Is desalination the answer to clean water crisis?

On a planet where 71% of its surface is covered by water, the unfortunate fact is that there is lack of clean, safe, potable water. This global issue is particularly concerning given that the population of the planet is set to increase to 9 billion by 2050. Currently, around 40% of the world’s population is already affected to some degree by water scarcity and a whopping 1.1 billion people still do not have access to safe water.

Fresh water sources

Planet Earth is estimated to hold 1.4 billion km³ of water¹. Of this, 97% or 1.35 billion km³ is saline water held in the world’s oceans, with 3% or 42 million km³ being fresh water. Of this, 35 million km³ (or 81% of all fresh water) is locked in the ice caps and glaciers. Ground and surface water, from which most of the world’s population derives its drinking water, accounts for some 8 million km³ (only 19% of all fresh water and 0.6% of all water on earth).

We can add to this a further 13,000 km³ of water in the form of water vapour and clouds in the atmosphere. This constantly changes, with precipitation moving the water around the planet driven by weather patterns and, moreover, by climate change. As climate change worsens, the atmospheric water content will rise.

The point here is that the amount of water on Earth is finite and, although the planet is very good at recycling the water we already have, there are no other water resources to draw from. Once it has gone, it’s gone for good. This begs the question, should we be looking elsewhere for our drinking water? Could the answer lie in the sea?

Saline solution

Desalination of seawater is a process that removes the dissolved minerals that make the water salty. We know it better as saline water. Desalination is actually a process that happens naturally during the rainwater cycle when water evaporates from the oceans leaving the salts and minerals behind to form clouds. It was first observed by the ancient Greek philosopher Aristotle and later by Leonardo de Vinci, who successfully removed the salt from seawater using a still.

On the surface of it, desalination to produce potable water seems like a great idea. However, the main issue is the cost of a desalination plant and the fact that some methods use significant amounts of energy.

The desalination processes

There are around 18,000 desalination plants operating in around 180 countries around the world. Some of these countries, such as the United Arab Emirates and Saudi Arabia, are almost totally dependent on desalination for their water supply. Saudi Arabia is one of the driest countries on Earth with an average annual rainfall of just 150mm².

So, how are the minerals removed? There are a number of processes used:

• Solar distillation – Otherwise known as the ‘man-made rainwater cycle’, solar distillation mimics the Earth’s natural hydrological cycle by using the sun to evaporate seawater in a controlled environment such as large containers. Here, the evaporated water is collected as fresh water. The main problem with this is that large areas of land are needed for the containment structures.
• Reverse osmosis – This process uses a semi-permeable membrane that allows the water to pass through but not the minerals it contains. The membranes are extremely thin and liable to bacterial contamination, so any water desalinated in this way must be treated before use. It is, however, the system that uses the least energy, so it is a very popular method.
• Electrodialysis – This involves passing water through membranes that are electrically charged. The membranes simply trap the salt ions that the water contains, allowing the fresh water to flow through, ready to be removed for use.
• Nanofiltration – Carbon nanotubes or CNTs are either single, open-ended tubes, or a film comprising many CNTs that are arranged in a similar way to a honeycomb structure. These can be used for many applications, including seawater and brine desalination, the removal of heavy metals and even the removal of organic pollutants.
• Gas hydrate formation – Gas hydrates are solid crystalline forms that are made by combining seawater with natural petroleum gases such as methane and propane. This process, conducted at high pressure and low temperature, removes all salts and other impurities and leaves behind the fresh water.

Pros and cons

Desalination seems an ideal solution to the water shortage problem. By using the abundant seawater already available, we can at least make inroads into providing clean water for all. However, desalination is not without risk. The processes mentioned here can produce highly salted brines, solutions and wastewater products that are in themselves dangerous to the planet's fragile marine eco systems. In some instances, at present, these highly salted solutions are simply discharged directly back into the sea making the seawater even saltier. There is also the risk of seepage from desalination processes affecting near-coastal fresh water aquifers, a problem that is already affecting some coastal aquifers around the world because of over-extraction of groundwater³.

Despite these problems, desalination is a process by which the water shortage problems of the world could be eased, but it needs investment on a massive scale to deliver results for some of the poorest nations on earth who just do not have the money to invest. It may even fall on the richer nations of the world to help in this regard and really make a difference.

References

1. https://www.usgs.gov/science/faqs
2. https://climateknowledgeportal.worldbank.org/country/saudi-arabia/climate-data-historical
3. Groundwater – a threatened resource – UNEP Environment library no. 15 (1996)