Forward Osmosis Glossary: FO, PRO & Key Terms

This glossary covers key terms related to osmotically-driven and salinity gradient processes. Each term is defined with technical accuracy for membrane researchers, engineers, and water treatment professionals. For the complete glossary covering all 80+ membrane technology terms, see our comprehensive membrane technology glossary.

Osmotically-Driven and Salinity Gradient Processes

Forward Osmosis (FO)

A membrane process that uses the natural osmotic pressure difference between a concentrated draw solution and a dilute feed solution to transport water across a semi-permeable membrane. Unlike RO, FO does not require high applied pressure, resulting in lower fouling propensity and energy consumption for certain applications. It is explored for desalination, food concentration, and emergency water purification. Tech Inc. provides FO test cells and draw solution sampling systems for research and pilot testing.

Pressure Retarded Osmosis (PRO)

A process that generates energy by exploiting the osmotic pressure difference between a high-salinity stream and a low-salinity stream across a membrane. Water permeates from the low-salinity side to the high-salinity side, and the resulting pressurised volume is used to drive a turbine. PRO is a key salinity gradient power technology being researched for renewable energy generation from river–sea water interfaces. Tech Inc. supplies PRO membrane test equipment for research institutions worldwide.

Osmotic Pressure

The minimum pressure that must be applied to a solution to prevent the inward flow of pure water across a semi-permeable membrane. Osmotic pressure is proportional to solute concentration and temperature and is a critical parameter in designing RO, FO, and PRO systems. Accurate measurement of osmotic pressure is essential for predicting membrane flux and energy requirements.

Draw Solution

A highly concentrated solution used in forward osmosis to generate the osmotic driving force that pulls water across the membrane from the feed side. Common draw solutes include sodium chloride, ammonium bicarbonate, and various organic compounds. The choice of draw solution impacts FO performance, regeneration energy, and potential for reverse solute diffusion.

Salinity Gradient

The difference in salt concentration between two solutions, which represents a source of chemical potential energy. Salinity gradients can be harnessed for energy generation through PRO or reverse electrodialysis (RED). Natural salinity gradients exist where rivers meet the ocean, and engineered gradients are used in closed-loop osmotic power systems.

Reverse Solute Diffusion (RSD)

The undesirable transport of draw solute molecules from the draw solution through the membrane into the feed solution in forward osmosis. RSD reduces the effective osmotic driving force, contaminates the feed stream, and increases draw solute replenishment costs. Membrane selection and draw solution chemistry are key factors in minimising RSD.

Internal Concentration Polarisation (ICP)

A phenomenon specific to FO and PRO where solute concentration builds up (concentrative ICP) or is depleted (dilutive ICP) within the porous support layer of the membrane. ICP drastically reduces the effective osmotic driving force and is one of the primary challenges limiting FO membrane performance. Membrane structural parameter (S) is a key metric for predicting ICP severity.

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Frequently Asked Questions

What is Forward Osmosis (FO)?

A membrane process that uses the natural osmotic pressure difference between a concentrated draw solution and a dilute feed solution to transport water across a semi-permeable membrane. Unlike RO, FO does not require high applied pressure, resulting in lower fouling propensity and energy consumption for certain applications. It is explored for desalination, food concentration, and emergency water purification. Tech Inc. provides FO test cells and draw solution sampling systems for research and pilot testing.

What is Pressure Retarded Osmosis (PRO)?

A process that generates energy by exploiting the osmotic pressure difference between a high-salinity stream and a low-salinity stream across a membrane. Water permeates from the low-salinity side to the high-salinity side, and the resulting pressurised volume is used to drive a turbine. PRO is a key salinity gradient power technology being researched for renewable energy generation from river–sea water interfaces. Tech Inc. supplies PRO membrane test equipment for research institutions worldwide.

What is Osmotic Pressure?

The minimum pressure that must be applied to a solution to prevent the inward flow of pure water across a semi-permeable membrane. Osmotic pressure is proportional to solute concentration and temperature and is a critical parameter in designing RO, FO, and PRO systems. Accurate measurement of osmotic pressure is essential for predicting membrane flux and energy requirements.

What is Draw Solution?

A highly concentrated solution used in forward osmosis to generate the osmotic driving force that pulls water across the membrane from the feed side. Common draw solutes include sodium chloride, ammonium bicarbonate, and various organic compounds. The choice of draw solution impacts FO performance, regeneration energy, and potential for reverse solute diffusion.

What is Salinity Gradient?

The difference in salt concentration between two solutions, which represents a source of chemical potential energy. Salinity gradients can be harnessed for energy generation through PRO or reverse electrodialysis (RED). Natural salinity gradients exist where rivers meet the ocean, and engineered gradients are used in closed-loop osmotic power systems.

What is Reverse Solute Diffusion (RSD)?

The undesirable transport of draw solute molecules from the draw solution through the membrane into the feed solution in forward osmosis. RSD reduces the effective osmotic driving force, contaminates the feed stream, and increases draw solute replenishment costs. Membrane selection and draw solution chemistry are key factors in minimising RSD.

What is Internal Concentration Polarisation (ICP)?

A phenomenon specific to FO and PRO where solute concentration builds up (concentrative ICP) or is depleted (dilutive ICP) within the porous support layer of the membrane. ICP drastically reduces the effective osmotic driving force and is one of the primary challenges limiting FO membrane performance. Membrane structural parameter (S) is a key metric for predicting ICP severity.