What is SWRO?

17th November 2020

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SWRO stands for seawater reverse osmosis. It is a filtration process, that can be applied at large-scale, to generate freshwater from seawater for industrial processes, irrigation and drinking water.

Osmosis is the phenomenon whereby water will flow from a solution of low concentration to one of high concentration, in order to balance the solutions concentrations. In plants, nutrients are held in greater concentrations at the roots, therefore water tends to flow to the roots from the surrounding soil.

In reverse osmosis, the opposite effect is possible, whereby water will flow from a higher concentration solution to a lower concentration solution. In order to achieve this process, a very high-pressure difference needs to be applied to overcome the traditional ‘osmotic pressure’ (the driving force of regular osmosis). In SWRO, seawater is pumped at high pressure towards a membrane. The membrane holds back the dissolved salts and inorganic molecules but allows the water molecules to pass through the membrane. This process is typically >95% efficient, although two passes may be required in order to generate fresh water suitable for drinking.

Desalination, the process of removing salt from seawater, can be undertaken by a variety of different processes. SWRO has developed in to the most energy efficient process, meaning that it has become the technology of choice for implementation at an industrial scale, meeting the freshwater requirements in an increasing number of locations across the globe. As well as the largest projects that are constructed to supply clean water for inhabitants of large conurbations in locations such as the Middle East, USA, North Africa, Spain and Australia, it can also be packaged into smaller units aboard ships, within factories, hotels or sports grounds to provide a consistent supply of freshwater where access may otherwise be limited.

On the outlet side of the membrane of an SWRO facility, the risk of corrosion is much reduced as you are effectively handling clean water. However, on the inlet side of the membrane, you are handling seawater or a more concentrated brine solution. When combined with the high pressures, stainless steel piping and fittings are widely specified. In fact, super duplex stainless steel has become the default specification for many operators given its cost-effective combination of high-strength and superior corrosion resistance. These family of alloys, with a pitting resistance equivalent number (PREN) >40 can be safely specified for long-term use, due to their strong resistance to pitting and crevice corrosion.

Langley Alloys can provide a complete package of stainless steel pipes, flanges and fittings. In addition, the combination of our bar stock and machining capability can be utilised to produce bespoke fittings, alongside our plate stock to support fabrication.