Never one to let a big idea pass him by, President John F. Kennedy signed an Appropriations Act into law in 1961 to pay for research and development into the extraction of fresh water from saline and brackish water. "There is nothing, really, that we can do in this country that can mean more in the long run to our people and to people all around the world than to be able to make an important and significant breakthrough in this area," said Kennedy. Sixty years later, in the US and globally, safe, energy-efficient desalination remains a key aspiration of governments.
Critical to the technology’s success is the need to reduce the energy expended on desalination, as well as to identify safe disposal or even commercial exploitation of brine as a by-product. There are many forms of inorganic matter besides sodium in brackish and seawater. Some can have commercial value. Others simply clog up the equipment. Most extraction of drinking water is in large plants. Smaller desalination centers powered by renewable energy, such as solar or geothermal, struggle to be energy efficient. Yet, such desalination plants could have an important part to play in rural areas where freshwater is scarce but brackish aquifers are present.
Patent Granted to Energy-Efficient Desalination Technology
What holds solar-powered desalination back is the low energy density in illuminations from the Sun. Ingenious efforts are underway to reduce the effects of that natural limitation. For example, in 2020 a US Patent was granted for a solar-powered method of separating a single cold fluid into at least two flows in an energy-efficient way. Clearly, that type of process could treat seawater to produce drinking water and brine. This patent goes beyond similar earlier efforts of solar-powered desalination.
A significant part of the originality in this patent is that the cold seawater is preheated before entering a treatment device by flowing beneath solar panels, while simultaneously cooling those panels. That water then enters a treatment device from which two flows emerge. In a desalination plant, those two flows would be drinking water and brine. In the device described, these two fluids flow back beneath the solar panels through two sets of extensive piping over which the incoming cold water flows.
The result is that the incoming seawater is preheated in three ways: by the solar panels and by the separated but warmer desalinated and fresh water in the pipes. Because of the preheating by three heat sources, the treatment device needs less energy than it would otherwise, and the solar panels are cooled, which further improves their efficiency. The patent was granted to Desolenator BV, a Dutch company. The inventor is Wilhelmus Jansen from Abu Dhabi, United Arab Emirates.
Water Quality Varies Drastically, Water Insecurity is Widespread
The scale of the problem which desalination can contribute to solving is huge. In 2021, twenty-six percent of the world’s population, some 2 billion people, still lack access to safely managed drinking water. In 2020, 771 million people did not have even basic water services. Remedying that situation by 2030 is one of the United Nation’s seventeen Sustainable Development Goals. Unfortunately, the world is not on course to meet the safe-water goal.
To achieve its aim, the UN says a four-fold increase in the rate of implementation of water projects is needed, as well as more funding. Importantly, too, the impact of each development goal on the others (energy, food, transport, among others) needs to be assessed. Fresh water is needed for drinking, washing, agriculture, industry, and commerce. For every case, rigorous assessment of water’s organic content and inorganic solutes is important.
Freshwater contains fewer than 1000 milligrams of salt per liter, while oceans have on average 35,000 milligrams per liter. Between the two extremes lie brackish waters in aquifers and surface water. Some brackish aquifers are not renewable sources, and scientists still do not know the source of all freshwater aquifers. As water is drawn from brackish sources, they become saltier. Surface freshwaters are found in rivers, sea ice, lakes, reservoirs, wetlands, creeks, and human-made canals. That water is part of the hydrological cycle. Much research remains to be done to understand this complex interconnected set of water resources.
Desalination Plants are Growing in Number Throughout the World
In the meantime, even with imperfect knowledge, nations are striving to provide safe water. In 2014, there were 16,000 desalination plants in the world, predominantly in Africa and the Middle East. In the intervening seven years, global and regional efforts have kicked in to address freshwater shortages, leading to significant increases in the number of plants. Their capacity ranges roughly from 6 gallons per day to 25 million gallons per day.
Although commercial and industrial desalination plants, even small ones, are technically sophisticated, the scientific principles they rely on are well known. Distillation is one key process. Going back to ancient times, it was the first way people extracted usable water from seawater. In essence, brine is left behind when plain water evaporates from heated salty water. The evaporated vapor is condensed to provide drinking water.
As knowledge of thermodynamics developed in the 19th century, the methods used in desalination plants to evaporate and condense water under different pressures increased in sophistication. Even modern methods of evaporation and condensation used in today’s desalination plants are open to further advancement. The downside is that energy is lost during phases changes from liquid to vapor and back, which means quite a lot of energy can be wasted.
One way of characterizing a desalination plant is to ask how many units of drinking water result from the amount input. That ratio gives the important parameter of the recovery rate. For example, extracting 300 units of drinking water from 900 units of seawater gives a 33.3% recovery.
Reverse osmosis is an alternate approach to producing potable water. In osmosis water flows through a semipermeable membrane into a saltier environment. In reverse osmosis, sufficient pressure is applied to overcome osmosis, and water is forced from brine through a membrane which prevents salts from following. Clogging the membrane with inorganic salts can be a problem. Electrodialysis has also been applied for desalination. In this method salts are left behind on charged membranes as water flows through, thereby purifying the water of salts. The method works with brackish water but not with more saline ocean water.
Desalination is one of many water purification technologies that needs to be employed if Earth’s abundant water is to be an asset in the fight to provide fresh water to everyone. Now the dilemma is where and how to apply these technical resources for the best outcome for people and their environments. This issue requires input from geologists, hydrologists, and other scientists, as well as political will and economic support from political leaders across the world. Ultimately, we need to protect our water resources and get safe, clean drinking water to where it is needed.
*Helen Gavaghan is a freelance journalist, and the founder, editor, and publisher of Science, People & Politics (ISSN 1751598X). She has written and edited for the major international science press and intergovernmental organizations, among others. Helen's books include the first official history of the European Organisation for the Exploitation of Meteorological Satellites and a history of application satellites, praised in Nature for bringing new material to the published literature. She has traveled widely as a reporter and lived and worked in Eindhoven, the Netherlands; London, United Kingdom; and Washington, D.C., in the United States. Helen now works and travels from West Yorkshire, UK.