Design of an Electrochemical Tubulated Bipolar Membrane Breadboard System for the Treatment of Spacecraft Waste Water

The removal of dissolved ions from waste water is essential for water repurification on long-term human space missions. Lynntech, Inc., has demonstrated a novel electrochemically driven purification method using tubulated bipolar ion exchange membranes for the separation of dissolved inorganic impurities as well as charged organic species from waste water. Generally, electrochemical separation methods have limited applications since they can only be applied to the purification of water that has a sufficiently high dissolved ion content to make the water conductive. The novel tubulated bipolar membranes composed of bilayers of oppositely charged ionically conducting polymers can be used to overcome this limitation. This paper deals with the scaling-up of such a device to increase the throughput to process about 100 liters of waste water per day. This is achieved by using stacks of tubulated bipolar membranes. The results of laboratory studies carried out using a 3 liter per day test system showing the effects of variations of current density and flow rate of fluid through the internal channels are presented. The operation of stacks under single pass as well as batch recirculation modes are discussed. Selection of membranes for bipolar membrane fabrication and the effect of thickness of the bipolar membrane unit consisting of the membrane and the concentrate compartments on the energy requirement are also discussed. The size and the power requirements of a unit to process 100 liters of water per day is presented. The data presented provide a technical foundation for further development and scale up for application in advanced life support systems.