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Desalination on the Sea of Cortez

Proponents of desalination tout its potential to quench the thirst of a water-deprived civilization. The logic is compelling. If fresh water is in short supply, why not remove the salt from the vast oceans? With an estimated volume of 1.1 million billion acre feet (an acre foot is the amount of water volume that would cover one acre, one foot deep) of seawater, there will always be enough ocean.

For all its potential, desalination has yet to be a game changer. Worldwide freshwater consumption is estimated at 7.5 billion acre feet per year. Of that total, roughly 20,000 desalination plants worldwide produce an estimated 30 million acre feet of fresh water per year. That’s an awful lot of water, but it’s less than 1 percent of global water consumption.

Nonetheless, desalination plays an outsized role in arid coastal regions around the world. In Israel, for example, five massive desalination plants on the shores of the Mediterranean Sea produce nearly a half-million acre feet of fresh water per year, an amount the nation plans to double by 2030. Israel’s Sorek Desalination Plant, located a few miles south of Tel Aviv, produces 185,000 acre feet of fresh water per year, from a highly automated operation that occupies only about 25 acres. Approximately 80 percent of Israel’s municipal water comes from desalination, and this nation of 9 million people is now exporting surplus water to Jordan.

Just over one year ago, desalination as a way to solve water problems was proposed in an unlikely place. A July 2021 report submitted to the Pima County, Ariz., board of supervisors explored the feasibility of desalinating seawater from the Sea of Cortez, the body of water that lies between the Baja California peninsula and the Mexican mainland, and piping it over the mountains and across the border with Mexico to Tucson. The challenges posed by this scheme — political, financial, technical, and environmental — exemplify the difficulties of desalination as a means of resetting the supply vs. demand equilibrium to yield abundant water in parts of the world desperately in need of it.

As described by the Arizona Daily Star, shortly after the proposal was submitted by engineers at the Pima County Regional Wastewater Reclamation Department, critics pounced, decrying the project as grandiose, impractical, costly, and an environmental disaster. Their first line of attack was the cost to ratepayers. It was estimated that the project could add $60 to $90 per month to the typical Tucson-area homeowner’s water bill.

The critics are not wrong. Because Tucson is so far from the Sea of Cortez, this project would be costly, even in comparison to other desalination projects. The cost of a 200-mile pipeline would be an estimated $3 billion, compared with an estimated construction cost for the desalination plant itself of $1.1 billion. Getting 90 million gallons of water per day (just over 100,000 acre feet per year) pumped from sea level to a peak elevation of over 3,800 feet and then back down into Tucson at an elevation of 2,300 feet is a massive undertaking.

The pipeline’s projected operating cost, estimated at $98 million per year, would be primarily for electricity for the pipeline pumps. They would require just under 100 megawatts of continuous power, which at $.10 per kilowatt-hour would total $85 million per year. Building desalination plants in locations far removed from the customer does not come cheaply. The cost to run the desalination plant, by comparison, was estimated at $73 million per year, with $45 million of that to pay for electricity to pump ocean water through the filtration membranes to remove the salt.

The projected cost for this plant to deliver 100,000 acre feet per year from the Sea of Cortez to Tucson also includes the cost to pay off the construction loans and the overall operating cost. Altogether, the report calculated the water would cost consumers $3,761 per acre foot. That’s expensive water. But these financial obstacles are not insurmountable.

A scenario put forth by the report’s authors proposed that 50 percent of the $4 billion price tag for construction take the form of a federal grant. If so, the retail price per acre foot would drop to $2,732. That’s still costly, but it moves into the range of other expensive solutions. If water sourced from desalination is part of an agency’s portfolio of various water-supply solutions, taking its place alongside treated wastewater, naturally recharged groundwater, and whatever supplies are still available from, for example, the Central Arizona Project that moves water from the Colorado River to Tucson, then the blended price can remain affordable to consumers.

Another challenge facing any proponent of desalination is the energy cost to desalinate the water. Here, however, critics of desalination may be overstating their case. Using existing, fully commercialized technology, seawater desalination requires — ocean salinity varies, so this is an average — about 3.5 kilowatt-hours per cubic meter of fresh water produced. Using the number the Tucson engineers relied on, 3.6 kilowatt-hours, and applying that worldwide, it would take 257 gigawatt years to desalinate 500 million acre feet of seawater per year, which is nearly 17 times current worldwide desalination output.

Supplying half a billion acre feet per year of new and perpetually available fresh water could eliminate water scarcity in every major coastal city on earth. Simultaneously investing in total reuse of interior urban wastewater and exploiting new indoor agriculture technologies would multiply this benefit. The total energy to desalinate this stupendous quantity of water, 257 gigawatt-years, is equivalent to 8.1 exajoules (the mega energy unit currently favored by economists). That sounds like a lot, and it is a lot, but according to the authoritative BP Statistical Review of Global Energy, it would be only 1.4 percent of the total energy produced worldwide in 2020.

Which brings us to the brine, the seawater that doesn’t make it through the filters, and is returned to the ocean. For every gallon of desalinated water, another gallon of brine, twice as salty as before, has to be processed. In practice, this usually means returning it to the ocean. Passionate debate rages as to just how harmful brine discharge is to maritime ecosystems, and here again, the Sea of Cortez does not align favorably. Compared, for example, with a site on the California coast, past which the robust California Current moves 250 trillion gallons per day, a desalination plant on the Sea of Cortez would discharge waste into a stagnant pond. If the dose makes the poison, and it always does, then the challenge of disbursing brine in the Sea of Cortez is magnified by its placidity.

When it comes to delivering an adequate supply of water, Pima County faces a quadruple threat: County authorities have already squeezed about as much rationing as they’re going to get out of their residents; the level of groundwater pumping they can sustain has been halved as the average precipitation has dropped from twelve inches per year historically to only six inches per year during the ongoing drought; the Colorado River allocations via the Central Arizona Project are threatened as never before, as Lake Powell and Lake Mead drop to historic lows; and the population is projected to increase from 1 million to 1.5 million water-consuming residents between now and 2050.

In the report on the feasibility of desalination as a solution for Tucson, one of the authors told me they “strategically bypassed the question of brine management,” based on the confidence that by the time a project of this magnitude completed its multi-decade planning process, innovative solutions to brine disposal would have been discovered. This is certainly possible. And since desalination is, ultimately, just another form of evaporation, a sufficiently distributed dispersal of brine into the Sea of Cortez ought to satisfy reasonable concerns about environmental impact.

A study prepared for the U.S./Mexico International Boundary and Water Commission in 2020 examined desalination opportunities in the Sea of Cortez. The study included a detailed assessment of emerging technologies. Conventional brine management solutions include ocean discharge and dispersion, which certainly ought to work in locations (unlike the Sea of Cortez) where there is a strong ocean current, along with evaporation ponds, which at scale would consume literally hundreds of square miles, and deep-well injection. But other solutions may be on the way.

For example, the so-called zero-liquid discharge solution to brine management involves extracting all the fresh water, leaving mineral solids that may have beneficial uses. Unfortunately, existing technology to accomplish this requires extreme amounts of energy, and innovative uses of the solid byproducts — such as turning sodium chloride into cement — are still in the concept phase of development.

Other innovations in desalination show more immediate promise. The energy required for reverse osmosis, which is the technology that modern large-scale desalination plants currently use, is 3.5 kilowatt-hours per cubic meter. Commercial desalination plants are pushing that limit. New mobile plants in Saudi Arabia have reduced the energy consumption to 2.3 kilowatt-hours per cubic meter. Just ahead, new filtration technologies and new ways to manage the filtration process promise to further lower the energy required to desalinate. The theoretical minimum amount of energy to desalinate seawater is just 1 kilowatt-hour per cubic meter. It is likely that 20 years from now, the energy required to commercially desalinate water from the ocean will be half what it is today.

In the meantime, maybe desalination is not yet a viable solution, however imaginative (and credit should be given to those investigating creative solutions for the region’s water problem), for landlocked Tucson, located 200 miles away and 2,000 feet above Pima County’s source of saltwater, a sea with minimal current to disburse the brine. But despite relentless litigation and lobbying by environmentalists to block desalination, no such excuse can be made in Southern California, where a water-hungry megapolis squats on the edge of a coast that boasts one of the strongest ocean currents in the world. Desalination at scale does work. It just depends on where you look.

This article originally appeared in the National Review.

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