Sustainable Operation of Membrane Distillation Under Supersaturation Conditions

As the demand for high-quality water increases, membrane technologies such as reverse osmosis and nanofiltration 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, like other membrane processes, MD has a small footprint and the ability to utilize low-grade heat; thus, making it an attractive process compared to conventional processes for resource recovery.

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 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 across the membrane: water evaporates from a heated feed stream of high salinity, vapors diffuse through the pores of the membrane, and the vapors condense into a cooler distillate stream on the opposite side of the membrane. MD can operate with feed streams at or above saturated concentrations; thus, separating dissolved solids from the brine (i.e., through simultaneous crystallization), and achieving close to zero liquid discharge. However, understanding of system performance under these extreme 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 distillate stream of deionized water and a feed solution of high salinity water from the Great Salt Lake (150 g/L – more than four times the salinity of seawater!). Optimal operating conditions were experimentally determined to be 50 and 30 °C for the feed and distillate streams, respectively. 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 reject 100% of inorganic salts and concentrate the feed solution to greater than twice its original concentration, recovering over 50% of the feed volume! During the experiments, the vapor flux declined up to 80% of its original value (11 to 2 L-m^-2-hr^-1) once the concentrated feed neared supersaturation. Results indicate that membrane scaling occurred due to increased concentration polarization, thus limiting heat and mass transfer through the membrane.

Scaling mitigation techniques were investigated to restore the vapor flux to its original level and sustain desalination. Real-time microscopy was used to elucidate the onset of membrane scaling, and revealed that membrane scaling contributes to the decline in vapor flux. A novel operating regime was employed in this process to prevent scale formation on the membrane surface, subsequently sustaining water flux and membrane integrity, and eliminating chemical consumption used in membrane cleaning. A system model was developed to evaluate the effect of operational and material parameters on system performance, and to optimize conditions for increased water production and reduced energy demand.