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Desalination: Priorities for research in the Pacific

By Colin A. Scholes

‘Desal’ technology has been in place on Pacific atoll nations since as early as the 1980s, so why did recent droughts invoke a state of emergency? Current reverse osmosis desalination research focuses on the needs of the industrial world, which are far removed from the challenges faced in developing tropical nations.

Recent droughts on the small island nations of Tuvalu and Tokelau have sharply focused the issue of water security in the Pacific. Given the idyllic location, it is hard to imagine Pacific islands suffering droughts, and for the larger volcanic islands such as those of Fiji and Hawaii, this is true. However, communities on flat coral atolls are heavily dependent on potable water extracted from the atolls’ freshwater lens – a body of fresh groundwater that essentially floats on the denser seawater under the islands, and is the result of accumulated rainwater that has percolated down. This is the only potable water reserve for the majority of coral atolls, and in times of severe droughts the freshwater lens shrinks rapidly, resulting in seawater seepage into islanders’ drinking wells.

Prolonged drought on some of the smaller islands caused by the 2010 La Niña weather event meant available potable water reserves were down to two days’ supply, and families were being forced to exist on only two buckets of water per day. Naturally, when Tuvalu declared a state of emergency recently the international community rushed to help, with New Zealand flying in two portable desalination plants to the Pacific island, and the International Red Cross shipping in another two portable desalination plants. These units are based on reverse osmosis membrane technology, which is the current technology of choice for seawater desalination, and is used in all of the desalination plants in operation or under construction in every Australian state except Tasmania.

In a normal osmosis process, a solvent (water) moves from a region of low solute (salt) concentration (freshwater) to a region of high solute concentration (seawater) across a solvent-permeable membrane barrier that is impermeable to the solute. The driving force for this movement of water is the chemical potential needed to equalise the solute concentration on each side of the membrane. In reverse osmosis, an external pressure is applied to the concentrated solution (seawater) to overcome the osmotic pressure and reverse the flow of water through the membrane. Hence, water is forced from the seawater to the freshwater side of the membrane, resulting in the production of potable water and leaving behind a concentrated brine solution. Development in membrane technology since the 1980s has meant that reverse osmosis desalination is less energy intensive than the traditional approaches of multistage flash distillation, and because of this it has rapidly gained a 57% market share in newly built desalination plants worldwide.

The distribution of reverse osmosis desalination plants to drought-affected areas, however, will only solve the acute short-term problem. The long-term problem of water security will remain because this is not the first time desalination technology has been implemented in the Pacific or even Tuvalu.

Almost anyone who has ever holidayed in the Pacific would have been exposed to desalinated water. Many of the resorts, especially those in Fiji, utilise reverse osmosis desalination to provide the high-quality water expected by resort patrons and to ensure water supply. Compared to those in Australia, these desalination units are tiny, producing on the order of 27 000 litres per day, which is 0.007% of the volume expected from the Victorian Wonthaggi desalination plant currently under construction.

The advantage of these small plants is there are no environmental concerns around the discharge of brine solution back into the ocean, which is the major environmental concern for many of Australia’s desalination plants, but they are two to three times more expensive to operate than Australian plants, mainly due to the cost of importing diesel to drive the process.

Importantly, desalination is not the exclusive domain of the tourist resorts. The Australian Government Investment Development Assistance Bureau (later AusAID) installed two reverse osmosis plants in Tuvalu in 1990, which were vital in averting a severe drought then, and other, Japanese-funded, desalination plants were installed in the 1990s and 2000s. All but one of these reverse osmosis plants are no longer working and so have been useless in assisting the current drought crises in Tuvalu. Only on Tuvalu’s island capital Funafuti is there a working desalination plant, which was installed in the early 1980s. However, this plant is based on multistage flash distillation technology, which is much more energy intensive than reverse osmosis desalination.

The Marshall Islands are in a similar situation, with all existing reverse osmosis desalination plants used to supply water to hotels and the local brewery. For community consumption, during the late 1980s the Japanese government donated three reverse osmosis units (totalling 22 000 litres per day) and the US government donated five, and in the 1990s an extra five were imported. All but one, on the island of Ebeye, has broken down due to lack of maintenance.

The situation on the island nation of Kiribati is another example: seven reverse osmosis plants were installed in the late 1990s in response to drought conditions across the nation’s three island chains. None is currently in operation, with two units logging only 72 and 128 working hours before malfunctioning. This is the most significant problem with desalination in the Pacific: once the crises have passed, desalination plants are too expensive to operate continually and so they are mothballed. However, reverse osmosis membrane plants cannot be left idle for long periods of time without maintenance, and inevitably those in the Pacific fall into disrepair. This isn’t just a Pacific problem: the Gold Coast desalination plant in Tugun is currently mothballed, yet it is still a cost to the state of Queensland.

The most significant problem is due to corrosion by the brine solution, which rusts the metal of the plant. This naturally attacks the weakest alloys of the plant and in locations where brine solution can reside even during shut-down if the system is flushed with cleaning water. Hence, important but vulnerable joints, valves and flanges deteriorate, which is exacerbated by the tropical climate. Biofouling is another important problem, especially on the reverse osmosis membrane surface, with the combination of fresh water, brine solution and generally an organic-based reverse osmosis membrane allowing the build-up of algae in the system. In a standard reverse osmosis desalination plant, flushing and cleaning cycles limit the build-up of biomatter in the system, but this is a significant enough problem to require replacement of the membranes every five years or so. If no cleaning cycle occurs and the plant is left idle for extended periods, then significant biofouling deposits will accumulate.

The problems in the Pacific are further compounded by the fact that contacting the manufacturer is difficult, costly and time-consuming, while obtaining the necessary replacement parts and service support can be almost impossible given that access to many islands is by supply boats every few months.

So, for the island nations in the Pacific, desalination problems are different from those of industrial nations. In Australia, the issues and focus of desalination research are to do with reducing the energy intensivity of the process, and improving the environment sustainability of the technology. For Pacific nations, desalination technology priorities are more about robust technology and equipment that can withstand regular start-ups and shut-downs, as well as extended periods of being idle. These are issues that current reverse osmosis desalination research largely ignores because in the industrial world maintenance and replacement parts are relatively cheap.

There are some desalination successes in the Pacific, with Nauru and Tonga operating successful desalination programs. Currently, Nauru operates three seawater reverse osmosis plants to provide 80% of household demand. They are container units leased from Veolia Water, and produce approximately

120 cubic metres per day, with extra capacity to spare. The Kingdom of Tonga, on the other hand, has no fixed reverse osmosis desalination units; instead it holds a large number of portable reverse osmosis units in reserve, which are deployed by the navy in times of emergency. This ensures the units can be delivered to locations when needed and concentrates the servicing and maintenance of the units in one location. The key difference is these desalination units are regularly maintained by a dedicated and skilled work force, which is not possible on many of the smaller islands.

These technology issues come about partly because of the different problems faced by the developed and developing world. The developed world has the finances and resources to undertake the research and development needed to solve their problems. The problems faced with adapting important technology in developing situations will continually be faced by those with the most need but the least resources.

Colin Scholes MRACI CChem is at the University of Melbourne. First published in Chemistry in Australia (www.raci.org.au/chemaust).