Harnessing buoyancy-driven instabilities for enhanced membrane desalination

Industrial activities in the United States generate large volumes of complex wastewaters that are challenging to treat, because they have a high concentration of dissolved salts. These wastewaters are often discharged to local surface waters, sewers, wells, and land surfaces, where they contaminate resources and ecosystems. This project uses a combination of computer simulations and laboratory experiments to develop an advanced water treatment process called membrane distillation, which can desalinate and recycle complex wastewaters. Specifically, we show that gravity can be harnessed to improve mixing within membrane distillation, and enable the use of renewable solar energy to reduce energy consumption. Integrated educational activities include summer internships for high-school students, undergraduate research activities, the development of free textbooks, and the organization of student research symposia. Membrane distillation is a thermal desalination process in which water evaporates and travels through a porous membrane that removes salts. The proposed work solves two long-standing technical challenges of membrane distillation. The first is cooling of the water near the membrane, which slows evaporation and the recovery of freshwater. The second is the accumulation of dissolved salts near the membrane, where they precipitate and clog the membrane. We address these issues by orienting the membrane so that gravity causes cool, salt-rich, water to sink away from the membrane, generating buoyant mixing within the wastewater. We also show that this mixing can be strengthened by actively heating the wastewater through a surface opposite the membrane. To that end, the proposed work uses computational fluid dynamics and bench-scale experiments to (1) elucidate the physics of the buoyant mixing; (2) explore optimal methods of actively heating the wastewater using electricity or renewable solar energy; and (3) validate the process against real-world municipal and industrial wastewaters of growing concern. We expect the work to broadly impact society by mitigating water scarcity and improving the environmental sustainability of desalination processes with critical applications to the energy-water-climate nexus. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.