Reverse osmosis

A reverse osmosis membrane acts in the form of a physical barrier for the hydraulic flow, allowing selective permeation of the solvent (mostly water) and partial or total retention of the other dissolved substances (mostly salt). Membrane material, manufacturing conditions and membrane morphology are decisive factors for the retention of dissolved substances and water permeability. However separation basically takes place on a thin, selective layer of the reverse osmosis membrane.

Osmosis is a natural phenomenon whereby water passes through a semi-permeable membrane from a solution with low salt concentration into a solution of higher salt concentration until an equilibrium in the chemical potential of the water is established. In the state of equilibrium, the differential pressure over both sides of the membrane is identical to the osmotic differential pressure. In order to reverse this osmotic flow of water, external pressure, greater than the osmotic differential pressure of both solutions, has to be applied. This additional external pressure enables the dissolved salt to be separated from the water. This phenomenon is defined as “reverse osmosis”.

Reverse osmosis processes are generally divided into three categories:

High-pressure reverse osmosis (5 – 10 MPa, as in seawater desalination)

Low-pressure reverse osmosis (2 – 5 MPa, as in brackish water desalination)

Nanofiltration (0.3 – 2 MPa, as in softening of drinking water)

While nanofiltration membranes are frequently considered as porous membranes, reverse osmosis membranes are defined as closed, nonporous films. The permeate is adsorbed by the membrane material and can diffuse through the structure. The driving force for mass transfer is the differential pressure between the external mechanical pressure and the osmotic pressure of the solution. Selectivity is mainly based on the preferential adsorption of solvent and dissolved salts.
Typical salt retention values for reverse osmosis are between 98% and 99.9%. A reverse osmosis membrane will completely retain all molecules which have a molecular weight of more than 150 Dalton. A difference is often made between integral asymmetric membranes (usually made of cellulose triacetate) and thin-film composite membranes (usually made of polyamide).
On a technical scale, this initially simple principle is implemented solely in the cross-flow mode using spiral wound elements. The permeability and stability of these membranes have improved considerably. Nevertheless, precise knowledge of the specific properties of the individual membrane products and the ability to scrutinise the properties specified, is paramount to installing optimum process engineering. For this purpose, FUMATECH BWT GmbH has built up an broad-based collection of data and developed independent characterisation sequences. Consequently, we can provide well-founded technical recommendations for the optimum membrane product, independently of the supplier, for use in the most different operations. Focussing on our position as a supplier of economical operational systems with long-term stability, we have developed a modular system, integrating the stages: pre-treatment, stabilisation, post-treatment and membrane cleaning which are finely tuned to each other and to the membrane properties.

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