Desalination and Water Recycling

Introduction

As we have seen when discussing outdoor farming and sustainability, we are running out of water. The search for this precious commodity to support large-scale agribusinesses has pushed this vital resource to the limit. Aquifers such as Ogallala in the US, the Arabian aquifer in Saud Arabia, and Sidi Bel Abbes in Algeria are all on the verge of drying up. Large-volume Rivers such as the Nile, Fuerte in Mexico, the Yellow River in China, and the Rio Grande frequently dry up before they reach the sea, or have their flow severely impacted by dams.

Seawater desalination plant in Qatar.

Recycling water

Domestically, we are using more water than ever. On average, every American uses 1100 liters of water daily, little of which is ever reused. Nascent efforts to recycle (also known as reclamation) water yield around 3.4 billion liters of water per day, or around 10 liters per person. This statistic includes recycled industrial wastewater, meaning that the amount of recycled domestic water is even less, with no more than 0.91% of the water consumed. This is mostly because of the technologies used in water recycling. They include microfiltration, reverse osmosis, and ultraviolet disinfection, all of which are power intensive and result in significant heat energy wastage.

A water treatment plant in Florida.

Rural and urban sprawl is another factor that inhibits the efficacy of water recycling. Supplying the necessary infrastructure to take water to and from centralized water treatment plants has proven to be challenging, especially for rural communities, but also for urban ones.

There exists a negative attitude towards recycled water in many parts of the world. The negative attitude stems from past interactions with poorly treated water and perception. In 2021, a water treatment plant in Oldsmar, Florida (shown in figure 2) was the victim of a cyber-attack. During the attack, hackers interfered with water chemical levels, making the water unsafe for human consumption. Such cases likely inflame public opinion and make people more apprehensive about using recycled water. In Australia, public opinion has severally defeated public authorities’ endeavors to recycle water in greater quantities. In 2011, residents of a town outside Brisbane defeated efforts to build a water treatment plant, despite being in the middle of a biting drought that saw the local dam’s levels drop to 11%.

Desalination as an alternative source of irrigation water

More than 50 years since Ernest Borlaug was awarded the Nobel Peace Prize for revolutionizing the way we grow our food, his methods, which made intensive farming viable, have led to the cultivation of the most marginal of lands – the Arabian, Egyptian and Mexican deserts, for instance, with the aid of groundwater and rivers. The grain produced from these areas is critical to feeding the world at a time when the population is rising steadily, ad other issues – soil pollution, climate change, and land repurposing in favor of industry and urban development – reduce the amount of arable land.

¹ Abandoning this land once the water dries up will not be a viable option, necessitating the need for alternative water sources. This paper considers the viability of desalination as a source of sufficient water to support increased grain production in the place of aquifers, rivers, and freshwater lakes.

What is desalination?

Desalination is a process by which salt and other minerals are removed from seawater so that it can be safe for domestic use and irrigation. ²

Desalination is a great plan on paper since the earth is covered 70% by water, presenting us with seemingly inexhaustible amounts of water. In practice, though, desalination is costly, with transport and actual purification costs ensuring that only a small amount of the global water supply is derived from desalination. Some of the most extensive uses of desalinated water are in the Mediterranean basin and the Middle East. ³

Types of desalination

Desalination processes there are three types of desalination. Thermal distillation uses heat from solar or conventional energy sources to heat seawater, capture the vapor, and condense it for clean water. It is most common in desalinating water with a high salt concentration or industrial water for recycling.  Membrane distillation is a process through which salt and other minerals are forcefully removed from the water by passing seawater through a membrane. Membrane desalination can easily produce water for domestic consumption.

Once the salt has been removed from the water, it can be processed further for domestic or industrial consumption or turned back into the sea. Desalination can also be large-scale or small-scale, depending on the need, energy availability, and the lay of the land. ⁴ Some desalination plants are hybrid, and use both membrane and thermal systems to desalinate water.

Ras al Khair plant in Saud Arabia, the biggest of its kind so far.

Thermal desalination

Thermal distillation is the most common form of desalination. There are three main thermal desalination types: multistage flash distillation, multi-effect distillation, and vapor compression. On average, a thermal desalination plant uses between 10 and 15 kWh to produce 1000 liters of desalinated water. ⁵ In more advanced plants, it is possible to generate power by having turbines at the entry of the seawater, thereby bringing down energy costs. Thermal desalination is favored in countries where energy costs are considerably low and water is extremely scarce. As such, Middle Eastern countries, with their vast fossil energy resources, have been able to extensively invest in thermal desalination.

Diagram of multi-stage flash distillation (MSF).

Multistage flash distillation

This process uses several stages (sometimes as many as 30) to progressively remove sea salt from water. The seawater is heated to various degrees during this process, while the surrounding (ambient) pressure is lowered. The process uses countercurrent exchange mechanisms to control heat and salt concentration, eventually resulting in water that has minimal or no salt and other minerals. Due to the process’s longevity, having been among the first desalination processes, its relative success rate, and high yield, it is very common. Some plants currently produce around 800 million liters of water per day. ⁶ However, it is expensive to set up and operate, due to the cost of the plant and the energy requirements.

Multi-effect distillation

Multi-effect distillation uses several stages, also known as effects, to heat water, and continually evaporate it, capturing steam, before discharging the brine back to sea. The water is heated by steam tubes, with each stage reusing heat from the previous stage. Each stage features lower temperature and pressure, a factor that considerably reduces the amount of energy needed to run the process. Due to its makeup, it can work for extended periods, under minimum supervision, presenting savings and enhanced performance. Since only the first cell is heated, as subsequent cells have lower temperatures, and therefore boil at a lower temperature, the method uses a much lower amount of energy. According to some estimates, multi-effect distillation uses as little as a third of the energy needed to produce the same amount of water as a multi-stage distillation machine.

Evaporators

Another way of desalinating water is through the use of evaporators, such the vapor compression and mechanical vapor compression systems. Evaporators use various methods to increase pressure on vapor that has been produced, in the process, releasing heat that is used to heat the original seawater concentrate. Evaporators are rarely used today. Among other reasons, they demand a much higher energy consumption – sometimes as much as three times other thermal processes. They are also less efficient, especially when the output demand is high or requires scalability.

 Membrane desalination

Membrane desalination processes use a semipermeable membrane to remove salt and other impurities from seawater. The two main processes are reverse osmosis and electrodialysis.

Reverse osmosis

In normal instances, osmosis involves the movement of water molecules from solutions of low concentrations to solutions of high concentration through a semipermeable membrane. Reverse osmosis reverses this process, by forcing seawater into a membrane through a process that removes salt and minerals from the water. In basic terms, reverse osmosis involves the application of higher osmotic pressure than what seawater is naturally exposed to (between 55-82 bar). The direction of water flow is reversed, meaning that through the membrane, water molecules behave counter to a normal osmosis process, and meaning that on the other side, water with a lower concentration of salt (pure water) is collected. The process is favored because of its capacity to reduce power consumption, and its huge potential to produce more water than thermal processes, without larger energy input.

Electrodialysis

Electrodialysis is a two-stage process. In the first stage, seawater is electrolyzed, while a selective membrane (which allows either anions, or cations, but not both to pass) separates the water from seawater concentrate.

Diagram showing electrodialysis and reverse electrodialysis.

Desalination and power consumption

One of the most prominent downsides against desalination, which is featured extensively in this paper, is the need for enormous amounts of energy. ⁷ Countries that do not have any reliable source of fresh water, and coincidentally have substantial fossil fuel resources can easily turn to desalination. Others, however, may not have enough fresh water, but do not have cheap energy at their disposal.

Thanks to a new technology being developed in Germany, known as capacitive deionization, ⁸ which involves the use of electrodes to extract ions from the water, generating clean water and ions. The ions are then put back into other brine water, in a process referred to as “desorption” in the process of regenerating the carbon electrodes. The method is in its early stages of development but shows great promise. One of its most important outcomes, once operational, is that it will drastically reduce the amount of energy spent on desalination. In an ideal situation, it could make desalination a net producer of electricity, rather than a consumer.

Alternatives

In recent years, nuclear power has been coopted in some desalination plants around the world. The energy and heat produced can be used in both thermal and membrane desalination plants. Nuclear power is cheaper, but the general political attitude toward the use of nuclear has compromised the use of this source of power. Solar power is also used in desalination. However, most solar plants are small and not easily scalable.

Other processes incorporate a desalination plant within a power generation plant. Waste heat from the plant is used to power a low-temperature desalination plant. This particular type of plant is chosen because it uses significantly less power than normal thermal or membrane desalination plants. Low-temperature desalination works by condensing water at a lower temperature than conventional desalination. The method does not require mechanical pumping or cooling. However, only a small amount of water can be desalinated in this way.

Desalination and the environment

Desalination is energy intensive and is usually powered with fossil energy. The resulting greenhouse emissions may exacerbate climate change, especially if it is done on a very large scale. As seawater is sucked into desalination machines, care must be taken to ensure that fish, and their larvae, as well as other marine animals, are not injured or killed in the process. Currently, desalination plants in California are responsible for the deaths of more than 70 billion fish larvae, a factor that significantly affects the state’s fisheries’ health and sustainability. This calls for well-constructed inlet areas that are clear of marine life.

A desalination plant inlet installed by Geiger.

Some environmental groups have also called for the use of desalination wastewater in the replenishing of aquifers. The process involves discharging the water into wells drilled into aquifers. The actions of soil and rocks filter the salt and help aquifers regain some of the water at a faster speed than natural processes.

Mitigation

In addition, the resulting highly concentrated seawater – brine, which is the byproduct of the process, should be disposed of properly. ⁹

Its concentration and temperature are not ideal for fish. Diffusers can be used to ensure that no negative impact on the environment is posed. In most desalination processes, every liter of clean water produces 1.5 liters of liquid contaminated with chlorine and copper. ¹⁰ This liquid depletes oxygen, and negatively affects organisms that are potentially part of coastal communities’ diets.

Once the water has been desalinated, other processes must be undertaken, such as transport and additional treatment. These processes may end up using more energy and resources, a factor that may be negative to the environment.

Other issues facing desalination

As we pointed out above, 70% of the earth’s surface is water. Out of this water, only 1% is readily available for drinking. The remaining water is either seawater or brackish, which is not extensively discussed in this paper but is still an important source of water for potential human consumption and irrigation. As rivers, aquifers, and other sources of freshwater dry up, why is desalination not an obvious alternative for farmers, local authorities, and governments? Indeed, since desalination is not a new technology, having been around on an industrial scale for more than 70 years, why is it only limited to a few countries that are highly water-insecure, and have enormous fossil fuel reserves? The answers to these questions are explored below.

A desalination plant in Israel, pictured in 1964.

Transport

Once the water has been desalinated, it needs to be transported to the people and areas that need it. This would be through the use of pipelines built from desalination complexes to farms inland and other parts along the coast.

China, South Africa, and the US show that it’s possible to move water across vast distances using a network of pipes and canals. In China, a giant network to move water from the south to the water-stressed north is in the works. The project will involve 11 provinces, many of which will be expected to give up their water and displace hundreds of thousands. The ecological and cultural aspects are also being downplayed – massive dams, canals, and tunnels through important cultural sites and buildings, are not featured in the conversation. In the US, similar attempts to have water flow from the Great Lakes region to the West have been thwarted by people who were concerned about their livelihoods, and an inclusive government that considers such concerns.

When desalination is factored into the equation, things become more complicated. For instance, to get the water needed to grow corn in Nebraska, for instance, we would need around 75.18km³ of water to grow its annual corn output of 1.7 billion bushels of corn alone. If this water was to be obtained from the ocean only, we would need more than 280 desalination plants on the scale of the Ras al Khair desalination plant.

On paper, such an endeavor is possible. China’s South-to-North Water Diversion Project plans to move 45 billion cubic meters of water across 2700 miles. This will cause a massive disruption in the socio-cultural and environmental spheres, in a way that would need years of negotiations among state governments and communities in the US to be actualized.

Scalability

The construction of a plant as big as the Ras al Khair plant costs around 7.2 billion dollars. 200 plants would be an enormous undertaking costing more than 1.5 trillion dollars. This investment would need to be justified. Some aspects that would need justification include the real need for a plant of such magnitude to grow food. However, when the corn produced by Nebraska is interrogated further, we find that much of it is used as animal feed, while some is used to produce corn syrup and ethanol. Such a project would need to be critically accessed too, to see whether Nebraska is best placed to be the recipient of this water.

The cost, scale, and financial implications would mean that scaling such a solution to other states in the Great Plains as well as marginal lands in other countries would be a difficult endeavor. This is especially so when not inspired by an existential threat, such as towns in the state running out of water for domestic consumption. Energy, as described above, may also prove to be prohibitive in attempts to set up a plant, especially in a country or continent where no unitary system of government exists, to muscle through unpopular but essential projects. As other desalination plants have shown, the larger a plant is, the more inefficient it is in land and energy use, as well as expensive in capacity building cutting-edge technologies would need to be deployed to make this economically and environmentally viable.

Examples

A reservoir on the South-to-North Water Diversion Project in China.

Scalability may not be an issue for long, however. Thanks to groundbreaking innovations at Hall Labs, it will soon be possible to acquire small desalination devices. The devices will use multi-effect distillation to clean grey and black water, in addition to converting seawater into clean water for drinking and domestic chores. Foreseeably, it will be possible to manufacture and use desalination plants for specific sizes of households or communities, as well as farms. In conjunction with other responsible water use practices, it is possible to use a minimal amount of water on farms, by minimizing evapotranspiration and improving irrigation efficiency. Any runoff can then be captured, and as needed, purified.  Communities that use extensive amounts of water can also recycle the water, and therefore reduce their water use footprint, while applying the water to two purposes at a time – domestic or industrial use, and irrigation.

Experimental design of a small household desalination machine by Hall Labs.

Conclusion

Desalination of ocean water for large-scale agriculture is an idea whose time may not yet have come, but will surely do. As the Ogallala steadily dries up, and groundwater in China recedes 3 meters every year, farmers are slowly appreciating the temporary nature of their enterprise if they do not obtain water from other sources. However, desalination is accompanied by massive capital, operational, and environmental costs that must be addressed before it can become a mainstream source of water.

Places like Nebraska and surrounding areas in the Great Plains were once open pastures. Today, they are a critical source of food for the US and much of the world. Unless another Ernest Borlaug moment comes up, and we grow even more food on less land, such areas will still be central to global food security. Water needs to be obtained from somewhere to farm these areas. However, where to get the water is still a big problem. In other articles, we will also interrogate the capacity of communities and governments to harvest stormwater for irrigation, as well as how large-distance water transport can work in addressing water shortage on farms in water-stressed areas.

1. Soil erosion and pollution, as a direct consequence of overgrazing, over cultivation, and improper industrial waste disposal have led to the contraction of arable land by more than 40%. This has caused the overuse of agrochemicals to boost food production on remaining land, further jeopardizing the existing land’s capacity to feed a growing population.[↩]
2. In its most basic form, desalination is an ancient practice. Sailors have used it for generations to remove salt from seawater and make it safe for drinking. The romans used clay filters to trap salt and produce clean water for consumption. The two methods – distillation and filtration, are the basic concepts used in today’s desalination plants[↩]
3. Saud Arabia obtains 60% of its water from desalination, and the rest from the Arabian aquifer. Since the aquifer is drying up fast, and the country needs even more water to sustain demand and agriculture, it has commissioned more plants in addition to the 33 it already operates.[↩]
4. Since desalination plants need so much energy to be viable, countries without fossil fuel resources may find it difficult to rely on desalination. Countries that are overly mountainous from the coast also have challenges in desalination, due to the logistics involved in taking the water inland.[↩]
5. Older plants take between 17 and 19kWh to produce 1m³ of water. Current ones produce water with marginally less power.[↩]
6. For example, the Shoaiba complex in Saudi Arabia produces 880 million liters per day, while Israel’s Soreq produces 540 million liters daily.[↩]
7. Since most desalination plants use fossil fuels, their impact on the environment is considered significantly negative. It is sometimes difficult to present desalination as an environmental-friendly alternative to groundwater mining, due to these concerns as well as other impacts on the ecosystem.[↩]
8. Capacitive deionization is an emerging technique, which may yet prove to be a game changer in the quest to run the desalination process with little or no energy.[↩]
9. In some instances, sodium hydroxide and hydrochloric acid can easily be removed from the brine for use in other functions. Though the technology to dispose brine properly exists, it is much better if it can be exploited as much as possible to pose the minimum risk possible to the ecosystem.[↩]
10. According to UNEP, the chemicals used during desalination can contaminate the water by reducing the amount of dissolved oxygen. This jeopardizes some marine organisms’ survival.[↩]