Thoughts From Engineers: Spinning Gold: Desalination Steps in to Alleviate World’s Water Crisis
A scenario reminiscent of Garret Hardin’s “Tragedy of the Commons” plays out quietly around the globe. Proof that swelling populations in bone-dry regions of the world can no longer survive on existing reserves of freshwater, technology (and money) step in to mute hard reality. According to the International Desalination Association (IDA), there are 20,000 desalination plants in operation today worldwide. Not surprisingly, more than 70 percent of them are located in the Middle East, a region with a mere 1 percent of the world’s freshwater supply but with more than 411 million people to support.
The year 2019 was projected to be another growth year for desalination in the Middle East, but additional plants are rapidly coming online in other parts of the world as well, including the United States. The city of San Antonio, for example, is in the process of building the largest desalination plant in the country for processing brackish water.
According to the IDA, a number of factors play into the surge in plant construction: inadequate supplies of freshwater is primary, and dropping energy costs associated with desalination also is key. For some countries now investing more aggressively in facilities (e.g., South Africa, Australia and the United States), desalination represented a remote Plan B. Today, as climate change continues to upset historic rainfall patterns and growing populations continue to test a region’s resources, countries and cities find themselves at varying stages of a familiar pattern. For a technology that’s likely to become more common in the near future, what is there to know?
The Beginnings of Desalination
The earliest desalination methods, going back centuries, originated when Greek sailors boiled water to create a drinkable vapor. But more-complex systems involving multi-stage flash distillation (MSF)—also involving a process of heat and condensation—began to be used on a commercial scale in the 1950s. The first large-scale reverse-osmosis (RO) plant for brackish water came online in the 1960s in California, following development of the first membrane prototype made out of cellulose acetate. The first large-scale plant for seawater became operational in 1974 in Bermuda.
What Does Large-Scale Desalination Look Like, and How Is It Changing?
There are two primary methods of large-scale desalination: thermal desalination, which uses heat to force the cycle of water evaporation and condensation, resulting in purified water; and RO, which uses pressure to force saltwater through a set of semi-permeable filters or membranes to trap salt, minerals and impurities. RO is the predominant form of treatment around the world with the exception of countries in the Middle East, where plentiful fossil fuels and other factors led to development of the more energy-intensive thermal desalination method. Change is underway, however, and the newest plants to come online in the Middle East are the more energy-efficient RO plants.
Costs associated with desalination have come down significantly in the last five years, and the process has become much more efficient, mainly due to improvements in membrane technology. In terms of reducing environmental impacts, there’s still a lot to be done.
The intake pipes are problematic in terms of bringing marine organisms in with the seawater. Changes in methodology have focused on drawing water up from a point well below the ocean’s surface to minimize marine-life disturbance. Proper disposal of the brine—rich in concentrated salt and toxins—in biologically rich coastal zones is a point of conflict for many. Some plants have tried to use several discharge points, several hundred feet from shore, to discharge brine away from ecologically sensitive near-shore zones. Others have diluted the brine or spread it over a larger area.
Critics also point to the massive energy needs of desalination as well as the significant carbon footprint of these plants, all of which can collectively aggravate climate issues. (Nearly 20 percent of Abu Dhabi’s carbon emissions come from desalination.) But inroads have been made on this front as well. The largest desalination plant in North America, the Carlsbad plant in Southern California, operational as of 2015 and supplying the city of San Diego with 10 percent of its water needs (50 million gallons of water per day), claims to use 144 different energy-recovery devices to recycle the hydraulic energy generated by the discharged brine. The plant’s website (www.carlsbaddesal.com) claims the process reduces total energy consumption by 46 percent (42,000 metric tons of carbon emissions).
The End Game: Solar-Powered and Pumping
The Carlsbad plant, loaded with the latest operational efficiencies, exemplifies a trend to make large-scale desalination “environmentally friendly,” ultimately moving it off conventional fuels entirely. Most experimentation on this front is taking place in the Persian Gulf, and small-scale trials have yielded positive results.
A project with the capacity to generate 120 million gallons of drinking water per day is under construction in Dubai and is expected to be operational in 2024. The plant, which is adjacent to and designed to be integrated with a 2MW photovoltaic facility, will not be completely solar-powered but will be connected to the grid for back-up energy. These developments are encouraging as the number of desalination facilities increases, whether processing seawater or brackish water. And there’s no doubt about it: the numbers will increase.
The Trajectory Towards Desalination: Cape Town’s Day Zero as a Tipping Point
In California, as in other arid parts of the United States, water-saving strategies form an important part of water policy. The technology needed to reuse wastewater efficiently is a booming area for innovation and development. The state as a whole still largely resists moving to desalination; it remains an option of last resort.
In contrast, according to the World Bank, countries in the Persian Gulf and North Africa are in a state of “absolute water scarcity.” Desalination provides 90 percent of the Gulf’s water supply; in Kuwait, desalination provides 100 percent. In this region, there are few options, and this has been known for some time. But for other parts of the world, the change in degree of water security has been more sudden, brought on by a more-abrupt change in circumstance.
In 2018, South Africa made world headlines as Cape Town grappled with fast-diminishing water supplies. Residents dug boreholes in backyards, washed cars under the cover of night, and dealt with severe water rationing as Day Zero loomed. The world watched in real time as a major metropolitan city, historically water-secure with reservoirs at near capacity, now struggled to provide a basic human necessity. Temporary desalination plants, constructed to avert panic during the height of the drought, are being replaced with permanent facilities.
Cases such as this will become more common as thriving, well-populated cities that were historically water secure come to terms with a new reality.