Desalination Methods and Corrosion

Corrosion failures in desalination plants can be very expensive. Avoiding corrosion starts with understanding the main process used in these plants and the pros and cons of each method.   Once understood, corrosion causes, solutions, and materials choice can be addressed. Following is a brief overview of the processes as well as their advantages and disadvantages, which help set the stage for avoiding quality problems that can plague desalination projects.

There are three main processes used in desalination plants, the oldest of which is multistage flash (MSF), where the water is essentially boiled at low pressure and the steam that flashes off is condensed for drinking water. The second process is multiple-effect desalination (MED), in which low pressure is used to force evaporation of seawater and the vapor is then condensed for drinking water. Although actual MSF and MED plants are land based, small-scale units have been fitted to large ships, such as cruise liners, to generate fresh water. The third process is seawater reverse osmosis (SWRO), where chloride is selectively removed from water by forcing it at high pressure through a special membrane. This method involves no heat transfer but requires enough electricity to power the high-pressure pumps that are required.

All three of these methods have advantages and disadvantages. MSF plants require a large quantity of steam and are usually associated with a power station, which can generate extra steam and electricity for the desalination plant. They can produce very large quantities of fresh water and have been widely used in the Middle East, where they are associated with oil-fired power stations. They use relatively large amounts of energy to generate fresh water and may not be competitive with MED or SWRO in some applications.

MED plants were traditionally smaller than MSF plants, but they only require a supply of low-pressure steam. With improvements in scale control chemicals, large plants of a similar size to MSF have become possible with a lower water production cost.

SWRO plants require no heat, but they do require significant quantities of electricity to drive all the high- and low-pressure pumps. Some recent plants have either constructed large solar arrays in the desert or have installed wind farms, so that they are powered by renewable energy. They do have the advantage that they can be constructed in a wide range of sizes and can be modularized so that extra modules—or “trains”—can be added as water demand increases. Because SWRO is relatively easy to turn on and off, they are also popular in vacation destinations where water demand varies throughout the year. Most of the new desalination plants currently being designed and built are SWRO.

As for energy requirements for each process, reverse osmosis currently has the lowest energy requirements. But this does not take into account the energy required to construct or refurbish items consumed in the process.

All three processes can be subject to corrosion unless suitable materials are used for the many components, and they are operated within their normal limits of use. Some of these corrosion mechanisms are common to two or all three types of desalination, while others are specific to one method.

Take the next step in managing expensive corrosion problems at desalination plants with Roger Francis’ book titled Corrosion in Desalination Plants.

Source: repurposed with permission from WaterCorr News, an AMPP digital publication.

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