"Water, water, every where,
Nor any drop to drink."
Samuel Coleridge's 1798 poem, The Rime of the Ancient Mariner, laments the Mariner's vast surroundings of undrinkable seawater. Little did Coleridge likely predict that these punishing conditions would affect even the land-dwellers over two centuries later.
An estimate from the United Nations states that 55 percent of the world's population was concentrated in urban centres in 2018, and that figure is likely to rise to 68 percent by 2050. Further to this, reports suggest that many urban centres are at risk of running out of drinking water, including some of the largest cities in the world such as London, Tokyo, and Beijing. Residents in London will likely already be acutely aware of potential water shortages owing to long implementations of hosepipe bans over the last few summers.
Not so much so as in Cape Town, however, which experienced a water crisis between 2015 and 2020 in which it came critically close to "Day Zero", the day on which municipal water would no longer be provided to taps in the city. Whilst Cape Town managed to avoid this unfortunate situation, Mexico City could be the first city to cross this particular "Rubicon", with predictions that "Day Zero" could come as early as this summer.
It is clear that fresh water availability and urban water resilience are becoming critical issues. Whilst an increase in urban population concentrates demand for fresh water, changes in climate are thought to be affecting supply, exacerbating the situation. In response, the COP28 presidency launched its water agenda at World Water Week which took place in Stockholm in August 2023. Additionally, some of the top priority areas of COP28 have been designated as conserving and restoring freshwater ecosystems, enhancing urban water resilience, and bolstering water-resilient food systems.
Fresh water from the ocean
Fresh water for our towns and cities comes from a variety of sources depending on where in the world the demand is located. Typically, a combination of underground aquifers, lakes, and reservoirs serve most urban centres. However, an increasing population is putting more pressure on these resources, and a changing climate is thought to be making these resources much more unpredictable, for example through changes in rainfall and snowmelt. Higher volume and more predictable methods of fresh water production are therefore necessary for sustained availability.
Initially, we can look for solutions in areas of the globe which would typically be considered to be water-scarce and yet harbour large cities and populations. Dubai, for example, would be impossible to sustain on natural fresh water supplies, such as aquifers, alone. In fact, Dubai's existence is made possible only by fresh water production through desalination, with around 90% of its fresh water supply being desalinated seawater from the Arabian Gulf. On paper, desalination sounds like an ideal solution: simply taking water from the most abundant resource on earth, the ocean, and removing the salt to make it drinkable. However, the energy consumption and waste products of current desalination approaches mean that these are an imperfect solution.
Increasing energy efficiency of desalination
Traditionally, desalination plants relied on simple distillation technology; in effect heating seawater to boiling point and collecting the resulting steam as fresh water, while a concentrated salt solution (brine) remained as waste. These systems, however, require a large amount of energy to run, typically in the form of burning gas or diesel for heat. These fuel sources are notoriously carbon intensive and, if implemented as a solution to climate induced water shortages, could end up exacerbating the problem they seek to fix.
However, there has been considerable innovation in distillation technology over several decades, including multi-stage flash distillation, which was filed as a patent application in 1957 (GB831478). This technology reduces the energy requirements of the process somewhat through evaporating water over multiple stages and improving thermal efficiency by use of concurrent heat exchangers. The principle of boiling large quantities of water using fossil fuels, however, was largely unchanged.
One natural solution to this drawback would be to harness renewable energy sources such as solar, wind, and/or tidal to power the desalination, with the latter making particular sense owing to the natural proximity of desalination plants to the ocean. Innovation in desalination technology has acknowledged this, and patents (such as EP2903938) have recently been granted for solar-powered thermal desalination systems.
Further innovation has come through changing the method of operation of desalination plants to use reverse osmosis rather than distillation. These systems utilise high applied pressure to overcome osmotic pressure, effectively pushing salts against their concentration gradient through a semi-permeable membrane, which may additionally remove other chemical and biological impurities. Whilst more efficient than distillation systems, the use of high pressures still requires relatively high energy expenditure. Recently, however, lower-energy forward osmosis desalination systems have been devised which utilise lower pressures than traditional reverse osmosis systems, and promise to require lower energy inputs and to bring higher water recovery. Examples of such systems are found in patent applications such as EP20157536 and EP21706693.
What can be done with the brine?
A further problem with any means of extracting fresh water from seawater that is largely yet to be addressed is what can be done about the waste from the process, namely highly saline brine. This brine may also contain further chemical contaminants, such as cleaning compounds and heavy metals leached from the desalination infrastructure. Returning this brine to the ocean can have adverse effects, particularly at the ocean floor where oxygen levels may be drastically reduced, and the high salt content can create a toxic environment.
The majority of innovation in this area focusses on the valorisation of the waste product, for example by extraction of bulk mineral compounds, such as sodium, magnesium, calcium and potassium salts. Other metals of interest, in particular lithium which is becoming increasingly relevant, can also be extracted (see our COP28 Series article on Lithium here).
This process could be undertaken using vast solar evaporation ponds, as is the case for lithium extraction from geothermal brine, however the time taken is on the order of months or years. New technologies such as nanofiltration can instead be implemented to treat desalination brine, and recover these valuable resources more rapidly. Several patent applications for such technologies have been filed and granted. One recent example is US11884567 which implements nano-filtration and membrane technology to concentrate desalination brine from which desirable materials can be more efficiently recovered.
Conclusion
In order to increase urban water resilience, not only do we need to be more conservative, but we need to find new and reliable sources of fresh water. The world's oceans would appear to be a natural solution for anyone looking for water, but making seawater potable has traditionally been both an energy intensive and environmentally damaging process. Recent innovation looks towards harnessing renewable resources, such as solar energy, reducing energy intensity, and minimising brine discharge back to the oceans.
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