Water Energy Matters

Issues related to the water-energy nexus

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Desalination, part II: A (relatively) short primer on the technology

In the last post, we looked at the tremendous growth of the desalination industry in the last few decades, as well as the benefits and drawbacks of desalination plants. This post will look at the technology of desalination, especially the major desalination methods in use globally today.

Desalination Technologies

Although the illustration shows the desalination system for a reverse osmosis process, the key elements are largely the same for all desalination methods. (Image courtesy of OnEarth Magazine)

Although the illustration shows the desalination system for a reverse osmosis process, the key elements are largely the same for all desalination methods. (Image courtesy of OnEarth Magazine)

The two main water sources for desalination are seawater and brackish water. The five key elements of a desalination system are largely the same for both sources. They consist of:

1) Intake — getting the water from its source to the processing facility;
2) Pretreatment — removing suspended solids to prepare the water for further processing;
3) Desalination — removing dissolved solids, primarily salts and other inorganic matter, from a water source;
4) Post-treatment — adding chemicals to the desalinated water to prevent corrosion of downstream infrastructure pipes; and
5) Concentrate management and freshwater storage — handling and disposing or reusing the waste from the desalination, and storing freshwater before it’s provided to consumers.

We will mainly focus on the third stage — desalination — where the majority of advancements in technology have happend. There are three categories of desalination methods: membrane, thermal or distillation, and ion exchange. The thermal and membrane methods are the two most widely-used today. The ion exchange process won’t be discussed here since ion exchangers are only economical in removing small amounts of salt. The process of ion exchange is very effective at producing ultrapure water, but is limited at desalting on a large scale.

Cumulative global capacity of installed desalination plants for thermal and membrane technology. (Image courtesy of Desalination: A National Perspective)

Cumulative global capacity of installed desalination plants for thermal and membrane technology. (Image courtesy of Desalination: A National Perspective)

Before 1998, most desalination plants were thermal. However, in recent years, technological improvements in reverse osmosis (RO) desalination, a membrane filtration method,  has made the number of plants using membrane technology surpass that of thermal. As of 2008, membrane processes accounted for 56 percent of desalination capacity worldwide while thermal processes accounted for 43 percent. Small-scale ion exchangers and hybrid processes accounted for the remaining one percent.

Below, I’ll provide an overview of the membrane and thermal methods because currently, they are the two primary categories of desalination used at the utility scale. Although a number of different desalination processes fall under each of these categories, for each category, I will focus more on the specific process that is most prevalently used in desalination plants worldwide. For the membrane process, this is RO filtration; for the thermal process, this is multistage flash distillation (MSF).

Membranes Methods
Membranes can be designed to selectively allow or prevent the passage of certain ions, including salts. Membranes play an important role in the separation of salts in natural processes (such as osmosis and dialysis), and this principle has been adapted for commercial use. Commercially-available membrane processes include RO, nanofiltration (NF), electrodialysis (ED), and electrodialysis reversal (EDR).

Membrane technologies can be used not only for desalting brackish water and seawater sources, but also for treating wastewater because of their ability to remove contaminants other than salts (e.g., organic contaminants, bacteria, and viruses). Typically, 35 to 60 percent of the seawater fed into a membrane process is recovered as product water. For brackish water desalination, water recovery can range from 50 to 90 percent.

Reverse Osmosis Filtration

Natural osmosis and reverse osmosis. (Image courtesy of www.filterfast.com)

Natural osmosis and reverse osmosis. (Image courtesy of http://www.filterfast.com)

In the natural process of osmosis, solvents (such as water) diffuse or pass through a semipermeable membrane (think cheese cloth) that blocks the passage of solutes (such as salts). More specifically, when solvents of different concentrations of solutes are separated by a membrane, the solvent wants to move from the low to the high concentration of solutes to achieve equilibrium. At this point, osmotic pressure across the membrane becomes equal (usually 350 pounds per square inch).

RO, as the name implies, is the opposite of what happens in osmosis. A pressure greater than the osmotic pressure is applied to saline water to cause freshwater to flow through the membrane while holding back the solutes, or salts. The water that comes out of this process is so pure that they have to add back salts and minerals to make it taste like drinking water.

As mentioned in the section above, new membrane desalination capacity has surpassed new thermal capacity mostly due to significant advances in RO technology. RO desalination is popular because of its sustainability, cost effectiveness, and simplicity. RO plants typically use less energy than thermal distillation, which has led to a reduction in overall desalination costs over the past decade.

The largest RO plant in the world, located near Ashkelon, Israel, produces 320,000 cubic meters of water daily (m3/day) — about 6% of the country’s total water needs.  The cost of producing one cubic meter of water is a bit more than $0.50 USD, one of the world’s lowest prices for desalinated water.

 The Ashkelon plant located on the Mediterranean coast is the world’s largest seawater reverse osmosis (SWRO) plant that is producing water at one of the lowest costs. (Video courtesy of YouTube)

Thermal Methods
The basic principle of the thermal processes is to apply heat to create water vapor, which then condenses into pure water, separating it from most of the salts and impurities. Thermal processes include multistage flash (MSF), multiple effect distillation (MED), and mechanical vapor compression (MVC). Thermal processes are configured to use and reuse the energy required to evaporate water.

Thermal distillation was the earliest method used to desalinate seawater on a commercial basis. Thermal processes are used across the Middle East and will continue to be a logical choice for the region for several reasons.

First, the regional seas are very saline, hot, and periodically have high concentrations of organics, which are challenging conditions for RO desalination technology. Second, RO plants have only recently approached the large production capacities required in this region, so much of the existing desalination capacity is thermal-based. Third, dual-purpose cogeneration facilities built there combine water production with electric power generation to take advantage of shared intake and discharge structures, as well as to improve energy efficiencies (usually by 10-15 percent). These cogeneration facilities allow the thermal desalination processes to use low-temperature waste steam from the power generation turbines. These reasons, combined with the artificially-low cost of energy in the region, make thermal processes the dominant desalination technology in the Middle East.

Multistage Flash Distillation

MSF is the most robust of all desalination technologies and is capable of very large production capacities. The number of stages used in the MSF process is directly related to how efficiently the system will use and reuse the heat that it is provided.

The general process of a multistage flash distillation plant. (Image courtesy of www.sidem-desalination.com)

The general process of a multistage flash distillation plant. (Image courtesy of http://www.sidem-desalination.com)

The MSF process consists of a series of stages, or chambers, maintained at decreasing pressures from the first stage (hot) to the last stage (cold). In this illustration of the process, seawater flows in on the right side through tubes in the upper part of the chambers where it is warmed by the water vapor produced in each stage. Its temperature increases from sea temperature to the temperature of the heater on the left as it travels in that direction. The seawater then flows through the heater (the squiggly line through the cloud, which represents steam) where it receives the  necessary heat for the process.

At the outlet of the heater, when entering the bottom of the left-most chamber (the first stage), the seawater is overheated compared to the temperature and pressure of that stage. It will immediately release heat (known as “flashing“), and thus vapor, to reach equilibrium with the conditions in that chamber. The vapor is then condensed into freshwater on the tubes at the top of the chamber. The process takes place again in the next stage, and so on until the last and coldest stage (the chamber on the right end). The freshwater builds up and is extracted from the coldest stage (the blue-colored distillate flow). Seawater slightly concentrates from stage to stage and builds up the brine flow at the bottom, which is also extracted from the last stage.

The state of desalination technology today: comparisons and areas for improvements
No single method of desalination is the “best” choice. Globally, both thermal and membrane technologies are used widely for seawater desalination. Both processes require energy for the separation of salts, and various energy sources can be used. Brackish water is typically desalinated using membrane processes (such as RO, NF, or ED).

The combined energy requirements of thermal technologies are greater than those of membrane technologies, but it is not so simple to compare the total energy use of these very different processes. Thermal processing such as MSF and MED are capable of using waste, or low-grade, heat (such as in cogeneration facilities mentioned above), which can significantly improve the economics of thermal desalination. For example, many of the largest modern cruise ships use the MED desalination process to make freshwater at sea; MED requires 20 to 33 percent of the energy required for RO and the ships’ propulsion engines can provide the required heat.

The major desalination technologies in use today are generally efficient and reliable, but the cost and energy requirements are still high. Ongoing research efforts are aimed at either reducing cost (by powering plants with less-expensive energy sources, such as low-grade heat) or overcoming operational limits of a process (by increasing energy efficiency).

Improvements will be incremental since the current technologies are relatively mature. Ultimately, no desalination process can overcome the thermodynamic limit of desalination, and we’re pretty close to approaching that limit.


Desalination, part I: The challenges of applying ethics in water scarcity

One of the most iconic movie of the 1990’s that foreshadows what the harsh environments of a resource-scarce future may look like is Waterworld. The movie opens with a voice-over narrator explaining that the polar ice caps have melted and the planet is covered by water. The camera pans from an image of Earth into a lone trimaran sailing in a vast, endless sea.

The harsh environmental future of Waterworld. (Video courtesy of YouTube)

Within a few shots, the opening scene has not only established a vibrant image of an extreme and dire future, but has illustrated the conspicuous lack of basic resources that most of us in developed countries take for granted — things such as potable water; land for growing food and raising animals; and means of electricity generation. The male protagonist Mariner, in his post-apocalyptic warrior dress, pees into a small container. He then pours the urine into a rudimentary, homemade filter of funnels and gizmos, and drinks what comes out the other side — a process called desalination.

Drinking one’s own (albeit filtered) urine signals a certain direness under conditions of extreme survival. But desalination — the process by which unpotable water, such as seawater, brackish water, and wastewater, is purified into freshwater for human consumption and use — is not some far-fetched technology we will eventually need in a distant future.

Desalination’s recent global development
Desalination technology has been used for centuries, if not longer, largely as a means to convert seawater to drinking water aboard ships and carriers. Advances in the technology’s development in the last 40 years has allowed desalination to provide water at large scale.

From a global perspective, desalination technology is applied for several purposes: providing freshwater for industrial sectors; supplying drinkable water for the domestic and public sectors; and acquiring water for emergency situations, such as army and refugee operations.

Desalination plays a particularly crucial role in sustaining life and economy in the Persian Gulf. According to Corrado Sommaria, the president of the International Desalination Association (IDA): “Some countries in the Gulf rely on desalination to produce 90 percent or more of their drinking water, and the overall capacity installed in this region amounts to about 40% of the world’s desalinated water capacity.” Much of this is in Saudi Arabia, Kuwait, the United Arab Emirates, Qatar, and Bahrain.

global desalination capacity

Global desalination capacity by country and total capacity. (Image courtesy of Desalination: A National Perspective)

The remaining global capacity is mainly in North America, Europe, Asia (which each have about 15 percent), and North Africa (which has six percent). (A facility’s rated capacity is the full output it is technically capable of, though in reality, it usually produces under that rated value.)  Australia‘s capacity is also increasing substantially. Global desalination capacity has been increasing dramatically since 1960 to its 2008 value of 42 million cubic meters of water daily (m3/day). Of this cumulative capacity, approximately 37 million m3/day is in use. From the above graph, we can see that worldwide desalination capacity more than doubled between 1993 and 2003, and continues to grow steadily today.

Proponents and critics of desalination
Estimates indicate that, by 2025, 1.8 billion people will be living in regions with absolute water scarcity, and two-thirds of the world population could be under stress conditions. Desalinated water is possibly one of the only water resources that does not depend on climate patterns. Desalination appears especially promising and suitable for dry regions.

In one of the country’s biggest infrastructure projects in its history, Australia’s five largest cities are spending $13.2 billion on desalination plants. In two years, when the last plant is scheduled to be up and running, these cities will draw up to one-third of their water from the sea.

Proponents of desalination, like IDA, argue that it sustains population growth, creates jobs, and even supports the development of  energy industries (such as the oil and gas industries in the Middle East). Desalination stops dependence on long-distance water sources and prevents local traditional water sources from being over-exploited. Furthermore, research and development has made great strides in making desalination plants increasingly energy efficient and cost-effective.

However, there are a number of desalination plants worldwide that have been described as uneconomical and unproductive.  Many environmentalists and economists oppose any further expansion of desalination because of its price and effects on the environment. Energy is the most expensive component of running a desalination plant; it is often responsible for one-third to more than half of the cost. Therefore, the cost of desalinated freshwater is more vulnerable to the fluctuation of energy prices than any other water source.

A desalination intake pipeline near Nuweiba, Egypt. (Image courtesy of prilfish)

A desalination intake pipeline near Nuweiba, Egypt. (Image courtesy of prilfish)

Environmentally, desalination plants emits large amounts of greenhouse gas emissions because they are so energy-intense. Furthermore, they degrade marine environments through both the intake and discharge processes. Marine organisms such as invertebrates, fish, and even mammals are killed on the intake screen and smaller organisms, such as eggs, larvae, and smaller fish, that are able to pass through the screen are killed during processing stages. After separating the impurities from the water, the plant discharges the waste, also known as brine, back into the sea. Because brine contains much higher concentrations of salt, it causes harm to the surrounding marine habitat.

In Australia, the mega infrastructure project is drawing fierce criticism and civic protests. Many citizens are angry about rising water bills and environmentalists are wary of the plants’ effect on the climate. Australia relies heavily on coal to generate most of its electricity and is already a major emitter of greenhouse gases — the principle cause of climate change. Ironically, one of the main reasons the country is in need of freshwater is because it’s still recovering from a decade-long drought that the government says was deepened by climate change. Therefore, desalination, which initially appears as an answer for providing freshwater, may in the long run exacerbate the intertwined energy- and water-scarcity cycle.

As scarcity increasingly becomes reality, an appeal to ethics will be challenging
The sentiments of the anti-desalination campaigner in the video below echoes this irony: “It is by a mile the most environmentally-unsound way toward security.” He and other critics say that more environmentally-friendly methods should be exhausted before resorting to desalination. These include mandating more efficient appliances, using less water, or recycling used water.

Australia’s desalination plant provides controversial solution to one of the world’s driest countries. (Video courtesy of Al Jazeera English)

When a society is accustomed to a certain level of access to a resource, it’s hard to ask its citizens to lower their consumption or reuse water based on the argument that it is an ethical choice. In many instances, we observe individual behaviors change in response to policy mandates or market costs. But when can we say that we’ve exhausted all other ways that are less environmentally-damaging? How much should consumption be reduced? How do we decide which water needs are necessary (e.g., water for drinking, agriculture, electricity generation) and which ones aren’t (e.g., water for golf courses) for a certain quality of life?

Waterworld highlights the harsh decisions people face in a scarce-resource future because of the heightened awareness for survival. Pirates raid small pockets of human settlements for resources, they have no qualms about kidnapping a child for the map tattooed on her back, and paranoid atoll residents are willing to kill the Mariner out of distrust. Violence pervades and there is little sense of civility or ethical codes of conduct.

Though the movie is suggested to take place in 2500, it is not hard to imagine that tensions and battling interests over resources will intensify in the not-so-distant future. Making ethical decisions about fair and equal distribution of resources is a challenge today, and will become increasingly more difficult as those resources diminish — even with the most sophisticated of technological developments.