Optimization of the Distiller Calcium Limiter (DCaL) System for Calcium Removal in Spacecraft Wastewater
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The Distiller Calcium Limiter (DCaL) system removes calcium scale precursors from spacecraft wastewater. Previous research has indicated that the DCaL system successfully removes calcium, preventing the formation of calcium scale on heat transfer surfaces. The objective of this study was to optimize the DCaL system; this includes completing a mass balance, determining the optimum ion exchange membranes (anion and cation), and determining the effectiveness of electrodialysis reversal. Three membrane pairs were tested: Astom Neosepta® AMX/CMX (anion/cation), Astom AHA/CMB, and proprietary research membranes AEM/CEM. Tests were conducted using three individual test stands with different cell stacks that contained the membranes. The feed used for testing consisted of CaCl2 (20 g/L) and NaCl (25 g/L). The results from the testing were used to determine which membrane was the most efficient at removing calcium. A chemical compatibility test was then conducted by completing permselectivity tests, which were used to compare new membranes versus membranes that were previously soaked in brine (a concentrated urine mixture containing chromic acid) for 99 days. SEM images were also taken of the membranes soaked in brine to view any physical changes that may have occurred. The effect of electrodialysis reversal was determined by completing tests using ISS simulated wastewater (US/Russian chromic acid ISS pretreatment) and the DCaL-WFRD system. Three material balance tests were conducted to distinguish the ion transfer rates and water transfer rates. A vacuum test was completed to determine whether the electrodialysis stack could hold vacuum. Based on testing, the results showed that the Astom Neosepta® AMX and CMX membranes provided the highest performance in terms of calcium removal and chemical compatibility. The results also showed that electrodialysis reversal improves calcium removal and prevents fouling of the membranes. The material balance confirmed that the DCaL system removes calcium; however, additional tests are necessary to obtain data with better resolution and to determine the effect of more complex feed mixtures.