By Meghan Cleary

On a planet covered by 70% water, it’s difficult to understand why an intensifying water availability crisis afflicts so many. In truth, only 0.5% of the Earth’s surface is covered by accessible freshwater, leaving a massive proportion of the world’s water supply seemingly untapped. While ocean water is apparently abundant and underutilized, it faces many challenges on its way to becoming pure water; the most difficult of which is desalination. But in the face of a global water shortage that already causes 1.1 billion people to lack access to water and is projected to impact two-thirds of the world’s population in just four years, the desalination of ocean water is a solution worth exploring.

To understand why investment in technologies to desalinate ocean water is necessary, one must first consider the extent to which the Earth’s freshwater supply is disappearing. The Earth’s already minuscule freshwater supply is diminishing further due to several factors that will only worsen into the future. According to the 2020 UN World Water Development Report, these factors include an increase in demand for freshwater that is growing at twice the rate of global population growth because of the excessive water requirements of industrial and commercialized agricultural processes. The expansion of commercialized agriculture and industrial processes not only creates a demand for water that exceeds the current supply, but it also perpetuates and exacerbates anthropogenic contributions to climate change that further reduce the supply of freshwater.

Climate change has many effects on local and temporal freshwater supplies. Melting glaciers and extreme precipitation events in tropical areas caused by climate change may increase local freshwater availability in the short term, eventually overwhelming and degrading streams leading to freshwater shortages in other seasons. Drier and hotter air in the midlatitudes resulting from climate change will reduce the flow of freshwater through streams, further depleting freshwater in those regions. As the effects of climate change intensify and the world’s population continues to increase, freshwater supplies will only become scarcer. Therefore, it is crucial to consider desalination methods as potential ways to tap into the massive saltwater supply on Earth.

One of the primary mechanisms by which seawater is desalinated is reverse osmosis. In reverse osmosis, saltwater is on one side of a semipermeable membrane, and pure water is on the other. Pressure is then applied to the saltwater side to counteract the natural osmotic pressure from the pure waterside and push water through the pores in the semipermeable membrane to the pure water side. Large salt particles cannot fit through the semipermeable membrane and are trapped on the saltwater side, thereby ridding the water of salt. At the local or regional scale, reverse osmosis can decontaminate wastewater or desalinate seawater to produce potable drinking water. It’s relatively simple design among many other key advantages make reverse osmosis an important contender in the hunt for the best desalination technique; however, reverse osmosis is also subject to a number of limitations that need to be examined before declaring it the best desalination technique.

As global freshwater supplies continue to decrease and billions are left without reliable drinking water, the desalination of seawater by reverse osmosis seems like the perfect solution to an existential threat. In addition to its capacity to tap into a vast water supply, reverse osmosis has many advantages that set it apart from other water purification techniques. Compared to other desalination methods, reverse osmosis yields the most drinking water. Other methods, including the boiling of seawater to collect and condense pure steam, require up to three times as much seawater as they produce. In contrast, the most efficient reverse osmosis systems can achieve yields of over 90 percent. Since reverse osmosis does not require a phase change, it is relatively energy efficient. In addition to the reduced operating costs of a reverse osmosis system that result from its ability to efficiently use energy, the initial investment in a reverse osmosis system is relatively low. Reverse osmosis requires a modular system that is easily installed and inexpensive compared to other evaporative desalination systems.

While reverse osmosis may seem like the most efficient and economical choice of desalination method when examining energy use and output alone, it has several important drawbacks that must be considered. Before saltwater can undergo reverse osmosis, it must be pretreated with chemicals to protect the semipermeable membrane from significant degradation or “fouling.” This pretreatment adds capital costs to the reverse osmosis operation in skilled labor, expensive chemicals, and time. Even with pretreatment making the feed solution more compatible with the membrane, solids will still accumulate on the membrane surface, thereby reducing the membrane’s capacity to filter out particulates and produce safe drinking water. This inevitable membrane fouling further reduces the efficiency and increases the cost of operating a reverse osmosis system, with the membrane’s permeability decreasing and the membrane itself having to be replaced or restored. Although new technological developments are making reverse osmosis systems more energy-efficient, the energy cost of these systems remains high and is especially sensitive to greenhouse gas emissions limits and energy prices. Once water has been pretreated and fed through a reverse osmosis system, the leftover residue of pretreatment and brine that were not passed through the membrane must be disposed of. These waste products of reverse osmosis pose a serious environmental threat to marine and terrestrial life if disposed of improperly. They require specialized and complicated disposal procedures such as discharge through submarine outfalls.

Overall, reverse osmosis has the potential to meet the mounting demand for a new way of obtaining potable drinking water. The incomparably pure water reverse osmosis produces and the modular systems that would allow it to be installed and mainly employed within existing water distribution infrastructures make it a viable option to supply drinking water at a larger scale in the future. However, costly pretreatment, membrane maintenance, and complicated waste disposal requirements serve as significant barriers to reverse osmosis’s wide-scale implementation. This desalination technique’s remarkable potential should inspire new research into ways of overcoming these limitations and making reverse osmosis one of the leading paths forward in achieving sustainable potable water.

References

American Membrane technology Association. (2010). Pretreatment for Membrane Processes. https://www.amtaorg.com/wp-content/uploads/12_Pretreatment.pdf

Arnell, A.W., Benito, G, Cogley, J.G., Döll, P, Jiang, T, Mwakalila, S.S. (2018). Freshwater Resources. International Panel on Climate Change. https://www.ipcc.ch/site/assets/uploads/2018/02/WGIIAR5-Chap3_FINAL.pdf

Bureau of Reclamation. (2020, November 4th). Water Facts – Worldwide Water Supply. https://www.usbr.gov/mp/arwec/water-facts-ww-water-sup.html

Joss A, Baenninger C, Foa P, Koepke S, Krauss M, McArdell CS, Rottermann K, Wei Y, Zapata A, Siegrist H. ( 2011). Water reuse: >90% water yield in MBR/RO through concentrate recycling and CO2 addition as scaling control. Pub Med. https://pubmed.ncbi.nlm.nih.gov/21959090/

Kershner, K. (2011). How Reverse Osmosis Works. How Stuff Works. https://science.howstuffworks.com/reverse-osmosis.htm#pt5

Koncagül, E, Tran, M, Connor, R. (2020). The United Nations World Water Development Report: water and climate change facts and figures. UNESCO World Water Assessment Programme. https://unesdoc.unesco.org/ark:/48223/pf0000372876.locale=en

Lazarides H.N.,Katsanidis E. (2003). Membrane Techniques: Principles of Reverse Osmosis. Encyclopedia of Food Sciences and Nutrition. https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/reverse-osmosis

Nicholas, N. (2019, August 29th). Pros and Cons of Seawater Desalination Using RO for Drinking Water. Water Online. https://www.wateronline.com/doc/pros-and-cons-of-seawater-desalination-using-ro-for-drinking-water-0001

Santos Sanchez, A. (2018). Desalination Concentrate Management and Valorization Methods. Sustainable Desalination Handbook, pp. 351-399. https://www.sciencedirect.com/science/article/pii/B9780128092408000095

Sarai Atab, M, Smallbone, A.J., Roskilly, A.P. (2016, November, 1st). An Operational and Economic Study of a Reverse Osmosis Desalination System for Potable Water and Land Irrigation. Desalination, Volume 397, pp. 174-184. https://www.sciencedirect.com/science/article/pii/S0011916416306658

World Future Council. (2012, October 21st). How Does Agriculture Contribute to Climate Change? https://www.worldfuturecouncil.org/how-does-agriculture-contribute-to-climate-change/

World Wildlife. Overview. https://www.worldwildlife.org/threats/water-scarcity

Can the Desalination of Ocean Water by Reverse Osmosis Solve the Freshwater Availability Crisis?