Membrane contactor processes for wastewater reclamation in space: Part I. Direct osmotic concentration as pretreatment for reverse osmosis

An innovative system that combines three different membrane processes for reclamation and reuse of wastewater in future space missions was evaluated. The direct osmotic concentration (DOC) system consists of an array of five reverse osmosis (RO) elements, a direct osmosis (DO) pretreatment process, and a combined direct osmosis/osmotic distillation (DO/OD) pretreatment process. Optimized operating conditions, including RO pressures, salt load in the brine loop, and flow velocities were determined for the three subsystems. Mass and heat transfer in the pretreatment processes were measured. Water flux in the DO process was found to be strongly dependent on the type of membrane used; it ranged from 10 to 25 l/(m2 h) for a cellulose triacetate membrane specifically designed for this application and from 0.5 to 2 l/(m2 h) for commercially available RO membrane. Water flux through the dual DO/OD process was also found to be highly dependent on temperature gradient across the membranes—increasing with increasing temperature gradient. The conditions for minimum energy consumption of the system were determined and used in estimating the specific energy cost of treating the wastewater generated in space. The weight of salt resupply for continuous operation was also estimated. When compared to alternative technologies, the DOC system provides high wastewater recovery (>95%), at low energy cost (<90 × 103 J/l (25 Wh/l)), with minimal resupply (<20 kg/year).