Desalination - is it a viable solution to reduce water stress?
Addressing the challenge of ensuring everyone has access to reliable and affordable energy and water is a core driver for our work at sustain:able.
Women and children are often disproportionately affected by lack of access to these essential resources.
As mums to young families, we are passionate about working with companies to deliver a better future for our children and grandchildren, where they won't need to worry about energy or water security.
When you hear that water stress is predicted to increase in the future, like us, you might think that desalination can be used to meet that demand.
Population growth and increasing stress on freshwater resources have led to an increased focus on desalination. Freshwater comprises only 3% of the global water budget, and desalination is seen as a key technology to unlocking further resources.
As of 2020, there were more than 20,000 desalination facilities in 150 countries producing more than 100 billion litres of water a day to over 300 million people.
But this is just a fraction of the future predicted global demand as water stress worsens, so we looked into whether desalination can be scaled-up to meet this demand without creating a host of new problems.
What is desalination?
Desalination is the process of removing dissolved salts and other contaminants from water to produce potable fresh water. Source waters for desalination can include brackish groundwater, domestic wastewater, seawater, process wastes and brines, water from hydraulic fracturing and fossil fuel production as well as wastewater recycling.
How is it done and where is it being done?
Thermal desalination and reverse osmosis are the two common methods for processing saline water.
Thermal desalination involves heating the source water before capturing the resulting water vapor against a surface and draining it into a collection vessel. This process accounts for roughly 25% of globally produced desalinated water and has a freshwater recovery rate of between 25% to 50%.
Reverse osmosis involves forcing the source water through fine membranes that prevent salt and other contaminates from passing through. Reverse osmosis accounts for around 69% of the world’s desalinated water and has a freshwater recovery rate of up to 85%.
Several countries, such as Bahamas, Maldives, Bahrain and Qatar already meet all their water needs through desalination, while Saudi Arabia (population 34 million) gets about 50% of its drinking water from desalination.
Currently, 70% of the world’s desalination capacity is in the Middle East, while North Africa (predominantly Algeria and Libya) accounts for a further 6%. Australia and Israel have also heavily invested in desalination, with Israel aiming to meet more than 50% of its water needs via desalination.
How is the waste (brine) disposed of?
Every day more than 100 billion litres of concentrated brine are produced globally.
The vast majority is pumped back out to sea, with care taken to reduce the environmental impact.
According to best practice, brine should be released in a strong sea current where possible so that it is readily mixed and diluted by the surrounding sea water.
Other alternatives include the use of diffusers along the brine outflow pipe or diluting the brine with either fresh water, wastewater or seawater prior to ejection.
In some areas waste brine is pumped into wellbores, although this is a less common practice.
Environmental, energy and cost challenges
There are multiple challenges associated with desalination and continued debate on the environmental impacts resulting from the process.
Concentrated brine has an increased density compared to seawater, so if not adequately dissipated it will sink and accumulate on the sea floor. An increase in salinity leads to a decrease in the amount of dissolved oxygen in the water which negatively impacts marine life. Thermal desalination plants also need to be conscious of the temperature of the discharged brine as it can be considerably warmer than the surrounding sea water.
Although power consumption for desalination has dramatically reduced in recent years, it is still an energy-intensive process. In most regions fossil fuels are used to generate the electricity required for desalination, which has led to criticism given the link between fossil fuels and climate change. An increase in global temperatures would worsen aridification, requiring more desalinisation and creating a dangerous positive feedback loop.
Furthermore, the desalination process often requires various chemicals which, if poorly regulated, can end up in the wastewater, impacting the area surrounding an outflow pipe for tens to hundreds of metres, as well as having the potential to be concentrated along the food chain.
The final environmental concern is in regard to the extraction of seawater. When sea water is extracted from the surface, an intake screen is used to try and prevent marine life being sucked into the supply. However, fish and other sea life can become injured or killed when sucked onto the screen and very small organisms such as larvae, eggs and plankton are killed when drawn into the desalination plant.
Reducing energy consumption for desalination plants has been a focus for some time. According to the IDA, development of new energy recovery devices, high pressure pumps, and membranes could bring the total energy use of desalination plants to less than 2.5 kWh/m3 from its current levels of 3.5 - 4.5 kWh/m3.
Part of this is due to a shift away from the thermal methods of desalination, to the electrically-driven reverse osmosis method. Although there are still improvements to be made:
state-of-the-art reverse osmosis desalination plants give rise to >1 kg of CO2 and >1 m3 of brine for each cubic meter of freshwater produced (University of Birmingham Dubai).
Innovative research is being carried out on membrane technology for the reverse osmosis method, including chemical-free methods, and use of nano-technology and graphine, which is hoped to improve efficiency of the desalination process.
Research is also ongoing to find beneficial uses of the brine concentrate, such as mineral extraction technologies, which can then be used to offset the costs for production of desalinated water. However, while this may offset the cost, the environmental impacts of treatment and disposal of the brine is still a challenge to be carefully managed.
Desalination is already being used to alleviate water stress, but in order for it to sustainably scale up to meet predicted future demand, developments in energy usage, efficiency, water withdrawal and brine disposal is needed. It is heartening to find so much research and development effort is currently going into tackling these challenges.
This has highlighted to us the critical importance of water conservation and good stewardship of any water resources we may impact, both in our personal and professional lives.
Do you have questions about the relationship of energy and water? Are you operating in areas where there are opportunities to enhance water supply and sanitation?
Yale 360: Accessed 17th February 2022
USGS: Accessed 17th February 2022
Water Tech Online: Accessed 17th February 2022
MIT: Accessed 17th February 2022
National Geographic: Accessed February 17th 2022
Scientific American: Accessed 17th February 2022.
UNEP: Accessed 1st March 2022.
International Desalination Association (IDA): Accessed 1st March 2022.
University of Birmingham Dubai: Accessed 1st March 2022.