Pressureretarded

Pressure retarded osmosis (PRO) is a membrane-based process that uses a high-salinity draw solution separated from a low-salinity feed water by a semipermeable membrane. Water moves across the membrane from the feed to the draw solution due to osmotic pressure. An external hydraulic pressure is applied to the draw solution to oppose this flow, thereby retarding it. The resulting diluted draw solution can then be processed to recover fresh water, and energy can be harvested by expanding the draw solution through a turbine or energy-recovery devices as the draw solution is reconcentrated.

The operation relies on the osmotic pressure gradient between the two solutions. By applying pressure on the draw side, PRO aims to control the net water flux while enabling energy extraction from the salinity gradient. In practice, PRO is often discussed as a potential complement to desalination and as a source of salinity-gradient energy, with pilot systems explored for integrating with reverse osmosis plants or for standalone power generation and water treatment.

Materials and design considerations include selecting membranes with low tendency for fouling and concentration polarization, and identifying draw solutes that are economical, stable, and easy to reconcentrate or separate. Draw solute regeneration and system integration are central technical challenges, as they influence overall efficiency and operating costs.

Status and outlook: PRO has been studied since the mid-20th century, with renewed interest in the 2000s due to advances in membrane science. While demonstrations and pilot projects show feasibility for energy production and water recovery, large-scale commercial deployment has been limited by membrane performance, draw-solute options, and economic viability. Ongoing research focuses on improving membranes, draw solutions, and system configurations to enhance practicality.