Membrane contactor processes for wastewater reclamation in space: II. Combined direct osmosis, osmotic distillation, and membrane distillation for treatment of metabolic wastewater

The improvement of an innovative dual membrane contactor process for treatment of combined hygiene and metabolic wastewater was investigated. Flux and solute rejection in the combined direct osmosis/osmotic distillation (DO/OD) process were enhanced by incorporating membrane distillation (MD) concepts into the process. Two new configurations were investigated: DO/MD, in which the driving force was temperature gradient only, and DO/membrane osmotic distillation (DO/MOD) in which the driving forces were temperature gradient and concentration gradient. Development of a temperature gradient across the membranes substantially enhances the flux of the dual membrane process. It was demonstrated that water flux could be increased by up to 25 times with only a 3–5 °C temperature difference across the membranes. Solutes in the feed wastewater, including urea, were completely rejected. It was demonstrated that complex wastewaters that cannot be treated by one process only could be well treated using a dual membrane process.