A safe, effective means to control solid waste is a critical need on long-term space missions. With current waste models, 1300 kg of waste occupying a volume 20 m3 will be generated in a 180-day mission to Mars. Unprocessed waste poses a biological hazard to crew health and morale. The waste processing methods currently under consideration include incineration, microbial oxidation, pyrolysis and compaction. Although each has advantages, no single method has yet been developed that is safe, recovers valuable resources including oxygen and water, and has low energy and space requirements. Thus, the objective of this project was to develop a low temperature oxidation process to convert waste cleanly and rapidly to carbon dioxide and water.
In this Small Business Innovative Research (SBIR) Phase I project, TDA Research Inc. (TDA) conducted tests to measure the rates of oxidation using ozone with five model waste components. We used wheat straw to represent food scraps, cellulose to represent waste paper, methionine to represent feces, urea for other biological waste, and high density polyethylene (HDPE) for plastic and plastic wrapping. We measured the oxidation rates of individual waste components and also a mixture of all five. At temperatures less than or equal to 200°C, we obtained a maximum waste oxidation rate of 68 grams per hour per liter of reactor volume with high selectivity for carbon dioxide and water. In addition we obtained very low concentrations of NOX. In all cases, we found that less than 1% of the nitrogen in the waste compounds was converted to NOX. Finally, we found that the highest oxidation rates obtained in this project approach those rates needed to design a reasonably sized system for use onboard spacecraft and the estimated equivalent system mass (ESM) for this method of waste control is similar to values estimated for other, less efficient methods.