Is Desalination The Future? Sure, But Take That With a Grain of Salt

Background

Desalination is nothing new: ancient civilizations across China, Egypt, Greece and Rome, all had their own ways of making salt water drinkable. Sailors, for example, have long boiled seawater and used sponges to capture what had evaporated; and in places like China, people were using specially woven bamboo and clayware to desalinate around 2000 years ago.  

Today’s desalination methods are essentially contemporary updates of these two processes: either the salt water is evaporated and recaptured, or it is filtered in some way. 

In the first of these two methods, referred to today as thermal desalination, salt water is boiled and the purer evaporate is captured, in a process imitating (in part) the natural water cycle. Heating the water to a boil takes an immense amount of energy; a challenge that practitioners are trying to address through several different means. In many plants, desalination plants are integrated with other industrial plants, and thereby make use of byproduct heat that otherwise may have been wasted.  And it’s now increasingly common to place seawater into low-pressure vessels, which reduces the boiling temperature of the water and therein reduces the amount of energy required. 

While it seemed, for the past several years, that this type of thermal desalination was waning in popularity due to its steep energy requirements (and associated costs), experts, including the president of the International Desalination Association, have recently indicated that the proliferation of solar power is helping reinvigorate thermal desalination efforts. 

Indeed, most new plants do not utilize thermal desalination processes—instead, they make use of membrane desalination, which is like a high-tech update to the very ancient method of filtration. 

In reverse osmosis, by far the more popular method of membrane desalination and now the most popular method of desalination in general, salt water is first pretreated in some way, depending on what it needs—that can mean filtration, chlorination, and/or coagulation. Then, the water is pushed through a semipermeable membrane, a material which allows some molecules to pass through, while blocking others. Salt water is pushed through these semipermeable membranes at a high pressure (which can require a significant amount of energy), and impurities such as salt are blocked while fresh water is able to flow through. 

The other membrane desalination method, electrodialysis, is similar, in that salt water is pushed across a semipermeable membrane—only instead of being pushed using pressure, as is the case in reverse osmosis, water is pushed using electrical currents.  Both membrane methods require significant amounts of energy (though they require less than thermal methods), which can get expensive. The high-tech equipment that the processes require, including the membranes themselves, can also drive up prices. 

Still, as a stand-alone process, membrane desalination tends to be the cheapest and most efficient method. But as a component of an integrated plant that capitalizes on byproduct heat, thermal desalination can also make a lot of sense, and innovations continue to make this method more appealing.  

In addition to the method being used to desalinate, the type of water being used is also important in determining the cost and efficiency of a system. Desalinating brackish groundwater, for example, typically requires much less energy—and is therefore cheaper—than desalting seawater, because of its lower salt content. But even desalting seawater is not an entirely consistent process—different bodies of water can have very different salt concentrations, while also containing different amounts of minerals like magnesium, sulfate, calcium, and potassium, which can impact processing methods. 

According to French company Sidem Veolia, which specializes in a type of thermal desalination, three kilograms of seawater are needed to produce one kilogram of fresh water.  For companies utilizing standard membrane technology, yields are often closer to 50 percent, while researchers at the New Jersey Institute of Technology (NJIT) have recently developed methods that obtain yields of 80 percent, and impressively, the San Francisco-based Water FX says that its latest technology is able to achieve yields of 93 percent. 

High costs

As of June 2015, according to the International Desalination Association, there were 18,426 desalination plants in 150 countries, supplying more than 300 million people with water for some of their daily needs through the 86.8 million m3 they produce each day.  In 2008, the total installed capacity was 47.6 million m3 per day, representing an 82 percent change over that 7-year period. 

And while capacity for desalinated water is growing, it still remains a relatively expensive option. Costs vary significantly from region to region and the exact nature of the processes being used, but can easily be several times the cost of traditional water resources. 

At the Carlsbad desalination project in California’s San Diego County, which opened in December 2015, operators projected the cost of its water in 2016 to be between $1.70 and $1.90 per m3, depending on demand.  These prices are about double those of the authority’s current most expensive water supply.  In Israel, where the technical capacity and scale of production is high, the cost of water from most of its plants is in the range of $0.65-$0.8 m,3 while IDE Technologies has been able to get the price down to $0.52/m3 at its cutting edge Sorek Plant, which opened in 2013. 

And while desalinated water is being used in a number of places to meet municipal demand, the high costs make it more difficult to justify for farms, whose consumption quantities are steep. 

In Australia, for example, another desalination hotspot, reports indicated that desalinated water costs farmers around $0.77 (AUS $1) compared to the $0.15 (AUS $0.20) they are used to paying. So, some researchers there are arguing that the most effective way to make desalinated water an attractive product to farmers could be to enrich it with nutrients: farmers, the argument goes, would be more likely to pay extra for their water if there was a tangible incentive for them.

Still desalination is becoming cheaper and more efficient thanks to technological advancements. Companies like WaterFX, mentioned earlier, are harnessing solar power rather than traditional, more expensive forms of energy to run their plants, and are also boasting of previously unheard of rates of recovery. This means more fresh water and more concentrated waste product, which could be re-sold and used for industrial applications. 

Another variable affecting the cost of desalination is the level to which water is desalted. Obviously, the standards in order for it to be consumable for humans are relatively strict, as are the requirements necessary in order for desalinated water to have broad agricultural uses. However, researchers are making strides forward in encoding salt-resistant traits in crops, which could have interesting implications for desalination for agriculture. 

Using desalinated water

Most desalinated water is for direct human consumption or for agriculture. And given that as a sector, agriculture tends to be the top consumer of water, it is important to understand the linkages between desalination and agriculture.

Greenhouses are one relatively popular application of desalination to agriculture.

Companies like Seawater Greenhouse (who we discussed briefly last year in a piece about agricultural technology in the Arabian Gulf) and Australia-based Sundrop Farms are both successfully utilizing salt water to grow crops within greenhouses. The companies evaporate and condense fresh water from salt water, and also use the salt water to keep temperatures within the greenhouse cooler in the process—especially important, given that many of these greenhouses are being set up in hot and arid climates. 

Given the high price tag of desalination, it makes financial sense for the technology to be applied to situations in which high-value crops are grown very intensely, like they are in a greenhouse. These types of greenhouses are operating and being set up around the world, but are particularly popular in deserts close to saltwater bodies.

When it comes to broader agricultural applications, the role of desalinated water becomes less clear. For one, the mineral composition of seawater tends to be different from that of fresh water: there can, unsurprisingly, be elevated levels of sodium and chloride, both of which can be damaging to crops, as well as the possibility for too-high levels of boron, which is also bad for soils.  But technological advancements can help desalinators deal with these problems: boron-rejecting membranes, for example, help desalinators using membrane desalination deal with that concern more effectively.  Furthermore, seawater can have deficiencies in minerals essential to crop health, like calcium and magnesium—although these can be added back into the soil by farmers with relative ease. 

In Israel, where desalination was originally developed to help meet domestic, rather than agricultural, demand, the challenges associated with using desalted water on crops became particularly obvious around a decade ago. Researchers discovered then that the mineral deficiencies were having a deleterious effect on tomatoes, basil, and certain types of flowers.  

But the solution to this challenge wasn’t to simply stop using desalinated water for agriculture—it was to set up a system through which Israeli farmers could be quickly notified of any changes to the mineral composition of desalinated water, which allows them to alter their practices accordingly. 

Indeed, Israel’s know-how when it comes to desalination is arguably unparalleled. Water is a vital, historical concern for the country—highlighted by the fact that Israel’s national water company, Merokot, was established in 1937—a decade before the State of Israel was declared. Longstanding water concerns paired with its proximity to the Mediterranean Sea have helped push Israel to become a natural leader in desalination. 

Today, roughly 35 percent of Israel’s drinking-quality water comes from desalination plants, compared to just 7.5 percent in 2006.  Decision makers are planning to increase this figure to 70 percent by 2050.  

Going forward, it seems likely that Israeli desalination will remain focused on providing its citizens with drinking water, rather than on supplying its agricultural sector—and the aforementioned challenges with using desalted water on farms represent only a small reason as to why that will be the case. 

In Israel, agriculture’s demand of fresh water is less than the global average, at 58 percent, while its domestic use is much higher than the 8 percent average, at 35.3 percent.  And Israeli agriculture’s demand for water is unlikely to grow too much more in the future, thanks in part to the efficiency of Israeli irrigation: it was scientists from the country, after all, who invented the now-popular, ultra-efficient system of drip irrigation. 

And not only is Israel’s agriculture sector a highly efficient user of water, but much of the water it does use is sourced from a very dependable and sustainable source: sewage. Israel recycles 86 percent of its wastewater, a proportion several times higher than any other country. About half of all water used for agricultural purposes in the country right now is recycled water.  

Challenges and limitations

Although the ability to finally harness the power of salt water is certainly exciting, there are still some challenges and concerns with the technology more long-term than its steep costs at present.

Perhaps most obvious is the fate of the salty byproduct, the brine. While some companies are able to recycle and/or re-sell their byproducts, more are forced to dump the salt-rich solutions back into seas, employing methods that they argue are environmentally responsible. Many environmentalists have expressed concern over what the potential long-term implications of the practice might be, while some have argued that the practice will negatively affect underwater ecosystems. 

Also important is how the rise of desalination could change our perception of the impending global water deficit. As a solution, desalination implies a problem that is strictly about supply. But in reality, the deficit is not only about limited supply, but also about too-steep demand. As most of the world approaches a water-uncertain future, it will become increasingly important to work hard to curtail excessive consumption, especially as it relates to agriculture. 

It is also critical that decision makers look toward the water resources they already have, like wastewater, and begin to aggressively prioritize recycling them. This will obviously require some reconditioning of public perceptions to dismantle the “ick-factor,” but encouraging the indirect consumption of such water, through agriculture, rather than the direct consumption of recycled water, could be a good place to start. 

Desalination can be an important and powerful part of a more water-secure future, but it is essential that it be considered as a part of a complex solution, rather than a panacea.