Desalination: can it solve water scarcity on Greece's islands?
Desalination turns seawater, which never runs out, into drinking water. The price is energy and brine. The real question is not whether it works, but when it is worth it.
Desalination: what it is and how it works
Desalination removes salt from seawater or brackish water so that it can be used. The dominant technology today is reverse osmosis (RO). According to a review in Frontiers in Sustainable Cities (Schunke et al., 2020), high-pressure pumps push seawater through a semi-permeable membrane. To overcome the osmotic pressure of seawater, the pressure is usually raised to 55–70 bar. Fresh water comes out on one side. On the other comes a dense, salty residue: brine.
The same review says reverse osmosis accounts for 65% of desalination production capacity worldwide and is the dominant form of large-scale seawater desalination. In a single pass through the membranes, it turns up to about 50% of the incoming water into fresh water.
The global scale is already large. A 2019 study in Science of the Total Environment (Jones et al.) counted 15,906 desalination plants in operation, producing about 95 million cubic metres of water a day. Almost half of it is produced in the Middle East and North Africa.
Why desalination is on the table now: seasonal water scarcity
Water scarcity is not just a lack of rain. The European Environment Agency (EEA) measures it with the water exploitation index WEI+, which is water use as a share of renewable freshwater resources. Above 20%, water scarcity conditions are considered to prevail. Above 40%, the stress is severe and use may be unsustainable. So scarcity has two causes at once: how much water we use, and how much is renewed.
The EEA reports that Greece, together with Romania, Portugal, Italy and Spain, had water scarcity mainly in spring and summer 2023. In general, scarcity intensifies between April and September. In 2023, water scarcity affected 28% of EU territory for at least one season. According to the European Commission, it affected 38% of the EU population in 2019.
The picture for Attica and EYDAP's reservoirs is in our article on water shortage in Greece. Here we focus on one of the proposed solutions.
Desalination energy use and cost: what we know
Desalination needs a lot of energy, and this is its main drawback. The table compares the values given in the Schunke et al. (2020) review, in kilowatt-hours per cubic metre of water (kWh/m³).
| Water source | Energy (kWh/m³) |
|---|---|
| Conventional surface water treatment | 0.2–0.4 |
| Indirect potable reuse | 1.5–2.0 |
| Seawater reverse osmosis | 3.5–4.5 (range 3–6.7) |
In other words, using the typical values in the table, a cubic metre of desalinated water takes about 9 to 20 times the energy of a cubic metre of surface water. The range depends on the feed water, the required quality and the plant's efficiency. High-pressure pumps and membranes take about 65% of the energy. Energy recovery devices, which reuse the pressure of the outgoing brine, can cut consumption by 25–40% compared with systems without them.
At scale, this matters. The International Energy Agency (IEA) expects desalination's share of total final energy consumption in the Middle East to rise from 5% to almost 15% by 2040. Globally, desalination is the largest source of growth in the water sector's energy use to 2040.
On cost per cubic metre, we did not find an open primary source with reliable figures for the Greek islands, so we give no number. What the literature says is qualitative. Schunke et al. write that seawater desalination is less cost-effective than other options for drinking water, and that pump energy weighs heavily on operating costs. If the power comes from fossil fuels, there is also an emissions cost: the same review reports that desalination in Australia emitted 1,193 thousand tonnes of CO₂ equivalent in 2015. Renewable energy is listed as one way to reduce that problem.
Brine: what happens to the leftover salt
Every desalination plant produces brine, which usually goes back to the sea. Jones et al. (2019) estimated global brine production at about 142 million cubic metres a day, roughly 50% more than earlier estimates. Four countries (Saudi Arabia, the UAE, Kuwait and Qatar) produce 55% of it. Dividing the study's two totals gives about 1.5 cubic metres of brine for every cubic metre of fresh water. That is our own calculation, not a figure from the study, and it varies from plant to plant.
The authors conclude that better brine management is needed to limit environmental harm and disposal costs. Research also looks at ways to dilute brine before discharge, to reduce the impact on marine life. For an island, the practical questions are where the outfall goes, how much the brine is diluted, and who monitors it.
Water scarcity solutions beyond desalination
Desalination is not the only answer. EU policy sets the priority as a water-efficient, water-saving economy. The recast Drinking Water Directive addresses leakage in supply networks. A 2020 regulation makes it easier to reuse water for irrigation. The European Climate and Health Observatory notes that Greece is one of only a few EU countries with drought management plans for some or all of its river basins.
A sensible order of options, as we read it, is:
- Cut demand through savings in homes, hotels and farming.
- Fix leaks, so that water already pumped and treated is not lost.
- Reuse treated wastewater, for irrigation or other uses, at an energy cost between surface water and desalination.
- Reservoirs and groundwater, where they exist and can take it.
- Desalination as a supplement for peaks, mainly where the other options fall short.
This order is not a rule. On a small, dry island, desalination may be the only realistic source. What matters is that every option is compared on the same terms: water gained, energy, emissions, brine and time to build.
How a scenario model compares the options
A scenario model does not "predict" water scarcity. It answers "what if" questions. What if the winter is dry and summer demand rises? How much water do we gain by cutting leaks, compared with a new desalination plant? How much extra power does the island need in August? The difference between scenario analysis and what-if analysis is explained in our article on scenario analysis.
Kassandra Labs is at concept stage. We have not run such a model for any island and have no results to show. What we describe is the method: every option on the same table, with the same data, and with the uncertainties in the open.
Frequently asked questions
What is desalination?
Desalination removes salt from seawater or brackish water so it can be used for drinking or other purposes. The most common method is reverse osmosis, where high-pressure pumps push water through a membrane. According to a 2020 review, it accounts for 65% of global desalination capacity. The salty residue left over is called brine.
How much energy does desalination use?
Seawater reverse osmosis typically needs 3.5–4.5 kilowatt-hours per cubic metre, within a range of 3–6.7 depending on the plant. Conventional surface water treatment needs 0.2–0.4. Energy recovery devices can cut consumption by 25–40%. These figures come from the review by Schunke et al. (2020) in Frontiers in Sustainable Cities.
Is desalination brine harmful to the sea?
Brine is saltier than seawater and, if it is not diluted properly, it can affect marine life. A 2019 study estimated global brine production at about 142 million cubic metres a day and called for better brine management. The local impact depends on where the outfall is placed, how well the brine is diluted, and how it is monitored.
What are the main solutions to water scarcity?
EU policy puts water efficiency and saving first. Other measures include cutting leaks in supply networks, reusing treated wastewater for irrigation, reservoirs and desalination. No single solution is enough on its own. The right mix depends on the place, the available energy and the pattern of demand, which is why options should be compared side by side.
Sources
- Schunke A.J., Hernandez Herrera G.A., Padhye L., Berry T.-A., "Energy recovery in SWRO desalination: current status and new possibilities", Frontiers in Sustainable Cities 2:9, 2020
- Jones E., Qadir M., van Vliet M.T.H., Smakhtin V., Kang S., "The state of desalination and brine production: a global outlook", Science of the Total Environment 657, 2019
- IEA, "Introduction to the water-energy nexus", 23 March 2020
- European Environment Agency, "Use of freshwater resources in Europe" (indicator, published 28 Nov 2025)
- European Commission, Water scarcity and droughts
- Climate-ADAPT / European Climate and Health Observatory, "Ξηρασία και λειψυδρία" (2024)