Sea change: as costs subside, the world relies more on desalination to quench its thirst

The ongoing drought in California has more or less forced the state to embrace desalination. Most of the upcoming Californian projects will probably be solarpowered, though this is not the case for the Carlsbad RO plant, the start-up of which was brought forward from 2016 to 2015. The solar-powered plant at Fresno will not only remove salt and metals from the water but recycle them for use in other products. Desalination units will also come to Santa Barbara and Huntington. All in all, between 13 and 15 plants have been proposed for the Sunshine State. In the North Africa and Middle East region the need for desalination is overwhelmingly obvious.

Many of the region’s recent projects are notable for their size (table). In fact, “the world’s largest” is an oft-used, but not always accurate, epithet associated with these projects. In 2005 Ashkelon was the largest SWRO plant, but it was soon overtaken. Fast forward to 2018, and we find that a desalination plant being built at Rabigh, Saudi Arabia, will be almost triple the capacity of Ashkelon. More important than sheer size, though, are the technical advances made possibly by applying economies of scale. At Sorek, for example, the pressure tubes will expand form 8 in. to 16 in., reducing the piping requirement by 75%. Advanced energy recovery technology has been applied at Hadera and in later projects.

The building will continue: Morocco has completed a pilot mobile plant fuelled by solar power. The Red Sea to Dead Sea (RSDS) desalination project, which aims to provide water to Israel, Jordan and Palestine, has been signed in Aqaba. Qatar is boosting its capacity in preparation for the 2022 World Cup. Egypt is planning projects in Hurghada, East Port Said and on the Suez Canal. The UAE is building a plant at Ghalilah in Ras Al Khaimah and is planning others. Other installations are planned in Iran and Oman.

In Asia the water shortage is especially acute. It is estimated that 29 out of the 48 countries in the Asia Pacific region lack water security. Despite some progress, demand continues to outstrip supply. It is reported that China, after increasing desalination capacity by 70% between 2006 and 2010, has missed its desalinated water production targets in recent years (1).

This is hampering industrial projects, which are forbidden by law to draw on local water tables and are therefore forced to supply their water. One of the major ongoing projects is taking shape in Tangshan in Bohai Bay, where a desalination plant is being constructed to supply potable water to Beijing, 170 miles away. In January 2016 Black & Veatch announced it has won contracts for two projects, one in Hong Kong and the other in Singapore. Elsewhere is Asia, desalination plants have been announced for in India, Kazakhstan and Bangladesh in the last year.

In Europe, the desalination leader is undoubtedly Spain. The country supplies much of the expertise that makes many of the world’s plants function. It was also the first European country to adopt desalination (Lanzarote, 1964). By 2013, 27 plants had been built, 13 were under construction and work on 11 more was still to start. However, the country has been suffering from over-capacity since the economic crisis of 2007, with desalination caught between rising fuel costs and lack of demand. Nevertheless, Campo Dalías, built by Veolia, was started up in October 2015. It is an RO plant with isobaric chambers that recover up to 95% of the brine pressure which is then transferred to the feed in order to reduce pumping requirements.

Other important technological trends include desalination plants powered by green energy. These are especially suitable in hot dry countries, or in remote locations, such as islands, where a smaller scale is required. Solar is the favourite renewable source, but wind and wave solutions have also been proposed or adopted. Wind has been considered as a candidate energy source for desalination, especially in dry regions such as Texas in the United States, where wind power is already used and there is ample brackish water waiting to be tapped. Wave power operates by converting kinetic energy from ocean swell into electrical power that fuels the RO desalination process.

For example the CETO system, developed by Carnegie Wave Energy in Perth, Australia, uses submerged buoys actuated by ocean swell to drive pumps that pressurize seawater delivered ashore by a subsea pipeline. Once ashore, the seawater can either drive a turbine to generate electricity to supply an RO plant. As of January 2016 six demo projects have been carried out.

In the desalination industry, several innovations are aimed at conserving energy, sustainability or reducing the

environmental footprint. Here are some: Energy recovery. Earlier RO plants relied exclusively on pumps to drive the seawater through the membranes. Now isobaric energy devices (energy recovery devices or ERDs) can recover enough waste energy to supply about 25-30% of the energy required. In modern plants, ERDs have become standard, and have been installed in SWRO plants such as Ras Al-Khair, Fujairah II and Sorek.

Valves and actuators in desalination

Chemical pre-treatment uses valves ranging from plastic diaphragm and check valves to more robust PTFE-lined plug, ball and butterfly valves. Desalination, in which high-chloride water is pushed at high pressure through the membrane, is the most demanding application. Super duplex stainless steel is required for all wetted parts of the valves, which are typically plug valves. In the final treatment, the brine discharge, which is full of salt, chemicals and metals, requires rubberseated butterfly valves with ductile iron bodies or full port ball valves.

In SWRO technology, a critical role is played by ERDs (energy recovery devices), which can reduce the energy consumed by pumps by as much as 60%. Control valves and various special valves can be installed on these systems. Since designers are constantly seeking to enhance flow efficiency and reduce costs, this may provide scope for innovative products in the field of valves, actuators and other control devices.

Materials used in valves need to be resistant to corrosion, fast flow and cavitation. In the severe environment of the membrane feed process, 300-series materials are inadequate and even materials such as duplex 2205 and austenitic 904L are at risk of failure. Better is a 6%Mo alloy such as 254 SMO, but in recent years it has been found that super duplex (such as SAF 2507) is more cost-effective. Kurt Hilegems, from the Spanish Castflow company which makes valves for desalination estimates his company consumes about ten to 15 tonnes of super duplex per year in valve manufacture. This, he says, is due to the trend of replacing austenitics and superaustenitics with duplex and super duplex steels.

Conclusion

The innovations outlined above show that desalination’s technological evolution is far from over. The advances made in energy and water recovery and in renewable energy use, the economies of scale brought about by large plants and the flexibility and modular structure of small-scale plants are all factors that increase the economic viability of desalination. From the point of view of suppliers, it has to be said that these types of innovation will result in less piping and fewer pumps and valves per output, but this will be offset by the number of projects as desalination is seen to be more attractive.

References

(1) Figures from the International Desalination Association (www.idadesal.org).