What Is a Membrane Process? | Fluence
Ultrafiltration and reverse osmosis

Two membrane processes — ultrafiltration and reverse osmosis — are used to treat silty river water at the Central Puerto in Buenos Aires, Argentina.

Membranes are commonly used at various stages in the water treatment process, but what exactly is a membrane process? It’s any method that relies on a membrane to filter or remove particles from water.

As explained in a paper by David C. Sammon on the International Union of Pure and Applied Chemistry website:

In general, membrane processes offer the possibility of separating water from various types of solute and of separating solutes either on the basis of size or because some are ionized and others are not. In addition to these cases where a high degree of separation is achieved, there are many instances where the composition of the dissolved material is altered. One example is in reverse osmosis, where the permeate has a considerably reduced salt content.

There are many types of filtering media used in water treatment, including clay, silt, and sand, but a membrane is able to separate smaller substances such as salts and ions from a liquid.

In a membrane process, fluid is passed through a barrier. The separation occurs because of the pressure difference between the two membrane surfaces, keeping contaminants on one side of the membrane and fresh water on the other side.

Reverse Osmosis

membrane process reverse osmosis

In reverse osmosis, pressure is used to force contaminated water through a membrane that captures contaminants and allows purified water through.

The process of reverse osmosis is based on that of osmosis. Jean-Antoine Nollet, a French scientist and cleric, first observed osmosis in 1748. The French word was coined based on the terms “endosmose” and “exosmose,” from the Greek. It is best known as the process cells use to transport water.

Reverse osmosis reverses the process. Pressure is used to force a pure solvent through a semi-permeable membrane. Water typically passes easily through membranes because its molecules are small. The pore size of a reverse osmosis filter is general 0.0001 of a micron.

Reverse osmosis is most often used for desalination of seawater and brackish water for potable and industrial applications, but also for wastewater treatment and water reuse. It can also be used to remove trace phosphates, calcium, heavy metals, and other substances.


Another membrane process is ultrafiltration. These membranes have a filter size of between roughly 0.1 and 0.01 microns. This allows for the retention of proteins, fatty acids, macromolecules, bacteria, and suspended solids in a liquid. In water treatment, the ultrafiltration process is able to remove bacteria, protozoa, and some viruses from the water.

Combined Treatments

Often these processes can be used in combination to provide a comprehensive water treatment solution.

One example is the Central Puerto facility in Buenos Aires, Argentina, which needed to treat river water before it was used in its industrial equipment. Ultrafiltration was used in combination with reverse osmosis to create demineralized water for the plant’s high-pressure boiler. The ultrafiltration process helped resolve issues the operation previously experienced with membrane fouling.

Mine water treatment is another example application for these combined processes. The wastewater from mining operations is extremely high in total suspended solids and colloids. Ultrafiltration can remove these particles to prepare it for treatment with reverse osmosis. In some cases, the water is passed through reverse osmosis twice to reach the needed final specifications for full water treatment.

Are you interested in using a membrane process for water or wastewater treatment? Contact RWL Water to learn more about our custom solutions or to discuss your specific water challenge.

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