High pressure membranes are typically classified into two types: nanofiltration and reverse osmosis membranes. These membranes operate at high pressures that can range from 73-1200 psi. Nanofiltration membranes are capable of removing divalent ions and dissolved organic matter from solution and are typically used to freshen brackish water. Reverse osmosis membranes, however, are capable of removing monovalent ions and are primarily used for desalination and to remove dissolved contaminants.

Desalination has been used extensively in the Middle East due to the limited supply of fresh water. In the United States, RO systems have been gaining interest due to recent water shortages in the coastal western and south-eastern states. In addition, the U.S. military uses portable RO units to supply drinking water free of nuclear, biological, and chemical contaminants during remote battlefield operations. Other medical and commercial applications of high pressure filtration include dialysis for kidney failure, concentrating liquids for the food industry, chemical reaction buffers in fuel cells and batteries, and hydrogen production.

Diffusion is the primary removal mechanism for high pressure membranes and departs significantly from the straining mechanism used with low pressure membranes. Diffusion is a transport phenomenon that occurs when a difference in chemical potential exists between two solutions. A solution that contains a high concentration of impurities such as salt will have a chemical potential that is lower than pure water. In order to minimize the free energy of the system, water moves from an area of low solute concentration to an area of high concentration to equalize the potential gradient. The overall entropy of the system is increased, thereby lowering the Gibbs free energy which is governed by the following equation:

G = H - TS

G: Gibbs Free Energy
H: Enthalpy
T: Temp
S: Entropy

When this occurs through a semipermeable membrane the process is referred to as osmosis. Unfortunately, this process dilutes the concentration of salt while reducing the amount of pure water on hand. In order to reverse this process, high pressure is applied to raise the chemical potential of saltwater above fresh water reversing the flow of water. The concentration of salt increases while additional pure water is gained. Large pressures are required to overcome the osmotic pressure that results from the potential difference between saltwater and pure water. As one might expect, producing these elevated pressure requires substantial amounts of energy.