Fig. 3: Challenges and processes for reverse-osmosis-based brine volume reduction.
From: Brine management with zero and minimal liquid discharge
a, High total dissolved solids (TDSs) brine is a challenge to achieving a high water recovery (WR) in reverse osmosis (RO). The high TDS brine has a high osmotic pressure and thus requires a high applied pressure to push water through a semi-permeable membrane. The osmotic pressure of the retentate continues to increase as water is recovered, depleting the driving force and resulting in negligible flux after a low-to-moderate WR is achieved. b, Mineral scaling is also a challenge to achieve a high WR in RO. In this illustrated example, the maximum WR is not limited by a high brine TDS or osmotic pressure, as the flux drops precipitously even when there is a large driving force (the difference between applied pressure and retentate osmotic pressure). Instead, recovering more water concentrates all ions in the brine, including those that can form precipitates that decrease the performance of RO membranes. Three different technologies can address the challenge of high brine TDS. c, High-pressure RO (HPRO) applies a pressure (ΔP) that exceeds the osmotic pressure of the brine (πb). With an RO membrane, the brine osmotic pressure roughly equals the transmembrane osmotic pressure difference (Δπm). HPRO requires redesign of membranes and modules to withstand high operating pressure. d, Counter-flow RO (CFRO) uses a draw solution to reduce the transmembrane osmotic pressure difference. e, Low-salt-rejection RO (LSRRO) uses a leaky membrane to reduce the transmembrane osmotic pressure difference. Both CFRO and LSRRO enable water permeation using an applied pressure that is lower than the brine osmotic pressure (πb > ΔP > Δπm).
