VWS Westgarth Office Contact

Orbital House
3 Redwood Crescent
East Kilbride
G74 5PR

+44 (0)1355 588 000
Fax: +44 (0)1355 588 001


Veolia Water Oil & Gas Angola Contact:

Viana Park
Pavilhão nº1 Quarteirão 9
Estrada do Calumbo
Pólo Industrial de Viana
República de Angola

Tel: +244 940 120 949
Tel: +244 949 229 399


Seawater Reverse Osmosis

How Reverse Osmosis Works

The phenomenon of osmosis occurs when pure water flows from a dilute saline solution through a membrane into a higher concentrated saline solution.

The phenomenon of reverse osmosis

Figure 1

The phenomenon of osmosis is illustrated in Figure 1.

A semi-permeable membrane is placed between two compartments.

'Semi-permeable' means that the membrane is permeable to some species and not permeable to others. Assume that this membrane is permeable to water but not to salt. Then, place a salt solution in one compartment and pure water in the other compartment. The membrane will allow water to permeate through it to either side but salt cannot pass through the membrane.

As a fundamental rule of nature, this system will try to reach equilibrium. That is, it will try to reach the same concentration on both sides of the membrane. The only possible way to reach equilibrium is for water to pass from the pure water compartment to the salt containing compartment to dilute the salt solution.

Figure 1 also shows that osmosis can cause a rise in the height of the salt solution. This height will increase until the pressure of the column of water (salt solution) is so high that the force of this water column stops the water flow. The equilibrium point of this water column height in terms of water pressure against the membrane is called the osmotic pressure.

If a force is applied to this column of water, the direction of water flow through the membrane can be reversed. This is the basis of the term Reverse Osmosis. Note that this reversed flow produces pure water from the salt solution, since the membrane is not permeable to salt.

Reverse Osmosis in Practice

With a high-pressure pump, pressurised saline feed water is continuously pumped to the module system. Within the module, consisting of a pressure vessel (housing) and a membrane element, the feed water will be split in a low saline product called permeate and a high saline brine called concentrate or reject. A flow regulating valve called concentrate valve, controls the percentage of feed water that is going to the concentrate stream and the permeate which will be obtained from the feed.

Seawater Reverse Osmosis

Over the last few decades, the process of desalination using membrane technology has been established as a flexible low cost solution to the production of potable and process water from seawater or brackish water.

Seawater Reverse Osmosis (SWRO) is typically the lowest capital and operating cost solution for most applications world-wide, given its relatively low overall energy consumption and ease of manufacture and construction.

With the growing trend for hybrid desalination plants i.e. a combination of thermal and membrane technology, to provide the optimum plant configuration in terms of running cost and flexibility, SWRO will continue to establish itself as a suitable technology in even the most difficult locations.

VWS Westgarth is continually developing SWRO technology and design and build plant with capacities ranging from 100m³/day to over 100,000m³/day. The design can be tailored to meet the requirements of any location.

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