(432b) MD for Mineral Recovery From the Great Salt Lake

As the demand for high-quality water increases, membrane technologies such as reverse osmosis are becoming widely used as treatment processes for tertiary wastewater treatment, water reclamation, and desalination. However, along with high-quality product water, a concentrated brine stream is produced as a byproduct of these treatment processes. Membrane distillation (MD) is an emerging membrane technology that has the capability to facilitate desalination and further concentrate brines. The advantages of MD are two fold: a high-quality water is produced for industrial and municipal uses and minerals can be extracted from the brine for agricultural and other beneficial uses (e.g., fertilizers, road salts). Additionally, MD has small footprint and ability to utilize low-grade heat, thus making it an attractive process over conventional processes for resource recovery.

Membrane distillation (MD) is a thermally driven membrane process in which the driving force is the difference in partial vapor pressures across a hydrophobic microporous membrane. One configuration of MD is direct contact MD (DCMD). In this process, brine solution and distillate streams are in direct contact on either sides of the MD membrane. The difference in partial vapor pressures of the two streams controls the mass transport of water vapors in MD: water evaporates from a heated feed stream of high salinity, diffuses through the pores of the membrane, and condenses into a cooler distillate stream on the opposite side of the membrane. MD has the potential to sustainably operate at or above saturation concentrations, thus possibly achieving close to zero liquid discharge. However, understanding of system performance under these conditions is limited. The current study aims at optimizing system performance through understanding the impact of membrane fouling/scaling (and their reversibility) on heat and mass transport through these membranes under supersaturated conditions.

DCMD experiments were performed with a feed solution of high salinity water from the Great Salt Lake (150 g/L, more than four times the salinity of seawater) and a distillate stream of deionized water. Optimal operating conditions were experimentally determined to be 50 and 30 °C for the feed and distillate streams, respectively, and the system was operated in a batch mode to closely monitor the performance of the membrane and vapor flux decline as the feed solution became more concentrated. DCMD was able to concentrate the feed solution to twice its original concentration, recovering 50% of the feed volume, and achieved 100% inorganic salt rejection! During the experiments, the vapor flux declined to 80% of its original value (11 to 2 L-m^-2-hr^-1), in which the concentrated feed neared supersaturation and scaling occurred at the membrane surface, indicating that increased concentration polarization limits heat and mass transfer through the membrane.

Scaling mitigation techniques were investigated to reverse the vapor flux to its original level and sustain the operations. Real-time microscopy was used to elucidate the onset of membrane scaling and revealed that membrane scaling contributes to the decline in vapor flux. Preventing scale formation on membrane surfaces sustained water flux and membrane integrity, and eliminated chemical consumption used for membrane cleaning. A system model was developed to evaluate the effect of operational and material parameters on system performance and to optimize operations, increasing water production and reducing energy demand