Electrochemical oxidation process for water condensates recycling in a shuttle orbiter

A pilot plant has been built for the regeneration of humidity condensates during shuttle missions. The plant is an automated experimental apparatus which includes an electrochemical cell, two reservoirs and a control system to maintain the temperature within a chosen range, so that the ionic exchange membranes are not damaged. The electrodes used to determine catalytic efficiency were: platinised titanium with ruthenium oxide and Sb and Sn coatings deposited on a Ti substrate, prepared with two different techniques, spray/brush coating and electrodeposition. The Ti/SnO2-Sb2O5 coating was characterized through the following techniques: X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS) and linear and cyclic voltammetry (CV). Among the different contaminant dissolved in the contaminant solution, urea was chosen to carry out the electrochemical tests. Several regenerative treatment runs were performed with the solution simulating the shuttle condensate water to determine the evolution, during the tests, of the total organic carbon (TOC) concentration, the nitrate and nitrite concentrations, the ammonia concentration, the pH and the conductivity. These runs were carried out at different current intensities to understand the influence of the anodic current on the organic decomposition rates, in particular of the urea in presence of an amount of sodium chloride. The contaminant removal efficiencies in the electro-oxidation test appeared to be depressed for both electrodes at high current density, whereas the inversion of the electrode polarity seemed to offer a further advantage. The best result reached in the direct electro-oxidation was the 24 ppm as TOC with Ti/SnO2-Sb2O5 at 0.2 A. This value is far from the limit concentration (0.5 ppm), which is the standard level for drinking water in space appliances. During the tests, a decrease in the pH was observed, due to the formation of refractory substances such as organic acids.