wasconsidered as the main reactant of the photocatalytic degradation of VOCs, and itsconcentration was measured by using nitrobenzene as probe. Half-lives for all VOCsexisted in the Ersatz wastewater proposed by NASA were estimated, and results indicatedthat the MP50 was capable of dissolving most of these VOCs at or under around 100seconds. The third and last study looked into the MP50 degradation performance with twocommon stubborn VOCs – ethanol and 1,4-dioxane. Results showed that MP50 eventuallyfully mineralized about 11.78% of ethanol. As for 1,4-dioxane, MP50 successfullydegraded around 78.83% of it with a reaction rate constant two to three orders of magnitudehigher than that of other bulk reactors reported in literatures. These findings demonstrate agreat potential for the micropillar PMFR to replace thermal catalytic reactor on ISS andeven 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.