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| Fig. 1: Desalination plant in Ras Al Khair, Saudi Arabia (Source: Wikimedia Commons) |
As freshwater consumption increases from growing urban populations, its availability from conventional sources wanes due to advancing climate change. Cities that are vulnerable to these trends, particularly in the drought-prone and rapidly growing western U.S., must either import water or diversify their supply from conventional surface water sources to confront these threats.
Though coastal cities have practically infinite access to ocean water, its salinity renders it unusable for drinking and irrigation. Desalination - a process that converts seawater into potable freshwater - offers a solution and is currently implemented in more than 16,000 plants worldwide, delivering over 97 million m3/day of freshwater. [1] However, the steep energy demands of desalination have prevented its widespread adoption because of both sustainability and cost concerns, even in regions with strained freshwater infrastructures.
Adopting seawater desalination over conventional surface water treatment methods presents significant energy challenges. The theoretical minimum specific energy for seawater desalination is given by
| Specific energy consumption | = | - | nRTln(r) r |
where n is the solute concedntratin (moles of solute ions per m3), R is the ideal gas constant, T is temperature, and r is desalination recovery rate. With total dissolved solids of 35,000 mg/L and a 50 percent recovery at room temperature, this value is
| Specific Energy Consumption | = | - | 35.0 kg m-3 ×
2 ×
8.314 J mole-1°K-1
× 298 °K × ln(1/2) (0.023 kg mole-1 + 0.035 kg mole-1) × 1/2 |
| = | 1.17 kWh m-3 |
which has been confirmed across the literature and is between 2.7 and 5.3 times the realized specific energy for conventional surface water treatment [2]. Furthermore, the energy required to realize a reverse osmosis desalination system is significantly larger, with specific energy consumption ranging from 2.5 to 4.0 kWh m-3. [3] Table 1 illustrates the spectrum of specific energy consumption required by various water treatment techniques.
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| Table 1: Specific energy consumption of various water treatment techniques. [3] |
Saudi Arabia, a country that faces extreme rainfall scarcity and limited freshwater resources, has long made desalination a national priority. It houses the world's highest capacity desalination plant, Ras Al Khair (see Fig. 1), which can produce 1,401,000 m3 of desalinated water per day. [4] Furthermore, supported by substantial government funding and abundant fossil fuel resources, Saudi Arabia's desalination infrastructure can meet 100 percent of its domestic freshwater demand. [5] This ensures a stable water supply that supports the needs of Saudi Arabia's rapidly diversifying economy and protects the country from turbulent regional conflicts over freshwater resources.
By 1991, San Diego was reliant on imported water from the Metropolitan Water District of Southern California (MWD) for over 95 percent of its freshwater demand. [6] One year prior, in the thick of a severe drought, the MWD adopted the Interim Interruptible Conservation Plan, a shortage allocation plan which would require San Diego to cut its overall municipal and industrial water use by 20 percent and agricultural use by 50 percent. While other Southern California population centers such as Orange County and Los Angeles relied on the MWD, they were much less dependent on importing water because their freshwater resources included local groundwater aquifers which San Diego does not have.
This period emphasized San Diego's dependency on external freshwater resources as backlash from the public prompted political discussions about diversifying water supply, culminating in the 2015 inauguration of the Claude Bud Lewis Carlsbad Desalination Plant, which produces 50,000,000 gallons of freshwater per day, satisfying 10 percent of the city's water demand. [7]
While the Carlsbad plant represents a significant step towards desalination, San Diego's path to diversifying freshwater resources is not as simple as Saudi Arabia's. In Saudi Arabia, desalination had been a national priority for many decades, essential for survival in a region devoid of freshwater. The energy trade-off was insignificant in the face of abundant fossil fuel resources. On the other hand, desalination occupies a more conflicting role in California's political and regulatory landscape. Williams frames the issue as a paradox between two parallel water governance paradigms - diversification and the water-energy nexus. [8] Indeed, backlash against reducing water consumption, such as after the MWD's 1990 plan, indicates a strong public imperative to invest in diversifying infrastructures such as desalination. However, standing in direct contrast is California's push to regulate the energy costs associated with producing freshwater. Given these paradoxical issues, the political discourse around desalination in California will continue to offer valuable insights into balancing resource independence with sustainable practices at a time when climate change is intensifying the political landscape around both issues simultaneously.
© Jack Glad. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. Eke et al., "The Global Status of Desalination: An Assessment of Current Desalination Technologies, Plants and Capacity," Desalination 495, 114633 (2020).
[2] J. Kim et al., "A Comprehensive Review of Energy Consumption of Seawater Reverse Osmosis Desalination Plants," Appl. Energy 254, 113652 (2019).
[3] N. Voutchkov, "Energy Use For Membrane Seawater Desalination - Current Status and Trends," Desalination 431, 2 (2018).
[4] E. Koutroulis and D. Kolokotsa, "Design Optimization of Desalination Systems Power-Supplied by PV and W/G Energy Sources," Desalination 258, 171 (2010).
[5] Y. Ibrahim et al., "The Sociopolitical Factors Impacting the Adoption and Proliferation of Desalination: A Critical Review," Desalination 498, 114798 (2021).
[6] B. D. Richter et al., "Tapped Out: How Can Cities Secure Their Water Future?," Water Policy 15, 335 (2013).
[7] T. Denson, "Desalination and California's Water Problem", Georget. Environ. Law Rev. 28, 713 (2016).
[8] J. Williams, "Diversification or Loading Order? Divergent Water-Energy Politics and the Contradictions of Desalination in Southern California," Water Altern. 11, 847 (2018).
