Scaling Behavior of Urine-Based Wastewaters in High Water Recovery Membrane Distillation Systems for Space Applications

One of the significant challenges facing future long-range space exploration is the need to provide adequate water supplies with minimal resource input from Earth. To meet this challenge, it is important to develop robust technology that can extract >90% of the water from the urine produced by crew members. Current technologies are challenged by the onset of inorganic mineral scaling and precipitation at such high recovery rates. Membrane distillation, which uses a thermally-induced vapor pressure difference to distill water vapor through a microporous hydrophobic membrane, has shown promise for treating concentrated wastewaters at high recovery rates. Therefore, the overall goal of this research effort is to develop robust, scaling-resistant membranes that will enable >90% water recovery from urine-based wastewaters via membrane distillation. This involves first exploring the chemistry behind urine scaling in membrane distillation, then assessing which membrane surface properties can be optimized to prevent scaling at high water recovery rates, and finally, developing robust methods to tune these surface properties. This specific study presents the results of membrane distillation tests of three urine-based wastewaters: 1) raw urine augmented with calcium to simulated microgravity urine, 2) calcium-augmented urine with phosphoric acid added to lower pH to 4, and 3) calcium-augmented urine with NASA’s standard chromium trioxide and phosphoric acid pretreatment. The water quality parameters of the feed, distillate, and brine were monitored, and the scaled/fouled membranes were analyzed using SEM, EDX, and XRD to determine which constituents were responsible for the scaling/fouling. Visual Minteq modeling was also used to identify at what point constituents should theoretically begin precipitating out of the bulk feed solution. These results provide insight into which critical chemical constituents and mechanisms need to be targeted to mitigate problematic membrane scaling.