Pressure-retarded membrane distillation for simultaneous hypersaline brine desalination and low-grade heat harvesting

Membrane distillation (MD), an emerging pre-concentration technology for zero liquid discharge to manage industrial wastewater, has become increasingly attractive when the low-grade waste heat is utilized as its driving force. Pressure-retarded membrane distillation (PRMD) – a derivative of MD by applying a hydraulic pressure lower than the membrane liquid entry pressure at the cold permeate side – has the potential to further recover the low-grade heat in terms of electricity, which is an added benefit for the use of conventional MD. Here, we for the first time experimentally evaluated the feasibility of using PRMD for simultaneous desalination of hypersaline water and harvesting of low-grade heat and explored the mechanisms governing the experimental PRMD performance. Using a commercial membrane, NaCl solution at the concentration as high as 4 M (233.76 g L⁻¹) is desalinated with the salt rejection >99.9% and additional energy is generated in PRMD at the same time. We showed that the membrane operated with its active layer facing a cold solution orientation in PRMD exhibits better mechanical stability at higher applied pressures, which is essential for achieving higher power output. However, in contrast to the other orientation that is used in conventional membrane distillation (MD) processes, the coupled effects of internal temperature polarization (ITP) and internal concentration polarization (ICP) lower effective driving force and membrane permeability, which significantly compromises the water vapor flux and peak power density. These results indicate that unlike MD processes, controlling internal scaling and fouling are critical for PRMD and our findings can serve as useful guides for creating high-performance PRMD membranes in practical environmental applications.