Emerging investigators series: Prospects and challenges for high-pressure reverse osmosis in minimizing concentrated waste streams

Reverse osmosis (RO) is the most common process for extracting pure water from saline water. RO is more popular than thermal processes such as multi-effect distillation and multi-stage flash due to its lower energy consumption and cost. RO is currently limited to treating streams with total dissolved solids (TDS) values of less than 50000 ppm. Zero liquid discharge (ZLD) processes involving pretreatment, RO, and thermal steps can concentrate and dispose of high-salinity waste brines with greater thermodynamic efficiency than purely thermal processes; however, ZLD processes are not yet widely practiced. Waste streams requiring ZLD typically have TDS values as high as 300000 ppm and include seawater RO (SWRO) brines, flowback and produced water from unconventional shale gas development, formation water from CO₂ sequestration, and flue-gas desulfurization (FGD) wastewater. The TDS levels of these streams can exceed those of seawater by nearly an order of magnitude, and even concentrating a stream with TDS levels similar to those of seawater requires a high-pressure RO process to achieve high water recovery. In this review, we consider a high-pressure RO (HPRO) process with applied pressures of 2400-5000 psi (compared to 800-1000 psi for SWRO) to reduce the volume of high-salinity brine wastes. We discuss the challenges amplified by the elevated pressure requirements and feed salinities, such as ion precipitation and scaling, biofouling, and RO module mechanical stability. We also propose solutions to address these limitations of HPRO.