Why Aren’t We Desalinating More Seawater?Water
It is no secret that clean water is one of the most precious resources we have.
Exponential population growth (especially in low-rainfall areas) has contributed to a severe depletion of freshwater and groundwater resources worldwide. According to the U.S. Geological Survey, groundwater supplies in the United States have dwindled over 1000 cubic kilometres in the past century alone, or equal to 400,000 Pyramids of Giza. Important reservoirs worldwide are shrinking and on the verge of drying up, as particularly evident in the rapid depletion of groundwater in Chennai, India. Combined with increased instances of drought and associated knock-on agricultural implications, some would say that we are on the edge of a crisis.
With this huge strain on our freshwater resources, the natural step towards ensuring water security would be to turn to the vast quantities of saltwater found covering our blue planet’s 1.35 billion cubic kilometers of oceans. While groundwater, lakes and rivers make up about 0.6% of Earth’s water, our oceans hold a staggering 97.39% of the total available supply. In this regard, the answer seems obvious.
The question is, can we harness this seemingly limitless supply of saline water to meet the mounting demands of our growing global population? If yes, then how?
Let’s take a deeper look at water desalination- it’s history, it’s challenges, and whether or not we can consider it the next logical step towards providing water security for our thirsty planet.
Looking to the past
Water desalination has existed (albeit in rudimentary forms) since prehistory, when early humans discovered that boiling seawater created vapour that could be captured and cooled into distilled drinking water. It has been referenced in many instances of early history, ranging from a nod in the Bible (Exodus 15:22) to a more comprehensive evaluation of water’s properties when boiled in Aristotle’s treatise Meteorolgica.
In every culture, desalination has been an important part of everyday life on the high seas, where sailors have used distillation to turn seawater into a sustainable source of drinking water aboard seafaring ships.
However, modern industrial desalination has its roots in post-WWII America, when the United States government started funding salt-to-freshwater initiatives as a way to combat increasingly devastating droughts in the American Southwest. Between the 1940s-60s, industrial plants were becoming more common and generally employed thermal (distillation) desalination techniques to produce clean water. By the 1980s, industrial desalination by reverse osmosis (RO) became the norm and is a trend that we continue to see worldwide today.
How does modern industrial desalination work?
There are 2 primary ways in which water is desalinated on an industrial scale: thermal distillation and reverse osmosis (RO).
Thermal Distillation is the oldest and most straightforward method of desalination. Inbound saline water is heated to the point of boiling, after which the steam vapor is collected and recondensed. Due to the heating and cooling processes, distillation is more energy-intensive than other desalination techniques, but advances in technology are finding ways to minimise this energy expenditure. What is produced is pure, clean water- all traces of contaminants are removed.
Reverse Osmosis (RO) is the process by which highly pressurized water is forced through thousands of tubes lined with semi-permeable membranes. While RO in itself can be somewhat more energy-efficient than desalination by distillation, the necessary pre-treatment can be energy-intensive, can use a greater usage of potentially damaging chemicals, and the final product is water that is safe to drink but not to the same purity standards as distilled water.
Seawater is pulled into the system by massive intake pipes deep in the ocean. For this reason, most desalination plants are in close proximity to the coast, although a growing number of inland plants are processing brackish (or slightly salty) water. Intake pipes are covered with a series of rakes and screens to keep out marine life and underwater waste.
Inbound water goes through a number of physical and chemical filtering steps which may vary from facility to facility, but can include filtering through special salt-absorbing sand and injecting air bubbles into specially treated water to separate coarse particles from liquids.
Pre-treated water goes through a distillation or reverse osmosis process, as described above
This clean water then goes through an industrial-grade mineralization process to improve the taste and mineral content before being distributed to homes, businesses, and agriculture.
Where is desalination used today?
At present, there are more communities surviving off of desalinated seawater than you might expect, with upwards of 20,000 desalination plants worldwide servicing over 30 countries across the globe.
While many desalination projects are designed to service smaller, regional needs (think island nations with limited freshwater supplies), large scale industrial desalination is big business in cash-rich, freshwater-poor regions such as the Middle East, Western Australia, and the American Southwest. In certain cases such as in Israel and Saudi Arabia, more than 50% of municipal water supply comes from desalinated seawater, and this number is projected to grow in the coming years.
In the United States, a growing number of desalination plants are being used not just for municipal water, but also as a standby in case of increasingly-common droughts in places like California, Texas, and Florida. Due to the high costs of producing desalinated drinking water, these plants are designed to operate only when freshwater supplies run dangerously low. In these instances, desalination plants are one of the final lines of defense against these crippling droughts.
So, what’s the problem? Why aren’t we all drinking treated seawater?
Given our dwindling reserves of available fresh water and vs. near-limitless supply of seawater, desalination can seem like the natural choice to meet our unquenchable water needs.
However, the truth behind industrial-scale desalination is considerably more complicated, and there are many challenges to overcome before we can view desalination as a viable alternative to freshwater.
The fact is, large scale water purification can be expensive, and no more so than in the formidable process of transforming huge amounts seawater to tap water. While the costs have lowered considerably since the early days of desalination in the 1960s, you can generally still expect desalinated water for a family of 5 water to run into the thousands of dollars per year, or more than double the cost of water from freshwater sources.
This price point is primarily due to the fact that desalination plants are pricey to build (with some of the larger projects costing up to $1B or more), the technologies are still developing, and massive quantities of energy are needed to run the intake and filtration mechanisms, particularly in the case of distillation plants.
A midsize desalination plant which uses the distillation method can burn up to 840 Megawatts of energy per day, or enough to power 30,000 American homes. Much of the developing world simply cannot afford water at this price point, leaving desalination as an option available only wealthy countries.
The price concerns about desalination are not only about the burdensome financial cost, but there are growing concerns about the environmental cost, specifically associated with the waste produced.
A Briney Environmental Situation
Desalination, by its very nature produces massive quantities of salt as a byproduct- but the type of byproduct is dependent on the method used to process the seawater.
Distillation produces a clean, condensed seasalt, which is not too logistically complicated to dispose of in a non-harmful, environmentally-safe manner.
On the other hand, desalination by reverse osmosis produces a staggering amount of liquid waste in the form of brine- an intensely salinated grey sludge. With this method, up to 50% of intake water coming out as briney waste, and the current prevailing method is slow-release dispersal back into the sea via a network of underwater pipes.
This brine is not in itself classified as toxic, but is often mixed with a cocktail of treatment chemicals before disposal, and there are questions being raised about the long-term effects of releasing highly concentrated saline into already-delicate marine ecosystems.
Another environmental point to consider is the extreme quantities of power used to desalinate water. While modern desalination plants are increasingly designed with sustainability in mind, many of the original plants from the 1960s-70s are still in operation today, and may still employ antiquated, inefficient technologies.
From a biodiversity perspective, there are also concerns about current water intake systems, during which smaller marine life and their eggs can be sucked into the pipes and destroy local sea-life populations.
The Future of Desalination
There are certainly a number of bold considerations to be taken into account before desalination can be implemented as the end-all water solution we are seeking. However, we are entering into a shining new era of industrial desalination, in which emerging technologies and sustainable designs are helping to streamline the process and give us a cleaner, more efficient way to produce freshwater from saltwater.
All of us in Team Mitte we are working towards building a machine for your home that reimagines drinking water. Mitte purifies via thermal distillation and is 4-5 times more energy-efficient than any other consumer distiller on the market. Our patent-pending (EU patent n° EP17154125) distillation chamber is based on the principle of a thermoelectric heat pump, driven by two Peltier elements. We have termed the system Peltier-driven-Distillation (PDD).
Other innovations include a device which utilises cutting-edge electrochemical techniques to desalinate water with unprecedented energy efficiency; however products such as these are still in their infancy and in its current form can only produce 720ml (24 fluid oz.), equal to a pint-and-a-half of water, per day.
Plenty of Water for our Thirsty Planet
Presently, desalination of seawater is seen as a compliment to (rather than a replacement for) current water resources. It is currently costly, complicated, and raises environmental concerns over its energy consumption and wasteful byproduct production. It does, however, extend a much-needed lifeblood to communities worldwide who rely on clean water to thrive.
Given the quantity of saltwater on earth, believing that this almost inexhaustible resource could be the singular answer to our freshwater needs is tempting. It is not an easy task, but intrepid new desalination solutions are on their way, and soon you too might be accessing the mighty Pacific through your taps at home.
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